Introduction to Bridge
Design
Components of a Bridge
Decking consisting of slab, girders, trusses, etc.
Bearings for decking
Abutment and piers
Foundation for abutment and piers
River training works like revetment for slopes at abutments, aprons at bed level, etc.
Approaches to the bridge to connect the bridge proper to roads on either side
Handrails, guard stones, etc.
Components of a Bridge
Components above the level of bearings are grouped as superstructure while those below the
bearing level are classified as substructure.
Classification of Bridges
According to function as aqueduct (canal over drain), viaduct (road or railway over a valley),
pedestrian, highway, railway, road-cum-rail or pipeline bridge.
According to material of construction of superstructure as timber, masonry, iron, steel,
reinforced concrete, prestressed concrete, composite or aluminium bridge.
According to type of superstructure as slab, beam, truss, arch, cable-stayed or suspension
bridge.
According to interspan relation as simple, continuous or cantilever bridge.
Classification of Bridges
According to the position of the bridge floor relative to the superstructure as deck, through, half
through or suspended bridge.
According to the method of connection of different parts of the superstructure, particularly for
steel construction, as pin-connected, rivetted or welded bridge.
According to road level relative to the highest flood level of the river below, particularly for
highway bridge, as high level or submersible bridge.
According to the method of clearance for navigation as high level, movable-bascule, movable
swing or transporter bridge.
Classification of Bridges
According to total length as culvert (less than 6m), minor bridge (6m to 60m), major bridge
(above 60m) or long span bridge (above 120m).
According to degree of redundancy as determinate or indeterminate bridge.
According to the anticipated type of service and duration of use as permanent, temporary,
military (pontoon, bailey) bridge.
Bridges
Canal Bridges on Bridges on
Bridges Drains rivers
Canal Bridges
•All bridges across canals are designed according to IRC
specifications. Bridges combined with irrigation structures are
normally designed to Class A loading. Unimportant road bridges are
designed for Class B loading.
•In small bridges, the width (parallel to the flow of the stream) should
be sufficient to give a minimum carriageway of 4.25m for a single
lane bridge and 7.5m for a two lane bridge between the inner faces
of kerbs or wheel guards. Extra provision should be made for
footpath, etc if required.
Clear waterway excluding width of pier shall not be less than B, where B is bed width.
The waterway should be rounded off to nearest meters.
Data Required
•Cross section of canal
•Ground level, bed level of canal
•Discharge of canal
•Existing top level of road and type of road
•Ground water level at site
•Safe bearing capacity
•Type of soil
Selection of Site
Normally selection of site for culverts and small bridges on canal is guided by road alignment.
However, where there is a choice, select a site
i. Which is situated on a straight reach. Sufficiently d/s of bends.
ii. Which is sufficiently away from confluence of two or more canals.
iii. Which make approach roads feasible on the straight.
iv. Which offers a square crossing
Selection of Bridge Site : Characteristics
of an Ideal Site
A straight reach of the river.
Steady river flow without serious whirls and cross currents.
A narrow channel with firm banks.
Suitable high banks above high flood level on each side.
Rock or other hard inerodible strata close to the river bed level.
Economical approaches which should not be very high or long or liable to flank attacks of the
river during floods; the approaches should be free from obstacles such as hills, frequent
drainage crossings, sacred places, graveyards, or built up areas or troublesome land acquisition.
Selection of Bridge Site : Characteristics
of an Ideal Site
Proximity to a direct alignment of the road to be connected.
Absence of sharp curves in approaches.
Absence of expensive river training works.
Avoidance of excessive underwater construction.
A number of bridge sites are investigated and site is decided to serve the needs of the bridge at
the least cost.
To the extent possible, it is desirable to align the bridge at right angles to the river, i.e., provide a
square crossing.
Skew crossing has to be provided in order to avoid costly land acquisition or sharp curves on the
approaches.
Design Discharge
The design discharge may be taken as the maximum value obtained from at least two of the
methods mentioned.
If the values obtained by the two methods differ by more than 50%, then the maximum design
discharge is limited to 1.5 times the lower estimate.
Freak discharges of high intensity due to the failure of a dam or tank constructed upstream of
the bridge site need not be catered for.
It may be adequate to design for a flood occurring once in 20 years in case of culverts and once
in 100 years for bridges and to ensure that rarer floods be passed without excessive damage to
the structure.
