Bridge Lecture Slide by Micotol

Part 2- Bridge Construction

1- Introduction & Investigation1

1.1. IntroductionBridge is a structure providing passage (highway, railway,

pedestrian, canal, Pipeline) over obstacle (river, valley, road, railway)

“Build bridges and you will have a friend”

Bridge engineering is one of the fascinating fields in civil engineering

calling for expertise in many areas: structural analysis and design,

geotechnique, traffic projection, surveying, runoff calculation and

methods of construction.

A bridge engineer has to have an appreciation of economics and

aesthetics besides ability in analysis and design.

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1.2. Investigation for Bridge1.2.1. Preliminary Survey for site selection

Preliminary data for site selection is collected from site visit for different alternative bridge sites.

The proposed road alignment

The local terrain and site conditions

The required design life of the bridge

The likely traffic volumes

The resources available for the project

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1.2. Investigation for Bridge1.2.2. Site Selection

Ideal site for bridge crossings

On straight reach of the river or nodal point for meandering rivers

Where the flow is steady and uniform

Beyond the disturbing influence of large tributaries

Has well defined & stable high banks above flood level

Has reasonable straight approach & permits square crossings

Has good foundation conditions

Has short span

Doesn’t requirement extensive river training work

Doesn’t requirement extensive underwater construction

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1.2. Investigation for Bridge1.2.2. Site Selection

Initial Considerations during site selection

Appropriate vertical and horizontal alignment

Soil strength to ensure stability of the structure

Should not have adverse impact on adjoining land or building

Location of bridge in relation to approach road alignment.

a) Total Span < 60m:- The alignment of approach govern

b) 60m < Total Span < 300m:- Both alignment & good bridge site selection

c) Total Span > 300m:- Good bridge site governs

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1.2. Investigation for Bridge1.2.3. Site Investigation

Factors that most often need to be confirmed by field inspection are

High-water marks or profiles and related frequencies.

Selection of roughness coefficients,

Evaluation of apparent flow direction and diversions,

Flow concentration (main stream),

Observation of land use and related flood hazards, and

Geomorphic relationships and soil conditions

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Elements of Site investigation

I- Catchment area and runoff data

Catchment size

Catchment grade

Catchment cover

Presence of artificial or natural storage

Change in nature of catchment (deforestation & afforestation)

Maximum recorded intensity & frequency of rain fall

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II- Drawings

A. Index map: general topographic map (1:50000)

B. Contour Map:

C. Site Plan: show detail of selected site 100-200m u/s and d/s of selected site

D. Cross section & longitudinal section of the river

E. Catchment area map:

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III- River Survey

A. General Information: River name, flood direction, nearest town…

B. Water Level

• OFL (Ordinary flood level)

• LWL (Lowest water level)

• HFL (Highest Flood Level)

• DFL (Design Flood Level)

C. Design Discharge is flood occurring no more than once every 10yrs (returning

period)

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Elements of Site investigation Design Discharge is maximum flow that can pass through the bridge without

• Causing unacceptable disruption to traffic

• Endangering the piers & abutment foundation with scours

• Damaging approach embankments

• Causing flood damage on upstream of embankments.

Calculation of Design Discharge

a) Rational Formula

b) Area – Velocity Method

c) Unit Hydrograph

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IV. Soil Investigation

It is required to get

Soil profile

Engineering property of the foundation material

Foundation level for abutments & piers for design of foundation

The above information is obtained by analyzing samples taken from boreholes,

test pit or geophysical survey.

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1.2. Investigation for Bridge1.2.4. Span Determination

A. Economic Span: Min span length where cost of superstructure = Cost of

substructure.

B. Hydraulic Requirement: bridges are designed to accommodate design discharge

at design flood.

C. Location of Piers: Piers should be located to cause minimum obstruction to flow

D. Free Board: water way below superstructure should be designed to pass design

flood & floating debris with back water effect.

