This document discusses load standards and the effective width method for bridge engineering according to the Indian Roads Congress (IRC). It outlines various loads that must be considered in bridge design like dead load, live load, impact load, and wind load. It also describes the IRC's standard load classifications for bridges and provides equations for calculating impact percentage and effective slab width. The effective width method per the IRC is described for slabs spanning in one or two directions and cantilever slabs.

1. BRIDGE
ENGINEERING
IRC Loading standards and Effective width
method

2. I.R.C. loading standards
 Loads on bridges :
The following are the various loads to be considered
for
the purpose of computing stresses, wherever they
are
applicable.
· Dead load
· Live load
· Impact load
· Longitudinal force
· Thermal force
· Wind load

3. Cont’d
· Seismic load
· Racking force
· Forces due to curvature.
· Forces on parapets
· Frictional resistance of expansion bearings
· Erection forces

4. Highway bridges:
In India, highway bridges are designed in
accordance with IRC bridge code.
IRC: 6-2014 – Section II gives the
specifications for the various loads and
stresses to be considered in bridge design.
There are three types of standard loadings for
which the bridges are designed namely, IRC
class AA loading, IRC class A loading and IRC
class B loading

5. Classification of loading
 Dead load – The dead load is the weight of
the structure and any permanent load fixed
thereon.
 Live load – Bridge design standards specify
the design loads, which are meant to reflect
the worst loading that can be caused on the
bridge by traffic, permitted and expected to
pass over it.
 In India, the Railway Board specifies the
standard design loadings for railway bridges in
bridge rules.

6. CLASSIFICATION OF
LOADING
Road bridges and culverts shall be divided into
classes according to the loadings they are
designed to carry.
 IRC Class 70R Loading: This loading is to be
normally adopted on all roads on which
permanent bridges and culverts are
constructed. Bridges designed for Class 70R
Loading should be checked for Class A
Loading also as under certain conditions,
heavier stresses may occur under Class A
Loading.

7. Cont’d
 IRC Class AA Loading: This loading is to be
adopted within certain municipal limits, in
certain existing or industrial areas, in other
specified areas, and along certain specified
highways. Bridges designed for Class AA
Loading should be checked for Class A
Loading also, as under certain conditions,
heavier stresses may occur under Class A
Loading.

8.  IRC Class A Loading: This loading is to be
normally adopted on all roads on which
permanent bridges and culverts are
constructed.
 IRC Class B Loading: This loading is to be
normally adopted for timber bridges.

9. LOAD, FORCES AND
STRESSES

10. COMBINATION OF LOADS AND
FORCES AND PERMISSIBLE
STRESSES

11. UNIT WEIGHT OF MATERIALS

12. LIVE LOADS

13. WHEEL ARRANGEMENT FOR
70R

14. Live load combination

15.  Footpath: For all parts of bridge floors
accessible only to pedestrians and animals
and for all footways the loading shall be 400
kg/m^2.
 For the design of foot over bridges the loading
shall be taken as 500 kg/m^2.
 Kerbs: 0.6 m or more in width, shall be
designed for the above loads and for a local
lateral force of 750 kg per metre, applied
horizontally at top of the kerb. If kerb width is
less than 0.6 m, no live load shall be applied in

16. Footway designed for live
load/m2
The footways shall be designed for the
following live loads per square metre for
footway area, the loaded length of footway
taken in each case

17. Impact load
 The dynamic effect caused due to vertical
oscillation and periodical shifting of the live
load from one wheel to another when the
locomotive is moving is known as impact load.
 The impact load is determined as a product of
impact factor, I, and the live load. The impact
factors are specified by different authorities for
different types of bridges. The impact factors
for different bridges

18. For Class A or Class B Loading
 The impact fraction shall be determined from
the following equations which are applicable
for spans between 3 m and 45 m.

19. VALUE OF IMPACT
PERCENTAGE

20. SPAN VS IMPACT %

21. WIND LOAD
Wind load on a bridge may act
· Horizontally, transverse to the direction of span
· Horizontally, along the direction of span
· Vertically upwards, causing uplift
· Wind load on vehicles
 Wind load effect is not generally significant in
short-span bridges; for medium spans, the
design of sub-structure is affected by wind
loading; the super structure design is affected
by wind only in long spans.

22.  The hourly mean wind speed and pressure
values given in Table 5 corresponds to a basic
wind speed of 33 m/s, return period of 100
years, for bridges
situated in plain terrain and terrain with
obstructions, with a flat topography.

24. INTENSITY OF WIND
PRESSURE

25. DESIGN WIND FORCE ON
SUPERSTRUCTURE
 The superstructure shall be designed for wind
induced horizontal forces (acting in the
transverse and longitudinal direction) and
vertical loads acting simultaneously.
 The transverse wind force (in N) shall be taken
as acting at the centroids of the appropriate
areas and horizontally and shall be estimated
from:
where, is the hourly mean wind pressure in
N/m2 (see Table 5), is the solid area in m2 G is

26. Cont’d
 The longitudinal force on bridge superstructure
(in N) shall be taken as 25 percent and 50
percent of the transverse wind load as
calculated as per Clause 209.3.3 for beam/
box/plate girder bridges and truss girder
bridges respectively.
 An upward or downward vertical wind load (in
N) acting at the centroids of the appropriate
areas, for all superstructures shall be derived
from:

27. Design Wind Forces on
Substructure
 The substructure shall be designed for wind
induced loads transmitted to it from the
superstructure and wind loads acting directly
on the substructure.
 Loads for wind directions both normal and
skewed to the longitudinal centreline of the
superstructure shall be considered.

