008 Seismic Analysis 30-06-13

Course # CE-5120

03 Credit Hours

Introduction to Bridge Engineering

Bridge Engg.Email: [email protected] 2Dr. Syed Mohammad Ali

Sequence of Presentation

• Examples on Distribution Factors

• Seismic Analysis

– Basic Steps

– Overview of various procedures

– Elastic Response Spectrum Procedure

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Example Problem

• Example 6.3

• Determine the AASHTO distribution factors for bridge shown in the figure.

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Example Problem



Example Problem

• Example 6.4

• Use the lever method to determine the distribution factors for the bridge shown in Figure 6.12(a).

• Exterior Girder

• Consider Figure 6.14. The deck is assumed to be simply supported by each girder except over the exterior girder where the cantilever is continuous.



Example Problem

• Considering truck 1, the reaction at A (exterior girder load) is established by balancing the moment about B:



Example Problem



Lateral Loads

• Seismic Loads

• The AASHTO provisions apply to bridges with conventional slab, girder, box girder, and truss superstructures whose spans do not exceed 500 ft (150 m) [A3.10.1].

• Bridges with spans exceeding 500 ft (150 m) and other bridge types, such as suspension bridges, cable-stayed bridges, movable bridges, and arches, are not applicable.

• SEISMIC DESIGN PROCEDURE


Waqas

Sticky Note

Regular means constant in properties stiffness etc


Lateral Loads

• Seismic Loads

Table 2.1-Seismic Zones

Seismic Zone Peak Horizontal Ground

Acceleration 1 0.05 to 0.08g

2A 0.08 to 0.16g

2B 0.16 to 0.24g

3 0.24 to 0.32g

4 > 0.32g



Lateral Loads


• The six steps in the seismic design procedure are discussed

• STEP-1:

• The first step is to arrive at a preliminary design describing:– the type of bridge,

– the number of spans,

– the height of the piers,

– a typical roadway cross section,

– horizontal alignment,

– type of foundations, and

– subsurface conditions.

• The nature of the connections between the spans of the superstructure, between the superstructure and the substructure, and between the substructure and the foundation are also important.



Lateral Loads


• For example, if a bridge superstructure has no deck joints and is integral with the abutments, its response during a seismic event is quite different from one with multiple expansion joints.

• There are also innovative energy dissipating connections that can be placed below the superstructure at the abutments and pier caps to effectively isolate the superstructure from the effects of ground shaking.

• These devices can substantially reduce the magnitude of the inertial forces transmitted to a foundation component and can serve as a structural fuse that can be replaced or repaired if a larger earthquake occurs.



Lateral Loads


Isolators being installed in Golden gate Bridge California (April 2006)



Lateral Loads


• STEP-2:

• The second step is to determine the acceleration coefficient, A.

• Contours of horizontal acceleration in rock expressed as a percent of gravity are illustrated on the map

• At a given location, the acceleration coefficient from the map has a 90% probability of not being exceeded in 50 years. This value corresponds to a return period of about 475 years for the design earthquake. There is a 10% probability that an earthquake larger than the design earthquake implied by the acceleration coefficient from the map will occur.

• In some cases, such as for bridges on critical lifelines, a larger acceleration coefficient corresponding to the maximum probable earthquake, with a return period of around 2500 years, must be used. 2% probability of being exceeded in 50 yrs.


Waqas

Highlight


Lateral Loads

• STEP-3:

• The third step is to determine the seismic performance zone for each bridge [A3.10.4].

• These seismic zones group together regions of the United States that have similar seismic risk.

• The greater the acceleration coefficient, the greater is the risk. The seismic zones are given in Table 4.11, and the higher the number the greater are the seismic performance requirements for the bridge in regard to the method of analysis, the length of bridge seats, and the strength of connections.


Waqas

Highlight


Lateral Loads

Table 2.1-Seismic Zones

Seismic Zone Peak Horizontal Ground

Acceleration 1 0.05 to 0.08g

2A 0.08 to 0.16g

2B 0.16 to 0.24g

3 0.24 to 0.32g

4 > 0.32g

Seismic Hazard Map BCP 2007



Lateral Loads

• STEP-4:

• The fourth step is to determine the importance category of a bridge [A3.10.3].



Lateral Loads

• STEP-4:

• Consideration should be given to possible future changes in the role of the bridge when assigning an importance category.



Lateral Loads

• STEP-5:

• The fifth step is to determine a site coefficient S, which is dependent on the soil conditions at the bridge site [A3.10.5].

• The acceleration coefficients given on the map are in rock that may be at some depth below the surface where the bridge is located.

• Depending on the nature of the soil overlying the rock, the acceleration at the surface can be amplified to as much as double the acceleration in the rock.

• The four soil profiles given in Table 4.13 are used to select an approximate acceleration coefficient modifier from Table 4.14.



Lateral Loads



Lateral Loads

• STEP-5:

• In locations where the soil conditions are not known in sufficient detail or the soil profile does not fit any of the four types, the AASHTO Specifications [A3.10.5.1] state that a type II soil profile shall be used.

• The use of this default site condition could be non-conservative and should not be used unless type III and type IV soil profiles have been ruled out by a geological or geotechnical engineer.


Waqas

Highlight


Lateral Loads

• ELASTIC SEISMIC RESPONSE SPECTRUM

• For design purposes, a curve is drawn through the average of the maximum response to give the elastic response spectrum shown by the smooth line in Figure 6.36(c).

• This response spectrum was developed for a single earthquake under one set of soil conditions.

• When a response spectrum is used for design purposes, it is usually based on more than one earthquake and includes the effects of different soil conditions.



Lateral Loads


Bridge Engg.Email: [email protected] 40Dr. Syed Mohammad Ali20-Jun-14

Q & A

Thanks!


008 Seismic Analysis 30-06-13

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