CHAPTER 9 SEISMIC DESIGN FOR RAILWAYThis document provides a framework of considerations and...

© 2018, American Railway Engineering and Maintenance-of-Way Association 9-i

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CHAPTER 9

SEISMIC DESIGN FOR RAILWAY

STRUCTURES1

TABLE OF CONTENTS

Part/Section Description Page

1 Seismic Design for Railway Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-11.1 Introduction (2004) R(2012) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-31.2 Post-Seismic Event Operation Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-31.3 General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-81.4 New Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-321.5 Existing Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-481.6 Other Facilities and Infrastructure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-501.7 Construction by Others (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-541.8 Retired Facilities (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-54

2 Commentary to Seismic Design for Railway Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-1C - Section 1.2 Post-Seismic Event Operation Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-2C - Section 1.3 General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-5C - Section 1.4 New Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-12C - Section 1.5 Existing Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26C - Section 1.6 Other Facilities and Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26

Chapter 9 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-G-1

Notations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-N-1

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-R-1

1 The material in this and other chapters in the AREMA Manual for Railway Engineering is published as recommended practice to railroads and others concerned with the engineering, design and construction of railroad fixed properties (except signals and communications), and allied services and facilities. For the purpose of this Manual, RECOMMENDED PRACTICE is defined as a material, device, design, plan, specification, principle or practice recommended to the railways for use as required, either exactly as presented or with such modifications as may be necessary or desirable to meet the needs of individual railways, but in either event, with a view to promoting efficiency and economy in the location, construction, operation or maintenance of railways. It is not intended to imply that other practices may not be equally acceptable.

© 2018, American Railway Engineering and Maintenance-of-Way Association

9-ii AREMA Manual for Railway Engineering

INTRODUCTION

The Chapters of the AREMA Manual are divided into numbered Parts, each comprised of related documents (specifications, recommended practices, plans, etc.). Individual Parts are divided into Sections by centered headings set in capital letters and identified by a Section number. These Sections are subdivided into articles designated by numbered side headings.

Page Numbers – In the page numbering of the Manual (9-2-1, for example) the first numeral designates the Chapter number, the second denotes the Part number in the Chapter, and the third numeral designates the page number in the Part. Thus, 9-2-1 means Chapter 9, Part 2, page 1.

In the Glossary and References, the Part number is replaced by either a “G” for Glossary or “R” for References.

Document Dates – The bold type date (Document Date) at the beginning of each document (Part) applies to the document as a whole and designates the year in which revisions were last made somewhere in the document, unless an attached footnote indicates that the document was adopted, reapproved, or rewritten in that year.

Article Dates – Each Article shows the date (in parenthesis) of the last time that Article was modified.

Revision Marks – All current year revisions (changes and additions) which have been incorporated into the document are identified by a vertical line along the outside margin of the page, directly beside the modified information.

Proceedings Footnote – The Proceedings footnote on the first page of each document gives references to all Association action with respect to the document.

Annual Updates – New manuals, as well as revision sets, will be printed and issued yearly.

© 2018, American Railway Engineering and Maintenance-of-Way Association 9-1-1

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Seismic Design for Railway Structures1

— 2018 —

TABLE OF CONTENTS

Section/Article Description Page

1.1 Introduction (2004) R(2012) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-3

1.2 Post-Seismic Event Operation Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-31.2.1 General (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-31.2.2 Guidelines (2017). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-3

1.3 General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-81.3.1 Approach (2004) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-81.3.2 Ground Motion Levels (2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-91.3.3 Performance Criteria (1998) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-32

1.4 New Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-321.4.1 Scope (2004) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-321.4.2 Design Approach (2001) R(2011) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-321.4.3 Conceptual Design (2001) R(2011) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-331.4.4 Structure Response (2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-341.4.5 Analysis Procedures (2003) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-391.4.6 Load Combinations and Response Limits (2002) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-411.4.7 Detailing Provisions (2001) R(2011) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-42

1.5 Existing Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-481.5.1 Scope (2012) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-481.5.2 Inventory (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-481.5.3 History (1995) R(2012) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-481.5.4 Assessment and Retrofit (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-49

1.6 Other Facilities and Infrastructure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-501.6.1 Scope (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-501.6.2 Inventory (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-511.6.3 Track and Roadbed (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-511.6.4 Culverts (2013) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-521.6.5 Retaining Walls (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-521.6.6 Tunnels and Track Protection Sheds (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-52

1 References, Vol. 94, 1994, p.110; Vol. 96, p. 64, Vol. 97, p. 113.

Seismic Design for Railway Structures


9-1-2 AREMA Manual for Railway Engineering

TABLE OF CONTENTS (CONT)


1.6.7 Buildings and Support Facilities (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-531.6.8 Utilities, Signal and Communication Facilities (2017) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-531.6.9 Rail Transit (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-54

1.7 Construction by Others (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-54

1.8 Retired Facilities (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-54

LIST OF FIGURES

Figure Description Page

9-1-1 100-year Return Period, Peak Ground Acceleration - United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-149-1-2 100-year Return Period, 0.2 Second Period Spectral Response Acceleration - United States. . . . . . . . . . . 9-1-169-1-3 100-year Return Period, 1.0 Second Period Spectral Response Acceleration - United States. . . . . . . . . . . 9-1-189-1-4 475-year Return Period, Peak Ground Acceleration - United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-209-1-5 475-year Return Period, 0.2 Second Period Spectral Response Acceleration - United States. . . . . . . . . . . 9-1-229-1-6 475-year Return Period, 1.0 Second Period Spectral Response Acceleration - United States. . . . . . . . . . . 9-1-249-1-7 2475-year Return Period, Peak Ground Acceleration - United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-269-1-8 2475-year Return Period, 0.2 Second Period Spectral Response Acceleration - United States. . . . . . . . . . 9-1-289-1-9 2475-year Return Period, 1.0 Second Period Spectral Response Acceleration - United States. . . . . . . . . . 9-1-30

LIST OF TABLES

Table Description Page

9-1-1 Specified Response Radii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-49-1-2 Damage Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-49-1-3 Seismic Performance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-89-1-4 Ground Motion Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-99-1-5 Weighting Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-119-1-6 Site Class Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-349-1-7 USGS Site Factor, Fpga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-359-1-8 USGS Site Factor, Fa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-369-1-9 USGS Site Factor, Fv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-369-1-10 GSC Site Factor, Fa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-379-1-11 GSC Site Factor, Fv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-379-1-12 Analysis Procedure Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-409-1-13 Load Combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-419-1-14 Response Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1-42



AREMA Manual for Railway Engineering 9-1-3

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SECTION 1.1 INTRODUCTION (2004) R(2012)

The railroad industry is vitally interested in maintaining reliability in its infrastructure to assure safety for its employees, passengers, customers’ goods and the public at large.

These guidelines have been developed specifically for railroad facilities to help reduce damage from earthquakes. While many structures, such as culverts, retaining walls and buildings, may not be substantially different because of use on railroads, North America’s railroad bridges are functionally and behaviorally different from highway and other types of bridges.

This document provides a framework of considerations and methodologies for seismic design of new bridges, roadbed and other railroad facilities. This document also addresses retrofit and post-seismic event response and inspection considerations.

Railroad bridges historically have performed well in seismic events with little or no damage. Contributing to this ability are several factors, unique to railroad bridges, which are consistent throughout North America. First, bridges are traversed by a track structure that functions as a restraint against longitudinal and lateral movement during earthquakes. Second, configurations and certain details of railroad bridges typically differ from other types of bridges. Third, the controlled operating environment permits different seismic performance requirements for railroad bridges compared to highway bridges.

SECTION 1.2 POST-SEISMIC EVENT OPERATION GUIDELINES

1.2.1 GENERAL (2018)

The responses of track and structures to seismic events vary greatly with respect to each other and to the various types of construction, geotechnical conditions and other seismic parameters. The risk factors, structural importance, public safety and value, etc. will all factor into post-seismic event inspection priority.

1.2.2 GUIDELINES (2017)

Unless more appropriate guidelines have been developed as a result of experience with significant earthquakes in the affected area and/or consideration of other local conditions, the following are recommended:

1.2.2.1 Operations1

Railroads should subscribe to a notification system that supplies continuous real time notification of seismic events, with magnitude and epicenter location. Notifications should be directed to the railroad’s train dispatcher or other designated operations controller. Immediately after an earthquake is reported to the Railroad, the train dispatcher or other designated operations controller should, using all available systems, notify all trains and engines within a 100 mile radius of the reporting area to run at restricted speed until magnitude and epicenter have been determined by proper authority. Inspection of track, structures, signal and communication systems should be initiated.

1 See Part 2 Commentary to Seismic Design for Railway Structures




1.2.2.2 Response Levels

Upon determination of the magnitude and epicenter, the following response levels should govern operations within the specified radius from the epicenter:

Proper authority shall be stipulated in the railroad’s emergency response plan. The associated damage philosophy with respect to the above operating procedures can be correlated with the damage criterion shown in Table 9-1-2.

EarthquakeMagnitude1

ResponseLevel

California and Baja California

Remainder of North America

0.0 - 4.99 I5.0 - 5.99 II 50 miles (80 km) 100 miles (160 km)6.0 - 6.99 III

II100 miles (160 km)150 miles (240 km)

200 miles (320 km)300 miles (480 km)

7.0 or greater IIIII

As directed, but not less than for 6.0 - 6.99.As directed, but not less than for 6.0 - 6.99.

I Resume maximum operating speed. The need for the continuation of inspections will be determined by proper authority.

II All trains and engines will run at restricted speed within the specified radius of the epicenter until inspections have been made and appropriate speeds established by proper authority.

III All trains and engines within the specified radius of the epicenter must stop and may not proceed until proper inspections have been performed and appropriate speed restrictions established by proper authority. For earthquakes of 7.0 or greater, operations shall be as directed by proper authority, but the radius shall not be less than that specified for earthquakes between 6.0 and 6.99.


Response Level

Ground Motion Level

Expected Damage to Track, Structure, Signal and Communications

I 0 Very low probability of damage or speed restrictions.II 1 Moderate damage which may require temporary speed restrictions.III 2 Heavy damage which can be economically repaired. Track or structures

may be out of service for a short period of time.III 3 Severe damage or failure requiring new construction or major

rehabilitation. Track or structures may be out of service for an indefinite period of time.

Table 9-1-1. Specified Response Radii

Table 9-1-2. Damage Criterion




1

3

4

The post-seismic event response will be affected by the individual Railroad’s operating requirements based in part on the risk factor, return periods, required factor of safety, structural occupancy, signal and communication systems and appurtenances such as highways, building types and waterways.

1.2.2.3 Post Earthquake Inspection

Inspection procedures and modifications of facilities to expedite the inspection process should be established before the seismic event. The following list provides a general guideline that may be used for developing an inspection procedure:

1.2.2.3.1 Track and Roadbed

Line, surface and cross level irregularities caused by embankment slides or liquefaction, track buckling or pull aparts due to soil movement, offset across fault rupture, etc.

Disturbed ballast

Cracks or slope failures in embankments

Slides and/or potential slides in cuts, including loose rocks that could fall in an aftershock

Scour due to tsunami in coastal areas

Potential for scour or ponding against embankment due to changes in water courses

1.2.2.3.2 Drainage

Blockage of cut ditches or other changes in drainage patterns. (While these conditions will not usually prevent restoration of service, they will require correction.)

1.2.2.3.3 Bridges

Following an earthquake, inspectors may need to travel by rail between bridges. Efficient use of time spent on bridge inspection is necessary in order to restore train service. Normally dry stream beds may be flooded or otherwise impassable when inspection is required. Therefore, provisions should be made beforehand to permit access to bearing areas and other load carrying components from the track rather than from the ground.

Upon arrival at the bridges, in all cases, the inspector should first visually verify profile and alignment of the track and look for signs of ground displacement before proceeding with more detailed inspection of bridge components. The inspectors should look for the following conditions in various bridge types:

a. Steel

Loose or misaligned walkway and handrails

Misaligned beams or girders

Loose or broken diaphragms or truss members

Distorted pins or gusset plates

Deformed bracing

Missing, loose or broken bolts or rivets




Displaced or damaged bearings

Deformed or broken anchor bolts

Towers, columns, piers or bents out of alignment (lateral, longitudinal or vertical)

b. Concrete

Span and walkway misalignment

Fresh cracks in beams or girders

Displaced bearing pads

Piers or bents out of alignment (lateral, longitudinal or vertical)

New cracks or changes in existing cracks in piers or abutments

Broken or freshly cracked piles in bents

c. Timber Trestles

Inadequate bearing areas

Displaced or split caps, piles, post or sills, particularly in framed bents

Piers or bents out of alignment (lateral, longitudinal or vertical)

Broken or freshly cracked piles in bents

Broken bracing (longitudinal and sway)

Missing, bent or broken bolts

d. Movable Spans

Tilt or settlement of lift towers

Damage to sheaves

Damage to counterweight and guides

Misalignment of track girders and segmental girders of rolling lift spans

Gear misalignment in swing spans

Damage or displacement at span locks, centering devices or movable rail joints and associated signal appliances

Misalignment of rest piers or pivot piers

1.2.2.3.4 Culverts

Line and surface of track




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3

4

Blockage of openings

Masonry arches: cracks, loose or dislodged bricks or stones

Concrete box culverts and arches: cracks, spalled concrete; joint separation in non-tied precast sections

Pipes: joint separation

1.2.2.3.5 Retaining Walls

Wall distortion, misalignment or rotation, cracking

1.2.2.3.6 Tunnels

Fallen material or loose material that may fall in an aftershock

New cracks or failures in lining

Offsets due to displacement across fault

Unusual flow of water within tunnel

1.2.2.3.7 Other Structures1

Structural and/or non-structural damage to essential buildings that would prevent or inhibit use.

NOTE: Inspect promptly, with concurrence of local building authorities, to prevent outside inspectors from “red tagging” buildings that are damaged but not unsafe.

Leaks and/or structural damage to fueling facilities, including tanks and pipelines. Look for evidence of leaks in buried fuel lines.

Catenary support structures and tension-regulating systems of electrified lines.

NOTE: Substations should be inspected by a qualified individual.

1.2.2.3.8 Structures That May Fall on Track

a. Overpasses

Reduced support for span at bearings

Column damage

Damage to any span restraint system

b. Adjacent Buildings

Structural damage affecting ability to resist aftershocks

Clearance infringements





Power lines that may be vulnerable to aftershocks

1.2.2.3.9 Signal and Communication Facilities

Signal and communications facilities must be inspected by qualified personnel. However, others involved in inspection should note damage to pole lines and other obvious damage to equipment. Signal masts, signal bridges or instrument housings observed to be out-of-plumb should be reported immediately.

1.2.2.4 Tsunamis1

After a tsunami warning is issued to the Railroad, the train dispatcher or other designated operations controller shall notify all trains and engines within the areas vulnerable to the tsunami to move out of those areas before the estimated arrival of the tsunami. To the extent possible all other equipment should also be moved. The movement should be to the closest location at an elevation deemed to be safe. This movement may be in reverse of the train’s normal direction.

Railroad offices within potential tsunami affected areas and railroad dispatch centers shall be included on the email notification system provided by The National Weather Service. All railroad employees in those offices and those working on line with equipment in such areas shall be notified by their respective offices to move out of areas vulnerable to the tsunami when a warning is received.

Following a large earthquake near the coast, trains should not enter areas vulnerable to tsunamis until it is determined that the tsunami danger has passed. Trains already in vulnerable areas should not be stopped if the track is passable, but should proceed to protected or higher areas if possible.

SECTION 1.3 GENERAL REQUIREMENTS

1.3.1 APPROACH (2004)2 R(2014)

Structures shall be designed to satisfy the specified performance criteria. The main objectives of the required performance criteria are to ensure the safety of trains and to minimize the costs of damage and loss of use caused by potential earthquakes.

In order to provide a framework for evaluating seismic effects on railroad structures, a three-level ground motion and performance criteria approach consistent with the railroad post-seismic event response procedures is employed. The ground motion levels, the structure performance requirements and the railroad response levels are as shown in Table 9-1-3.

1 See Part 2 Commentary to Seismic Design for Railway Structures2 See Part 2 Commentary to Seismic Design for Railway Structures

Table 9-1-3. Seismic Performance Criteria

Railroad Response Level Ground Motion Level Performance Criteria Limit State

II 1 ServiceabilityIII 2 UltimateIII 3 Survivability




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1.3.2 GROUND MOTION LEVELS (2014)

The ground motion levels reflect the seismic hazard at the site. They are defined in terms of peak ground and spectral response acceleration levels associated with a given average return period.

The average return period for each ground motion level may be determined based on seismic risk considerations (see Article 1.3.2.1) and structure importance classification (see Article 1.3.2.2), using the range of average return periods shown in Table 9-1-4.

Level 1 Ground Motion represents an occasional event with a reasonable probability of being exceeded during the life of the structure. Level 2 Ground Motion represents a rare event with a low probability of being exceeded during the life of the structure. Level 3 Ground Motion represents a very rare or maximum credible event with a very low probability of being exceeded during the life of the structure.

1.3.2.1 Risk Factors1

Earthquakes are extreme events associated with a great amount of uncertainty and risk factors are an integral part of seismic design. To achieve a balance between seismic risk and costs associated with risk reduction, a certain amount of risk must be accepted. If there is a severe social penalty associated with structure failure, the acceptable level of risk will be greatly reduced.

The greatest amount of uncertainty is associated with the seismic hazard at the site. Therefore, the overall seismic risk of a bridge is strongly affected by the design ground motion used.