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2- Bridge type and Selection

2.1. Bridge Types13

2.1. Types of Bridge14

Classification is mainly based on Superstructure

a) Material

b) Span Length

c) Span Arrangement

d) Functionality

e) Structural form

f) Span Length

g) Movement

2.1. Types of Bridge Construction Materials

Timber Bridges

Masonry Bridges

Reinforced Concrete Bridges

Pre-stressed Concrete Bridges

Steel Bridges

Composite Bridges

Span Arrangement

Simply Supported

Continuous

Cantilever

Traffic type/functionality

Road bridge

Railway bridge

Pedestrian bridge

Span length

L ≤ 6m (Culvert)

7m < L ≤ 15m (Small span bridges)

16 ≤ L ≤ 50m (Medium span Bridges)

50 ≤ L≤ 150m (Large Span Bridges)

L≥150m (Extra Large Span Bridges)

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2.1. Types of BridgeStructural Arrangement or Form

Slab Bridges

T - Girder (Deck girder Bridges)

Box Girder

Arch Bridges

Truss Bridges

Cable Stayed Bridges

Suspension Bridges

Movements

Movable Bridges

(Bascule, Lift, Swing)

Fixed Bridges

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Slab Bridge17


Slab Bridges are the simplest and least expensive structures that can be

built for small spans up to 12m.

They normally requires more concrete and reinforced steel than Girder

Bridge of the same span but the formwork is simpler and less expressive,

hence they are economical when these cast factor balance favorably.


These bridges can be built on ground supported false work or

constructed of precast elements.

The load carrying mechanism is by plate action, i.e., by bending and

twisting due to continuity in all directions.

Application of a load on the portion make the slab deflect into a dish

shape locally, causing a two-dimensional system of bending and

twisting moments.

Slab Stringer (T-Girder) Bridge

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Pre- Stressed Girder BridgesSteel girders with open intermediate bent

diaphragmsPre-stressed I-girder intermediate bent

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Pre- Stressed Girder Bridges22

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Pre- Stressed Girder Bridges

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Pre- Stressed I-Girder Pre- Stressed Slab deck

Pre-stressed Double Tee GirdersSteel Plate Girder Bridge

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2.1. Types of BridgeT-Girder Bridge26

2.1. Types of BridgeT-Girder Bridges27

They are used for bridges spanning from about 10 meters-25 meters.

These usually consist of equal1y spaced beams or girders or stringers (generally with

spacing of 1.8-3.6m) spanning longitudinally between supports.

The slab is structural1y continuous across the top.

The slab serves dual purpose of supporting the live load on the bridge and acting as the

top flange of the longitudinal beams.

Diaphragms are provided transversely between the beams over the supports and

depending on the span, at mid span and other intermediate locations.

The purpose of providing diaphragms is to ensure lateral distribution of live loads to

various adjacent stringers, the magnitude of the share of each stringer depends on the

stiffness of the diaphragms relative to the stringers and on the method of connectivity.

Design of T- girder bridges consists of deck slab analysis and design, and the T-girder

analysis and design.

Box Girder Bridge28

2.1. Types of BridgeBox-Girder Bridge

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2.1. Types of BridgeBox-Girder Bridges30

Concrete box girder bridges are economical for spans of above 25 to 45m.

They can be reinforced concrete or pre-stressed concrete. Longer span than 45mwill have to be pre-stressed.

They are similar to T-beams in configuration except the webs of T-beams are all interconnected by a common flange resulting in a cellular superstructure.

The top slab, webs and bottom slab are built monolithically to act as a unit, which means that full shear transfer must be provided between all parts of the section.

The interior webs resist shear and often only a small portion of girder moments. Consequently they are usually thinner than the webs of T-beams.

In the case of continuous T-beams, the webs must resist the negative girder moments as well as all the shear, and contain all the reinforcement for positive moments.


The bottom slab (soffit) contains reinforcement for the positive moment and also

acts as a compression flange in the negative moment regions of continuous spans.

