Bridges [Compatibility Mode]

7/28/2019 Bridges [Compatibility Mode]

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NNPC FSTP Engineers

Introduction to Civil

Engineering StructuresCourse Code:


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Contents

Bridges


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Bridges


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Bridges

Symbolize the work of the civil engineer

Purpose is to bridge a gap

The structural design and construction canbe one of the most challenging and complexaspects of modern civil engineering.


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Origin of Bridges

From the time human beings felt theneed to cross obstacles without

closing the way or gap beneath.


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Progress of Bridge Engineering

Initially Bridges were in the form of accidentallyfallen trees.

Later deliberately felled trees.

Presently complex suspension, cable-stayed andsegmentally erected bridges are common.

Design and construction of bridges is a multi-

disciplinary process.


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Scope of Lecture

General introduction to HighwayBridge designs and construction.


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Activities in a Bridge Project

Establishing a need

Feasibility studies

Conception of solutions

Design of selected solution

Construction

Inspection Repair and maintenance


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Establishing a Need

In highway system development there may be

need:

To Carry traffic over a river, valley or ravine.

Carry traffic over existing road that should not bedisturbed.

Carry traffic over important service mains thatcannot be economically re-routed or relocated.

Construct roadway over swampy areas that maynot be economically back-filled.


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Possible Viable Alternatives

Tunnel

Helicopter shuttle

Ferry service or pontoon service

Earth or concrete dam

Note:

1) Choice of appropriate solution depends on the result

of a feasibility study.2) Assume bridge emerged the best option.


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Feasibility Study for a Bridge:

Details

Possible access problems to site including

height and load restrictions, gradient, trafficand obstructions.

Local ground features that might give an

indication of soil strata, drainage, andgroundwater level.

Nature, distribution and condition of nearby

or onsite structures and buildings which maybe affected during the site work.

Topographical and geological maps.


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Feasibility Study for a Bridge:

Details (Contd)

Presence of service mains.

Uses of adjacent buildings that maypreclude certain types of construction, e.g.piling.

Borehole records of nearby sites, and natureof mining activities in the area if applicable.

Information on present and former uses of

site and adjacent sites.

Rights of way requirements.

Etc., etc, etc.


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Conception of Solution

Requirements: The bridge has to be

Structurally sound.

Economical.

Aesthetically pleasing. Easy to maintain and repair.

To meet these conditions may require movingthe bridge site upstream or downstream.


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Ideal Sites for Bridges over

Rivers

Where the reach of the river is straight.

Where the channel is narrowest. Where bridge can easily be linked to approach

roads.

Where soil strata are competent enough tosupport substructure.

Where underwater construction is minimal.

Where pier heights are not expensive. Where construction site is not easily liable to

flooding.


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Aesthetic Qualities of the Bridge

Harmony with its neighbours and

surrounding. Well proportioned in terms of lengths, deck

thickness and support arrangement.

Visual stability to give confidence to users. Appropriate surface finishing - rough or

smooth.

If painted, colouring must be chosen tocreate harmony and avoid excessivereflection.


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Selection of Material of

Construction

Any of the following options can be selected:

Steel

Normal weight concrete (reinforced/pre-stressed)

Lightweight concrete (reinforced or pre-stressed)

Aluminium

Composite construction (steel/concrete)

Timber

Glass reinforced plastics


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Method of Construction

This influences the selection of the bridge typeand may be any of the following:

Construction on formwork and falsework.

Construction using permanent forms.

Incremental launching - span by span.

Segmental construction - cast in place orusing pre-cast units.


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Possible Foundation Types

Spread footings

Rafts

Pile foundations; Pier foundations

Mass concrete abutments Reinforced earth abutments

Anchored walls

Box structures

Cellular abutments


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Bridge Types

1. Beam bridge (flat)2. Arch bridge

3. Suspension bridge4. Cable-stayed bridge


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Selection Criteria for Bridge

Types

There are no hard and fast rules for theselection of bridge types.

In general however, the available rule-of-the-thumb guide which is currently widelyused is based on span ranges.


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Span and Bridge Types

.Span (m) Bridge type

0 - 45 I type pre-tensioned girder

40 - 90 Pre-cast segmental constantdepth box

85 - 135 Pre-cast segmental variable

depth box120 - 185 Cast-in-place segmentalbridges

185 - 365 Cable stayed bridges

365+

Steel suspended bridges


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Beam Bridges

Can be divided into two main groups:

1- Simple beam:Transmits the loads vertically through piersor abutments and is horizontally self

supporting.

Economical for spans up to 50 metres.


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Beam Bridges (Contd)

2- Cantilever:

This transmits the loads through piers which arenormally central to the beam.

It provides bridges with spans of up to 540meters.

There are many variations in beam design,ranging from steel truss design to pre-stressedconcrete units.


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.


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.


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Beam Bridge Examples


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Beam Bridge


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Beam Bridge


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Arch Bridges

Used since Roman times in Europe.

