advanced concrete bridge design
TRANSCRIPT
University Of Anbar Lecture 1 /Assit. Pr. Dr. Yousif A. Mansoor 1
Advanced Concrete Bridge Design
Assit. Pr. Dr. Yousif A. Mansoor
05:51:43
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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
05:51:43
Assit. Pro. Dr. Yousif A. Mansoor
University OF Anbar
Assit. Prof. Dr. Yousif A.Mansoor
05:51:43
Assit. Pro. Dr. Yousif A. Mansoor
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
Assit. Pr. 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
05:51:43
University OF Anbar
Assit. Pr. Dr. Yousif A.Mansoor
05:51:43
University OF Anbar
Assit. Pr. Dr. Yousif A.Mansoor
05:51:44
05:51:44
Assit. Pro. Dr. Yousif A. Mansoor
University OF Anbar
Dr. Yousif A.Mansoor
05:51:44
University OF Anbar
Assit. Pr Dr. Yousif A.Mansoor
05:51:44
05:51:44
Assit. Pro. Dr. Yousif A. Mansoor
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
University OF Anbar
Dr. Yousif A.Mansoor
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.
University OF Anbar
Dr. Yousif A.Mansoor
05:51:44
Components of a Girder bridge (Beam Bridge)
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Assit. Pro. Dr. Yousif A. Mansoor
DESIGN PHILOSOPHY
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Assit. Pro. Dr. Yousif A. Mansoor
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.
University of Anbar
Dr. Yousif A. Mansoor
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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
Philosophies of Design
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
Assit. Pro. Dr. Yousif A. Mansoor
An attempt to address the variability in loads
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Philosophies of Design
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 .
Assit. Pro. Dr. Yousif A. Mansoor
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Philosophies of Design
ASD does not recognize different variabilities of different load types.
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Philosophies of Design
φ - Strength Reduction Factor
In LFD, load and resistance are not considered simultaneously.
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Philosophies of Design
• The LRFD philosophy provides a more uniform, systematic, and rational approach to the selection of load factors and resistance factors than LFD.
Assit. Pro. Dr. Yousif A. Mansoor
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.
University of Anbar
Dr. Yousif A. Mansoor
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
University of Anbar
Dr. Yousif A. Mansoor
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
University of Anbar
Dr. Yousif A. Mansoor
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
University of Anbar
Dr. Yousif A. Mansoor
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
University of Anbar
Dr. Yousif A. Mansoor
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
University of Anbar
Dr. Yousif A. Mansoor
Basic Design Equation
Resistance => Load Effect
University of Anbar
Dr. Yousif A. Mansoor
Design Equation
h - load modifier
g - load factor
Q - load or force effect
f - resistance factor
Rn - nominal resistance
ShigiQi < fRn
University of Anbar
Dr. Yousif A. Mansoor
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
LRFD
1.25 (ft)DL + 1.75 (ft)LL ≤ Φ Fy
Φ and load factors by calibration
New, larger live load model University of Anbar
Dr. Yousif A. Mansoor
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Philosophies of Design - LRFD Fundamentals
• Variability of Loads and Resistances:
• Suppose that we measure the weight of 100 students…
Assit. Pro. Dr. Yousif A. Mansoor
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Philosophies of Design - LRFD Fundamentals
• Variability of Loads and Resistances:
• Suppose that we measure the weight of 100 students…
x :- MEAN(AVERAGE )
Assit. Pro. Dr. Yousif A. Mansoor
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Philosophies of Design - LRFD Fundamentals
• Variability of Loads and Resistances:-
Assit. Pro. Dr. Yousif A. Mansoor
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Philosophies of Design - LRFD Fundamentals
• Variability of Loads and Resistances:-
Assit. Pro. Dr. Yousif A. Mansoor
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Philosophies of Design - LRFD Fundamentals
• Variability of Loads and Resistances:-
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Philosophies of Design - LRFD Fundamentals
Reliability Index:
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Philosophies of Design - LRFD Fundamentals
Reliability Index:
Resistance Factor (uncertainties)
Material Properties
Prediction Equations
Workmanship
Quality Control
Consequence of Exceeding Limit State
University of Anbar
Dr. Yousif A. Mansoor
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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
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Chapter 1: Introduction
AASHTO-LRFD
Chapter 1: Introduction
Assit. Pro. Dr. Yousif A. Mansoor
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Chapter 1: Introduction
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 .
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Chapter 1: Introduction
1.3.2: Limit States
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Chapter 1: Introduction
1.3.2: Limit States - Load Modifiers
These modifiers are applied at the element level, not the entire structure.
Assit. Pro. Dr. Yousif A. Mansoor
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Chapter 2 – General Design and Location Features
AASHTO-LRFD
Chapter 2: General Design and Location Features
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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
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Chapter 2 – General Design and Location Features
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
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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
used.
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2.5.2 - Serviceability
o 2.5.2.6.2 Criteria for Deflection
• 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.
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2.5.2 - Serviceability
o 2.5.2.6.2 Criteria for Deflection
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2.5.2 - Serviceability
o 2.5.2.6.2 Criteria for Deflection
• 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
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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
LRFD
Probability and reliability are the basis for a safe design.
Components and connections proportioned to satisfy all applicable
LIMIT STATES and LOAD COMBINATIONS.
University of Anbar
Dr. Yousif A. Mansoor
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.
University of Anbar
Dr. Yousif A. Mansoor
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
University of Anbar
Dr. Yousif A. Mansoor
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.
University of Anbar
Dr. Yousif A. Mansoor