BRIDGE LOADING
(Ref. IRC: 6-1966)
While designing the bridges the following loads, forces and stresses should be considered
where applicable.
1.
4.
5.
6.
7.
8.
11.
Dead load
2.
Live load
3.
Dynamic load
Longitudinal forces
a.
Longitudinal forces by the tractive effort of vehicles
b.
Longitudinal forces by braking of vehicles
b.
Longitudinal forces due to frictional resistance of expansion bearings
Wind load
Centrifugal forces due to curvature
Horizontal forces due to water currents
Buoyancy
9.
Earth pressure
10.
Temperature stresses
Secondary stresses 12.
Erection stresses
13.
Earth quake force
1.
DEAD LOAD:
The dead load carried by a bridge consists of its own weight and permanent
stationary load such as that of wearing coat, kerb, parapets etc. dead load invariably form
a relatively large loading component and result in significant design forces and
deformations. It is however can be estimated fairly accurately.
2.
IRC STANDARD LIVE LOAD:
According to the IRC classification, the live loads for road bridges can put into
the following four classes.
Class A loading
Class A loading consists of a wheel load composed of a driving vehicle and
trailers of specified axle spacing and loads as shown in fig. This load is normally adopted
on all roads on which permanent bridges and culverts are constructed.
Class B loading
The Class B loading is identical to Class A loading as far as positions of axles are
concerned but the magnitude of axle loads is 60% of the corresponding loads in Class A
vehicle. This load is intended for temporary structures, timber bridges and bridges in
specified areas.
Class AA loading
This loading consists of either a tracked vehicle of 700 KN or a wheeled vehicle
of 400 KN as shown in fig. Normally structures on National Highways and State
Highways are provided for these loading. Structures designed for Class AA loading
should be checked for Class A loading. Since under certain conditions, heavier stress
might be obtained under Class A loading. One train of Class AA vehicle is considered for
every two lanes of roads.
Class 70R loading
This is the Revised Version of Class AA loading and consists of tracked and
wheeled loading. The track dimensions are slightly different than those of class AA track
loading. 70R-wheel loading is of two types: a) 70R bogie loading weighing 400 KN
through two axles and b) 70R train loading weighing 1000 KN through seven axles. In
this loading, there are three alternate arrangements of wheels termed as col. 'l', 'm' and 'n'.
The details of arrangements are shown in fig. For multi lane bridges, one lane of Class
70R loading is considered for every two lanes of carriageway and it is allowed as an
alternate to Class A loading.
3.
DYNAMIC LOADS:
Moving live loads produce higher stresses than those, which would be caused if
loading vehicles were stationary. In order to take into account the increasing stresses due
to dynamic action, an impact allowance is made. The theoretical estimation of this load is
quite complex. It depends upon many factors such as roughness of the surface, spring
system of the vehicle, condition of expansion joints at the entry to bridge etc. The IRC
code however recommends definite values of impact factors for the vehicles for
simplifying the analysis. The impact allowance is expressed as % of the live load and
depends on the material used in the construction of deck of the bridge, type of loading
and the bridge span.
For class A and B loading
Impact factor fraction for RCC bridge
Impact factor fraction for Steel bridge
= 4.5/(6+L)
= 9/(13.5+L)
For Class AA and Class 70R loading
I.
For span less than 9 m
a.
b.
For tracked vehicles: 25% for span upto 5m linearly reducing to 10% for
spans of 9 m
For wheeled vehicles: 25%
II.
4.
For spans of 9 m or above
a.
For tracked vehicles:
10% upto a span of 40 m and in accordance with the curve in the code for
spans greater than 40 m
- RCC Bridge
10% for all span
- Steel Bridge
b.
For wheeled vehicles:
25% for spans upto 12 m and in accordance with the curve in the code for
spans greater the 12 m- RCC Bridge
25% for spans upto 23 m and in accordance with the curve in the code for
spans greater the 23 m- Steel Bridge
LONGITUDINAL FORCES
Longitudinal forces are caused in road bridges due to
a.
b.
c.