The acceptable risk criteria with respect to Level 1 Ground Motion shall consider the safety and continuing operation of trains with speed restrictions. For Ground Motion Levels 2 and 3, the acceptable risk criteria may be based mainly on economic considerations unless the bridge has a high passenger train occupancy rate. Train traffic is stopped per Railroad Response Level III for Ground Motions Levels 2 and 3 until bridge inspections are completed.

1.3.2.2 Structure Importance Classification2

The purpose of the structure importance classification system is to assist the engineer in determining the appropriate average ground motion return period for each of the three limit states: serviceability, ultimate and survivability. The importance of a structure is determined by three measures: Immediate Safety, Immediate Value and Replacement Value. These three measures are combined in Article 1.3.2.2.4 to determine the appropriate return period for each of the limit states.

1.3.2.2.1 Immediate Safety3

Immediate safety is a measure of the magnitude of earthquake a structure should be able to survive without any interruption of service. Factors to be considered are occupancy, hazardous material and community life lines. These factors should be summed to obtain the immediate safety factor. The immediate safety factor should not exceed 4.

Table 9-1-4. Ground Motion Levels

Ground Motion Level Frequency Average Return Period (Yrs.)

1 Occasional 50-1002 Rare 200-4753 Very Rare 1000-2475

1 See Part 2 Commentary to Seismic Design for Railway Structures2 See Part 2 Commentary to Seismic Design for Railway Structures3 See Part 2 Commentary to Seismic Design for Railway Structures




a. Occupancy Factor

Freight Service only . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Less Than 10 Passenger Trains per Day. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2More than 10 Passenger Trains per Day. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

b. Hazardous Material Factor

The value of the hazardous material factor should be determined by the engineer by considering the type of material being handled, the volume and the proximity of the structure to population. The hazardous material factor should be a value between 0 and 4.

c. Community Life Lines Factor

The community life line factor should reflect the danger to community if the structure fails during a seismic event. The community life line factor should be a value between 0 and 4. The nature of the structure should be taken into account when determining the community life line factor. If the structure is over a route that is critical for post seismic evacuation, a high community life line factor should be used. A high community life line factor should also be used when the structure is over a community’s water supply. The potential disruption of telephone, electric, and water lines attached to the bridge and the importance of continued rail service should also be considered when determining the community life line factor.

1.3.2.2.2 Immediate Value1

Immediate Value is a measure of the magnitude of earthquake a structure should be able to survive with an interruption of service but with the ability to return to service after minor repairs. The factor is based on the railroad’s utilization of the structure and the ability to detour around the structure. The utilization of the structure by others should also be taken into account.

a. Railroad Utilization Factor

Under 10 million gross tons annual traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Between 10 million and 50 million gross tons annual traffic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Over 50 million gross tons annual traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

b. Detour Availability Factor

No Detour Available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.00Inconvenient Detour Route. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.50Detour Route Readily Available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.25

The Immediate Value factor should be determined by multiplying the railroad utilization factor by the detour availability factor. Usage by outside parties should be taken into account after this railroad utilization and detour availability is taken into account.

1.3.2.2.3 Replacement Value2

Replacement value is a measure of the magnitude of the ultimate earthquake the structure should be able to survive. The factor is determined by the difficulty of replacing the structure.

a. Span Length Factor





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Span length less than 35 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1Span length between 35 feet and 125 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2Span length between 125 feet and 250 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3Span length greater than 250 feet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

b. Bridge Length Factor

Bridge length less than 100 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0Bridge length between 100 feet and 1,000 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5Bridge length greater than 1,000 feet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.0

c. Bridge Height Factor

Bridge height less than 20 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.75Bridge height between 20 feet and 40 feet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.00Bridge height greater than 40 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.25

The replacement value factor is determined by multiplying the span length, bridge length and bridge height factors, but should not exceed 4.0. The replacement value should be increased for conditions that would increase the difficulty of replacement such as multiple track, movable structures, difficult foundation and substructure reconstruction conditions, urban location and difficult access.

1.3.2.2.4 Conversion of Factors to Return Periods

The importance classification factor for each limit state is calculated using the following weighting factors. Individual railroads may decide to change the weighting factors to better represent the conditions that they operate under.

To calculate the importance classification factor for each limit state, add the Immediate Safety, Immediate Value and Replacement Value factors together after multiplying them by the appropriate weighting factor.

a. Return Periods

The return period for each limit state is calculated using a linear relationship between the appropriate average return period limits shown in Table 9-1-4. To calculate the return period, multiply the importance classification factor by the

Table 9-1-5. Weighting Factors

Weighting FactorsLimit State

Immediate Safety Immediate Value Replacement Value

0.80 0.20 0.00 Serviceability0.10 0.80 0.10 Ultimate0.00 0.20 0.80 Survivability




difference between the maximum and minimum return periods and divide by 4. Add this result to the minimum return period to get the final value.

1.3.2.3 Base Acceleration Coefficient Maps1

Several base acceleration coefficient maps are provided in this Article to help define the seismic hazard. Figures 9-1-1 through 9-1-9 show peak ground, short-period (0.2 second) and long-period (1.0 second) accelerations in the United States for return periods of 100 years, 475 years and 2475 years. These maps are mainly for illustration purposes and more accurate acceleration coefficients may be determined using web-based interactive tools found on the United States Geological Survey (USGS) website. Acceleration coefficients for sites located in Canada may be determined using the tools found on the Geological Survey of Canda (GSC) website. Other sources or site-specific procedures may be used to define the base accelerations as long as they are based on accepted methods.

Base acceleration coefficients with return periods other than 100 years, 475 years or 2475 years may be determined based on the following formulas:

• Peak ground acceleration for return period, R, less than 475 years

• Peak ground acceleration for return period, R, between 475 years and 2475 years

PGAR = en

n = ln(PGA475) + [ln(PGA2475) - ln(PGA475)] x [0.606 x ln(R) - 3.73]

PGAR = Base peak ground acceleration coefficient for return period = R

PGA100 = Base peak ground acceleration coefficient for return period = 100 years



• Short-period (SS) and long-period (S1) spectral response accelerations for return period, R, may be determined based on the formulas above by substituting the appropriate variables (SS or S1) for PGA.

SS,R = Base short-period (0.2 second) spectral response acceleration coefficient for return period = R

SS,100 = Base short-period (0.2 second) spectral response acceleration coefficient for return period = 100 years



S1,R = Base long-period (1.0 second) spectral response acceleration coefficient for return period = R

S1,100 = Base long-period (1.0 second) spectral response acceleration coefficient for return period = 100 years


PGAR PGA475R

475--------- n

=

n

PGA100PGA475-------------------- ln

1.558–-------------------------------=




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Figure 9-1-1. 100-year Return Period, Peak Ground Acceleration - United States




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Figure 9-1-1. 100-year Return Period, Peak Ground Acceleration - United States (Continued)




Figure 9-1-2. 100-year Return Period, 0.2 Second Period Spectral Response Acceleration - United States




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Figure 9-1-2. 100-year Return Period, 0.2 Second Period Spectral Response Acceleration - United States (Continued)




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(Continued)




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Figure 9-1-8. 2475-year Return Period, 0.2 Second Period Spectral Response Acceleration - United

States (Continued)




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1.3.3 PERFORMANCE CRITERIA (1998) R(2014)1

The requirements for each of the following limit states shall be satisfied.

1.3.3.1 Serviceability Limit State2

The serviceability limit state contains restrictions on bridge stresses, deformations, vibrations and track misalignments due to a Level 1 Ground Motion. Critical members shall remain in the elastic range. Only moderate damage that does not affect the safety of trains at restricted speeds is allowed. The structure shall not suffer any permanent deformation due to deformations or liquefaction of the foundation soil.

1.3.3.2 Ultimate Limit State3

The ultimate limit state ensures the overall structural integrity of the bridge during a Level 2 Ground Motion. The strength and stability of critical members shall not be exceeded. The structure may respond beyond the elastic range, but displacement, ductility and detailing requirements shall be satisfied to reduce damage and loss of structure use. The damage should occur as intended in design and be readily detectable and accessible for repair. The structure shall not suffer any damage which threatens the overall integrity of the bridge due to deformations or liquefaction of the foundation soil.

1.3.3.3 Survivability Limit State4

The survivability limit state ensures the structural survival of the bridge after a Level 3 Ground Motion. Extensive structural damage, short of bridge collapse, may be allowed. Structural and geometric safety measures that add redundancy and ductility shall be used to reduce the likelihood of bridge collapse. Failures of the foundation soil shall not cause major changes in the geometry of the bridge. Depending on the importance and the replacement value of a bridge, an individual railroad may allow irreparable damage for the survivability limit state, and opt for new construction.

SECTION 1.4 NEW BRIDGES

1.4.1 SCOPE (2004) R(2014)

This article applies to bridges with spans not exceeding 500 feet in length. Movable bridges, arch type bridges and bridges with spans exceeding 500 feet in length may require additional analysis and design considerations, which are beyond the scope of this article.

1.4.2 DESIGN APPROACH (2001) R(2011)

Bridge design for seismic loads should start with conceptual considerations to select the appropriate bridge type and configuration. The conceptual phase should be followed by analysis for Level 1 Ground Motion to size the various structure members. Finally, appropriate detailing provisions should be incorporated to allow the bridge to respond well during the Level 2 and 3 Ground Motions. Structures located in areas of low ground motion levels need not meet the conceptual design requirements and detailing provisions provided they are capable of withstanding the full Level 3 Ground Motion loadings within the elastic range.

1 See Part 2 Commentary to Seismic Design for Railway Structures2 See Part 2 Commentary to Seismic Design for Railway Structures3 See Part 2 Commentary to Seismic Design for Railway Structures4 See Part 2 Commentary to Seismic Design for Railway Structures




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1.4.3 CONCEPTUAL DESIGN (2001) R(2011)1

Conceptual design provisions contained herein should be followed as much as practical. The design should maintain a balance between functional requirements, cost and seismic resisting features.

1.4.3.1 Configuration2

The preferred configuration should be incorporated as shown below when possible. Special design and detailing considerations may be necessary for other configurations.

1.4.3.2 Superstructure3

The preferred superstructure characteristics should be incorporated as shown below when possible. Special design and detailing considerations may be necessary for other superstructure characteristics.

1.4.3.3 Substructure4

The preferred substructure characteristics should be incorporated as shown below when possible. Special design and detailing considerations may be necessary for other substructure characteristics.


PREFERRED CONFIGURATION SPECIAL CONSIDERATIONStraight bridge alignment Curved bridge alignment

Normal piers Skewed piersUniform pier stiffness Varying pier stiffnessUniform span stiffness Varying span stiffness

Uniform span mass Varying span mass


PREFERRED SUPERSTRUCTURE SPECIAL CONSIDERATIONSimple spans Continuous spansShort spans Long spansLight spans Heavy spansNo hinges Intermediate hinges


PREFERRED SUBSTRUCTURE SPECIAL CONSIDERATIONWide seats Narrow seats

Seat bent caps Integral bent capsMultiple column Single column




1.4.3.4 Ground Conditions1

Structures should be founded on competent, stable soils or otherwise designed to satisfy the performance requirements during soil instability.

1.4.4 STRUCTURE RESPONSE (2014)

1.4.4.1 Site Effects2

The effects of site conditions on the response spectrum shall be determined according to Article 1.4.4.1.1 and Article 1.4.4.1.2 based on the foundation soil characteristics.

1.4.4.1.1 Site Class3

A site shall be classified as A through F in accordance with Table 9-1-6. Sites shall be classified by their stiffness in the upper 100 feet (30 m) of the soil profile.

vs = average shear wave velocity for the upper 100 feet (30 m) of the soil profile

N = average Standard Penetration Test (SPT) blow count (blows/ft (blows/0.3 m)) for the upper 100 feet (30 m) of the soil profile


Table 9-1-6. Site Class DefinitionsSite Class Soil Type and Profile

A Hard rock with measured shear wave velocity, vs > 5,000 ft/s (1,500 m/s)B Rock with 2,500 ft/s (760 m/s) < vs < 5,000 ft/s (1,500 m/s)C Very dense soil and soft rock with 1,200 ft/s (360 m/s) < vs < 2,500 ft/s (760 m/s),

or with either N > 50 blows/ft (blows/0.3 m), or su > 2.0 ksf (100 kPa)D Stiff soil with 600 ft/s (180 m/s) < vs < 1,200 ft/s (360 m/s), or with either 15 < N <

50 blows/ft (blows/0.3 m), or 1.0 ksf (50 kPa) < su < 2.0 ksf (100 kPa)E Soft soil with vs < 600 ft/s (180 m/s), or with either N < 15 blows/ft (blows/0.3 m),

or su < 1.0 ksf (50 kPa), or any profile with more than 10 feet (3 m) of soft clay defined as soil with PI > 20, w > 40 percent and su < 0.5 ksf (25 kPa)

F Soils requiring site-specific evaluations, such as:

• Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, and collapsible weakly cemented soils.

• Peats or highly organic clays (H > 10 feet (3 m) of peat or highly organic clay where H = thickness of soil)

• Very high plasticity clays (H > 25 feet (7.6 m) with PI > 75)

• Very thick soft/medium stiff clays (H > 120 feet (36 m) with su < 1.0 ksf (50 k Pa)




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su = average undrained shear strength in ksf (kPa) for the upper 100 feet (30 m) of the soil profile

PI = plasticity index

w = moisture content

1.4.4.1.2 Site Factors1

Site factors shall be determined from Table 9-1-7 through Table 9-1-11 based on the Site Class determined from Table 9-1-6 and the values of the acceleration coefficients. Separate tables are provided for United States Geological Survey (USGS) and Geological Survey of Canada (GSC) based accelerations and must be used accordingly.


Table 9-1-7. USGS Site Factor, Fpga

USGS Site

Class

Peak Ground Acceleration Coefficient (PGA)1

PGA <0.10

PGA =0.20

PGA =0.30

PGA =0.40

PGA >0.50

A 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.2 1.2 1.1 1.0 1.0D 1.6 1.4 1.2 1.1 1.0E 2.5 1.7 1.2 0.9 0.9

F2 * * * * *

Notes: 1Use straight-line interpolation for intermediate values of PGA. 2Site-specific hazard analysis should be performed for all sites in Site Class F.




Table 9-1-8. USGS Site Factor, Fa

USGS Site

Class

Spectral Acceleration Coefficientat 0.2 second period (Ss)1

Ss <0.25

Ss =0.50

Ss =0.75

Ss =1.00

Ss >1.25

A 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.2 1.2 1.1 1.0 1.0D 1.6 1.4 1.2 1.1 1.0E 2.5 1.7 1.2 0.9 0.9

F2 * * * * *

Notes: 1Use straight-line interpolation for intermediate values of Ss.

2Site-specific hazard analysis should be performed for all sites in Site Class F.

Table 9-1-9. USGS Site Factor, Fv

USGS Site

Class

Spectral Acceleration Coefficientat 1.0 second period (S1)1

S1 <0.10

S1 =0.20

S1 =0.30

S1 =0.40

S1 >0.50

A 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.7 1.6 1.5 1.4 1.3D 2.4 2.0 1.8 1.6 1.5E 3.5 3.2 2.8 2.4 2.4

F2 * * * * *

Notes: 1Use straight-line interpolation for intermediate values of S1.





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Table 9-1-10. GSC Site Factor, Fa

GSC Site Class

Spectral Acceleration Coefficientat 0.2 second period (Ss)1

Ss <0.25

Ss =0.50

Ss =0.75

Ss =1.00

Ss >1.25

A 0.7 0.7 0.8 0.8 0.8B 0.8 0.8 0.9 1.0 1.0C 1.0 1.0 1.0 1.0 1.0D 1.3 1.2 1.1 1.1 1.0E 2.1 1.4 1.1 0.9 0.9

F2 * * * * *

Notes: 1Use straight-line interpolation for intermediate values of Ss.


Table 9-1-11. GSC Site Factor, Fv

GSC Site Class

Spectral Acceleration Coefficientat 1.0 second period (S1)1

S1 <0.10

S1 =0.20

S1 =0.30

S1 =0.40

S1 >0.50

A 0.5 0.5 0.5 0.6 0.6B 0.6 0.7 0.7 0.8 0.8C 1.0 1.0 1.0 1.0 1.0D 1.4 1.3 1.2 1.1 1.1E 2.1 2.0 1.9 1.7 1.7

F2 * * * * *

Notes: 1Use straight-line interpolation for intermediate values of S1.





1.4.4.2 Damping Adjustment Factor1

The Damping Adjustment Factor, D, may be calculated from the following formula. In the absence of more definitive information, a damping adjustment factor of 1.0 shall be used.

1.4.4.3 Seismic Response Coefficient2

The Seismic Response Coefficient, Cm, to be used in the methods of analysis recommended in Article 1.4.5, shall be calculated from the following formula. For areas with soft soil conditions and high seismicity, or close proximity to known faults, use of a site-specific response spectrum is preferred.

1.4.4.4 Low Period Reduced Response3

a. The seismic response of the bridge may be reduced in accordance with Paragraph 1.4.4.4b if the following provisions are satisfied.

(1) The period, T, of the bridge is determined using the effective moment of inertia, Ie, for reinforced concrete substructure members. The effective moment of inertia may be calculated using EQ 2-12 in Chapter 8, Part 2, Paragraph 2.23.7c.

(2) The period, T, of the bridge is determined including the effects of foundation flexibility.