Determination of Design Discharge
The maximum discharge which a bridge across a natural stream is to be designed to pass can be
estimated by the following methods:
a) By using any one of the empirical formulae applicable in the region.
b) By using a rational method involving the rainfall and other characteristics for the area.
c) By area-velocity method, using hydraulic characteristics of the stream such as cross-sectional
area, and the slope of the stream.
d) By unit hydrograph method.
e) From any available records of the flood discharges observed at the bridge site or at any other
site in the vicinity.
Determination of Design Discharge
It is desirable to estimate the flood discharges by at least two of the above methods.
Determination of Design Discharge
Empirical Formulae
Empirical formulae from flood discharge from a catchment have been proposed of the form:
𝑄 = 𝐶𝐴
Where Q = maximum flood discharge in m^3/s
A = catchment area in square kilometers
C = constant depending on the nature of the catchment and location
n = constant
Ryves formula
𝑄 = 𝐶𝐴 ⁄
The value of C is taken as 6.8 for flat tracts within 25km of the coast, 8.5 for areas between 25
and 160km of the coast and 10 for limited areas near the hills.
A reliable value for C for any particular region can only be derived by a careful statistical analysis
of a large volume of observed flood and catchment data.
The reliability of an empirical formula of this nature is extremely limited.
Rational Method
A rational method for flood discharge should take into account the intensity, distribution and
duration of rainfall as well as area, shape, slope, permeability and initial wetness of the
catchment.
A typical rational formula is
𝑄 = 𝐴𝐼 λ
Where Q = msximum flood discharge in m^3/s
A = catchment area in square kilometer
𝐼 = peak intensity of rainfall in mm per hour
λ = a function depending on the characteristics of the catchment in producing the peak run-off
.
λ=
tc = time of concentration in hours
.
tc= 0.87
L = distance from the critical point to the bridge site in kilometres
H = difference in elevation between the critical point and the bridge site in meters
P = coefficient of runoff for the catchment characteristics
f = a factor to correct for the variation of intensity of rainfall 𝐼 over the area of the catchment
Value of P in Rational Formula
Surface P
Steep bare rock and also city pavements 0.90
Rock, steep but with thick vegetation 0.80
Plateaus, lightly covered 0.70
Clayey soils, stiff and bare 0.60
Clayey soil, lightly covered 0.50
Loam, lightly cultivated 0.40
Loam, largely cultivated 0.30
Sandy soil, light growth 0.20
Sandy soil, heavy bush 0.10
Value of f in Rational Formula
Area km^2 f Area km^2 f
0 1.000 80 0.760
10 0.950 90 0.745
20 0.900 100 0.730
30 0.875 150 0.675
40 0.845 200 0.645
50 0.820 300 0.625
60 0.800 400 0.620
70 0.775 2000 0.600
Area-velocity Method
Based on hydraulic characteristics of the stream
The velocity in the stream under flood conditions is calculated by Manning’s or similar formula
The discharge Q is given by equation
𝑄 = 𝐴𝑉
Where Q = discharge in m^3/s
V = velocity of flow in m/s
. .
𝑉= 𝑅 𝑆
n = coefficient of roughness
S = slope of bed
R = hydraulic mean depth in meters = Area of cross section/wetted perimeter
Unit-Hydrograph Method
A hydrograph is the graphical representation of discharge in a stream plotted against time due to
rain storm of specified intensity, duration and areal pattern.
For any given drainage basin, the hydrographs of runoff due to two rain storms will be similar,
their ordinates being proportional to the intensity of the rainfall.
A unit hydrograph is defined as the runoff hydrograph representing a unit depth (1mm) of direct
runoff as a result of rainfall excess occurring uniformly over the basin and at a uniform rate for
specified duration.
Unit-Hydrograph Method
Steps for computing peak discharge:
i. The storm hydrograph for the basin for a particular rainfall excess is plotted from documented
data of runoff rates in m^3/hour and time in hours.
ii. The base flow is separated from the direct runoff.
iii. The volume of direct runoff is computed from the area under the storm hydrograph.
iv. The volume divided by the area of the basin gives the direct runoff in terms of depth of flow d
(expressed in mm) over the basin.
v. The ordinates of the unit hydrograph are obtained by dividing the corresponding ordinates of the
strom hydrograph by d.
vi. Direct runoff for any given storm can be calculated by multiplying the maximum ordinate of the
unit hydrograph by the depth of runoff over the area.
vii. The maximum runoff rate is obtained by adding the base flow to the maximum direct runoff rate.
