The document provides guidance on loads and forces that should be considered when designing bridges, including: 1. Dead loads, live loads, dynamic loads, longitudinal forces, wind loads, centrifugal forces, horizontal water currents, buoyancy, earth pressures, temperature effects, and seismic loads. 2. It describes the various live load models (Class A, B, 70R, AA) and provides details on load intensity, wheel/track configuration, and load combinations. 3. Design recommendations are given for calculating impact factors, braking forces, wind loads, water current pressures, earth pressures, and seismic forces.

1. While designing the bridges the following loads and forces should be
considered where applicable.
1. Dead load
2. Live load
3. Dynamic load
4. Longitudinal forces
a. Longitudinal forces by the tractive effort of vehicles
b. Longitudinal forces by braking of vehicles
c. Longitudinal forces due to frictional resistance of expansion bearings
5. Wind load
6. Centrifugal forces of vehicle due to curvature of bridge
7. Horizontal forces due to water currents
8. Buoyancy
9. Force exerted by earth pressure
10. Load induced by temperature variation effect
11. Load induced by creep, shrinkage and other secondary effect
12. Erection load
13. Loads induced by earthquake
Bridge Loading
Refer IRC 6-2014

2. IRC Bridge
Live Load Vehicular load
Pedestrian load
Class A load
Class B load
Class AA load
Class 70R load
• Wheeled load
• Tracked load
• Single, Two and Seven
Axel wheeled load
• Tracked load
Normal load
Represents normal
vehicular traffic
Abnormal load
Represents industrial or
military vehicular traffic

3. Class A/B Loading
CLASS A LOADING (KN)
Total load = 554 KN
CLASS B LOADING (KN)
68 68 68 68
27 27 114 114
16 16 68 68 41 41 41 41
20 1.1 3.2 1.2 4.3 3 3 3 20 C/C distance of axle (m)
Total length of a train = 18.8m
g
f
Cross section
1.8 m
B
W
1.1m
3.2m
1.2m
Plan
Carriageway
Width
G F
5.3 to 6.1m
Uniformly increasing
from 0.4 to 1.2.m
150 mm
Above 6.1m 1.2 m 150 mm

4. Class 70R Loading
Class 70R tracked vehicle
Total Weight 700 KN
4.57m
7.92m
90m 90m
Cross-section of Class 70R tracked vehicle
0.84m
C
1.22m
2.06m
0.84m
350KN 350KN
Carriage way
width
(m)
Minimum
value of C
(mm)
≥5.3m 1200

5. Cross-section of Class 70R two
axel wheeled load 400KN
C
Class 70R Loading
80 120 120 170 170 170 170 KN
70R seven axel wheeled load 1000 KN
1.22m
2.79 m
Plan
 Value of C is same as of 70R
tracked loading
 Min. distance between wheeled
loads of Class70R is 30 m
2.79 m
3.96 1.52 2.13 1.37 3.05 1.37
Plan

6. Wheel Arrangement of 70R Wheeled Load
‘L’ Type
2.79 m
0.86 m
0.41 m
0.61 m
‘M’ Type
2.79 m
0.41 m
0.61 m
0.38 m
‘N’ Type
2.79 m
0.51 m
0.51 m
0.25 m
0.23 m
Contact area of tyre may be obtained from
the corresponding tyre load, tyre pressure
and tyre tread width. Tyre tread width may
be taken as overall tyre width minus 25 mm
up to tyre 225mm and 50 mm for tyres over
225 mm width.
Maximum tyre pressure = 5.273 Kg/cm2
Minimum Wheel Spacing and Tyre Size of Heaviest Axle

7. Cross-section of Class AA
wheeled load 400KN
C
37.5 62.5 62.5 37.5 KN
Class AA Loading
[Refer Annex]
Total Weight 700 KN
3.6 m
7.2m
90m
Cross-section of Class AA tracked
vehicle
1.2m
2.05m
0.85m 0.85m
C
350KN
350KN
1.2m
Plan
0.6m 1m 0.6m
0.3m
0.15m
Carriage way
width (m)
Minimum
value of C
(mm)
Multi lane bridge
≥5.3m 1200

8. • 70R loading is adopted on all roads on which
permanent bridges are constructed. Bridges designed
for 70R loading should be checked for Class A loading.
• Class AA loading is adopted on specified location on
which permanent bridges are constructed. Bridges
designed for Class AA loading should be checked for
Class A loading.
• Class A loading is adopted on all roads on which
permanent bridges are constructed. Bridges designed
for Class A loading should be checked for Class AA/70R
loading.
• Class B loading is adopted on specified location on
which temporary bridges are constructed.