D= Damping Adjustment Factor

= Percent Critical Damping (e.g. 5%)


Cm= Seismic Response Coefficient for the mth mode

SS= Short-Period (0.2 second) Spectral Response Accleration Coefficient determined in accordance with Article 1.3.2.3

S1= Long-Period (1.0 second) Spectral Response Accleration Coefficient determined in accordance with Article 1.3.2.3

Fa= Site Factor for short-period range of acceleration spectrum determined in accordance with Article 1.4.4.1

Fv= Site Factor for long-period range of acceleration spectrum determined in accordance with Article 1.4.4.1

D= Damping Adjustment Factor determined in accordance with Article 1.4.4.2Tm= Period of vibration of the mth mode in seconds


D 1.50.4 1+

------------------------- 0.5+ =

CmFvS1D

Tm---------------- FaSSD=




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(3) The bridge response considers the lateral flexibility of the spans between piers.

(4) The effects of foundation rocking are accounted for if the moment due to seismic loads exceeds the overturning moment of the footing.

b. The seismic response coefficient, Cm, for bridge structures with periods less than the initial transition period, To, may be determined as follows:

Cm = Seismic Response Coefficient for the mth modePGA = Peak Ground Acceleration Coefficient determined in accordance with Article 1.3.2.3FPGA = Site Factor for peak ground acceleration determined in accordance with Article 1.4.4.1 (for GSC based acceleration Fpga shall be replaced with Fa)D = Damping Adjustment Factor determined in accordance with Article 1.4.4.2To = Initial transition period = 0.2(FvS1/FaSS) in secondsTm = Period of vibration of the mth mode in seconds

1.4.5 ANALYSIS PROCEDURES (2003) R(2014)

1.4.5.1 General

1.4.5.1.1 Serviceability Limit State1

Methods based on elastic analysis shall be used to determine stresses and deformations for the serviceability limit state. The methods recommended include: (1) Equivalent Lateral Force Procedure that is applicable to regular bridges and (2) Modal Analysis Procedure for multi-span irregular bridges.

1.4.5.1.2 Ultimate and Survivability Limit State2

Conceptual design methods shall be used to ensure satisfactory performance for both the ultimate and the survivability limit states. Recommendations for the selection of an appropriate bridge type, geometry and materials and requirements for ductility, redundancy and good detailing, as described in Article 1.4.2, Article 1.4.3, and Article 1.4.7, shall be incorporated.

Non-ductile, non-redundant primary load carrying elements of structures shall be designed to satisfy the performance criteria with respect to Level 2 and/or Level 3 Ground Motions. The design forces shall be the lesser of the seismic loads or the maximum forces which can be transmitted to the element. The seismic loads may be computed by increasing the Level 1 Ground Motion forces by the ratio of the Seismic Response Coefficients.

1.4.5.2 Procedure Selection3

The selection of the analysis procedure for the serviceability limit state shall be based on the bridge configuration as shown in Table 9-1-12.


0.03 < Tm < To seconds

Cm = FpgaPGA for Tm 0.03 seconds

Cm FpgaPGATm 0.03– FaSSD FpgaPGA–

To 0.03– ---------------------------------------------------------------------------------for+=




1.4.5.3 Equivalent Lateral Force Procedure1

The Equivalent Lateral Force Procedure may be used for two-span bridges or multi-span regular bridges as described in Article 1.4.5.2. The procedure is described below.

a. Calculate the Seismic Response Coefficient (Cm) for each of the two principal directions of the structure as follows.

(1) Calculate the natural period of vibration (Tm) for each of the two principal directions of the structure using any commonly accepted method.

(2) Calculate the Seismic Response Coefficient (Cm) for each of the two principal directions of the structure from Article 1.4.4.3 “Seismic Response Coefficient.”

b. Perform static analysis on the bridge in each of the two principal directions.

(1) Calculate the distributed seismic load in each direction from the following formula.

(2) Distribute the seismic load to individual members based on the stiffness and support conditions.

c. Combine the loads in each of the two principal directions of the structure to get the final seismic design loads.

(1) Combination 1: Combine the forces in principal direction 1 with 30% of the forces from principal direction 2.

(2) Combination 2: Combine the forces in principal direction 2 with 30% of the forces from principal direction 1.

Table 9-1-12. Analysis Procedure Selection

Bridge Configuration Analysis Procedure1

Single-span No analysis requiredTwo-span ELF or MA ProcedureMulti-span regular2 ELF or MA ProcedureMulti-span irregular2 MA ProcedureNotes:1. ELF denotes Equivalent Lateral Force Procedure, MA denotes Modal Analysis Procedure.2. Irregular bridges are those structures with significantly irregular configuration or support stiffness.


p(x) = distributed seismic load per unit length of bridgeCm= Seismic Response Coefficient

w(x) = distributed weight of bridge per unit length

p x Cmw x =




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1.4.5.4 Modal Analysis Procedure1

The Modal Analysis Procedure may be used for any structure configuration. The procedure is described below.

a. Develop elastic response spectra from Article 1.4.4.3 “Seismic Response Coefficient.”

b. Perform dynamic analysis on the structure in each of the two principal directions using the elastic response spectra to determine the individual member loads.

(1) A mathematical model should be used to calculate the mode shapes, frequencies and member forces. The model should accurately represent the structure mass, stiffness and support conditions.

(2) An adequate number of modes should be included so that the response in each principal direction includes a minimum 90% mass participation.

c. Combine the loads in each of the two principal directions of the structure using one of the following methods to get the final seismic design loads.

(1) SRSS Method - Combine forces in individual members using the square root of the sum of the squares from each principal direction.

(2) Alternate Method - Perform two load combinations for investigation.

(a) Combination 1: Combine the forces in principal direction 1 with 30% of the forces from principal direction 2.

(b) Combination 2: Combine the forces in principal direction 2 with 30% of the forces from principal direction 1.

1.4.6 LOAD COMBINATIONS AND RESPONSE LIMITS (2002)2 R(2014)

a. The loads shall be combined in accordance with the formulas in Table 9-1-13 based on the structure material. These combinations shall be used in lieu of those specified in Chapter 8 Concrete Structures and Foundations, Part 2 Reinforced Concrete Design and Chapter 15 Steel Structures, Part 1 Design for seismic loads.

Table 9-1-13. Load Combinations


Material Design Method Combination1, 2

Steel Allowable Stress Design D + E + B + EQConcrete Load Factor Design 1.0D + 1.0E + 1.0B + 1.0PS + 1.0EQ

D= Dead LoadE= Earth PressureB= Buoyancy

PS= Secondary Forces from PrestressingEQ= Earthquake (Seismic)




NOTE:

(1) Effects of other loads, such as stream flow pressure, live load and friction shall be included if they have a significant likelihood of acting concurrently with earthquake loads.

(2) Buoyancy loads should be based on the water level that has a significant likelihood of occurring concurrently with earthquake loads and produces the most conservative load combination.

b. The response limits given in Table 9-1-14 shall be satisfied for each structure material.

Table 9-1-14. Response Limits

1.4.7 DETAILING PROVISIONS (2001) R(2011)1

Appropriate detailing provisions shall be incorporated into the structure to meet the performance requirements for the Level 2 and 3 Ground Motion.

1.4.7.1 Continuity Provisions2

The structure shall be designed with an uninterrupted load path to transfer lateral forces from the superstructure to the ground.

1.4.7.1.1 Superstructure3

The superstructure shall be designed to carry the lateral forces to the bearings or shear connectors. The lateral forces from the span may be carried to the end supports by the following load paths:

a. Lateral bracing system.

b. Lateral bending of the girders, including torsional effects as applicable.

c. Diaphragm action of concrete decks or steel ballast pans provided that the deck is adequately connected to the girders.

End cross frames or diaphragms shall be designed to carry the lateral forces to the bearings or shear connectors.

1.4.7.1.2 Bearings4

The bearings shall be designed to transfer the lateral forces to the substructure. Bearings may be supplemented by shear connectors to help transfer the lateral forces provided that the movement required to engage the shear connectors does not cause failure of the bearing device.

Material StressSteel The allowable stresses used in Chapter 15, Steel Structures,

Part 1, Design may be increased by 50%.Concrete The design strengths should be used as specified in Chapter

8, Concrete Structures and Foundations.





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1.4.7.2 Ductility Provisions1

The ductility provisions contained herein shall be incorporated into the structure design.

1.4.7.2.1 Longitudinal Reinforcing Confinement2

Longitudinal reinforcing in concrete columns, pier walls and piles shall be adequately confined to allow the member to respond in the post-yield range. This requirement may be met by the following provisions.

a. Concrete columns and concrete piles fixed at the pile cap shall meet the following requirements:

(1) The volumetric ratio of spiral or circular hoop reinforcement in the plastic hinge zone shall not be less than:

(2) The total cross-sectional area of rectangular hoop reinforcement in the plastic hinge zone shall not be less than:


Ach = cross-sectional area of a member measured out-to-out of confinement reinforcement.

Ash = total cross-sectional area of hoop reinforcement, including cross-ties.

hc = cross-sectional dimension of member core measured center-to-center of confinement reinforcement.

s 0.12fcf y------

s that required by Chapter 8, Article 2.11.2

Ash 0.3 shcfcf y------

AgAch--------- 1–

Ash 0.09shcfcf y------




(3) The longitudinal spacing of the confinement reinforcement in the plastic hinge zone shall not be greater than:

(4) The transverse spacing of hoop or cross-tie legs in the plastic hinge zone shall not exceed 14 inches (350 mm).

(5) The length of the plastic hinge zone from the joint face shall not be less than:

(6) The longitudinal spacing of the column confinement reinforcement outside the plastic hinge zone shall not be greater than:

(7) The design shear force shall be determined from consideration of the maximum forces that can be generated at the faces of the joints at each end of the member. These joint forces shall be determined using the member strength defined in Paragraph 1.4.7.3.1.b.

(8) The confinement reinforcement in the plastic hinge zone shall be proportioned to resist shear assuming the nominal concrete shear strength is zero when the shear force determined in Paragraph 1.4.7.2.1.a.(7) is greater than one-half the maximum required shear strength in this area and the factored axial compressive force for the seismic load condition is less than Agf 'c/20.

hx = maximum transverse spacing (inches or mm) of hoop or cross-tie legs

lo = length of plastic hinge zone from the joint face


s one-quarter of the minimum member dimension

s six times the diameter of the longitudinal reinforcement

s 6” (150 mm)

s 414 hx–

3----------------- inches s 100

350 hx–3

-------------------- mm+

+

lo the depth of the member

lo one-sixth of the clear span of the member

lo 18” (450 mm)

s six times the longitudinal reinforcement diameter

s 6” (150 mm)





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b. Reinforced concrete pier walls with axial loading below the lesser of 0.4Pb or 0.1f 'cAg may be exempted from the column transverse reinforcing requirements if the ratio of the Level 3 Ground Motion acceleration to the Level 1 Ground Motion acceleration is less than or equal to 2. The reinforcing shall meet the following requirements:

(1) Minimum percent of horizontal reinforcing is 0.25%.

(2) Cross ties shall have a minimum cross sectional area of 0.2 in2 (129 mm2) with a 135 hook on one end and a 90 hook on the opposite end and shall be placed so that the 90 and 135 hooks of adjacent ties shall be alternated both horizontally and vertically.

(3) Spacing of all horizontal bars and cross ties shall not exceed 12 inches (300 mm) in any direction, except vertical spacing shall not exceed 6 inches (150 mm) in plastic hinge zones.

1.4.7.2.2 Splices in Reinforcing1

Lap splices are not allowed in a main load carrying member within a distance “d” (effective depth) of any area designed to respond in the post-yield range.

1.4.7.3 Provisions to Limit Damage2

The following provisions shall be incorporated into the design to limit damage.

1.4.7.3.1 Weak Column Provisions3

Reinforced concrete columns which are designed to respond in the post-yield range shall be detailed to prevent damage to adjacent superstructure, bent cap and foundations. This requirement may be met by the following provisions:

a. Concrete column longitudinal reinforcement shall comply with ASTM A706. ASTM A615 reinforcement shall be permitted if the actual yield strength based on mill tests does not exceed the specified yield strength by more than 18000 psi (124 MPa) and the ratio of the actual ultimate tensile strength to the actual tensile yield strength is not less than 1.25.

b. The bent cap and foundation shall be designed for the lesser of 1.3 times the nominal column strength or the Level 3 ground motion load.

c. The plastic hinge zone should be designed to occur in locations that can be inspected.

1.4.7.3.2 Concrete Joints4

The joint shall be configured and reinforced to reduce the likelihood of damage to the superstructure and bent cap and foundation. This requirement may be met by the following provisions:

a. Concrete column joints with superstructure, bent cap and foundation shall be designed in accordance with the following provisions:

(1) Column longitudinal reinforcement shall extend as close as practical to the far face of the adjoining member, but not less than:





For hooked bars in tension:

For straight bars:

(2) Confinement reinforcement shall be provided throughout the joint to the end of the longitudinal column reinforcement in an amount equal to the greater of that specified in Article 1.4.7.2.1a or Paragraph b of this Article.

(3) The nominal shear strength of the joint shall not be taken greater than:

b. Concrete column joints where the column is integral with the bent shall meet the following requirements:

(1) Vertical stirrups with a total area of 0.16 times the area of longitudinal column reinforcement shall be placed on each moment resisting side of the column within a distance of half the column width from the column face.

(2) Vertical stirrups with a total area of 0.08 times the area of longitudinal column reinforcement shall be placed within the column width.

(3) The top and bottom bent cap and integral superstructure flexural reinforcement in the area of the joint shall be increased by 0.08 times the area of longitudinal column reinforcement and adequately developed or hooked beyond the columns at the ends.

(4) The volumetric ratio of column transverse reinforcement carried into the cap shall not be less than 0.4 times the area of longitudinal column reinforcement divided by the square of the longitudinal column reinforcement embedment length into the cap.

1.4.7.3.3 Steel Joints

Joints in main lateral load carrying steel members shall be designed to be stronger than the adjoining member. This requirement may be met by designing the connections for the lesser of 1.3 times the connecting member yield strength or the Level 3 ground motion load. Slip-critical bolts may be designed to carry the higher ground motion loads by bearing rather than friction.

ldh that required by Chapter 8, Section 2.17

ldh 8db

ldh 6 150mm

ldhf ydb

65 fc----------------inches ldh

f ydb

5.4 fc------------------mm

ld that required by Chapter 8, Sections 2.14 through 2.16

ld 2.5 times that required in this Article for hooked bars in tension

20 f cpsi 1.7 fcMPa




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1.4.7.4 Redundancy Provisions1

The redundancy provisions listed below are suggested to increase survivability during the higher level ground motion events.

1.4.7.4.1 Bearing Seats2

Bearing seats should be proportioned to accommodate the maximum relative movements caused by earthquakes. This requirement may be met by the following provision:

Bearing seats supporting the ends of girders which are allowed to move relative to the seat during an earthquake shall be designed to provide a minimum support width, N, measured normal to the face of the abutment or pier, not less than that specified below:

N = (12 + 0.03L + 0.12H)(1+0.000125S2) inches {N = (305 + 2.5L + 10H)(1+0.000125S2) mm}

1.4.7.4.2 Shear Connectors3

Shear connectors may be provided to resist the maximum seismic loads. The shear connectors should be positioned so that they are engaged prior to failure of the bearing device.

1.4.7.4.3 Span Ties

Span ties may be used to reduce the likelihood of unseating during the higher level ground motion events. The spans may be tied together by alternate means through the bent caps such as by anchor bolts, shear rods or common bearing plates provided the load path is adequately verified. The span ties shall be designed to allow for the effects of thermal movement of the span.

1.4.7.4.4 Foundation Rocking4

Foundation rocking response may be used to satisfy the performance requirements for the Level 3 Ground Motion for non-ductile single pier foundations. The analysis should be conducted in accordance with well established procedures. New bridge design using rocking response shall have bearing blocks at the toe and heel of the footing with elastomeric material placed between the footing and bearing blocks.

1.4.7.4.5 Continuous Welded Rail5

Continuous welded rail (CWR) may be evaluated as a redundant load path for seismic loads or to increase bridge damping provided the following requirements are satisfied:

a. No expansions joints are allowed in the CWR over the bridge length and at least 200 feet (60 meters) onto the embankments.


L = length (ft or m) of the bridge deck to the adjacent movement joint, or to the end of the deck.S = angle of skew (degrees) measured from a line normal to the span.H = At abutments, H is the average height (ft or m) of piers supporting the bridge deck to the next

movement joint, or H = 0 for single span bridges. At piers, H is the pier height (ft or m).





b. CWR shall be adequately anchored to the ties over the bridge length and at least 200 feet (60 meters) onto the embankments.

SECTION 1.5 EXISTING BRIDGES

1.5.1 SCOPE (2012)

This part of the chapter addresses the extent to which existing bridges should be reviewed for resistance to seismic forces. In those areas where the horizontal acceleration shown in Figure 9-1-4 exceeds 10% of gravity, existing bridges should be reviewed for resistance to seismic forces.

1.5.2 INVENTORY (2018)1

The Engineer should prepare an inventory of existing bridges in areas subject to seismic events. The Engineer should also identify those bridges owned by others which, because of seismic response or subsequent damage, could potentially impact the operating property. This may include bridges that are over, under, or adjacent to railroad operations.

The accumulation of this information is found, or best contained, in inventory or inspection records. All such records, not so noting, should be modified to provide for indicating the bridge is in a seismic activity zone. Further, these records should note bridges which have been designed, or analytically shown, to be seismic resistant. A reference to the level of resistance might be included.

1.5.3 HISTORY (1995)2 R(2012)

Existing bridges in areas of seismic activity can be expected to have a history of response to various levels of seismic activity. To a large extent, the need for and direction of analytical investigation can be based on the response of the bridges to past events.