The unit hydrograph method assumes that the storm occurs uniformly over the entire basin and
that the intensity of rainfall is constant for the duration of the storm.
These assumptions may be reasonable for small basins, they are not normally satisfactory for
large catchments of over 5000 square kilometer
Estimation from Flood Marks
If flood marks can be observed on an existing bridge structure near the proposed site, the flood
discharge passed by the structure can be estimated reasonably well by applying an appropriate
formula.
It is desirable to locate these marks as soon after the flood as possible.
Linear Waterway
When the water course to be crossed is an artificial channel for irrigation or navigation, or when
banks are well defined for natural streams, the linear waterway should be the full width of the
channel or stream.
For large alluvial streams with undefined banks, the required linear waterway should be
determined using Lacey’s formula given in equation as follows:
𝐿=𝐶 𝑄
Where, L = Linear waterway in meters
Q = the designed maximum discharge in m3/s
C = a constant, usually taken as 4.8 for regime channels but may vary from 4.5 to 6.3
according to local conditions.
It is not desirable to reduce the linear waterway below that for regime condition.
If a reduction is effected, special attention should be given to afflux and velocity of water under
the bridge.
Afflux
Afflux is the heading up of water over the flood level caused by constriction of waterway at a
bridge site. It can be computed from formula:
𝑋= −1
Where, X = afflux
V = velocity of normal flow in stream
g = acceleration due to gravity
L = width of the stream at HFL
L1 = Linear waterway under the bridge
C = coefficient of discharge through the bridge, taken as 0.7 for sharp entry and 0.9 for bell
mouthed entry
The afflux should be kept minimum and limited to 300mm.
Afflux causes increase in the velocity on downstream side, leading to greater scour and requiring
deeper foundations.
The increased velocity under the bridge should be kept below the allowable safe velocity for the
bed material. Typical values of safe velocities are as below:
Loose clay or fine sand : upto 0.5 m/s
Coarse sand : 0.5 to 1.0 m/s
Fine gravel, sandy or stiff clay : 1.0 to 1.5 m/s
Coarse gravel, rocky soil : 1.5 to 2.5 m/s
Boulders, rock : to 5.0 m/s
Economical Span
The most economical span length is that for which the cost of superstructure equals the cost of
substructure.
Let A = cost of approaches
B = cost of two abutments including foundations
L = total linear waterway
l = length of one span
n = number of spans
P = cost of one pier including foundation
C = total cost of bridge
Economical Span
Assuming that the cost of superstructure of one span is proportional to the square of span
length, total cost of superstructure equals n.k.l^2, where k is a constant.
The cost of railings, floorings, etc., is proportional to the total length of bridge and can be taken
as k’L
C = A+B+(n-1)P+ n.k.l^2+ k’L
For minimum cost dC/dl should be zero.
Substituting n = L/l, and differentiating and equating the result of differentiation to zero, we get
P = kl^2
Therefore, for an economical span, the cost of superstructure of one span is equal to cost of
substructure of the same span.
Location of Piers and Abutments
Piers and abutments should be so located as to make the best use of the foundation conditions
available.
As far as possible, the most economical span as above should be adopted.
If navigational or aesthetic requirements are to be considered, the spans may be suitably
modified.
Number of spans should be kept low as piers obstruct water flow.
If piers are necessary, an odd number of spans is to be preferred.
Location of Piers and Abutments
For small bridges with open foundations and solid masonry piers and abutments, the
economical span is approximately 1.5 times the total height of piers or abutments, while that for
masonry arch bridges it is about 2.0 times the height of keystone above the foundation.
The alignment of piers and abutments should be, as far as possible, parallel to the mean
direction of flow in the stream. If any temporary variation in the direction and velocity of the
stream current is anticipated, suitable protective works should be provided to protect the
substructure against the harmful effects on the stability of the bridge structure.
Vertical Clearance Above HFL
For high level bridges, a vertical clearance should be allowed between the HFL and the lowest
point of the superstructure.
This is required to allow for any possible error in the estimation of HFL and design discharge.
It also allows floating debris to pass under the bridge without damaging the structure.
Minimum Vertical Clearance Above HFL
Discharge (m^3/sec) Minimum Vertical Clearance (mm)
Below 0.3 150
0.3 to 3.0 450
3.1 to 30.0 600
31 to 300 900
301 to 3000 1200
Over 3000 1500
Subsoil Exploration
The determination of a reasonably accurate soil profile at each of the proposed bridge sites is
essential for correctly deciding the location and type of foundation.
