Advanced Concrete Bridge Design

University Of Anbar Lecture 1 /Assit. Pr. Dr. Yousif A. Mansoor 1

Assit. Pr. Dr. Yousif A. Mansoor

05:51:43

Assit. Pro. Dr. Yousif A. MansoorLecture 1/ university of Anbar 5

What is a Bridge?•Bridge is a structure which covers a gap

•Generally bridges carry a road or railway across a natural or artificial obstacle such

as, a river, canal or another railway or another road

•Bridge is a structure corresponding to the heaviest responsibility in carrying a free

flow of transport and is the most significant component of a transportation system

in case of communication over spacings/gaps for whatever reason such as aquatic

obstacles, valleys and gorges etc.

Bridge is the KEY ELEMENT in a

Transportation System

05:51:43

Assit. Pro. Dr. Yousif A. Mansoor

University OF Anbar

Assit. Pro. Dr. Yousif A.Mansoor

05:51:43

University OF Anbar

Assit. Pro. Dr. Yousif A.Mansoor

05:51:43

University OF Anbar

Assit. Prof. Dr. Yousif A.Mansoor

05:51:43

University OF Anbar

Assit. Prof. Dr. Yousif A.Mansoor

05:51:43

University OF Anbar

Assit. Pr.Dr. Yousif A.Mansoor

05:51:43

University OF Anbar

Assit. Pr.Dr. Yousif A.Mansoor

University Of Anbar

Assit. Pr. Dr. Yousif A.Mansoor

05:51:43

University OF Anbar

05:51:43

University OF Anbar

05:51:43

University OF Anbar

05:51:43

University OF Anbar

05:51:43

University OF Anbar

05:51:44

University OF Anbar

Dr. Yousif A.Mansoor

05:51:44

University OF Anbar

Assit. Pr Dr. Yousif A.Mansoor

05:51:44

General Bridge Components: These are supports on a bridge pier, which carry the

weight of the bridge and control the movements at the bridge supports,

including the temperature expansion and contraction. They may be

metal rockers, rollers or slides or merely rubber or laminated rubber (

Rubber with steel plates glued into it).

: Bridge dampers are devices that absorb

energy generated by earthquake waves and lateral load

: A wide column or short wall of masonry or plain or

reinforced concrete for carrying loads as a support for a bridge, but in

any case it is founded on firm ground below the river mud

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05:51:44

General Bridge Components

Bridge Deck: The load bearing floor of a bridge which

carries and spreads the loads to the main beams. It is either of

reinforced concrete., pre-stressed concrete, welded steel etc.

Abutment: A support of an arch or bridge etc which may

carry a horizontal force as well as weight.

Expansion Joints : These are provided to accommodate the

translations due to possible shrinkage and expansions due to

temperature changes.

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05:51:44

Components of a Girder bridge (Beam Bridge)

05:51:44

DESIGN PHILOSOPHY

A general statement for assuring safety in engineering design is that

Resistance (of material & x-section) ≥ Effect of applied load

• When applying this principle ,it is essential that both sides of inequality are evaluated for the same condition. For example if the effect of the applied load is to produce compressive stress on soil, then it should be compared with bearing capacity of soil.

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Dr. Yousif A. Mansoor

Philosophies of Design

For decades the allowable stress design (ASD), also called the workingstress design (WSD) or the straight line design, was used for designing

steel, concrete, wood, and masonry structures; it is still being used for

designing those structures .

For concrete structures, WSD was replaced by the strength designmethod in the 1960s .

Design of steel structures began with ASD, which was followed by the

load factor design (LFD); this was followed by yet another design

method—the load and resistance factor design (LRFD)—which was

introduced in the United States in the 1980s and which has been gaining

wider acceptance by structural engineers for design purposes.Assit. Pro. Dr. Yousif A. Mansoor

ASD - Allowable Stress Design

FS based on experience and judgement

LFD - Load Factor Design

LRFD - Load and Resistance Factor Design

Considers the statistical variability in loads and

resistance. Provides a mathematical basis for

establishing safety 31

An attempt to address the variability in loads

The foundation of every design philosophy is the known stress–strain

relationship of the materials .

Unless otherwise mentioned, the following assumptions are implied in the

context of material properties:

1 The material is homogeneous .

2 The material is isotropic .

3 The material is Hookean; that is, it obeys Hooke’s law, meaning the

material is linearly elastic .

ASD does not recognize different variabilities of different load types.