Tractive effort caused through acceleration of driving wheels
Braking effort due to application of brakes to the wheels
Frictional resistance offered to the movement of free bearings due to change of
temperature
Braking effort is invariably greater than the tractive effort. It is computed as follows.
a.
b.
For single and two lane bridges braking loads taken as 20%of the first train load
and 10% of the loads of succeeding trains or parts thereof on any one lane only.
For multilane bridge braking load is taken as in (a) above for the first two lanes
and 5% of the loads on the lanes in excess of two.
The longitudinal force due to friction at a free bearing is to be limited to the sum of the
dead load and love load reactions at the bearing multiplied by the appropriate coefficient
of friction as follows.
Roller bearings
0.03
Sliding bearing
0.15 - 0.25
For other types of bearings, it is decided as per the specification of bearings.
REVIEW OF IRC LOADING
In fact the IRC loadings have little relation to the vehicles currently used in the India. The
Class AA tracked vehicles of load 700KN is by no means an accurate representation of present
military tanks and a specified tail to nose distance of 90 m is not likely to be observed in the
event of emergency. The internal stresses assessed from these hypothetical loadings after
laborious calculation can at best be only approximate.
The existing IRC loadings are complicated in their application, especially if various types
of live loadings are to be considered alternately in the design to determine the severest effects.
Class 70R loading is a newly introduced live loading and can be taken as a replacement of IRC
Class AA loading. But this load is also not simple.
Basis for IRC dynamic load is not clear. No systematic study has been made to derive
realistic impact factor for road bridges. In the current practice impact effect is considered for the
full length of the load instead of applying impact effect to the heaviest axle.
The loading of different countries vary considerably both qualitatively and quantitatively.
In a comparative analysis of loading specified in different countries, it is found that IRC loading
is the most severe for a single lane bridge, but it is less severe than the British, French, Germany
and Japanese for a two lane bridge.
In view with the wide variations in highway loadings specified in various countries and to
develop simplified standard loading which can really represent the composition of vehicular
traffic there is a need of revision of IRC loading.
IRC Class AA Loading
Class AA tracked vehicle
Total Weight 700 KN
90000
90000
3600
7200
Cross section of Class AA
tracked Vehicle
850
Cross section of Class AA
wheeled vehicle
850
1000
2200
37.5KN
62.5KN
1200
350KN
37.5KN
350KN
2050
300
Carriage way
width (m)
Minimum value
of C (mm)
Single lane
bridge 3.8m and
above
300
Multi lane bridge
< 5.5m
>5.5m
600
1200
300
300
300
150
1200
600
1000
600
IRC Class A and B Loading
18500
3000
20000
8200
1200
1100 3200 1200 4300
4900
1200
3000 3000
4900
18500
3000
20000
68
68
27 27 114 114
68
68
68
41
41
16 16
41
41
41 41
68
68
1100 3200 1200
68
27 27
114 114
16 16 68 68
CLASS A
LOADING
( in KN)
CLASS B
LOADING
(in KN)
Section Of Class A and B Loading
1800
Ground Contact Area
Axial Load (KN)
B
1100
3200
1200
114
68
41
27
16
Clear
Carriageway
Width
5.5 to 7.5m
Above 7.5m
B long span
(mm)
W short
span(mm)
250
200
150
150
125
500
380
300
200
175
Minimum Clearances
g
uniformly increasing
from 0.4 to 1.2.m
1.2 m
f
150 mm
150 mm
All distances are in mm except indicated
IRC Class 70R Loading
Class 70R tracked vehicle
Total Weight 700 KN
30000
30000
4570
7920
Cross-section of Class 70R
bogie loading 400KN
Cross-section of Class 70R
tracked vehicle
840
840
2380
37.5KN
62.5KN
1220
350KN
37.5KN
350KN
2060
IRC 70R train loading 1000KN
W
1220
80
KN
120 120 170
KN KN KN
170
KN
170
KN
170
KN
+
a
+
a
+
3960
1520 2130 1370 3050
1370
a = 450; b = 1480
for column 'l'
a = 795; b = 790
for column 'm'
Value of C is same as for Class AA loading
All distances are in mm