Borings have to be taken over the length of the bridge and approaches at suitable intervals,
including preferably at probable location of abutments and piers.
The data required are:
(i) nature of soil deposit
(ii) depths and thicknesses of soil strata
Subsoil Exploration
(iii) location of ground water table
(iv) depth of rock bed
(v) engineering properties of soil and rock
If hard rock is available at shallow depths, trial pits may be dug by open excavation, and the soil
overlying the rock may be examined and classified.
Scour Depth
Normal depth of scour may be computed for natural streams in alluvial beds as under
.
𝑑 = 0.473
Where, d = normal depth of scour below HFL for regime conditions in a stable channel in meters
Q = design discharge in m^3/sec
f = Lacey’s silt factor for a representative sample of bed material
𝑓 = 1.76√𝑑𝑚𝑚
where, dmm is particle size in mm
Lacey’s Silt Factor
Type of bed Size of particle Silt factor, f
material (mm)
Very fine silt 0.05 0.4
Fine silt 0.12 0.6
Medium silt 0.23 0.8
Standard silt 0.32 1.0
Medium sand 0.50 1.2
Coarse sand 0.73 1.5
Scour Depth
When the effective linear waterway L is less than the regime width W, the value of d computed
is to be increased by multiplying the same by the factor (W/L)^0.67
Maximum Depth of Scour
The maximum depth of scour is to be taken as below:
i. In a straight reach 1.27d
ii. At a moderate bend 1.50d
iii. At a severe bend 1.75d
iv. At a right angled bend 2.00d
v. At noses of piers 2.00d
vi. At upstream noses of guide bunds 2.75d
The minimum depth of foundation below HFL is kept as 1.33d for erodible strata.
Maximum Depth of Scour
If the river is of flashy nature and bed does not submit readily to the scouring effects of floods,
the maximum depth of scour should be assessed by observation and not by the above
calculations.
Standard Specification of Road Bridges
Indian Roads Congress (IRC) has provided various codes for the design of bridges.
•IRC:6-2017 Standard specifications and code of practice for road bridges section : ii loads and
load combinations
•IRC:112-2020. CODE OF PRACTICE. FOR. CONCRETE ROAD BRIDGES.
•IRC:SP:13-2004. GUIDELINES FOR THE. DESIGN OF SMALL BRIDGES. AND
CULVERTS.
Loads
Dead load
Superimposed dead load
Live load
Impact or dynamic effects of live load
Wind load
Longitudinal forces caused by the tractive effort of vehicles or by braking of vehicles
Longitudinal forces due to the frictional resistance of expansion bearings
Loads
Centrifugal forces due to curvature
Horizontal forces due to water currents
Buoyancy
Earth pressure
Temperature stresses
Secondary stresses
Erection stresses
Forces and effects due to earthquake (seismic forces)
Live Load
Class AA
Class A
Class B
An additional loading known as class 70R is sometimes specified to be used instead of class AA.
Village Road Bridge, VRB
These shall be constructed on village road crossing and shall be safe for IRC Class A loading.
The roadway shall be kept as 4.25 m.
Skew VRBs if required should be avoided but can be provided on specific approval of
Superintending Engineer.
The details of reinforcement of skew bridges shall be provided as per MOT drawings.
The pipe culverts should generally be not provided.
DRBs/PRBs
These shall be constructed on roads maintained by PWD and shall have a roadway as per norms
of PWD.
These bridges shall be designed for IRC Class AA loading.
Angle of crossing of DRBs/PRBs shall be decided in consultation of PWD.
On curved metalled/tarred roads, possibility of utilizing existing roads as diversion should be
considered.
Waterway
The total waterway between inner faces of abutments will be kept as:-
(i) B+2D+2F for Q upto 5.60 cumecs with a minimum of 2.5 m.
(ii) B+2D for Q more than 5.60 cumecs.
(iii) The waterway shall be rounded off to the nearest meters.
Where, B, D and F are bed width, depth of water and free board in meter.
The clear waterway excluding width of piers shall not be kept less than B.
Free Board
The free board shall be provided as below:-
Free Board Discharge
0.45 m Q < 5.60 cumecs
0.60 m Q > 5.60 cumecs
Spans
The number of bays can be odd or even depending on the spans
Simply supported slab bridges be provided up to clear span of 8 m. For spans above 8 m,
multiple number of spans of equal width be provided.