9. Carriage
Way (m)
No of
lane
Live loads
<5.3 1 Class A loading for 2.3m width and for remaining width 500 Kg/m2
≥5.3
<9.6
2 One lane of Class70R/AA loading or two lanes of Class A loading
≥9.6
<13.1
3
One lane of Class 70R/AA for every two lanes with Class A loading
for remaining lanes or three lanes of Class A loading
≥13.1
<16.6
4
One lane of Class 70R/AA for every two lanes with Class A loading
for remaining lanes or one lane of class A for each lane
≥16.6
<20.1 5
≥20.1
<23.6 6

10. 0.4 m 1.8 m 1.7 m 1.8 m
Class A
Class A
1.8 m
0.4m
5 KN / m2
2.3 m
0.15m 0.5m
Class A
1.2m
Class 70R (W/T)
1.2m
1.8 m
0.4 m
0.15m
0.5m
Class A Class 70R (W/T)
Combination of live loads
For Single Lane Bridge
For Two Lanes Bridge
For Two Lanes Bridge
For Three Lanes Bridge

11. 1.2m
1.8 m
0.4 m
0.15m
0.5m
Class A
1.8 m
0.5m
Class A
1.8 m
0.5m
Class A
1.2m
1.2m
Class 70R (W/T)
1.2m
Class 70R (W/T)
1.2m
1.8 m
0.4 m
0.15m
0.5m
Class A
1.8 m
0.5m
Class A
1.8 m
0.5m
Class A
1.2m
1.8 m
0.5m
Class A
1.2m
1.2m
1.8 m
0.4 m
0.15m
0.5m
Class A Class 70R (W/T)
1.8 m
0.5m
Class A
1.2m
Combination of live loads
For Three Lanes Bridge
For Four Lanes Bridge
For Four Lanes Bridge
For Four Lanes Bridge

12. Live load information required for
analysis of bridge deck
• Type of load
• Number of axle of vehicle
• Magnitude of load on each axle
• Spacing of axle
• Contact area of wheel /track
• Spacing of vehicle in transverse and longitudinal direction
• Maximum lane load
• Reduction of live load in excess of two lanes
• Arrangement of wheel in case of 70R wheeled and train loading
• Combination of live loads
Pedestrian Load
Length of bridge ≤7.5 m ; Intensity of load = 4 or 5 KN/m2
>7.5m; Intensity of load ≤ 4 KN/m2
P = P’ – (40L – 300)/9 for up to 30 m span
P = (P’ – 260 + 4800/L) × (16.5 – W)/15 for greater than 30 m span
P’ = 4 or 5 KN/m2
P – Intensity of load
W – Width of foot way
L – Span of bridge

13. For class A and B loading
• Impact factor fraction for RCC bridge = 4.5/(6+L)
• Impact factor fraction for Steel bridge = 9/(13.5+L)
For Class AA and Class 70R loading for span less than 9 m
• For tracked vehicles: 25% for span up to 5m linearly reducing to 10% for spans of 9 m
• For wheeled vehicles: 25%
For tracked vehicles for spans of 9 m or above
• 10% up to a span of 40 m and in accordance with the curve in the code for spans greater than 40
m of RCC Bridge
• 10% for all span of Steel Bridge
For wheeled vehicles for spans of 9 m or above
• 25% for spans up to 12 m and in accordance with the curve in the code for spans greater the
12 m RCC Bridge
• 25% for spans up to 23 m and in accordance with the curve in the code for spans greater the
23 m Steel Bridge
Impact Load Moving live load with its dynamic effect.
Dynamic effect of live load is calculated by the impact factor.
Impact load = static value of live load × Impact factor