In order to take advantage of past experience, it is necessary to develop and correlate event and results histories. A detailed history of seismic events, based on public records, could be developed for each area of interest to the railway, The length of the history would be determined by the oldest in-service structure within the area. Statistical analysis of the data might be used to reduce the volume to more manageable ranges of values.

A history of the results of seismic activity would be assembled from railway inventory records or inspection reports, and other sources such as news media archives and witness oral accounts. Further, current inspection routines could be modified to specifically make observations designed to detect evidence of past seismic events.

An investigator developing a seismic history would be expected to have experience in the field of seismology. An investigator correlating seismic history and results records would be expected to have experience in the field of engineering forensics.





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1.5.4 ASSESSMENT AND RETROFIT (2018)

1.5.4.1 General

Existing bridges that are over, under, or adjacent to railroad operations should be screened, evaluated or analyzed for seismic performance in areas where the peak ground acceleration for the 475-year Return Period event (as shown in Figure 9-1-4) is greater than 10% of gravity. Bridges should include, but not be limited to, the following:

a. Railroad bridges (i.e., bridges supporting track);

b. Other bridges over, under or adjacent to railroad operations (such as vehicular, pedestrian, utility, and overbuild facilities)

c. whose failure during a seismic event may impact safe operations of the railroad

1.5.4.2 Timber Trestles Exclusion

Timber trestle railroad bridges or viaducts that are in a state of good repair, or with defects that do not compromise the short term primary load carrying capacity of the trestle, may be screened and eliminated from further seismic evaluation. For timber trestles that have significant deficiencies, seismic evaluation should focus on how deficiencies may compromise the seismic performance of the timber bridge.

A timber trestle, as with all other bridges, may be located at a site where surrounding geotechnical conditions present seismic vulnerabilities.

1.5.4.3 Investigation of Railway Owned Bridges and Structures1

The analysis of an existing bridge for its seismic performance should be conducted in accordance with the applicable provisions of Section 1.4, New Bridges. The results of this investigation will determine the seismic performance expected for various event levels.

The Engineer may, with support from historic event/results data, prioritize the use of resources for evaluation and analysis of existing bridges based on historic performance, safety risks, economic importance and operations impacts.

1.5.4.4 Investigation of Bridges Owned by Others

The Engineer may request that the Owner of a bridge over, under, or adjacent to railroad operations right-of-way of that may affect railroad safety or operations during a seismic event certify that the structure meets a prescribed seismic performance criterion. These bridges may include railroad, vehicular, pedestrian, utility, and overbuild facilities. Such certification should be furnished in a form determined by the Engineer and should be prepared by a qualified, licensed professional.

1.5.4.5 Retrofit Designs

Railroads may decide to retrofit bridges to reduce the potential for casualty or economic impacts in the event of an earthquake, or to expedite restoration of service following an earthquake. It is recognized that few bridges can be made totally resistant to the effects of an earthquake of great magnitude, especially where site geotechnical conditions make a bridge particularly vulnerable to major disturbance from seismic activity. The likelihood and severity of loss must therefore be balanced against the cost of retrofit.

a. Many different schemes of retrofit are available for various types of bridges. These schemes generally accomplish their purposes by one or a combination of the following:

1 See Part 2, Commentary to Seismic Design for Railway Structures




(1) Changing characteristic resonant frequencies and/or amplitudes (damping) of response to reduce seismic forces in the structure.

(2) Strengthening components of the structure to accommodate the seismic loads.

(3) Providing alternate paths for seismic forces within the structure.

(4) Accommodating displacements with catchers, stoppers, enlarged bearing areas or other devices (see Paragraph c below).

(5) Providing for "yielding type response" (such as "plastic" hinges) at non-critical points of the structure to relieve seismic stresses.

b. The following factors should be considered in any bridge retrofit design:

(1) Retrofit design should be site specific and should consider the condition and stability of the existing structure, including soils and foundation.

(2) Attachments of substructure to superstructure should permit normal movement of the structure required for temperature, creep and shrinkage, or other design accommodations.

(3) Behavior of the retrofit system should not cause damage to the primary structure that would preclude promptly returning the structure to service after a seismic event.

(4) Retrofits should permit both routine and post-seismic inspection, repair, and component replacement.

(5) Seismic response effects of secondary and non-structural components of the structure should be considered when considering the effectiveness of retrofits.

c. Retrofit designs for bearings may include the following:

(1) Adequate bearing seat area to accommodate potential displacements.

(2) Retainer blocks or catchers to limit bearing movements and prevent structural collapse.

(3) Guides or other means for readily returning bearings to their original positions after a seismic event thereby restoring structure geometry.

SECTION 1.6 OTHER FACILITIES AND INFRASTRUCTURE

1.6.1 SCOPE (2018)

Considerations for seismic effects on new and existing railroad and rail transit facilities and infrastructure, other than bridges, are provided in this section. These facilities and infrastructure include, but are not limited to, track and roadbed, culverts, retaining walls, tunnels, track protection sheds, stations, office and shop buildings, locomotive fueling facilities, utilities, signal and communication facilities.

General considerations include assumptions of seismic resistance, areas of seismic vulnerability and recommendations to improve seismic performance. Detailed procedures for performing seismic design of other railroad facilities and infrastructure are beyond the scope of this section.




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In those areas where the horizontal acceleration shown in Figure 9-1-4 exceeds 10% of gravity, existing structures and facilities should be reviewed for resistance to seismic forces.

1.6.2 INVENTORY (2018)

Railroads may have a good inventory of their own facilities. However, in several earthquakes where damage to railroad facilities was minor, structures owned by others, have collapsed. The presence of any structures whose collapse could adversely affect operations should be determined and recorded. Underground structures subject to seismic failure and buried utilities, including pipelines, should also be identified.

1.6.3 TRACK AND ROADBED (2007)1 R(2016)

The largest potential danger to track and roadbed in an earthquake is from failures in the subgrade due to slumping or liquefaction of the soils. This potential can be significantly reduced by eliminating excess water from ballast pockets and saturated embankments. French drains or drainpipes can be very effective.

The track and ballast can also be disturbed in earthquakes, but the potential for extensive damage to track is low. During the shaking process the stability of the ties and ballast will be momentarily weakened and if the rail is in compression it can buckle. The shaking may also result in surface and alignment deviations, loss of welded rail neutral temperature, jointed rail gapping or the loss of superelevation in curves. Primarily, the nature of concern with track following an earthquake is the availability of equipment to reestablish surface and line and welded rail neutral temperature or jointed rail gaps where track has been disturbed.

1.6.3.1 Track Structure

The existing track structure and all manner of special trackwork, including the rail, cross ties, other track material, and the ballast section is presumed resistant to all levels of seismic forces, but not to displacements caused by offset across a fault or other gross ground movements, including liquefaction.

Existing track facilities constructed by direct fixation of rail to a continuously reinforced concrete slab is presumed equally resistant to all levels of seismic forces.

1.6.3.2 Fills and Earth Cuts

Variations in soil materials and soil moisture contents found within existing fills and earth cuts, in general economically preclude adequate data collection for analysis of the site conditions. The Engineer may, based on the geometry, applicable standards of construction and a conservative estimate of existing soil properties, make an analysis of slope stability for the general case.

The magnitude of the seismic force should be calculated as a function of the vertical acceleration component of the design event. The combination will affect both magnitude and direction of the resultant force exerted by the mass above the failure (sliding) surface. This load would be applied as a uniform dead load surcharge at the level of the centroid of the mass. The Factor of Safety against sliding would be determined based on risk factors, and a value close to unity may be acceptable.

Fills founded on sloping strata or on strata of high moisture content should be given special attention.

Retrofit designs for fills would include stabilization by piling, toe berms and revised side slope run-to-rise ratios. Earth cut retrofit designs include stabilization by piling and revised side slope run-to-rise ratios.





1.6.3.3 Rock Cuts

Analytical investigation of rock cuts, as groups or as individual structures, is generally not practical. The Engineer should review the history of rock scaling programs for evidence of an extraordinary frequency of work at a specific site.

Retrofit designs include increased scaling efforts, rock stabilization by bolting or other means, increasing existing bench catchment capacity and selective rebenching.

1.6.4 CULVERTS (2013)

Drainage structures are subject to damage from distortion of the soils in which they are embedded. The most important consideration with culverts is that they maintain their ability to function following an event. Slumping and slope failures of the embankments can result in the ends of culverts becoming constricted, obstructed and/or buried. Consideration should therefore be given on new construction or during major maintenance projects to protect or lengthen the ends where this appears to be of practical benefit.

Culverts are presumed to be of a design generally resistant to seismic forces, but not to displacements due to fault rupture at the site, and to other large ground movements such as those caused by soil liquefaction. One method to improve resistance to failure due to ground displacement is the provision of flexible joints. Retrofit designs include installation of structural linings throughout the culvert. New construction may be required to improve seismic resistance.

1.6.5 RETAINING WALLS (2007)1 R(2016)

There are few railroad-specific issues related to retaining wall seismic design. There are a number of precautions to be taken in designing and constructing earth retaining structures in high seismic areas.

The primary need is to minimize potential for the retained earth to absorb and retain excess moisture. If the soil moisture increases appreciably above the optimum level used for good compaction, there can exist a potential for the soil to liquefy in an earthquake. This would immediately increase lateral loads which could result in lateral displacement, tilting or complete failure of the retaining wall.

Gravity-type structures should be designed to fail by sliding rather than by overturning, thereby taking advantage of active earth pressures developed by the sliding, and also thereby reducing the seismic induced earth pressures. Rigidly fixed structures could be subjected to very high soil forces that could only be reasonably predicted through an intensive soils investigation and analysis. Unless supported by a pile foundation, cantilever walls should be designed so that the design failure mode is sliding rather than overturning or collapse.

In summary, designers should minimize any potential for tilting in their design, take full advantage of active earth pressures and drain the retained earth or use other methods, such as capping, to minimize or eliminate any potential for liquefaction.

1.6.6 TUNNELS AND TRACK PROTECTION SHEDS (2007) R(2016)

1.6.6.1 Tunnels2

Tunnels are presumed to be of a design generally resistant to seismic forces, but not to displacements due to fault rupture at the site or other large ground movements such as those caused by soil liquefaction. Existing tunnel conditions should be reviewed to determine susceptibility to damage in a seismic event. Specific attention should be paid to the design of and conditions at the portal structure. The Engineer should review the history of tunnel maintenance programs for evidence of an extraordinary frequency of work at specific locations.





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New tunnel design is beyond the scope of this chapter.

Retrofit designs include increased scaling efforts, rock stabilization by bolting or other means, and the installation or strengthening of linings.

1.6.6.2 Track Protection Sheds

The superstructures of track protection sheds are, by the nature of their function, presumed to be of a design generally resistant to seismic forces. In active seismic regions, consideration should be given to review existing sheds for resistance to seismic forces, particularly in the transverse direction, applying the appropriate design accelerations.

Primary retrofit designs would provide catchment areas with stop blocks to limit the dislocation of column and beam bearing areas. The design should consider guides to return vertically separated members to the foundation area, and purchase points or jacking blocks for returning the structure to its design location.

1.6.7 BUILDINGS AND SUPPORT FACILITIES1 (2007) R(2016)

Seismic design loads and other requirements for railroad building and support facilities should be governed by the Uniform Building Code or other applicable local, state or federal regulations.

Building codes address the structural adequacy of the building with regard to life-safety but do not necessarily address functionality of railroad facilities. In addition to the safety of occupants, continuing function of the building and the equipment, which it contains, can be of great importance to the railway.

The fact that a structure situated in a seismic activity zone currently exists in an acceptable state of maintenance does justify the presumption that a level of seismic-resistant design is inherent to the construction. It does not, however, permit the presumption that the structure has been subjected to the maximum seismic loading anticipated for the zone.

The fact that a structure of a specific structural design performed successfully at a given level of seismic loading does not justify the presumption that all structures of that design will perform equally at that level of loading. The foundation conditions of a structure are of primary importance in determining resistance to seismic forces.

Seismic load analysis of a structure is site specific. The results of one analysis may not be transferred to a second structure except in the case where each and every design parameter is exactly equal.

Appurtenances associated with these facilities, such as storage racks, tanks, machinery, and stand-by generators, need specific attention. These need to be attached to the structure to resist overturning and shear in order to remain safe and operable.

1.6.8 UTILITIES, SIGNAL AND COMMUNICATION FACILITIES (2017)

Seismic design and maintenance of railroad utility services shall be governed by the Uniform Building Code or other applicable local state or federal regulations.

Utility services includes, but is not limited to, electric power supply, water, gas, fuel pipe lines, fire sprinkler system, heating and air conditioning, waste water treatment, water treatment, fuel storage, oil storage and distribution systems.

Design and maintenance of environmental facilities should consider seismic forces and other requirements as provided for by the Uniform Building Code and the applicable environmental regulatory agency. Additional consideration shall be made with respect to failure-risk factors and potential impact in high environmentally sensitive areas. Some facilities may be required to have spill prevention, containment and countermeasure plans in case of a seismic event.





Components within instrument bungalows should be secured with restraining devices so as to minimize the possibility of displacement and damage to the signal system.

1.6.9 RAIL TRANSIT (2007) R(2016)

AREMA Committee 12, Rail Transit, deals primarily with transit systems. As with other topics, Chapter 12 Rail Transit, will include references to this chapter on seismic guidelines for bridges, buildings, support facilities, track and roadbed items. The Structure Importance Classification of Rail Transit Facilities will be high due to a maximum value for Immediate Safety and Immediate Value.

SECTION 1.7 CONSTRUCTION BY OTHERS (2018)

Proposed construction by others on the operating right-of-way should be reviewed for compliance with seismic code governing the type of construction involved.

The Engineer may require that the Owner of any construction over, under or adjacent to trailroad operations, and that may affect the operation of the railroad during a seismic event, certify that the structure meets a prescribed seismic performance criterion. Such certification should be furnished in a form determined by the Engineer and should be attested to by a licensed professional qualified to render such judgment.

SECTION 1.8 RETIRED FACILITIES (2007) R(2016)

To the extent possible, abandoned railroad right-of-way structures, such as bridges, buildings and facilities should be removed to their foundation level as soon as possible after the time they are removed from service. Economic justification of expenditures for this work should include avoidance of analytical costs necessary to show the structure is stable and the reduced exposure to liability arising from failure of the retired construction during a seismic event.

© 2018, American Railway Engineering and Maintenance-of-Way Association 9-2-1

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9 Part 2

Commentary to Seismic Design for

Railway Structures1

— 2018 —FOREWORD

The purpose of this part is to furnish the technical explanation of various Articles in Part 2, Commentary to Seismic Design for Railway Structures. In the numbering of Articles of this Section, the numbers after the “C-” correspond to the Section/Article being explained.

TABLE OF CONTENTSSection/Article Description Page

C - Section 1.2 Post-Seismic Event Operation Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-2C -1.2.2 Guidelines (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-2

C - Section 1.3 General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-5C -1.3.1 Approach (2004) R(2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-5C -1.3.2 Ground Motion Levels (2017) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-5C -1.3.3 Performance Criteria (2006) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-11

C - Section 1.4 New Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-12C -1.4.3 Conceptual Design (2001) R(2011) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-12C -1.4.4 Structure Response (2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-14C -1.4.5 Analysis Procedures (2003) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-19C -1.4.6 Load Combinations and Response Limits (2002) R(2014) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-21C -1.4.7 Detailing Provisions (2013) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-22

C - Section 1.5 Existing Bridges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26C -1.5.2 Inventory (2018). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26C -1.5.3 History (1995) R(2012) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26C -1.5.4 Assessment and Retrofit (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26

C - Section 1.6 Other Facilities and Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26C -1.6.3 Track and Roadbed (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-26C -1.6.4 Culverts (2013). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-27

1 References, Vol. 94, 1994, p.110; Vol. 96, p. 64, Vol. 97, p. 113.




TABLE OF CONTENTS (CONT)


C -1.6.5 Retaining Walls (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-27C -1.6.6 Tunnels and Track Protecting Sheds (2007) R(2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-28C -1.6.7 Buildings and Support Facilities (2007) R(2016). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-28

LIST OF FIGURES

Figure Description Page

9-C-1 Peak Ground Acceleration vs. Return Period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-119-C-2 Example Response Spectra with Low Period Reduced Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-189-C-3 Example Response Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-21

LIST OF TABLESTable Description Page

9-C-1 Damping Values for Structural Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-159-C-2 Exceptions to Seismic Response Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-169-C-3 FRA Horizontal Track Alignment Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-22

C - SECTION 1.2 POST-SEISMIC EVENT OPERATION GUIDELINES

C - 1.2.2 GUIDELINES (2018)

C - 1.2.2.1 Operations

Railroads may receive notifications of seismic events from varying sources such as USGS, weather alert services, etc. Railroads should ensure that the selected notifications are from a source that is reliable, has rapid response time to an event and can supply all required information.

A railroad’s means of communications with trains can vary. Each railroad should have a response plan to notify all trains operating in the response area of a seismic event. Train dispatchers, control operators, or other positions designated for control of train operations should use all available systems to notify trains of a seismic event. PTC systems may be more reliable to stop or restrict the movement of trains, but may not be operating dependably due to damages caused by a seismic event.

C - 1.2.2.2 Response Levels

The post-seismic event operation guidelines are intended for use where experience or adequate knowledge of regional attenuation rates is not available. The response guidelines are based primarily on decades of experience with earthquakes in California. They provide the basis for a policy for areas where attenuation rates are relatively high, such as California. A more conservative policy is appropriate in areas where seismic experience is limited and/or attenuation rates are relatively low. These conditions exist in most of central and eastern North America. Seismic attenuation models were used to extend the California guidelines to cover other areas of North America. Where justified by adequate experience and/or analysis, a less conservative policy may be appropriate.