28. DRAG COEFFICENTS FOR
PEIRS

29. RACKING FORCE
 Racking force – This is a lateral force
produced due to the lateral movement of
rolling stocks in railway bridges.
 Lateral bracing of the loaded deck of railway
spans shall be designed to resist, in addition to
the wind and centrifugal loads, a lateral load
due to racking force of 6.0 kN/m treated as
moving load.
 This lateral load need not be taken into
account when calculating stresses in chords or
flanges of main girders.

30. Forces on parapets
 Forces on parapets - Railings or parapets
shall have a minimum height above the
adjacent roadway or footway surface of 1.0 m
less one half the horizontal width of the top rail
or top of the parapet. They shall be designed
to resist a lateral horizontal force and a vertical
force each of 1.50 kN/m applied
simultaneously at the top of the railing or
parapet.

31. Seismic load
 If a bridge is situated in an earthquake prone
region, the earthquake or seismic forces are
given due consideration in structural design.
 Earthquakes cause vertical and horizontal
forces in the structure that will be proportional
to the weight of the structure.
 Both horizontal and vertical components have
to be taken into account for design of bridge
structures.
 IS:1893 – 1984 may be referred to for the
actual design loads.

32. ERECTION FORCES
 There are different techniques that are
used for construction of railway bridges, such
as launching, pushing, cantilever method, lift
and place.
 In composite construction the composite action
is mobilised only after concrete hardens and
prior to that steel section has to carry dead
and construction live loads.
 Depending upon the technique adopted the
stresses in the members of the bridge
structure would vary. Such erection stresses

33. SNOW LOAD
 The snow load of 500 kg/m2 where applicable
shall be assumed to act on the bridge deck
while combining with live load as given below.
Both the conditions shall be checked
independently:
 a) A snow accumulation upto 0.25 m over the
deck shall be taken into consideration, while
designing the structure for wheeled vehicles.
 b) A snow accumulation upto 0.50 m over the
deck shall be taken into consideration, while
designing the structure for tracked vehicles.

34. EFFECTIVE WIDTH METHOD
AS PER IRC
 The effect of concentrated load on deck slabs
spanning in one or two directions or cantilever
slabs may be computed using any rational
method.
 The value of Poisson's ratio for concrete be
0.15.
 The method of assessment of effective width
given in clause 305.16.2 of IRC Bridge code.
 For Precast slabs, the actual width of each
precast unit should be taken as the width of
slab.

35. Cont’d
 For slabs spanning in two directions, either
Pigeaud’s method or westergaard’s method
can be used.
SLAB SPANNING IN ONE DIRECTION:
 The maximum BM caused by a wheel load
may be assumed to be resisted by an effective
width of slab.
be=kx(1-x/L)+bw

36. Cont’d
Where,
be= effective width
L=effective span in the case of SS slab and
equal to clear span in the case of continuous
slab
bw=Breadth of the concentration area of load
K=Constant (Depending on the ratio L’/L)
L’=width of the slab

37. CANTILEVER SLAB
 The effective width of dispersion, measured
parallel to the supported edge for concentrated
loads on a cantilever solid slab is
be=1.2 X + bw
Where
be=Effective width
X=distance of the C.G of the concentrated load
bw =Breadth of the concentration area of the load

38. Cont’d
 The effective width should be limited to 1/3rd
the length of the cantilever slab measured
parallel to the support.
 When two or more loads act on slab, the
effective width for one load overlaps the
effective width of the adjacent load, then the
resultant effective width should be taken as
sum of the respective effective widths for each
load minus the width of overlap.

39. DISPERSION OF LOADS
ALONG THE SPAN
 The effective length of slab on which a wheel
load shall be equal to the dimension of the tyre
contact area over the wearing surface of slab
in the direction of the span plus twice the
overall depth of the slab including thickness of
wearing course.

40. K Constant Values

41. DISPERSION OF LIVE LOAD

42. Moment coefficient m1 & m2 for
K=0.4

43. Moment coefficient m1 & m2 for
K=0.5

44. Moment coefficient m1 & m2 for
K=0.6

45. DISTRIBUTION OF
REINFORCEMENT
 Slab spanning in one direction shall be
provided at right angles to the main
reinforcement.
 0.3 times the live load moment and 0.2 times
the dead load moment
 Cantilever slabs is computed to resist a
moment equal to 0.3 times the live load
moment and 0.2 times the dead load moment.
 The steel is provided half at the top and half at
the bottom of the slab.

46. SPANNING IN TWO
DIRECTIONS
 The moments in the two directions can be
obtained by rational method. By the use of
curve given by Pigeaud is recommended.
 It is applicapable to rectangular slabs
supported freely on all four sides and
subjected to a symmetrically placed load.
 The dispersion of load may be 45 degree
through wearing course and deck slab.

47. Cont’d
 When the slabs are continuous, the same
procedure be adopted.
 The maximum midspan moment may be 0.8
Let L & B – the span lengths in the long and
short span directions.
a and b- dimension of tyre contact area in the
long and short directions
u & v-load spread after allowing dispersion
through deck slab, M1 and M2- Moments

48. Limitations:
 Only loads placed at centre can be
considered.
 While the other loads will be non central, some
approximation will have to be considered.
 When v/L is small, the reading of values m1
and m2 from the curves become less accurate.
 This method is most useful when K is more
than 0.55.

49. Thank you