φ - Strength Reduction Factor

In LFD, load and resistance are not considered simultaneously.

• The LRFD philosophy provides a more uniform, systematic, and rational approach to the selection of load factors and resistance factors than LFD.

LOAD & RESISTANCE FACTOR DESIGN

To overcome the deficiencies of ASD, the LRFD method was developed which is based on

a) The strength of material

b) Consider variability not only in resistance but also in the effect of loads.

c) Provide a measure of safety related to probability of failure.

Thus the safety criteria is:

ΦRn ≥ η Σ γ Qi

Where Φ is the resistance factor, Rn is the nominal resistance, γ is the statistically based

load factor and Qi is the effect of load and η is the load modification factor.

This equation involves both load factors and resistance factors.

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In the general equation for LRFD method of design

ΦRn ≥ η Σ γi Qi

η is the load modification factor that takes into its account the ductility, redundancy and operational importance of the bridge.Itis given by the expression

η = ηd ηr ηi ≥ 0.95

Where ηd is the ductility factor, ηr is the redundancy factor and ηi is the operational importance factor.

LOAD & RESISTANCE FACTOR DESIGN

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Ductility Factor:

• Ductility is important to the safety of the bridge.

• If ductility is present overloaded portion of the structure can redistribute the load to other portions that have reserve strength.

• This redistribution is dependent on the ability of the overloaded component and its connections to develop inelastic deformations without failure.

• Brittle behavior is to be avoided, because it implies a sudden loss of load carrying capacity when the elastic limit is exceeded.

• The value to be used for the strength limit state, ductility factors are

ηd = 1.05 for non-ductile components and connections

ηd = 0.95 for ductile components and connections

DUCTILITY FACTOR

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Redundancy Factor:

• A statically indeterminate structure is redundant, that is, it has more restraints than necessary to satisfy conditions of equilibrium.

• For example, a three span continuous bridge girder would be classified as statically indeterminate to second degree. Any combination of two supports or two moments or one support and one moment could be lost without immediate collapse, because the loads could find alternative paths to the ground.

• Redundancy in a bridge system will increase its margin of safety and this is reflected in the strength limit state redundancy factors given as

ηR = 1.05 for non-redundant members

ηR = 0.95 for redundant members

REDUNDANCY FACTOR

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Operational Importance Factor:

• Bridges can be considered of operational importance if they are on the shortest path between residential areas and a hospital or a school or provide access for police, fire, and rescue vehicles to homes, businesses, industrial plants, etc.

• It is difficult to find a situation where a bridge would not be operationally important.

• One example of a non important bridge could be on a secondary road leading to a remote recreation area, that is not open year around.

• In the event of an earthquake, it is important that all lifelines, such as bridges remain open. Therefore, following requirements apply to the extreme event limit state as well as to the strength limit state.

ηi = 1.05 for non-ductile components and connections

ηi = 0.95 for ductile components and connections

For all other limit states: ηi = 1.0

OPERATIONAL IMPORTANCE FACTOR

Advantages of LRFD

Accounts for variability in resistance and load

Achieves fairly uniform levels of reliability

Consistent method of design

Better document format

Matched and maintained commentary

References

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Disadvantages of LRFD

Different to present practice

Some understanding of statistics is required

Need statistical data for loads and resistances

Few computer programs are available

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Basic Design Equation

Resistance => Load Effect

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Design Equation

h - load modifier

g - load factor

Q - load or force effect

f - resistance factor

Rn - nominal resistance

ShigiQi < fRn

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Comparison of 3 Methods

Allowable Stress Design

(ft)DL + (ft)LL ≤ 0.55 fy

Or 1.82 (ft)DL + 1.82 (ft)LL ≤ Fy

Load Factor Design

1.3[(ft)DL + 5/3 (ft)LL] ≤ Φ Fy

Φ and load factors by judgement

Old live load model

1.25 (ft)DL + 1.75 (ft)LL ≤ Φ Fy

Φ and load factors by calibration

New, larger live load model University of Anbar

Philosophies of Design - LRFD Fundamentals

• Variability of Loads and Resistances:

• Suppose that we measure the weight of 100 students…

• Variability of Loads and Resistances:

• Suppose that we measure the weight of 100 students…

x :- MEAN(AVERAGE )

• Variability of Loads and Resistances:-

Reliability Index:

Resistance Factor (uncertainties)