The main load-carrying component of thebridge, the arch, is in a state of direct

compression throughout its length, hencematerials that are good in compression butpoor in tension, such as masonry or

brickwork, can be used to construct the arch.


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Arch Bridges (Contd)

Can carry greater loads than the beambridges because the load-carrying member,the arch, is in a state of compression

throughout. The arch supports the traffic either above or

below the main structural form (the arch).


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Illustration of Arch Bridges

f


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Illustration of Arch Bridges

(Contd)


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Arch Bridge Bridge Examples

A h B id


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Arch Bridge

A h B id


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Arch Bridge

A h B id


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Arch Bridge

S i B id


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Suspension Bridges

A Suspension Bridge consists of a cable-hung decking supported by towers.

The general layout comprises a central

suspended span with side spans; the lattermay take the form of a simply supportedbeam over short spans. The towers are

secured by main cables which arecontinuous between anchorages.

S i B id (C td)


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Suspension Bridges (Contd)

The foundations of the towers areconstructed by caisson or cofferdammethods and the cable anchorages or

foundations are taken throughanchorage tunnels to suitable grounds.



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The deck of the suspension bridge must bestiffened to prevent undue deflection and toprovide aero-dynamic stability. This is

achieved either by introducing a continuoustruss alongside or below the deck.

The supporting towers of many large span

suspension bridges are constructed in acellular design, in either steel or concrete.

Suspension Bridges


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Suspension Bridges

Simple Illustration

Suspension Bridge


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Suspension Bridge

Suspension Bridge


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Suspension Bridge

Arch and Suspension Bridges


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Arch and Suspension Bridges

Arch Bridge

Suspension Bridge

Moveable Bridges


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Moveable Bridges

Swing bridges: pivot on a central pier. Pivoted cantilever bridges: e.g. Tower

Bridge, London.

Vertical lift bridges: consist of simplebeams or girders which are supported and

raised by cables from high towers.

Choice of Bridge Systems


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Clear span requirement:

long span i.e. over 300 meters, steelconstruction is most likely solution. Concretearch bridges are developed for increasingly

longer spans. Steel construction may be cantilever girder,

arch or suspension. The cantilever arms can

be built without form-work and the centresections can be floated out and lifted intoposition.



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(Contd)

Steel arch form is suitable for spans of up to500 meters, but is difficult to construct.

The suspension bridges are the best known

form for spans over 600 meters.



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(Contd)

For a given span and a given load the main

girder weight decreases in the followingorder:

Cantilever

Arch

Suspension



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(Contd)

Foundation costs:

Foundations for cantilevers are simple by comparison with theother forms, owing to their vertical loading, and normally thecheapest. Foundations for suspension bridges are usually thevery extensive and costly.

Suspension

Arch

Cantilever

Design of Bridges


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Design of Bridges

The design process can be divided into four

broad areas, namely:

1. Preliminary proportioning and loadassessment.

2. Structural analysis.

3. Structural Design.

4. Detailing.

Proportioning and Loading


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p g g

Assessment

Generally the loads considered include:

Dead load or self weight

Live load or imposed load

Wind load Impact loading

Centrifugal force due to curvature

Secondary forces due to effects of creepand shrinkage.



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p g g

Assessment (Contd)

Thermal effects due to temperature variation

Construction load

Forces due to breaking and acceleration ofvehicles

Earthquake or seismic forces

Support settlement



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p g g

Assessment (Contd)

Horizontal forces on piers due to water

current,waves and debris. Impact on piers due to collision (e.g. ships)

Buoyancy effect

Earth pressure on abutments, etc.

Note:

The loads above can be satisfactorilyestimated for any bridge using code of practicerecommendations.

Note


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Note

Dead load tends to dominate the design of long

span bridges compared to live loads. Live load components are subdivided into:

uniformly distributed loads

point load due to wheel loads from abnormalvehicle loading.

Codes of practice provide adequate factors of safetyto be applied to the load and material properties.

Wind load is very important in long span bridgeshence they are checked for aerodynamic stability.

Structural Analysis


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a a ys s

Relevant equations relating the applied

loads to the material constant and internalreactions are written.

Solution of the equations with appropriate

boundary conditions yields the internalreactions.

Depending on the complexity of the method

of analysis used, the system of simultaneousequations may run into tens of thousands.

Methods of Structural Analysis


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y

Othotropic plate theory

Simple beam and related theories

Grillage analysis

Space frame analysis

Folded frame analysis

Finite strip method Finite element method

Structural Design


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g

Involves proportioning the various members of

the structure such that: permissible stresses are not exceeded;

deflection is acceptable;

cracking is not excessive; and

fire resistance is adequate.

Note: An important requirement of a design isthat it must be executable in the locality.

Detailing


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g

Most important phase of the design process.

Detailer must understand real intentions of thedesigner.

Detailer must have an idea of how components

are assembled or constructed on site. Should have adequate knowledge of structural

behaviour to ensure that reinforcements are

placed at appropriate positions. On major bridge projects it is preferable for an

engineer to do the draughting.

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Bridges [Compatibility Mode]