Depth of Foundations
1 (a) the minimum depth of foundation for abutments and piers in case of VRBs shall be kept as
below:-
Discharge in Depth of Thickness of Offset of
cumecs foundation foundation foundation
below bed or concrete (m) concrete (m)
ground (m)
Upto 1 1.00 0.30 0.15
1 to 3 1.20 0.30 0.15
3 to 14 1.50 0.45 0.20
Depth of Foundations
(b) Above 14 cumecs and in cases of National Highways and PRBs or clayey foundations, detailed
design shall be worked out.
2. Depth of foundation from scour considerations shall not be less than 1.2 m below the
anticipated maximum scour, which in turn shall be arrived at by multiplying normal scour depth R
by a factor of 1.5.
Width of abutments, wings, piers
1 (a) Top width at slab level
Spans Top Width
Abutment Pier Land wings Thickness of
Foundation
Concrete
Upto 3m 0.58m - 0.38m 0.30m
3m to 6m 0.69m 0.69m 0.38m 0.30m
Above 6m 0.92m 0.79m 0.38m 0.30m
(b) The side edges of abutments above bed level shall be chamfered in 30 cm to reduce head
loss.
(i) Wings
The width of the wing wall shall be kept 0.38m at top and 0.5m at GL.
The water face of the wing wall shall be kept vertical above GL. Providing 150mm offsets upto the
level of bottom concrete
(ii) Abutments
The width of abutment at bed level shall be kept as below:
Span Width at bed level
Up to 3 m 0.90 m
More than 3 m As per actual design
Water face of the abutment shall be kept vertical upto bed and shall be sloped at ½ : 1 (H:V)
below bed to the concrete level.
The earth face shall be kept vertical below bed.
The length of the abutment shall be kept equal to width of decking slab.
Upstream and downstream faces of the abutment shall be vertical to the concrete level.
(iii) Piers
For spans upto 8 m, faces of piers shall be vertical.
One step of 60 mm be kept at bed level upto a depth of 0.30 m and thereafter a batter of 3 : 4
(H:V) be provided on both faces.
For spans above 8 m, the design of the pier shall be worked out.
Ratio of height of the pier to its width shall not exceed 6.
The length of straight portion in upstream and downstream shall be 150 mm extra than decking
slab.
Thereafter, equilateral 60 degree nose shall be provided on downstream whereas upstream
portion shall be made semi-circular.
(iv) Land Wings
Its foundation should be kept at 0.60 m below GL and the triangular portion between abutment
concrete and land wing concrete be filled with lean concrete 1:6:12
(v) Water Wings
Water wings shall not be provided on bridges
Camber
Camber of 7.5 cm to 5 cm shall be given from centre to the edge.
Wearing Coat
Wearing coat having average thickness 75 mm shall be laid in cc 1:1.5:3 after seven days of
removal of shuttering.
Wire mesh of 50mm x 50mm shall be provided in wearing coat on bridges where temperature
reinforcement has not been provided.
Parapet
They shall be constructed after removal of shuttering.
Road Level
Attempt should be made to keep road level lowest possible to avoid longer ramps but it should
be at least 0.3 m above GL.
Approach Slab
No approach slab shall be provided on VRBs. Brick on edge in full road width may be provided
according to requirement.
On PWD roads, approach slab shall be provided as per PWD requirements (10mm Tor at 150 mm
mesh at top and bottom or 12 mm Tor at 200 mm mesh at top and bottom).
It shall be provided in full road width, resting on one side on abutment and in a length not
exceeding 3.5 m.
Ramps of bridge should be designed as double vertical curve.
Expansion Joints
VRB: 25 mm thick joint shall be kept at piers and abutments between two slabs with the help of
thermocol. The thermocol can be removed later to fill the joint with bitumen and saw dust upto
8.0 m span.
PRBs/DRBs: as per MOT drawings.
Construction Joint
12 mm between abutment and land wings should be provided from foundation to parapet.
Steel Reinforcements
Steel reinforcements shall conform to Ministry of Shipping and Transport specifications.
Pier Cappings, Bed Block
Shall be constructed with M15 grade concrete and 150 mm thickness over piers and abutments
for slab support.
Nomotex (Tar Mastic) board in two layers of 2 cm thickness each shall be laid over abutment and
pier bed blocks before laying slab.
Drainage
For drainage of superstructure, 75 mm diameter steel pipes shall be provided vertically at the
end of roadway near kerbs on both sides.
The spacing of these pipes shall not be more than 3.0 m and these shall be staggered.
