Introduction to Bridges

Bridge Engineering
 What is a bridge?

 A bridge is a structure that spans a divide such as:

• A stream/river/ravine/valley
• Railroad track/roadway/waterway
 The traffic that uses a bridge may include:
• Pedestrian or cycle traffic
• Vehicular or rail traffic
• Water/gas pipes
• A combination of all the above
 Function of a Bridge
• A bridge has to carry a service (which
may be highway or railway traffic, a
footpath, public utilities, etc.) over an
obstacle (which may be another road
or railway, a river, a valley, etc.) and to
transfer the loads from the service to the
foundations at ground level.
General Span Types
 Basic Components of a Bridge
The two basic parts are:
 Substructure - includes the piers, the abutments and
the foundations.
 Superstructure - consists of the deck structure itself,
which support the direct loads due to traffic and all
the other permanent and variable leads to which
the structure is subjected.
 The connection between the substructure and the
superstructure is usually made through bearings.
However, rigid connections between the piers (and
sometimes the abutments) may be adopted,
particularly in frame bridges with tall (flexible) piers.
A Typical Single Span Bridge
 COMPONENTS OF A BRIDGE

Figure 2:
1 January 2014 7
Components of a Bridge
Typical Beam/Girder Bridge
 Bridges which Carry Loads
Mainly in Flexure

• By far the majority of bridges are of this type. The

loads are transferred to the bearings and piers
and hence to the ground by slabs or beams
acting in flexure, i.e. the bridges obtain their
load-carrying resistance from the ability of the
slabs and beams to resist bending moments and
shear forces.
• Only for the very shortest spans is it possible to
adopt a slab without any form of beam.
Bri dges w hi ch Carry thei r Loads Mainly as
Axial Forces

• This type can be further subdivided into those

bridges in which the primary axial forces are
compressive (arches) and those in which these
forces are tensile (suspension bridges and
cable-stayed bridges). Such forces normally
have to be resisted by members carrying forces
of the opposite sense.
• It must not be thought that flexure is immaterial in
such structures. Certainly, in most suspension
bridges, flexure of the stiffening girder is not a
primary loading in that overstress is unlikely to
cause overall failure; however, in cable stayed
bridges (particularly if the stays are widely
spaced) flexure of the girder is a primary loading.
Bridges which Carry their Loads
Mainly as Axial Forces
Bridges which Carry their Loads
Mainly as Axial Forces
Classification of Bridges
Materials Usage
Concrete Pedestrian
Steel Highway
Wood Rail
Hybrid
Stone/Brick
Structural form Span
Slab Short
Girder Medium
Truss Large
Arch Long
Suspension
Cable Stayed
 Basic Types of Bridges
• Girder/Beam Bridge
• Truss Bridge
• Rigid Frame Bridge
• Arch Bridge
• Cable Stayed Bridge
• Suspension Bridge
Girder/Beam Bridge

• The most common and basic type

Truss Bridge

• Truss is a simple skeletal structure.

Forces in a Truss Bridge

In design theory, the individual members of a simple truss are only

subject to tension and compression and not bending forces. For
most part, all the beams in a truss bridge are straight.
Arch Bridges

• Arches used a curved structure which

provides a high resistance to bending
forces.
• Both ends are fixed in the horizontal
direction (no horizontal movement
allowed in the bearings).
Hinge-less Arch • Arches can only be used where
ground is solid and stable.
• Hingeless arch is very stiff and
suffers less deflection.
• Two-hinged arch uses hinged
bearings which allow rotation and
most commonly used for steel
arches and very economical design.
Two hinged Arch
Arch Bridges
 The three-hinged arch
adds an additional hinge at
the top and suffers very
little movement in either
foundation, but
experiences more
Three-hinged Arch deflection. Rarely used.
 The tied arch allows
construction even if the
ground is not solid enough
to deal with horizontal
forces.
Tied Arch
Forces in an Arch
 Arches are well
suited to the use of
stone because they
are subject to
compression.
 Many ancient and
well-known examples
of stone arches still
stand to this today.
 Cable Stayed

 A typical cable-stayed bridge is a continuous deck with

one or more towers erected above piers in the middle of
the span.
 Cables stretch down diagonally from the towers and
support the deck..
Cable Stay Arrangements
Cable Stayed Bridges
 Suspension Bridge

 A typical suspension bridge is a continuous deck with one or

more towers erected above piers in the middle of span. The
deck maybe of truss or box girder.
 Cables pass over the saddle which allows free sliding.
 At both ends large anchors are placed to hold the ends of the
cables.
Forces in Suspension Bridge
 Difference between Cable-stayed
and suspension bridges

Cable-stayed bridges may look similar to suspension

bridges—both have roadways that hang from cables and
both have towers. But the two bridges support the load of
the roadway in very different ways. The difference lies in
how the cables are connected to the towers. In suspension
bridges, the cables ride freely across the towers,
transmitting the load to the anchorages at either end. In
cable-stayed bridges, the cables are attached to the
towers, which alone bear the load.
 If one car pass on the bridge, here is the sequence of force
transmission for the cable-stayed bridge:
1. to the deck;
2. to the stay cable;
3. to the bridge tower;
4. and also is the last, to the foundation.
 The sequence of force transmission for the suspension bridge:
1. to the deck;
2. to the suspender;
3. to the main cable;
4. to the bridge tower;
5. and also is the last, to the foundation.
 Small span bridges (upto 15m) Medium span bridges (upto 50m)