Material Properties

Prediction Equations

Workmanship

Quality Control

Consequence of Exceeding Limit State

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AASHTO-LRFD Specification

• Contents

• 1. Introduction

• 2. General Design and Location Features

• 3. Loads and Load Factors

• 4. Structural Analysis and Evaluation

• 5. Concrete Structures

• 6. Steel Structures

• 7. Aluminum Structures

8. Wood Structures

9. Decks and Deck Systems

10. Foundations

11. Abutments, Piers, and Walls

12. Buried Structures and Tunnel Liners

13. Railings

14. Joints and Bearings

15. Index

Chapter 1: Introduction

AASHTO-LRFD

1.3.2: Limit States

Service:

o Deals with restrictions on stress, deformation, and crack width under regular service conditions.

o Intended to ensure that the bridge performs acceptably during its design life.

Strength:

o Intended to ensure that strength and stability are provided to resist statistically significant load combinations that a

bridge will experience during its design life.

o Extensive distress and structural damage may occur at strength limit state conditions, but overall structural integrity

is expected to be maintained.

Extreme Event:

Intended to ensure structural survival of a bridge during an earthquake, vehicle collision, ice flow, or foundation scour.

Fatigue:

o Deals with restrictions on stress range under regular service conditions reflecting the number of expected cycles .

1.3.2: Limit States

1.3.2: Limit States - Load Modifiers

These modifiers are applied at the element level, not the entire structure.

Chapter 2 – General Design and Location Features

AASHTO-LRFD

Chapter 2: General Design and Location Features

Contents

o 2.1 – Scope

o 2.2 – Definitions

o 2.3 – Location Features

• 2.3.1 – Route Location

• 2.3.2 – Bridge Site Arrangement

• 2.3.3 – Clearances

• 2.3.4 – Environment

o 2.4 – Foundation Investigation

• 2.4.1 – General

• 2.4.2 – Topographic Studies

Contents

o 2.5 – Design Objectives

• 2.5.1 – Safety

• 2.5.2 – Serviceability

• 2.5.3 – Constructability

• 2.5.4 – Economy

• 2.5.5 – Bridge Aesthetics

o 2.6 – Hydrology and Hydraulics

• 2.6.1 – General

• 2.6.2 – Site Data

• 2.6.3 – Hydrologic Analysis

• 2.6.4 – Hydraulic Analysis

• 2.6.5 – Culvert Location and Waterway Area

• 2.6.6 – Roadway Drainage

2.5.2 - Serviceability

o 2.5.2.6.2 Criteria for Deflection

Principles which apply

• When investigating absolute deflection, load all lanes and assume all components deflect equally.

• When investigating relative deflection, choose the number and position of loaded lanes to maximize the

effect.

• The live load portion of Load Combination Service I (plus impact) should be used.

• The live load is taken from Article 3.6.1.1.2 (covered later).

• For skewed bridges, a right cross-section may be used, for curved bridges, a radial cross section may be

• In the absence of other criteria, these limits may be applied to steel, aluminum and/or concrete bridges:

• For steel I girders/beams, the provisions of Arts. 6.10.4.2 and 6.11.4 regarding control of deflection

through flange stress controls shall apply.

• Vehicular load on wood planks and panels (extreme relative deflection between adjacent edges)..... 2.5 mm.

• Vehicular load on ribs of orthotropic metal decks (extreme relative deflection between adjacent ribs)..... 2.5 mm

2.5.2 – Serviceability

• 2.5.2.6.3 Optional Criteria for Span-to-Depth ratios

• Table 2.5.2.6.3-1 Traditional Minimum Depths for Constant Depth Superstructures

Probability and reliability are the basis for a safe design.

Components and connections proportioned to satisfy all applicable

LIMIT STATES and LOAD COMBINATIONS.

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Limit State

An event, or circumstance, under which a bridge, or a component,

ceases to satisfy the provisions for which it was designed.

The limit, or boundary, of structural usefulness.

The point at which a component is no longer able to fulfill its intended

(original) purpose.

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Limit State:“A limit state is a condition beyond which a structural

system or structural component ceases to fulfill the function for which it is designed”.Bridges shall be designed for specified limit states to achieve the objectives of constructability, safety and serviceability.

Generally the limit states that are considered in bridge design are:

1. Strength limit state2. Service limit state 3. Fatigue and fracture limit state 4. Extreme Event limit state

LIMIT STATES

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Strength Limit State

Intended to ensure that strength and stability, both locally and

globally, are adequate for statistically significant load combinations.

Distress or damage may occur, but without failure.

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Advanced Concrete Bridge Design

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