The bottom slab also affords a superstructure considerably thinner than a T- beam

bridge of the same span and permits even longer spans to be built.

Exterior Girder/web Interior Girder/web


Concrete box girder bridges have several advantages over other types;

1. The relatively shallow depth of box girders is all advantage where headroom is

limited like in urban overpasses.

2. Monolithic construction of the superstructure and substructure offers structural as

well as aesthetic advantage. The pier caps for continuous box girders can be

placed with in the box, facilitating rigid connection to the pier.

3. They provide space for utilities such as water and gas lines, power, telephone and

cable ducts, storm drains and sewers, which can be placed in the hollow cellular

section.

4. Reinforced concrete box girders have high torsional resistance due to their

closed shape and are particularly suitable for structures with significant

curvature. This construction also lends itself to aesthetic treatment.

Truss Bridge

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2.1. Types of BridgeTruss Bridge34

It creates a very rigid structure & one that transfers the load from a single point to

a much wider area

Loads members in tension and compression.

Members are pinned at joints (Moment = 0).

Triangles provide stability and strength.

Ways to strengthen members in bending.

Decrease overall length (deflections).

Cross section design (moment of inertia)

Use stronger materials (elastic modulus).


Truss bridges are used for larger spans for which the depth of girder

bridges is not practical due to fabrication, erection and transportation

limitation or due to economy in the case of concrete girders.

The larger the height is compared to the span, the greater its strength

Every bar in this bridge experiences either a pushing or pulling force, The bars rarely bend.

Ability to support weight relies on the strength of the joints

Reasons for its un popularity

• Lack of aesthetics

• High life time cost

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Arch Bridge

Sydney, Australia

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Eads Bridge, St. Louis

2.1. Types of BridgeArch Bridge39

A true arch transfers loads to its foundation by pure compression, however, the variable position of the live load always causes super imposed bending.

Arch bridges are

Very Strong if well designed

Can be very beautiful

Tend to be heavy

Need strong abutment

Economical for medium & long span bridges

Reduce bending on members

2.1. Types of BridgeClassification of Arch Bridges40

1. Position of deck

a. Deck Arch

b. Half through Arch

c. Through Arch

2. Based on nature of ribs

a. Truss Arch

b. Solid Rib Arch

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Cantilever Bridge

Quebec Bridge

2.1. Types of BridgeCantilever Bridge42

Cantilever bridge consists of two simple spans (anchor spans)

with cantilever on each side of either shore supporting a short

suspended span in the middle of the stream or river. This

arrangement results in substantial reduction of moments or forces, in

the suspended span.

Cantilever span can be erected without a false work, river navigation is not

impeded during construction.

They reduce moment & force in the suspended span, decreasing mid span

deflection so they have strength, rigidity & sturdiness required to carry

heavy rail road traffic.

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Cable stayed Bridge

2.1. Types of BridgeCable Stayed Bridge44

Cable-stayed bridges are unique in that the superstructure is supported

(or hung) at several intermediate points by inclined cables, or stays,

radiating from and continuous over the towers, instead of being supported

from underneath by conventional piers or bents.

The girder can be steel or pre-stressed concrete

Aesthetically pleasing

Easier and faster to build

Economically competitive for medium and large spans

Need strong towers

46 Suspension Bridge

2.1. Types of BridgeSuspension Bridge47

Can span distance for longer than any other kind of bridge

Aesthetic, light, and strong

Consists four essential parts (Tower, Anchorage, Cable, Deck)

The main cables are curved & continuous b/n towers.

The deck usually supported on stiffening trusses is hung from

suspension cables. It consists of a central main span flanked on each

side by a side span that is separated from the main span by towers. The

ends of the suspension cables are secured at the anchorage, which are

usually built of massive masonry or concrete.


The distinction between cable-stayed and suspension bridges is the profile of the

cable.