14. 5 10 20 25 45
0
0
10
25
50
55
A and B ( Steel bridge )
A and B ( Concrete bridge )
Class AA/70R tracked ( Concrete bridge )
Impact Factor
Span of bridge, m
IF in %
Class AA/70R tracked ( steel bridge )
Class AA/70R wheeled ( Concrete bridge )

15. 1. Externally applied longitudinal forces
• Tractive effort caused through acceleration of driving wheels
• Braking effort due to application of brakes to the wheels
• Frictional resistance offered by free bearings due to change of
temperature, shrinkage and creep
Force due to braking effort
Braking effort is invariably greater than the tractive effort so taken as a design longitudinal
force. It is computed as follows.
• For single or two lane bridge, braking loads taken as 20%of the first
train load and 10% of the loads of succeeding trains.
• For multilane bridge, braking load is taken as in (a) for the first two
lanes and 5% of the loads on the other lanes.
• The force due to braking effort shall be assumed to act 1.2m above
the roadway.
LONGITUDINAL FORCES

16. Forces due to frictional resistance offered by bearing
Fh/2 or µW
Fh/2 or µW
µW Fh - µW
Fh /2+ sδ Fh /2+ sδ
I. Simply supported deck on
unyielding support
Span without bearing
Span with fixed
and free bearing
Span with
elastomeric bearings

17. 2. Self induced longitudinal forces
Forces induced by Creep, Shrinkage or Effect of Temperature
Variation
II. Simply Supported/ Continuous
deck on flexible support
1 2 3
µW CL Sn + Fh X Sn /∑S CL Sn + Fh X Sn /∑S

18. Wind load = Wind load on the structure
+ Wind load on the live load
FT = PZ × A × G × CD
FL = 0.25 FT for beam type bridge
= 0.5 FT for truss type bridge
FT - Wind load in transverse direction
FL - Wind load in longitudinal direction of bridge
PZ - Design wind pressure
A - Exposed area of structure / live load to wind
G - Gust factor ; G = 2 for 150 m span
CD - Drag coefficient CD ≥ 1.3 depending upon b/d ratio and type of superstructure
Wind load on live load = Length of live load × 3m × FT
In the case of live load G is taken equal to 1.2m and point of application of wind load is 1.5 m.
Described method of wind load calculation is valid for bridges of
span upto150m and height of pier upto 100m
WIND LOAD

19. Pressure of water current P = 52 KV2 [kg/m2]
Where K- shape factor of the pier ( k= 0.5 -1.5)
V- velocity of the water current at the point, where
pressure intensity is to be calculated. [m/sec]
Intensity of pressure due to water current depends on
• Direction of current
• Velocity of water current
• Shape factor of the pier
• Maximum scouring depth
200 deviation of river course shall be considered in the calculation of the
pressure due to water current
HORIZONTAL FORCES
DUE TO WATER CURRENT
Horizontal forces due to water current =
Pressure of water current X Area of structure exposed to water

20. FORCE EXERTED BY EARTH PRESSURE
PA=1/2KAγH2
PP=1/2KPγH2
PP
PA
δ
ì
0.42H
H
Coulomb’s Theory

21. Seismic Force
Method of computation of Seismic Force
• Elastic Seismic Acceleration Method
In this method static analysis is made and seismic force is obtained for
acceleration corresponding to the fundamental mode of vibration.
• Elastic Response Spectrum Method
In this method dynamic analysis is made to first and higher modes of
vibration and forces are obtained for each mode by using of response
spectrum.
In elastic seismic acceleration method force due to
earthquake is calculated as follows.
Feq = Ah × (Dead load + Partial Live load)
where,
Ah = Z/2 × I/R × Sa/g

22. Bridges need not be checked for seismic effects
Bridges need special investigation for seismic effects

23. FORCE EXERTED BY
DYNAMIC EARTH PRESSURE
PA=1/2KAγH2
PP=1/2KPγH2
PP
PA
δ
ì
0.6H
H
Mononobe Okabe Theory
(Modified Coulomb’s Theory)