For earthquakes of 6.0 (Richter) and greater, a two-level response is recommended. In areas closer to the epicenter, operations are more restricted. In areas further from the epicenter, a zone of less restrictive response is recommended. This less

Commentary to Seismic Design for Railway Structures



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restrictive zone may be useful for moving trains away from the affected zone. Further information on the response levels is found in Reference 23.

In 1998, the Association of American Railroads (AAR) conducted a study of seismic attenuation rates in various regions in North America, primarily the United States and Canada (Reference 23). The study reviewed the response policies of four railroads, the various seismological regions in North America, and the corresponding seismic attenuation models. The seismic attenuation models were used to extend California-based policies to cover other areas of North America, based on equivalent levels of acceleration. The development of one railroad’s response policies, including extension of California-based policies to other regions, is described in Reference 6. Examples of findings in post-earthquake inspections of railroad infrastructure can be found in Reference 7, 15, 18 and 22.

For many years, the United States Geological Survey (USGS) reported earthquake magnitude using the Richter (ML) scale and the event magnitude incorporated in the chapter referenced that scale. Currently, the USGS reports events using a Moment Magnitude (MW) scale. In the range of magnitudes within these response recommendations, the two scales produce similar results.

C - 1.2.2.3.7 Other Structures

It may be desirable to have an arrangement with a qualified inspector to inspect essential buildings immediately after an earthquake so that their safety can be determined and certified to avoid unnecessary evacuations and/or restrictions on building use. Essential buildings would include, among others, dispatching centers, yardmaster’s towers, shop facilities, fueling facilities, buildings containing certain communications facilities, and, for lines with commuter service, passenger stations.

C - 1.2.2.4 Tsunamis

Tsunamis are associated with large offshore, and some near-shore, earthquakes. In some cases, they have been a primary source of earthquake-associated damage. Of over 1101 earthquakes known to have damaged railroads, fewer than 40 have occurred in locations where they could possibly have caused tsunamis. Tsunamis associated with nine of these earthquakes have caused significant railroad damage. They washed out embankments, washed spans off bridges and overturned rolling stock along coasts near the earthquakes. In addition to these events, two other tsunamis caused lesser damage.

Because of its very long wavelength, a tsunami behaves as a shallow surface wave. Its amplitude in mid-ocean is very small; as it approaches land, the amplitude builds up and all the energy of the original disturbance is concentrated into a few wavelengths with devastating results, erroneously called a tidal wave.

In addition to damage in the immediate area of an earthquake, tsunamis have caused damage at large distances from an earthquake. The tsunami generated by the December 26, 2004 magnitude 9.0 earthquake off the coast of Sumatra washed a train off a track adjacent to the coast in southern Sri Lanka, killing a large number of passengers. Alaskan earthquakes have caused damage and loss of life in Hawaii and California and significant damage in Oregon and Washington. Earthquakes near Chile have caused damage and loss of life in Japan. Hawaii, Japan and some other islands in the Pacific appear particularly vulnerable to tsunamis from distant earthquakes.

Evaluation of the potential hazard to railroad lines along the coasts of Alaska, Washington, California, and a few locations in Mexico is appropriate. A portion of the Alaska Railroad was severely damaged by a tsunami in 1964. The Washington coast and west coast of Mexico are subject to earthquakes that could generate large tsunamis. Tsunamis have been generated by submarine landslides due to earthquakes in California. There is a small, but definite, risk of tsunamis affecting the Atlantic coast.

Tsunami hazard can be considered in two scenarios: tsunamis generated by nearby earthquakes, and those generated by distant earthquakes. In 1812, an earthquake near Santa Barbara, CA caused a tsunami that produced a run-up (increase in the water surface elevation) of about 10 feet at the coast. Tsunamis from northern California earthquakes in 1859 and 1868 caused local run-ups of between 10 and 15 feet along the coast and in San Francisco Bay. The March 28, 1964 Alaska earthquake caused a

1 As of 2012




tsunami that produced run-ups in the range of 10 to 15 feet at locations in Washington, Oregon and northern California. About 1100 years ago, a tsunami with wave heights in the range of 15 to 20 feet apparently occurred in Puget Sound due to a magnitude 7 or larger, earthquake on the Seattle fault (Reference 16). The 2011 Tohoku, Japan earthquake caused a tsunami with wave heights exceeding 30 feet that destroyed a number of trains and other railroad facilities.

A study prepared for the U.S. Nuclear Regulatory Commission indicates maximum tsunami wave heights for distant earthquakes of about 4 feet, in deep water off the coast, for both Washington and southern California, and up to 7 feet for northern California, with the height increasing dramatically as the wave moves into more shallow coastal waters. In the case of Washington, the narrow channels and islands between the open ocean and the Puget Sound coast could reduce the wave height from the height at the outer coast. In the case of California, the coast is exposed. Advance warnings related to distant earthquakes would be issued by the West Coast & Alaska Tsunami Warning Center of the National Weather Service one to several hours before arrival of the first wave, with identification of vulnerable coastal areas.

The waves generated by a nearby earthquake will arrive shortly after the earthquake occurs. The time between the earthquake and the arrival of the tsunami is dependent on the distance between the location of landfall and the source of the tsunami. For a large earthquake, the wave height could be much greater than for a comparable distant earthquake. The strike-slip earthquakes that occur in California are relatively unlikely to produce a large tsunami unless they cause submarine landslides. On the other hand, a large interplate subduction zone earthquake, similar to the 1964 Alaska earthquake, which can occur near the coast in Washington, is likely to produce a major tsunami. There is good evidence that a large earthquake near the Washington-Oregon coast caused a tsunami in Japan in January of 1700. A very crude estimate of the time interval between such an earthquake off the Washington coast and the arrival of the first wave at the coast in Puget Sound is in the order of one hour. If the generating earthquake occurred in Puget Sound, the travel time would be a matter of minutes.

Characteristics of the tsunami generated by the June 23, 2001 magnitude 8.4 earthquake in southern Peru, although affecting the open coast, which is a different environment from the rail lines along Puget Sound in Washington, are of interest as the generating earthquake is similar to what could occur in Washington. At a location near the earthquake, the first wave arrived about 6 minutes after the earthquake. At a location near the end of the tsunami damage, the first wave arrived about 35 minutes after the earthquake. At most locations, the second and third waves were larger than the first wave. The earthquake occurred at low tide, which resulted in a smaller area of damage than would have been produced at high tide. The maximum run-up was about 30 feet. In a relatively flat area where the run-up was about 16 feet, inundation extended nearly a mile inland from the coast (Reference 10). The tsunami produced extreme scour. If a tsunami-generating earthquake and landslide were to occur off the California coast, travel times would be similar to those observed in Peru.

The appropriate response for a tsunami with a distant source would be movement of trains and, to the extent possible, other equipment out of designated tsunami warning areas before the estimated arrival of the tsunami. In the case of a nearby earthquake, advance warning may not be possible. Although most earthquakes do not cause tsunamis, the possibility does exist for large earthquakes in coastal areas.

Vulnerable areas are close to the coast and have relatively low elevations. Wave run-up heights are rarely greater than 30 to 35 feet, although extreme values in the order of 100 feet have been estimated and local variations due to ocean floor topography and focusing effects can be large. The crest to trough measurements for the December 26, 2004 tsunami at points on the Pacific coast of North and South America were generally in the range of 6 to 20 inches but the crest to trough at Manzanillo, Mexico was about 9 feet (Reference 24). Although most information on tsunami effects is related to vertical run-up, distance from the coast without significant increase in elevation would provide a degree of protection. A first approximation of the maximum inland penetration of a tsunami wave in a very flat region, based on the 2001 southern Peru earthquake and 30 to 35 feet of vertical run-up, would be in the order of two miles.




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C - SECTION 1.3 GENERAL REQUIREMENTS

C - 1.3.1 APPROACH (2004) R(2014)

The vulnerability of a bridge is determined by the risk associated with the earthquake ground motion and the specified performance criteria. The risks associated with the magnitude of the ground motion at a given location are defined by the acceleration coefficient maps in Article 1.3.2.3.

The performance criteria specified in this Section is consistent with the post seismic event operating procedures described in Section 1.2, Post-Seismic Event Operation Guidelines. Together, they aim to minimize consequences of earthquakes.

C - 1.3.2 GROUND MOTION LEVELS (2017)

C - 1.3.2.1 Risk FactorsThere are many sources of uncertainty involved in seismic design. The greatest source of uncertainty is associated with the regional seismicity and the expected ground motion characteristics at the site. The response of the bridge, which is affected by both the soil and the structure dynamic characteristics, and also the methods of analysis used, add to the overall degree of uncertainty.

Even when conservative earthquake magnitudes and assumptions are used in design, during its life a bridge may be subjected to maximum earthquake loads that exceed the desired performance criteria. The design of a bridge for extreme ground motion is economically undesirable, unless there is a severe social penalty associated with bridge failure. Therefore, a certain amount of risk must be accepted so that a balance between the probability of large earthquakes and the costs of overdesign can be achieved.

Determining “acceptable seismic risk” is a very complex task that must consider both social and economic aspects. Obviously the amount of risk that may be accepted for some bridges is greater than for others. Factors such as the volume and the type of train traffic, the value and the importance of the bridge and the cost of loss of use have to be considered when establishing acceptable seismic risk levels (see Article 1.3.2.2). The acceptable seismic risk levels must also be consistent with the risks due to other extreme events such as flood waters, fire and ship collision.

A relatively simple approach is to adjust the acceptable seismic risk levels used by seismic design codes of other structures, such as buildings and highway bridges to railroad bridges. Buildings and highway bridge design codes put a major emphasis on life safety. This is primarily due to their high occupancy rate and the social implications of a large loss of life at one location. Also, some highway bridges are part of lifelines that must remain open even after severe earthquakes. When the occupancy rate of most railroad bridges is compared to the occupancy rate of buildings and highway bridges the very large difference between the levels of risk of loss of life becomes apparent. In addition to this, the movement of trains is controlled by signalization and dispatchers, so that in the event of an earthquake trains may be stopped. Thus, lower ground motion return periods may be used for railroad bridge design, and more emphasis can be put on the economic aspects that are more rational and easier to express in a quantitative way.

Another approach is to perform a probability-based overall seismic risk or cost-benefit analysis. A probabilistic approach can account for uncertainties in the ground motion, the performance of the bridge during a given ground motion and the methods of analysis used. Seismic risk analysis may be performed in three steps: (1) Seismic Hazard Analysis, that yields a probability distribution function of ground motion parameters at the bridge site, (2) Seismic Performance Analysis, that yields probabilistic statements of the risks of the bridge exceeding the specified limit states, conditioned upon specified levels of ground motion, (3) Seismic Risk Analysis, that integrates the first two steps to yield the overall risk of the bridge exceeding the specified limit states. This approach, however, is only recommended for special bridge projects and is limited by the uncertainty involved in seismic hazard estimates.

C - 1.3.2.2 Structural Importance Classification

Examples of Determining Structural Importance Classification




Example 1

The proposed structure is a 130’ concrete trestle consisting of 5 spans each 26’ long. The bridge has a height of 30’. The structure is located on a branchline that has 12 million gross tons of traffic a year. There is no detour around the bridge. Approximately 25% of the traffic is hazardous material. There is not any passenger service on the line and the structure does not cross a community life line.

Immediate Safety Replacement ValueOccupancy Factor= 1 Span Length Factor = 1

Hazardous material Factor = 1 Bridge Length Factor = 1.50Community Life Line Factor = 0 Bridge Height Factor = 1.00

2 1.50

Immediate ValueUtilization Factor = 2

Detour Factor = 1.00 2

Serviceability Weighing Factor Weighted ValueImmediate Safety 2 0.80 1.60Immediate Value 2 0.20 0.40

Replacement Value 1.5 0.00 0.002.00

Return Period = 50 +2.00(100-50)/4 = 75 years

Ultimate Weighing Factor Weighted ValueImmediate Safety 2 0.10 0.20Immediate Value 2 0.80 1.60


Return Period = 200 + 1.95(475-200)/4 = 334 years

Survivability Weighing Factor Weighted ValueImmediate Safety 2 0.00 0.00Immediate Value 2 0.20 0.40






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Example 2

The proposed structure is a 500’ steel bridge consisting of 2 trusses each 250’ long. The bridge has a height of 50’. The structure is located on a mainline that has over 50 million gross tons of traffic a year. There is no detour around the bridge. Approximately 25% of the traffic is hazardous material There are two passenger trains per day on the line. The structure does not cross a community life line.

Immediate Safety Replacement ValueOccupancy Factor= 2 Span Length Factor = 3

Hazardous material Factor = 1 Bridge Length Factor = 1.50Community Life Line Factor = 0 Bridge Height Factor = 1.25

3 5.63Note: the factor cannot exceed 4

Immediate Value Replacement Value = 4.00Utilization Factor = 4

Detour Factor = 1.00 4

Serviceability Weighing Factor Weighted ValueImmediate Safety 3 0.80 2.40Immediate Value 4 0.20 0.80

Replacement Value 4 0.00 0.003.20

Return Period = 50 +3.20(100-50)/4 = 90 years

Ultimate Weighing Factor Weighted ValueImmediate Safety 3 0.10 0.30Immediate Value 4 0.80 3.20



Survivability Weighing Factor Weighted ValueImmediate Safety 3 0.00 0.00Immediate Value 4 0.20 0.80






C - 1.3.2.2.1 Immediate Safety

Immediate safety is divided into three factors; occupancy, hazardous materials and community life lines to represent the three most likely risks during and immediately after a seismic event.

These risks are:

The engineer should factor in any additional hazards that may be caused by the structure becoming unserviceable during a seismic event. The immediate safety factors are added together because the threat that each factor represents to railroad personnel and the public is independent of each other.

C - 1.3.2.2.2 Immediate Value

Immediate Value evaluates the railroads’s need to return the structure to service after a seismic event. The utilization factor is multiplied by the detour factor because the need to return a structure to service is reduced when a detour route is available. The engineer should examine the possibility of the detour route also being damaged in a seismic event when determining the detour availability factor.

C - 1.3.2.2.3 Replacement Value

Replacement Value evaluates the costs associated with replacing the structure. Replacement Value accounts for three of the major factors that affect replacement cost: span length, bridge length and bridge height. These factors are designed to be multiplied together to obtain a value which reflects the difficulty associated with replacing the structure. These factors may not represent the total cost to replace the structure. Other factors that should be considered are double track structures, movable structures, urban location, difficult access, environmental and political concerns.

C - 1.3.2.3 Base Acceleration Coefficient Maps

Acceleration coefficient maps reflect the seismic hazard at a site. They account for both maximum ground motion intensity expected and frequency of occurrence. The maps give ground acceleration levels with a uniform probability of being exceeded in all areas of the country. The steps involved in the development of these maps include: (1) the definition of the nature and location of earthquake sources, (2) magnitude-frequency relationships for the source, (3) attenuation of ground motion with distance from the source, and (4) determination of ground motion parameters at the site having the required probability of exceedance.

The base acceleration maps for return periods of 100 years, 475 years and 2475 years in the United States were prepared by the United States Geological Society (USGS) for AREMA. These maps are included mainly for illustrative purposes. Procedures for determining design accelerations for sites located in the United States and Canada are described in the following paragraphs.

Accelerations for sites in the United States may be estimated from the maps or, more accurately, determined by using the interactive tools found on the USGS website at http://earthquake.usgs.gov/hazards/. Determination of accelerations for sites in Canada will require the use of a web-based hazard calculator found on the Geological Survey of Canada (GSC) website at http://www.earthquakescanada.nrcan.gc.ca/hazard-alea/. Example procedures for each website are shown below. The acceleration values shown are for example purposes only and should not be used for design.