In suspension bridges the main cables are curved and continuous between the

towers. The deck and other vertical loading are suspended from these

cables at relatively short intervals. Being relatively flexible, the main cable

develops funicular shape, which is a function of the magnitude and position of

the loading. On the other hand,

In cable-stayed bridges, the cables are straight and extend from one tower and

connected to the deck directly at discrete points. Being, taut, they furnish

relatively inflexible support along the span at several points and provide

a bridge with relatively greater stiffness than that achievable in suspension

bridges.


It is believed that the greater

stiffness provided by cable-

stayed systems makes their

limit span less susceptible to

wind-induced vibrations,

compared to the limit span of

suspension bridges.

Suspension bridges are most

expensive to build and susceptible

to ‘wobbles’ due to wind, if badly

designed.

2.2. Selection of Bridge Types50

a. Geometric condition of the site (Road Alignment, Design flood and highest water mark)

b. Aesthetics,

c. Traffic capacity,

d. Need for future widening,

e. Structural stability,

f. Foundation (sub-surface) conditions, and strength of abutments.

g. Erection procedures,

h. Available Material

i. Knowledge(skill) and Equipment(capacity) of the contractor

j. Clearance requirement above and below the road way

k. General civic requirements with respect to location, financing and community values.

2.2. Selection of Bridge Types51

For Curved Bridges continuous box girder and slab bridges are good choices because

They have pleasing appearance

Can readily be built on a curve

Have relatively high torsional resistance

Structural Type Material Range of Spans (m)

Slab Concrete 0-12

Girder Concrete, Steel 12-250 (Concrete), 30-260(Steel)

Arch Concrete 90-500

Truss Steel 90-550

Cable Stayed Concrete, Steel <250

Suspension Steel 300-1400

2.2. Selection of Bridge TypesContinuous reinforced concrete bridge

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Less number of bearings than simply supported bridge since on line of

bearings are used over the piers.

Reduced width of pier, thus less flow obstruction and less amount of material.

Requires less number of expansion joints due to which both the initial

cost and maintenance cost become less.

Better architectural appearance.

Lesser Vibration and deflection.

Additional strength from moment redistribution due to continuity & rigid.

Smaller cross section of bridge components both superstructure & sub structure

Analysis is laborious and time consuming.

2.2. Selection of Bridge TypesSimple Span reinforced concrete bridge

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Elastic moment capacities are used for design resulting in large cross sections

Analysis and design is simple

High maintenance cost

Many construction joints at the discontinuities

2.2. Selection of Bridge TypesConcrete Construction54

Advantage

Adaptable to wide variety of structural shapes and loads

Low cost of maintenance (less than 1% of construction cost per year).

Long life and better resistance to temporary overloads and dynamic loads than steel bridges.

Cast-in-place reinforced concrete structure are continuous and monolithic

Easy construction, low cost and good seismic resistance.

They can also be given the desired aesthetic appearance.

Disadvantage

Large dead weight that require large foundation

Difficulty to widen or rebuild

Longer construction time

Expensive formwork and false work

2.2. Selection of Bridge TypesSteel Construction55

Advantage

Steel bridges can be built faster than reinforced concrete or pre stressed concrete bridge.

They can be erected with ease and this minimizing construction costs.

Steel superstructures are usually lighter than concrete superstructures which translate into reduced substructures costs, which can be significant when soil conditions are poor.

Steel superstructures can be designed with shallower depth than RC, which is an important consideration when overhead clearance is required.

Steel bridges are easy and faster to repair than RC.

Disadvantage

Corrosion of steel is the major drawback which requires prohibitively high maintenance cost. Corrosion can reduce cross section of structural members and weaken the superstructure.

The second disadvantage is that steel fatigues under repeated loading (its strength decreases under repeated loading at high number of cycles of loading)

3- Sub-Structure

3.1. Abutment56

3.1. Abutment57

Bridge Lecture Slide by Micotol

Education

Transcript of Bridge Lecture Slide by Micotol