24. FORCE EXERTED BY HYDRODYNAMIC PRESSURE
R
Ground
Shaking
Enveloping
cylinder
Pier
R Ground
Shaking
Enveloping
cylinder
Pier
F = C Ah W W- Weight of water bound in enveloping cylinder
W = πR2H × Unit wt. of water
R – Radius of enveloping cylinder
H – Submerged height of pier
Ah- Horizontal acceleration coefficient
C – Hydrodynamic coefficient
H/R C
1 0.39
2 0.58
3 0.68
4 0.73

25. Load Combinations
in the Design of Bridge [WSDM]
Combination
of loads
I
II A
II B
III A
III B
IV
V
VI
VII
VIII
IX
Increase in
permissible stress
according to load
combination
0 - 50 %

26. Load Combination [WSDM]

27. IRC 6 define four cases separately i.e. foundation, stability, limit state of
strength and limit state of serviceability to be considered in Limit State
Design Method. In each cases, there are further three combinations of loads
to be considered.
 Three combinations of limit state of strength and stability are
• Basic combination
• Seismic combination
• Accidental combination
 These combinations are given separately for serviceability check and
foundation design.
 Partial safety factors for loads for different combinations and for different
works are not similar. They are chosen as specified in code
 Refer IRC 6 – 2010, Table 3.1, 3.2, 3.3 and 3.4 for combination of loads
Load Combinations
in the Design of Bridge [LSDM]

28. • HS loading
It consists of truck with semi-trailer or the corresponding lane
loading. Lane load consists of a uniform load per unit length of
traffic lane combined with a concentrated load (one concentrate
load for simply supported span and two concentrated load in case of
continuous span).
HS loading may be HS 20-44 and HS 15-44.
It consists of a two-axle truck or the corresponding lane load. Lane
load consists of a uniform load per unit length of traffic lane
combined with a single concentrated load (two concentrated load in
case of continuous span).
H loading may be H 20-44 and H 15-44.

29. Lane Loading
H 20-44
HS 20-44
HS 15-44
H 15-44
18000 lbs for bending moment
26000 lbs for shear force
13500 lbs for bending moment
19500 lbs for shear force
640 lbs/ft
480 lbs/ft
8000 lbs
6000 lbs
32000 lbs
24000 lbs
H 20-44
H 15-44
8000 lbs
6000 lbs
32000 lbs
24000 lbs
32000 lbs
24000 lbs
HS 20-44
HS 15-44
Truck Loading
6'
14'
6'
14'-30’
14'
AASHTO
Live Loading

30. Maximum bending moment (KN-m)
Span
(M)
IRC loading
AASHTO BS
(HS20-44) HA HB
One lane Two lane One lane Two lane One lane Two lane
One
lane
Two
lane
5 687 687 231 462 243 488 756 838
10 1548 1548 573 1146 694 1388 1863 2095
15 2725 2725 1073 2146 1336 2672 3331 3776
20 4198 4198 1552 3104 2175 4350 5654 6379
25 5680 5680 2022 4044 3156 6312 7862 8914
30 7058 7058 2481 4962 4151 8302 10085 11468
35 8412 8412 2935 5870 5184 10368 12315 14043
40 9739 9739 3379 6758 6340 12680 14550 16663
45 11059 11059 3863 7726 7501 15002 16788 19288
50 12496 12496 4597 9194 8656 17312 19029 21914
Responses of bridge in different codal standard

31. Maximum Live Load Shear Force
for Two Lane Simply Supported Bridge (KN-M)
596
716 738 750
1020
1509
1619
1694
0
200
400
600
800
1000
1200
1400
1600
1800
10m 20m 25m 30
IRC
AASHTO
BS

32. Maximum Live Load Bending Moment
for Two Lane Simply Supported Bridge (KN-M)
1146
3104
4044
4962
2095
6379
8914
11468
0
2000
4000
6000
8000
10000
12000
14000
10m 20m 25m 30
IRC
AASHTO
BS