Procedure for sites in the United States

Occupancy: Risk to train crews and passengers due to damage to the structureHazardous Materials: Risk to the community caused by the possible release of hazardous

materialsCommunity Life Lines: Risk to the community caused by the damaged structure disrupting a

community lifeline

http://www.earthquakescanada.nrcan.gc.ca/hazard-alea/

http://www.earthquakescanada.nrcan.gc.ca/hazard-alea/




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a. Navigate to http://earthquake.usgs.gov/hazards/

b. Select the 'Hazard Maps and Site-Specific Data' link, then select the '2008' link under the Hazard Curves heading

c. Select 'Launch Application' (a map of the United States will appear)

d. Type in site location information (for this example Latitude = 33.06277, Longitude = -115.759)

e. Select the 'UHRS' (Uniform Hazard Response Spectra) tab. A graph will appear with several default response spectra plotted on it. The default Curve Selection is the Site Class B/C boundary which corresponds to Site Class B in the Site Factor Tables shown in Article 1.4.4.1.2.

f. Add custom return periods as needed (for this example a return period = 100 years was added)

g. The graph will update automatically with any custom return periods added. Acceleration values may be determined graphically by using the Value tooltip and scrolling across the vertices of the response spectra curves. Acceleration values for this example are as follows:

Lat: 33.06277, Lon: -115.759

Notes:

2% in 50 yrs = 2475-year Return Period

10% in 50 yrs = 475-year Return Period

~40% in 50 yrs = 100-year Return Period

Procedure for sites in Canada

a. Navigate to http://www.earthquakescanada.nrcan.gc.ca/hazard-alea/

b. Locate the ‘Hazard Maps and Calculations’ and select ‘Hazard Calculators – Determine seismic hazard at your site’

c. Select ‘Get 2010 hazard values’ from available selections

d. Enter the Latitude and Longitude of the site under consideration (for this example Latitude = 49.26, Longitude = -123.11)

e. Select the ‘Number of closest points for interpolation’ from the pull-down menu (for this example 15 points was selected, it is recommended to run the calculation on all 3 available options and select the highest acceleration values for design)

f. Enter additional optional information as desired (for this example no additional information was entered)

PGA ~40% in 50 yrs 0.2498 (g)PGA 10% in 50 yrs 0.4659 (g)PGA 2% in 50 yrs 0.7672 (g)

0.2 sec ~40% in 50 yrs 0.5974 (g)0.2 sec 10% in 50 yrs 1.146 (g)0.2 sec 2% in 50 yrs 1.936 (g)1.0 sec ~40% in 50 yrs 0.1696 (g)1.0 sec 10% in 50 yrs 0.3383 (g)1.0 sec 2% in 50 yrs 0.5757 (g)

http://earthquake.usgs.gov/hazards/




g. Click on CALCULATE

The page will reload after the calculation is complete. Scroll down to the acceleration values which will appear similar to this:

2%/50 years (0.000404 per annum) probability

40%/50 years (0.01 per annum)



Notes:

2%/50 years = 2475-year Return Period



40%/50 years = ~100-year Return Period

Future earthquakes and earthquake research will continue to improve the overall understanding of the seismic hazard and will result in revisions to the acceleration maps. The 2008 edition of the USGS maps and the 2010 edition of the GSC maps were used in the examples above. More recent maps, maps from different sources, or site-specific procedures may be used as long as they are based on accepted methods and are consistent with the site factors and response spectra equations in Article 1.4.4.

Formulas are included to determine base accelerations for return periods other than those shown on the maps. These formulas are based on the procedure shown in Article 2.6.1.3 of Reference 13. The FEMA 273 formulas were simplified for use with the AREMA base acceleration maps. The FEMA 273 formula for return periods less than 475 years has an exponent that is based on the acceleration level and site location. This exponent can be determined more directly using the accelerations for

Sa(0.2) Sa(0.5) Sa(1.0) Sa(2.0) PGA0.932 g 0.642 g 0.333 g 0.173 g 0.462 g







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return periods of 100 and 475 years. Section C2.6.1.3 of Reference 14 indicates that the acceleration-return period curves are nearly linear on a log-log plot between return periods of 475 years and 2475 years, therefore a single formula is used in this range. Example peak ground acceleration vs. return period curves, developed using the formulas shown in this Article, are shown in Figure 9-C-1 for various cities throughout the United States. These curves were developed for example purposes only using specific latitude and longitude values and should not be used for design.

C - 1.3.3 PERFORMANCE CRITERIA (2006) R(2014)

A three-level ground motion and performance criteria approach is employed to ensure train safety and structure serviceability after a moderate earthquake, minimize the cost of damage and loss of structure use after a large earthquake and prevent structure collapse after a very severe earthquake. Considering all the limit states can account for the unique structural and operating characteristics of railroad structures, and the specific needs of railroad bridge owners. Also, the performance based format used allows for future updates as the state of the art in earthquake engineering advances. Railroad bridge owners may use alternate seismic design criteria or waive certain requirements contained herein provided that adequate precautions are taken to protect the safety of trains and the public following an earthquake.

C - 1.3.3.1 Serviceability Limit State

The primary aim of the serviceability limit state is to ensure the safety of trains. After Level 1 earthquakes, trains are allowed to proceed at a reduced speed until inspections are completed, and the track is cleared. The stresses and deformations are limited to immediate use of the structure after a Level 1 earthquake. The allowable deformations of the structure and track may be related to the train speed restrictions after a Level 1 earthquake. Vibration of flexible bridges with natural periods in the transverse direction around 1 second may cause derailments even in the elastic response range.

Figure 9-C-1. Peak Ground Acceleration vs. Return Period

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 500 1000 1500 2000 2500

Peak

Gro

und

Acce

lera

tion

(G's

)

Return Period (years)

Los Angeles, CASeattle, WA

Butte, MontanaMemphis, TN




C - 1.3.3.2 Ultimate Limit State

The primary aim of the ultimate limit state is to minimize the extent of damage and to ensure the overall structural integrity of the bridge. After Level 2 earthquakes, trains are stopped until inspections are completed. Structural damage that can be readily detected and economically repaired may be allowed. By allowing the structure to respond beyond the elastic range and undergo inelastic deformations, the earthquake resistance capacity of bridges with good ductility is significantly increased.

C - 1.3.3.3 Survivability Limit State

The survivability limit state aims to prevent overall bridge collapse. After Level 3 earthquakes, the expected track damage would prevent immediate access to the bridge. The performance of the bridge during such earthquakes will mainly depend on the ductility and redundancy characteristics of the bridge and on the additional safety measures designed to prevent bridge collapse.

C - SECTION 1.4 NEW BRIDGES

C - 1.4.3 CONCEPTUAL DESIGN (2001) R(2011)

The behavior of bridges during past earthquakes has shown that the structure type, configuration and structural details have a significant effect on seismic performance. At many locations certain bridge types have survived earthquakes with relatively minor damage, while other bridges in the same vicinity have sustained extensive damage or collapsed. The survival or failure of bridges of a similar type has been linked to their configuration and the particular design and detailing criteria used. For example, bridges with skewed or irregular configurations have experienced extensive damage, often at locations where other bridges remained unharmed.

The conceptual approach recommended for satisfying the ultimate and survivability limit states consists of seismic design guidelines based on conceptual principles regarding structure type, configuration and details. Incorporating conceptual seismic design principles, especially during the early stages of bridge planning and design, can significantly improve seismic behavior at low additional costs. Also, such an approach is less sensitive to the uncertainties involved in the ground motion description, the numerical analysis of structure response in the post-yield range, and the limited analytical and experimental seismic research data on railroad bridges that is currently available.

The recommendations provided are intended to reduce the seismic demands by selecting an appropriate structure type for the existing site conditions. Following basic requirements for simplicity, symmetry and displacement capability will increase the seismic resistance by providing adequate strength, stability, ductility, redundancy, energy dissipation and deformation capability. Strength and stability are important attributes for satisfying the serviceability limit state, while ductility and redundancy have a significant effect on the ultimate and the survivability limit states. Displacement and deformation capacity is quite important for structures on poor soil conditions or near a fault line.

The conceptual design phase for railroad bridges should consider the soil conditions and the seismic hazard at the site and incorporate appropriate means to cope with the seismic induced forces that affect superstructure, substructure (including foundation) and load bearing strata. Since the nature and direction of gravity and seismic induced forces are significantly different, it is incumbent upon the design engineer to consider both types of loading conditions in the conceptual design phase in order to meet the performance requirements of the structure.

C - 1.4.3.1 Configuration

Bridge vulnerability to seismic effects is determined by the ability to resist earthquake forces and/or to withstand large relative movements. The selection of an appropriate structure type and configuration should take into account the seismic hazard at the site, the soil conditions and the bridge performance requirements. In general, sites near active faults, sites with potentially liquefiable or unstable soil conditions, and sites with unstable sloping ground conditions should be avoided, if practical, and measures to improve the soil conditions should be considered as an alternative. Conventional bridge structures are difficult to design to resist the load magnitudes generated by large ground displacements and possible settlement or shifting of




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foundations. Therefore, where the extent of poor soil conditions is relatively large, a structure type that can accommodate large ground displacements is recommended. For example, simple span structures with ample bearing support length can accommodate large movements, without accumulating loads.

Criteria for determining adequate structure configuration and layout include simplicity, symmetry and regularity, integrity, redundancy, ductility and ease of inspection and repair. Bridges should be simple in geometry and structural behavior. Simple structures provide a direct and clear load path in transmitting the inertial forces from superstructure to ground. The bridge behavior under seismic loads can be predicted with more certainty and accuracy with fewer dominant modes of vibration. To the extent possible, the preferred configuration characteristics of Article 1.4.3.1 should be incorporated. The horizontal strength and stiffness of substructure elements should not vary much along the bridge and the placement of the fixed and expansion bearings should be such that a balanced seismic load distribution to all piers can be achieved. Severe skews should be avoided even at the expense of providing longer spans or making changes in alignment.

Bridges with features such as extreme curvature or skew, varying stiffness or mass and abrupt changes in geometry require special attention in analysis and detailing to avoid premature damage or failure. The use of integral crash walls with piers in high seismic areas requires special considerations, since it creates an abrupt change in the pier stiffness. Alternative crash wall configurations, such as separate walls or piers of heavy construction as defined in Chapter 8, Article 2.1.5.1c, are recommended.

Redundancy and ductility considerations should also be taken into account when establishing the bridge configuration. In addition, it is desirable to have a certain degree of deformation capability within the seismic load transfer path, since seismic demands are reduced when controlled movements are allowed. Bridges with rigid superstructures and rigid substructures could benefit from some allowance for movements at the bearing location. However, adequate bridge seat widths are needed to ensure that movements can be accommodated without potential for span loss. A strong and stiff superstructure to substructure connection is more appropriate when the substructure is not too rigid or when the end diaphragms or cross frames of spans are designed and detailed to undergo ductile deformations during a strong earthquake.

C - 1.4.3.2 Superstructure

Simple spans of standard configuration are preferred by railroads since they have performed well during past earthquakes and can be returned to service or replaced. Continuous spans may reduce the likelihood of unseating at the piers. This feature can be incorporated in simple spans by providing wider seat widths or span ties. Long spans produce higher load demands on fewer foundations which will increase foundation vulnerability and reduce redundancy. Heavy ballasted concrete spans will produce higher load demands on the foundation with subsequent increases in foundation cost. These costs should be compared to the increase in material and maintenance costs of steel to determine the optimum superstructure type. Excessive ballast and other non-structural weight should be avoided as much as practical. Intermediate hinges attract high seismic demands and require special detailing to provide the lateral load paths required to withstand seismic loads.

C - 1.4.3.3 Substructure

Wide seat widths at the abutments and piers allow for large displacements without unseating the bridge spans. Integral bent caps have performed poorly during large earthquakes and require extensive detailing to reduce the likelihood of superstructure damage. Multiple columns provide redundancy in the substructure which is needed to survive the higher level ground motions. Battered piles tend to attract most of the lateral load during an earthquake.

C - 1.4.3.4 Ground Conditions

The foundation soil should be investigated for susceptibility to liquefaction and slope failure during the seismic ground motion. To the extent possible, bridges in regions of high seismicity should be founded on stiff, stable soil layers. Consideration should be given to ground improvement techniques when the extent of soil instability threatens the performance of the bridge or approach embankments. It may be possible to satisfy the performance requirements by other means, such as designing the foundation to survive the soil instability. Large diameter pile foundations may be used to withstand a slope failure or carry the bridge loads through liquefiable soil layers to competent material.




In some cases, ground improvement or design for soil instability may be impractical. Approach embankments may be allowed to fail during the higher level ground motion events provided that they can be quickly repaired using earth moving equipment. Retaining walls founded on deep liquefiable soils may require costly ground improvement to ensure stability. The effects of wall failure on rail operations should be carefully evaluated and weighed with the costs to improve the soils.

C - 1.4.4 STRUCTURE RESPONSE (2014)

C - 1.4.4.1 Site Effects

The behavior of a bridge during an earthquake is strongly related to the soil conditions at the site. Soils can amplify ground motions in the underlying rock, sometimes by factors of two or more. The extent of this amplification is dependent on the profile of soil types at the site and the intensity of shaking in the rock below. Sites are classified by type and profile for the purpose of defining the overall seismic hazard, which is quantified as the product of the soil amplification and the intensity of shaking in the underlying rock.

C - 1.4.4.1.1 Site Class

The site classes are consistent with those in Reference 4 and Reference 19. The stiffness of a site may be classified by the average shear wave velocity, average Standard Penetration Test (SPT) blow counts or average undrained shear strength of soils in the upper 100 feet (30 m) of the soil profile. Several methods to determine these average values are presented in Reference 4, along with steps that may be followed to classify a site. A default site class is not given, as this would require a judgment based on little to no knowledge of the soils.

Experience has shown that most railroad bridge failures that have occurred in seismic events were due to soil failures such as lateral spreading or liquefaction. Because of this, it is recommended that the foundation investigation should include soil borings or test pits taken to an adequate depth to determine the soil profile. It should be emphasized that an adequate foundation investigation is necessary to determine the appropriate foundation type for the structure.

C - 1.4.4.1.2 Site Factors

The site factors are consistent with those in Reference 4 and Reference 19. Site Class B is the reference site class for the USGS acceleration coefficient maps, and is therefore the site condition for which the USGS site factor is 1.0. Site Class C is the reference site class for the GSC acceleration coefficient maps, and is therefore the site class condition for which the GSC site factor is 1.0. Other Site Classes have separate sets of site factors which generally increase as the soil profile becomes softer. Except for USGS Site Class A and GSC Site Classes A and B, the factors also decrease as the ground motion level increases, due to the strongly nonlinear behavior of the soil.

Caution should be exercised when applying site factors if acceleration maps other than those discussed in Article 1.3.2.3 are used. In this case it would be appropriate to use site factors consistent with the maps being used.

C - 1.4.4.2 Damping Adjustment Factor

The Damping Adjustment Factor provides a simplistic method for scaling the seismic response coefficient to account for different structure types and conditions. The seismic response coefficient is given for 5% critical damping without the damping adjustment factor. The percent critical damping varies based on the structure material and system, effect of structure attachments (i.e., track and ballast), whether the structure responds in the elastic-linear or post-yield range, and whether or not the structure response is dominated by the foundation or abutment response.

The percent critical damping (preferably should be based on actual test data from similar structure types. A table of damping values for different structural (building) systems from Reference 11 is included below for information and guidance.




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C - 1.4.4.3 Seismic Response Coefficient

The Seismic Response Coefficient is the basis for determining the structure design loads for both the Equivalent Lateral Force Procedure and the Modal Analysis Procedure. The Equivalent Lateral Force Procedure only uses a single value based on the natural period of vibration of the structure for each of the two principal directions of the structure. The Modal Analysis Procedure combines values for multiple modes of vibration in each of the two principal directions of the structure.

For areas with soft soil conditions and high seismicity, or close proximity to known faults, or for special bridge projects, a site-specific hazard analysis is preferred. The analysis should be based on accepted practice using the ground motion return period determined in accordance with Article 1.3.2.2 “Structure Importance Classification.” A good discussion of site-specific hazard analysis is contained in Reference 4 and Reference 11.

The formula for the Seismic Response Coefficient is adopted from Reference 4, rearranged to more closely resemble previous editions of this chapter and modified by the Damping Adjustment Factor from Reference 11. The coefficient is based on 5% critical damping. There are exceptions to the formula; however, they were not included since the exceptions differ from code to code and unnecessarily complicate the Seismic Response Coefficient. The values obtained using the basic formula are conservative compared to the exceptions. The exceptions from various codes are listed below for information. If the bridge designer believes that the exceptions are needed for a particular site, they may be included or preferably use site-specific response spectra.

Table 9-C-1. Damping Values for Structural Systems

Structural System Elastic-Linear Post-Yield

Structural Steel 3% 7%Reinforced Concrete 5% 10%Masonry Shear Walls 7% 12%

Wood 10% 15%Dual Systems See note 1 See note 2

Notes:1. Use the value of the primary, or more rigid, system. If both

systems are participating significantly, a weighted value, proportionate to the relative participation of each system, may be used.

2. The value for the system with the higher damping value may be used.




C - 1.4.4.4 Low Period Reduced Response

Railroad bridges are often more rigid than typical multi-level buildings or highway bridge structures. Therefore the response of railroad bridges in the low period range needs to be thoroughly addressed. Most general response spectra curves, such as those defined in Reference 13 have reduced responses in the low period range. Typically, these curves vary linearly from the peak ground acceleration at zero period to a maximum constant acceleration response at the initial transition period, To as shown in Figure 9-C-2. Other response spectra curves, such as those given in Reference 5 show a flat region for very low periods that represent perfectly rigid response. The AREMA seismic response coefficient defined in Article 1.4.4.3 does not include the reduced response at low periods since it was felt that typical railroad bridge analysis underestimates the actual period of the bridge. Underestimation of the structure period can result in unconservative response for low period structures when the reduced response region of the response spectra is used. This section was developed to allow the bridge designer to take advantage of the reduced response for low period structures when appropriate. The provisions listed in Article 1.4.4.4 account for the most common sources of flexibility in the structure, however, the bridge designer should consider any other component that will increase the structure period.

Typical railroad bridge analysis uses the gross moment of inertia for reinforced concrete members to determine the stiffness and load distribution. Use of the gross moment of inertia for a reinforced concrete substructure member will underestimate the structure period when the flexural tension stress exceeds the concrete modulus of rupture. The effective moment of inertia, as determined from EQ 2-12 in Chapter 8, Part 2, Article 2.23.7c, of reinforced concrete members will provide a more representative structure period. The cracked moment of inertia used in EQ 2-12 may be determined from moment-curvature analysis of the member using the following relationship.