Culvert Bridge Precast concrete box Beam Bridge

Slab Bridge Precast Concrete I-Girder Bridge

T-beam Bridge Composite rolled steel beam bridge

Wood Beam Bridge Composite steel plate Girder Bridge

Precast concrete box Beam Bridge Cast-in-place RCC box girder bridge

Precast Concrete I-Girder Bridge Cast-in-place post tensioned concrete box girder
bridge

Rolled steel beam bridge Composite steel box girder bridge

Large span bridges (50m to 150m) Long span bridges (over 150m)

Composite steel plate Girder Bridge Cable stayed bridge

Cast-in-place post tensioned concrete box girder Suspension bridge

Post-tensioned concrete segmental construction

Concrete arch and Steel arch bridges

 Need for Investigation
 Before a bridge can be built at a particular site, it is essential to consider
many factors, such as
 The need for a bridge
 The present and future traffic
 Stream characteristics
 Subsoil conditions
 Alternative sites
 Aesthetics and cost
The aim of the investigation is to select suitable site at which a bridge can be built
economically, at the same time satisfying the demands of traffic, the stream, safety
and aesthetics.
 SELECTION CRITERIA FOR BRIDGE SITE
• For selecting a suitable site for a major bridge, the investigating engineer should
make a reconnaissance survey to get impression of the landscape and to
decide on the type of the structure to the site.

• Care should be taken to investigate a number of probable alternative sites and

then decide on the site which is likely to serve the needs of the bridge at the
least cost.

• A brief description of the reasons for the selection of a particular site should be
furnished in the investigation report along with sailent details of alternative sites
investigated and rejected
The characteristics of an ideal site for a bridge across a river are:
a. A straight reach of the river.
b. Steady river flow without cross currents.
c. A narrow channel with firm banks.
d. Suitable high banks above high flood level on each side.
e. Rock or other hard in erodible strata close to the river bed level.
f. Economical approaches, danger of floods, the approaches should be free
from obstacles such as hills, frequent drainage crossings, scared places,
graveyards or built up areas or troublesome land acquisition
g. Absence of sharp curves in the approaches;
h. Absence of expensive river training works;
i. Avoidance of excessive underwater construction.
 SELECTION CRITERIA FOR BRIDGE SITE
Different studies performs
PRELIMINARY during PRELIMINARY
STUDY(Techno-economic feasibilitySURVEY
survey) are:
 Topography
 Catchment area
 Hydrology
 Geo-technical data
 Seismology
 Navigation
 Construction resources
 Nearby bridges
 Traffic data
 Topography

Topographical details obtained from relevant 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
• Drawings marked as A & B can be obtained from Survey of India Map.
• In addition, one cross section each across the river at the selected sites
should be taken.
 CATCHMENT AREA

 This also can be obtained from the same map (Survey of

India)
 Used mainly for the flood analysis.
 HYDROLOGIC PARTICULARS

 Study about the low water level, highest flood level , slope
of surface of water, flood velocity and discharge of river.
 Data obtained from local enquiries or from the data
available for the nearest gauging site from irrigation or
flood control dept.
GEO-TECHNICAL AND SEISMIC DATA

 Study performed to get the stability of the river, location of

faults, their activity and their likely repercussions on a major
structure to be put up and particulars of past earthquakes
in the site vicinity.
 Study perform also to get soil classification, grain size and
depth at which hard strata is likely to be met with.
 Mainly Augur boring test is used to collect the soil samples
and further to study its engineering properties.
11

Soil Investigation is carried out to obtain following information

 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.
 NAVIGATIONAL REQUIRMENTS

 Some kind of navigation will exist on almost all major rivers.

 Study investigate about the size, density and volume of
traffic of vessels and boats so that it can be safely cross the
river without any nuisance to both the vessels and bridge.
CONSTRUCTION RESOURCES

 Investigation includes availability of quarry, skilled labor

and need for special equipment such as crushers,
batching plants, handling equipment etc.
DETAILS OF OTHER BRIDGES ACROSS THE
RIVER

 Study conducting mainly to correlate general criteria used

for selection of sites and design of that bridge.
 Also to get the behavior of the river at existing bridges on
either reach. Will help considerably in determining the
protection works, depth of foundation, type of foundation
etc required at each site.
 TRAFFIC STUDY

 If the alternative locations can be separated by a

considerable distance, the volume and type of traffic that
will pass at each location may be different in some cases.
 Economic Point of View.
 Detailed traffic survey have to be conducted to get
awareness of growth of traffic, density, volume and future
possibilities of expansion of traffic lanes etc.
 Selection of Bridge Types
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.
 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
 Selection of Bridge Types
Simple Span reinforced concrete bridge
 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
 Selection of Bridge Types
Continuous reinforced concrete bridge
 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.
 Selection of Bridge Types
ConcreteConstruction
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
 Selection of Bridge Types
Steel Construction
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)