My1 = Moment at first yield of reinforcing steely1 = Curvature at first yield of reinforcing steelEc = Concrete modulus of elasticity (Chapter 8, Part 2, Article 2.23.4)

It is common practice to model bridge foundations as either pinned or fixed. If the foundation stiffness is overestimated, then the structure period will be underestimated. Foundation flexibility for spread footings may be accounted for by including a rotational footing stiffness calculated in accordance with accepted procedures, such as those defined in Section 5.3 of Reference 17. Lateral translation flexibility of a spread footing need not be considered provided that the base soil friction is not exceeded. Foundation flexibility for pile footings may be accounted for by using accepted procedures, such as including a rotational pile cap stiffness that is derived from realistic pile load-deflection (t-z) data. When vertical piles are used, the lateral translation foundation stiffness should be determined from realistic pile lateral load-deflection (p-y) data, supplemented, if appropriate, by lateral soil resistance on the pile cap. If either of these foundation types is founded on sound rock, the effects of foundation flexibility can be neglected.

Lateral flexibility of the bridge spans may amplify the seismic response between the bridge piers. For example, a point in the middle of the span may have a higher response acceleration than the point at the top of the pier. This effect is typically accounted for by performing modal analysis of the bridge using a model with at least four elements making up the span length.

Table 9-C-2. Exceptions to Seismic Response Coefficient

Source ExceptionReference 2 For long period bridges (greater than about 3 seconds) response accelerations are proportional to

1/T2.Reference 12 For structures where any modal period of vibration (Tm) exceeds T1, the long period transition

period (varies between 4.0 seconds and 16.0 seconds), the Seismic Response Coefficient for that mode is permitted to be determined by the following equation:

CmFvS1DTL

Tm2

------------------------=




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Foundation rocking is a response that occurs when the applied moment on a spread footing exceeds the overturning moment resistance. Rocking response will increase the period of the foundation and most likely take it out of the low period reduced response range.

The low period reduced response defined in this Article has been developed based on review of the response spectra from other codes along with visual inspection of a number of response spectra generated from actual strong motion records. The perfectly-rigid period limit of 0.03 seconds corresponds to a frequency of 33 Hz and has generally been considered appropriate for this type of response. Evaluation of response spectra generated from actual strong motion records indicates that this is conservative except for sites very close (< 10 miles or 16 km) to the fault. The only structures that are expected to fall in the perfectly-rigid range are rigid piers with spread footings or piles founded on rock. Other rigid piers will generally fall in the low period linear transition region due to foundation flexibility.




0.00 < T 0.03 Perfectly-rigid region

0.03 < T To Low period linear transition region

To < T Ts Constant acceleration region

To = Initial transition period = 0.2(FvS1/FaSs)

Ts = Constant acceleration transition period = FvS1/FaSs

T = Period of vibration

SS = Short-period (0.2 second) Spectral Response Acceleration Coefficient determined in accordance with Article 1.3.2.3

S1 = Long-period (1.0 second) Spectral Response Acceleration Coefficient determined in accordance with Article 1.3.2.3

Fa = Site Factor for short-period range of acceleration spectrum determined in accordance with Article 1.4.4.1

Fv = Site Factor for long-period range of acceleration spectrum determined in accordance with Article 1.4.4.1

Figure 9-C-2. Example Response Spectra with Low Period Reduced Response




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C - 1.4.5 ANALYSIS PROCEDURES (2003) R(2014)

C - 1.4.5.1 General

C - 1.4.5.1.1 Serviceability Limit State

Within the serviceability limit state the response of a bridge is limited to its elastic range. Therefore, methods based on elastic analysis are most appropriate. The methods specified depend on the bridge configuration. The Equivalent Lateral Force Procedure expresses earthquake loads in terms of structure mass and Seismic Response Coefficient for the site. It will probably be applicable to the majority of the existing railroad bridges. The Modal Analysis Procedure is a more accurate approach that can evaluate irregular bridges, effects of higher modes of vibration and specific ground motion characteristics. Other analysis procedures such as time-history analysis or deformation-based methods may be appropriate for certain structures and/or site conditions, but are not addressed herein.

C - 1.4.5.1.2 Ultimate and Survivability Limit State

The response of a bridge near its ultimate limit state is highly nonlinear and uncertain due to incomplete knowledge of inelastic structural action. Seismic highway bridge design codes specify the use of elastic analysis for the ultimate loads, and response modification factors that account for nonlinear behavior. Satisfying the ultimate state criteria is practically the main requirement of these codes, and there is on-going research to improve the analysis models and to get more reliable estimates of the response modification factors recommended. Using a similar approach for the evaluation of railroad bridges for the ultimate limit state would require more research into nonlinear response of railroad bridges to extreme horizontal loads. Also, for railroad bridges, satisfying the serviceability limit state, that is concerned with the continuing operation of trains after a seismic event, is the main design condition. The serviceability limit state criteria is associated with very low risk levels of being exceeded, and it will most likely be more restrictive than the other limit states. Using a conceptual design approach for the ultimate and the survivability limit states can overcome the high level of uncertainties involved in numerical analysis of the nonlinear bridge response. Conformance with the ultimate and the survivability limit states is based on requirements for type, geometry, materials, ductility and redundancy. The conceptual design methods recommended to ensure satisfactory performance for the ultimate and the survivability limit states are based primarily on experience from past earthquakes and from research and testing results applicable to railroad bridges. Commonly accepted detailing provisions and guidelines for a specific seismic region which are consistent with railroad practices may be used until more specific requirements for adequate details, connections, ductility and redundancy are developed herein.

The requirement for non-ductile, non-redundant primary load carrying elements of structures to be designed for higher seismic loads is necessary to ensure survivability of some structures during an extreme event. The design forces to be used in this case are the lesser of the seismic forces or the maximum load which can be transmitted to the element. Non-ductile, non-redundant primary load carrying elements are bridge components whose failure can cause structure collapse. An example of such a component is a poorly reinforced single column concrete bent.

C - 1.4.5.2 Procedure Selection

The procedure used to analyze the structure is based on the bridge configuration. Single-span bridges do not require formal analysis, however they should be investigated using commonly accepted empirical formulations to ensure that the abutment seat widths are adequate to prevent span collapse. Two-span bridges are considered regular since they have only one bent, which precludes stiffness irregularity. Irregular bridges may be those with bridge vulnerability aspects as listed in Article 1.4.3.1. A more specific description of bridge irregularity may be found in other codes such as Reference 2.

C - 1.4.5.3 Equivalent Lateral Force Procedure

The Equivalent Lateral Force Procedure is included as a simple method of analysis that may be used for regular bridges. The calculations for this procedure are appropriate for hand calculation methods in most cases, though static computer analysis may be used to determine the load distribution to the individual members.




The two principal directions of the structure are typically the longitudinal and transverse directions of the bridge. For curved bridges, the longitudinal direction may be taken as a straight line connecting the centerline of the bridge at the beginning and end.

The natural period of vibration (Tm) for each of the two principal directions of the structure may be calculated using any commonly accepted method. The following simple formulation may be used.

The seismic response coefficient, Cm, applied to the substructure of single level bridges may be reduced to the average of the Cm value calculated in Paragraph 1.4.5.3a for the superstructure and the peak ground acceleration coefficient multiplied by the appropriate site factor, FpgaPGA, determined in accordance with Article 1.3.2.3 for the ground, but shall not be less than the peak ground acceleration coefficient, multiplied by the appropriate site factor, FpgaPGA. The actual seismic response coefficient, Cm, varies throughout the structure in proportion to the relative lateral movement. A common method of equivalent lateral force analysis assumes that one-half the weight of the substructure is lumped at the superstructure level for the period calculation and the foundation load is calculated using the complete bridge weight with the seismic response coefficient determined for the superstructure. This analysis approach is accurate when the substructure weight is small relative to the superstructure weight, but may be too conservative for heavy pier substructures. Rather than using the more accurate modal analysis approach, a simple modification to the equivalent lateral force procedure may be used to minimize the foundation demand for bridges supported by large pier substructures. It is conservative to assume that the actual seismic response coefficient, Cm, varies linearly from the peak ground acceleration coefficient, multiplied by the appropriate site factor, FpgaPGA, at the ground level to the seismic response coefficient calculated at the superstructure level as long as the response at the superstructure level exceeds the peak ground acceleration coefficient multiplied by the appropriate site factor. Therefore the average of these two acceleration values may be applied to the weight of the pier to more accurately determine the demand at the foundation.

The seismic load should be distributed to the individual members based on the stiffness and support conditions. For a regular structure with uniform weight per unit length and simple supports, this reduces to a simple beam calculation for the superstructure between supports and a single lateral load calculation for the supporting bents.

C - 1.4.5.4 Modal Analysis Procedure

The Modal Analysis Procedure is included as a general method of analysis that may be used for any bridge configuration. The calculations for this procedure are appropriate to be performed by any commonly available finite element computer program.

The response spectra is developed from Paragraph 1.4.4.3 “Seismic Response Coefficient.” The value of the Seismic Response Coefficient (Cm) should be calculated for a range of period (Tm) values to adequately define the spectral shape for the range of period (Tm) values needed to represent the structure. Figure 9-C-3 gives an example spectral shape for values of Fv, F1, S1 and D all equal to 1.0 and SS equal to 2.5.

W= Total weight of the bridge.g= Acceleration due to gravity (length/time2)K= The total structure stiffness including the stiffness of the superstructure, supporting members and

surrounding soil.

Tm 2 WgK-------=




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C - 1.4.6 LOAD COMBINATIONS AND RESPONSE LIMITS (2002) R(2014)

The load combination used for the level 1 ground motion should be consistent with the probability of occurrence of the earthquake. For this reason, live load is usually not included in the load combination. Certain situations, such as long viaducts with high traffic volume or bridges in yard and terminal areas, may require consideration of combinations which include live loading. Extreme loads, such as wind and stream flow pressure, are not normally combined with the seismic loading. In cases where a certain minimum level of stream flow is constant, that minimum level should be included in the earthquake load combination. Friction forces can vary significantly due to contact surface conditions and vertical earthquake accelerations, therefore the use of friction should be carefully considered if it reduces the effects of the earthquake load.

The stress limits are provided to satisfy the performance requirements of the serviceability limit state. The seismic loads are calculated at the yield level rather than at the working stress level, so it is appropriate to use a 50% allowable stress increase for steel and a 1.0 load factor for concrete.

Specific lateral deflection limits are not provided, however, the bridge must satisfy the performance requirements of Article 1.3.3. P effects should be considered if they are significant enough to affect the performance of the bridge. Columns designed in accordance with Article 1.4.7.3.1 may account for P effects using conventional methods for the level 1 earthquake, however, this is not appropriate for the higher level earthquakes. The only reliable way to account for P effects in the inelastic range of the columns for the higher level earthquakes is to perform nonlinear time history analysis. A practical limit from Reference 9 may be used which requires that the P moment should not exceed 20% of the plastic moment capacity of the column for the maximum credible earthquake. To perform a similar comparison, the column P moment for the level 1 earthquake should be multiplied by the ratio of the level 3 seismic response coefficient divided by the level 1 seismic response coefficient and should not exceed 20% of 1.3 times the nominal moment capacity of the column.

Figure 9-C-3. Example Response Spectra




The lateral deflection of the bridge must not preclude train operations outlined in Article 1.2.2.1. Because the fixed steel rails are the riding surface over which rail equipment operates, railroad bridges have inherently strict limitations on the tolerable, permanent displacement and distortion they can undergo in a seismic event, and still remain serviceable. After a level 1 ground motion event, the trains are allowed to continue at restricted speed. For most bridges, there is a very low probability that the train will be on the bridge during the earthquake. Therefore the track deflection to be considered is the permanent deflection which will remain after the earthquake has occurred. It is the responsibility of the bridge designer to determine how much permanent track deformation will result from the elastic deflection of the structure. For bridges where a train is considered to be on the bridge during the earthquake, the deflection limitations must be satisfied directly. The Code of Federal Regulations, Title 49, Part 213, Section 55 (49 CFR 213.55) provides alignment requirements based on the class of track and 49 CFR 213.9 defines the speed limits for each class of track. Table 9-C-3 includes the information from the 49 CFR and is provided herein as an indication of order of magnitude limits to track misalignment tolerable for 'safe' conditions at various speeds. However, the designer must establish with the railroad(s) the tolerable limits for permanent track deformations used in the design. The individual railroads may have maintenance limits on various railways for horizontal, vertical and superelevation alignments that are more restrictive than the FRA standards. Table 9-C-3 can be used to determine the track alignment requirements for a given train speed. For example, the track on a bridge supporting a freight train operating at a restricted speed, which cannot exceed 20 mph (32 kph) after an earthquake, would have to satisfy the alignment requirements of a class 2 track, which is no more than a 3" (76 mm) mid-offset on a 62 ft. (18.9 m) long tangent section of track.

Table 9-C-3. FRA Horizontal Track Alignment Requirements

NOTE:

(1) The deviation of the mid-offset from 62 foot (18.9 m) line. The ends of the line must be at points on the gage side of the line rail, five-eighths of an inch (16 mm) below the top of the railhead.

(2) The deviation of the mid-ordinate from 62 foot (18.9 m) chord. The ends of the chord must be at points on the gage side of the outer rail, five-eighths of an inch (16 mm) below the top of the railhead.

C - 1.4.7 DETAILING PROVISIONS (2013)

The detailing provisions are required to meet the performance requirements of the Level 2 and 3 Ground Motion. These provisions are based on accepted practice in high seismic areas and recent research. The structure design need not meet the required provisions provided that the structure is capable of resisting the Level 3 Ground Motion loadings in the elastic range.

C - 1.4.7.1 Continuity Provisions

Continuity provisions for transferring lateral forces from the superstructure to the ground are necessary to ensure structural integrity during a seismic event. All portions of the load path must be investigated to see that the lateral forces can be transferred. This is especially true for the load path from the superstructure span to the substructure, which is often not investigated for static loads. Friction should be neglected as a means to transfer lateral forces where there is a potential for uplift. At locations where movements are allowed, they should be accommodated or limited.

Class of Track

Maximum Operating Speedmph (km/h)

Maximum Horizontal Track Deviation from Alignment, in (mm)

Freight Passenger Tangent track1 Curved track2

1 10 (16) 15 (24) 5 (127) 5 (127)2 25 (40) 30 (48) 3 (76) 3 (76)3 40 (64) 60 (97) 1.75 (44) 1.75 (44)4 60 (97) 80 (129) 1.5 (38) 1.5 (38)5 80 (129) 90 (145) 0.75 (19) 0.625 (16)




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C - 1.4.7.1.1 Superstructure

Critical members which transfer lateral forces from the superstructure to the substructure and are non-ductile must be designed for the Level 3 Ground Motion forces or the maximum loads which can be transmitted to the member. Lateral bending of the girders is the load path for concrete box girders. Lateral bending resistance may also be used for other structures as long as the loads are investigated. For example, shorter open deck steel girders will often have the capability to transfer lateral loads without additional bracing since the live load is usually not combined with the seismic load.

C - 1.4.7.1.2 Bearings

Bearings are often the critical component in transferring seismic loads to the substructure. They shall be configured to transfer the lateral loads to the substructure or accommodate movement while allowing access for maintenance and replacement.

Seismic isolation bearings are not commonly used in railroad bridges due to the restraint or limits in span or bearing translation induced by other railroad bridge components such as continuous rail. If these bearings are used, the design of the superstructure, substructure, and bearings must consider the translation restraining effects of the railroad bridge components that limit the span or bearing displacements that seismic isolation bearings are intended to accommodate.

C - 1.4.7.2 Ductility Provisions

The importance of ductility during bridge response to large magnitude earthquakes is well recognized. During large earthquakes stresses in bridge members and connections exceed the elastic range and structures could experience large inelastic deformations. The ductility of a structure is usually defined in terms of the ratio between maximum deformation without failure and yield deformation. It depends on the individual member ductility and their loading condition, the ductility of the connection details and also on the structure configuration. For example, nonductile and poorly braced members loaded in compression may experience sudden failure even prior to reaching yield stresses. A ductile structure can undergo large inelastic deformations without significant strength degradation.

Ductile behavior reduces seismic loads and provides an energy dissipation mechanism. To achieve good ductility, locations that are expected to experience plastic deformations need to be adequately designed and detailed, and instability or brittle failure modes need to be prevented. At the same time the structure should have sufficient stiffness to maintain stability and avoid excessive drift.

The ductility provisions are required to ensure that the structure will meet the performance requirements of the Level 2 and 3 Ground Motion. These provisions are based on accepted practice in high seismic areas and recent research. The requirements for structure ductility for reinforced concrete, steel or timber structures are different, since they must take into account the inherent material properties and the typical structural configurations.

The requirements for reinforcement details in concrete structures in seismically active regions are well established in design codes and State guidelines for seismic design of highway bridges. These requirements should be followed in a manner consistent with railroad design and detailing practices. In general, these requirements are intended to increase ductility and reduce the likelihood of brittle shear failures.

The ductility requirements for steel structures are intended to prevent buckling and fracture and provide adequate connections and details. Due to differences in geometry, stiffness, ductility, mass and damping characteristics, the seismic behavior of steel bridges is fundamentally different from that of concrete bridges. One main difference is that steel bridges can yield and dissipate energy at various locations throughout the structure, and therefore plastic hinge regions do not need to be restricted only to the columns. Also, in steel members, the shear yielding mechanism is preferable, since it provides substantial stable energy dissipation, which is different from concrete members where flexural failure modes are desired and shear failure is avoided.

Seismic design and detailing requirements for steel bridges are not as well established and codified as those for concrete bridges. This is probably because of the inherent ductility of structural steel and the relatively good performance of steel




bridges during past earthquakes. In addition, by following relatively simple design and detailing guidelines, significant ductility levels can be achieved. Such guidelines include the following recommendations:

• Limit the width to thickness (b/t) ratios for plates in compression;

• Limit the slenderness ratio for main compression and bracing members;

• Avoid using details susceptible to fracture in areas expected to respond in the plastic range;

• Avoid field welds and other fatigue prone details;

• Design steel members such that yielding of the gross section occurs before local buckling or fracture;

• Avoid triaxial tension stress conditions that may occur at locations such as near the intersection of welds in thick elements. They can inhibit the ability of steel to exhibit ductility.

• Use stiffeners that are more rigid than the minimum needed to prevent buckling.

• Limit the axial compression load in columns to a percentage of their yield capacity;

• Provide means for an alternative load path in case of damage;

• Ensure that when damage occurs, the damage is confined to secondary, non-gravity carrying elements, such as bracing members;

• Consider using the end diaphragms or cross frames as locations for ductile behavior.

C - 1.4.7.2.1 Longitudinal Reinforcing Confinement

The provisions in this Article were adapted from Sections 21.6.4 and 21.6.5 of Reference 3 with minor changes in notation and terminology to be consistent with Chapter 8 notation and railroad bridge terminology. Notation which is not defined in this section is defined in Chapter 8, Article 2.2.1 and additional commentary is contained in Sections R21.6.4 and R21.6.5 of Reference 3.

Longitudinal reinforcing confinement is critical to ensuring that the concrete column will respond well in the post-yield range. Concrete piles with fixed heads will develop high bending moments at the cap interface, therefore they should be adequately confined to reduce the possibility of permanent damage. Extended concrete piles should be treated as regular columns above the ground. The reduced requirements for concrete pier walls with low axial loading have been shown by testing to exceed a ductility factor of 2.

C - 1.4.7.2.2 Splices in Reinforcing

The concrete cover tends to spall off of concrete members responding in the post-yield range. This eliminates the load transfer of lap splices and can cause premature failure.

C - 1.4.7.3 Provisions to Limit Damage

To limit damage during Level 2 Ground Motion, the distribution of strength and stiffness should be such that damage occurs at predetermined locations, and certain critical load carrying members are “protected” from inelastic response. The predetermined damage locations must be well detailed to sustain large inelastic deformations without strength degradation, and at the same time they should be the weakest links within their respective load paths in order to restrict damage to other members. In addition, the distribution of stiffness and strength should be such that plastic response or damage does not occur in locations inaccessible for inspection and repair.




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Since seismic demands are reduced when movements and ductile deformations are allowed, such damage control criteria can achieve good and reliable seismic performance at relatively low costs. The use of sacrificial elements, which could be easily replaced in the event of damage, may also offer a cost-effective way of enhancing the bridge seismic response and providing protection to other members. Knowledge of likely failure locations and modes also allows for the design of connection details and jacking locations for temporary support during repairs.

C - 1.4.7.3.1 Weak Column Provisions

Bridges in high seismic areas are typically designed so that plastic hinging is allowed in the reinforced concrete columns. The provisions for reinforcing steel material with maximum yield strength are adapted from Chapter 21, Section 21.1.5 of Reference 3, and are necessary to limit the post-yield loads delivered to the adjacent bent cap and foundation. The bent cap and foundation may be designed for 1.3 times the nominal column strength to ensure that they will not be damaged during plastic hinging. This requirement is also applicable for the superstructure when it is built integrally with the bent cap, as with cast-in-place box girder structures. Extended pile columns are not allowed to yield below the ground, since the area is inaccessible for inspection and repair.

C - 1.4.7.3.2 Concrete Joints

The provisions in this Article were adapted from Section 21.7 of Reference 3 with changes in notation and terminology to be consistent with Chapter 8 notation and railroad bridge terminology. Some of the ACI 318 provisions were modified or omitted to be consistent with the other provisions of this Chapter. For example, provision 21.7.2.1 of Reference 3 is omitted since it conflicts with the column overstrength requirements of Paragraph 1.4.7.3.1b and provision 21.7.4.1 of Reference 3 is modified since the joint shear reinforcement requirements of this Article allow for higher joint stresses. Notation which is not defined in this section is defined in Chapter 8, Article 2.2.1 and additional commentary is contained in Section R21.7 of Reference 3.

Concrete joints must be adequately detailed to reduce the likelihood of damage extending into the superstructure and bent cap. The additional so called “joint shear” requirements for integral bent caps and superstructure have been used on California bridges since the Northridge earthquake. Further details on these requirements may be obtained from Reference 20.

C - 1.4.7.4 Redundancy Provisions

Redundancy provisions are suggested to provide additional safety against failure during the Level 3 ground motion event. These provisions are particularly important when the Level 3 ground motion acceleration is much greater than the Level 1 ground motion acceleration.

C - 1.4.7.4.1 Bearing Seats

The provisions in this Article were adapted from Division I-A, Section 7.3.1 of Reference 1 with minor changes in terminology to be consistent with railroad bridge terminology. Some of the AASHTO provisions were omitted since they are already addressed with the other provisions of this Chapter. For example, the AASHTO linkage provisions are omitted since they are already addressed in Article 1.4.7.4.3.

Wide bearing seats will provide additional redundancy if bearing anchor bolts or shear rods fail during a high level ground motion. The AASHTO requirements provide an empirical equation for determining the minimum seat width as a function of the bridge length, height and skew. Seismic analysis of the bridge may also be used to determine the maximum relative movements. The bearing seat width requirements are not necessary if the superstructure is adequately connected to the substructure to prevent relative movement.

C - 1.4.7.4.2 Shear Connectors

Shear connectors are often used in high seismic areas to transfer the seismic loads from the superstructure to the substructure. Reinforced concrete shear keys should be placed as close to the girders as practical so that the bearings do not fail before shear key engagement. Shear connectors may also take the form of rods or pipes embedded through the superstructure of concrete box girder structures supported on elastomeric bearings.




C - 1.4.7.4.4 Foundation Rocking

Rocking response is a form of seismic isolation which reduces the response frequency of the bridge while dissipating energy. Bearing blocks are required on new bridge construction to reduce the permanent soil deformation which will result at the toe and heel of the rocking footing. This response mode is especially useful for evaluating existing bridges with large, non-ductile, single pier foundations. Further information on foundation rocking may be obtained from Reference 21.

C - 1.4.7.4.5 Continuous Welded Rail

The presence of track on railroad bridges has long been considered a distinguishing characteristic between the seismic response of highway bridges and railroad bridges. Properly detailed continuous welded rail will provide a continuous load path for longitudinal loads on the bridge. Further research into the load transfer mechanisms is required to adequately quantify the effect of CWR at this time, however, the presence of CWR is considered a desirable feature to add redundancy and increase damping in the longitudinal direction of short, straight bridges. Reference 8 allows an increase in damping of between 10 and 15 percent for straight bridges less than 300 feet if the abutments are capable of mobilizing the soil and are well tied into the soil. This increase in damping may be applied to straight railroad bridges less than 300 feet in length with CWR to reduce the seismic loading.

C - SECTION 1.5 EXISTING BRIDGES

C - 1.5.2 INVENTORY (2018)

Most railroads have a good inventory of their own bridges. However, in several earthquakes where damage to railroad bridges was minor, bridges owned by others, including those over, under or adjacent to railroad operations, have collapsed. The presence of any bridges whose collapse could adversely affect operations should be determined and recorded.

C - 1.5.3 HISTORY (1995) R(2012)

Areas with frequent significant seismic activity are more appropriate for historical analysis than areas that have rare, but severe, earthquakes, such as parts of central and eastern North America.

C - 1.5.4 ASSESSMENT AND RETROFIT (2018)

C - 1.5.4.3 Investigation of Railway Owned Bridges

Previous wording allowed that the Engineer could declare a bridge resistant to a specific level of seismic load when justified by historic event/results data and not perform an analysis in accordance with Section 1.4. Given the variables in earthquake magnitude, proximity, depth and wave propagation, the historical data may be insufficient to definitively prove a bridge completely seismic resistant. The revised wording allows the Engineer to consider bridges that have such a historical record to be lower in priority for evaluation or analysis.

C - SECTION 1.6 OTHER FACILITIES AND INFRASTRUCTURE

C - 1.6.3 TRACK AND ROADBED (2007) R(2016)

Although the track structure, with the possible exception of the ballast, is rarely affected by shaking, the distortion of the underlying ground may severely impact track geometry. Longitudinal distortion can cause track buckling, or high tensile stresses in the rails with the resulting risk of the track pulling apart. Lateral movements and/or settlement due to liquefaction or embankment failure can cause serious defects in line, surface and cross-level.

Fills supporting track are subject to two types of failures as a result of seismic activity. They are horizontal or vertical misalignment of the embankment and loss of fill materials by soil liquefaction.




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Misalignment could result from:

a. Movement associated with tectonic plate acceleration differentials at, or near, fault lines.

b. Local soil shear failure from forces generated by earth mass acceleration differentials.

c. Slope failures of the fill embankment.

d. Soil liquefaction. (Liquefaction requires the existence of a specific set of soil grain sizes and soil moisture conditions at the time a vibratory energy source is applied.)

e. Water damage caused by failed retaining structures, distribution systems or redirected water courses.

Track in earth cuts is subject to the same misalignment from tectonic plate movements as is track on fills. Local soil shear failures and liquefaction may also occur, resulting in covering the track structure with debris.

It is suggested that efforts to analytically predict these failures is of little value, as there is no practical retrofit design that would prevent the movement.

Whatever the type of movement imposed on the track it is likely the disturbance will affect the rail’s neutral temperature, for continuously welded rail (CWR), or the joint gapping for jointed rail, thus reducing the rail’s resistance to buckling at high temperatures (sun kinks). When realigning the track to the pre-earthquake alignment, CWR must be cut and stressed to the neutral temperature, and jointed rail regapped to the requirements specified by the railroad.

C - 1.6.4 CULVERTS (2013)

Culverts fall into three general types; rigid pipe, flexible pipe (including pipe arches) and box. The mechanisms by which these different types resist seismic loading are also different. Rigid pipe culverts primarily resist loads through their bending strength. Flexible pipes and pipe arches are primarily supported by passive pressures exerted on the surrounding soils under loading. Structurally, boxes do not depend on soil interaction for design capacity.

The primary modes of failure for culverts under seismic loading are;

• Failure of the structure through buckling or distortion.

• Pulling apart of joints through movement or displacement of surrounding earth.

• End failures through slope failure either burying the end of a culvert or the separation of end sections because of slope movement.

Hence, the integrity of the barrel and the support provided under the culvert at the time of construction are key factors in performance. Culverts should be installed in accordance with the AREMA Manual for Railway Engineering, Chapter 1, Part 4.

Partially collapsed or damaged culverts can be difficult to repair; thus, where the barrel is in good enough condition to accept a structural sleeve and hydrolic conditions permit this is the recommended repair method. Structural sleeves should be inserted and then grouted in place to assure the flow of water through the sleeve.

C - 1.6.5 RETAINING WALLS (2007) R(2016)

Design of retaining walls to fail by sliding instead of overturning or failure of the stem of cantilever walls is analogous to the use of strong column-weak beam moment resisting frames in buildings. If a wall supporting the railway embankment slides during an earthquake, a large amount of energy is absorbed and track damage is limited to loss of line and surface in amounts




that may be readily corrected. If a wall supporting a hillside above the track slides, the resulting reduction of clearance may be corrected by realigning the track. In either case, restoration of service will likely be considerably faster than in the case of collapse or overturning of the wall.

C - 1.6.6 TUNNELS AND TRACK PROTECTING SHEDS (2007) R(2016)

C - 1.6.6.1 Tunnels

Tunnels usually are subjected to less severe loading from earthquakes than structures on the surface of the ground. However, they have been damaged by shaking and severely damaged by displacements at locations where they were intersected by fault ruptures.

Tunnel lining damage, possibly due to earthquake accelerations even at some distance from the seismic event, has occurred where the tunnel floor slab has been removed to increase vertical clearances within the tunnel.

C - 1.6.7 BUILDINGS AND SUPPORT FACILITIES (2007) R(2016)

Structures located near the fault rupture are likely to suffer serious damage in a major earthquake. Safety of operation ultimately depends on post-event inspection of facilities in areas subjected to major ground movements and/or severe shaking. Proper design can reduce, but not totally eliminate, the probability of significant damage.

© 2018, American Railway Engineering and Maintenance-of-Way Association 9-G-1

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Chapter 9 Glossary

The following terms are used in Chapter 9 Seismic Design for Railway Structures and are placed here in alphabetical order for your convenience.

AmplitudeMaximum value of a function as it varies with time.

AttenuationA decrease in amplitude of the seismic waves with distance due to geometric spreading, energy absorption and scattering.

CollapseMajor change in the geometry of a bridge rendering it unfit for use.

DampingResistance which reduces vibrations by energy absorption.

DuctilityProperty of a member or connection that allows inelastic response.

Ductility RatioThe ratio between the maximum displacement for elastoplastic behavior and the displacement corresponding to yield point.

Dynamic MagnificationAn increase in the induced lateral forces in a structure due to frequency matching between the ground and structure.

ElasticityThe ability of a material to return immediately to its original form or condition after removal of the loads.

ElastoplasticImplies elastic behavior for a force that does not exceed a maximum value and plastic behavior above this maximum.

EpicenterThe point on the Earth’s surface located vertically above the point where the first rupture and the first earthquake motion occur.

FaultA fracture or fracture zone in the earth along which there has been displacement of the two sides relative to one another and which is parallel to the fracture.



9-G-2 AREMA Manual for Railway Engineering

Flexible StructureA structure that will sustain relatively large displacements without failure.

Fundamental PeriodThe longest period (duration in time of one full cycle of oscillatory motion) of vibration of a structure which has several modes of vibration, each with a different period.

Ground MovementTerm that refers to all aspects of ground motion, e.g. particle acceleration, velocity, displacement due to earthquakes.

Hoop ReinforcementCircular or rectangular transverse reinforcement capable of confining the concrete core after the concrete cover has spalled off. Circular hoop reinforcement shall either be welded or mechanically coupled with no lap splices. Rectangular hoop reinforcement shall consist of single or multiple overlapping stirrups which are closed by 135 hooks around a longitudinal reinforcing bar with no lap splices and cross-ties consisting of single-leg stirrups with a 90 hook around a longitudinal reinforcing bar on one end and a 135 hook around a longitudinal reinforcing bar on the other end. Cross-ties shall be alternated end for end along the longitudinal reinforcement.

Inelastic BehaviorBehavior of a member beyond its elastic limit.

IntensityQualitative or quantitative measure of the severity of seismic ground motion at a specific site. The most common intensity scale used in the United States today is the Modified Mercalli, 1956 version.

Limit StateA condition beyond which a bridge, member or connection ceases to satisfy the performance requirements for which it was designed.

LiquefactionTransformation of a granular soil from a solid state into a liquefied state as a consequence of increased pore-water pressure induced by vibrations.

MagnitudeQualitative measure of the size of an earthquake, related indirectly to the energy released, which is independent of the place of observation, e.g. Richter Magnitude Scale.

Mean Return Period, TThe average time (in years) between occurrences of an event of a given size or a condition associated with a given severity. The inverse of the mean return period is the average annual probability of exceedance. For an estimate of the probability of exceedance, p, during an exposure time, t (in years), the following relation may be used: p = 1–(1–1/T)t. An event with a particular mean return period has a 63% probability of being exceeded during an exposure time equal to that return period.

Moment Magnitude Scale (MW)A measure of the size of an earthquake based on the area of fault rupture, the average amount of movement, and the force that was required to overcome the friction holding the rocks together.

Glossary


AREMA Manual for Railway Engineering 9-G-3

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Natural FrequencyThe frequency (number of cycles per second) of free vibration of a structure if damping effects are neglected. Sometimes expressed in radians per second.

Natural PeriodThe time interval (in seconds) for a vibrating structure in free vibration to do one oscillation. The inverse of the natural period is the natural frequency.

Occupancy RateAverage number of persons occupying a structure each 24-hour day of the year.

Predominant PeriodsThe most significant periods of the earthquake ground motion.

Regular BridgeA bridge that has no abrupt or unusual changes in mass stiffness or geometry along its span and has no large differences in these parameters between adjacent supports.

ResonanceA state of maximum amplitude of vibration caused by the matching of the excitation frequency with the natural frequency of the structure itself.

Response SpectrumA plot showing maximum earthquake response with respect to natural period or frequency of the structure for a given damping. It reflects the response of an infinite series of single-degree-of-freedom systems subjected to a time history of earthquake ground motion.

Richter Magnitude Scale (ML)A measure of the size of an earthquake, also known as Local Magnitude. The measure is determined by taking the common logarithm (base 10) of the largest ground motion amplitude observed and applying a standard correction for distance to the epicenter.

Seismic HazardThe probability that given ground motion parameters at the site of a given bridge will be exceeded during a specified exposure time. May also be expressed in terms of average annual probability of exceedance of mean return period.

SeismicityFrequency of occurrence of earthquakes per unit area in a given region.

Serviceability Limit StateLimit state that relates to maximum stresses and deformations within the elastic range that ensures safety of trains traveling at reduced speeds.

Survivability Limit StateLimit state that relates to bridge collapse.



9-G-4 AREMA Manual for Railway Engineering

TsunamiA sea-wave caused by an earthquake, or a submarine landslide or eruption.

Ultimate Limit StateLimit state that relates to ultimate strength of material and stability of critical members. Structural damage that can be repaired within a short period of time is allowed.

VulnerabilityAmount of damage induced by a given degree of hazard.

Lifeline Earthquake Engineering,

Building an International Community of Structural Engineers,


Bulletin,

Earthquake Spectra,

Development of Seismic Response Criteria for North American Railroads,

Bulletin,


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