Civil Bhai

1. LIVE LOAD

As per IRC recommendations the live loadings are divided into following four categories.
1) Class AA loading
2) Class A Loading
3) Class B loading
4) Class 70R loadings
* Class AA loading- it considers a heavy military vehicles rolling on bridge. Its a usual practice to
design the structure for Class AA loadings on national highways and state highway. Its also
desirable for checking the structure designed for Class AA loading for Class A loading also.
Class AA loading considers following two types of vehicle.
TRACKED VEHICLE

WHEELED VEHICLE
Its assumed that no other live load will cover any part of a carriageway of a
bridge, when a train of tracked vehicle or wheeled vehicle is passing on it. Such two vehicles
should be spaced at distance of 90 meters.
* Class A loading – Its based on heaviest commercial vehicles which are going to run on roads.
Thus all important bridges on NH and SH which are not covered under type Class AA loading,
should be designed for Class A loading.
Its designed for a train moving with one engine and two bogies, such that minimum 18.4 m distance
clearance is maintained between two successive trains.
As shown in figure the axle loads will be acting simultaneously to create worse scenario.
* Class B Loading – its design is same as that of Class A loading. Its adopted for design of temporary
structures such as timber bridges.
* Class 70 R loading- Sometimes Class AA loading is replaced by Class 70 R loading, where letter R
indicates revised classification and its based on hypothetical vehicle consideration
It also considers tracked and wheeled vehicle except that ground contact length is 4.57m, length of
vehicle is 7.92 m and minimum spacing between successive vehicle is 30m.
*) Live load on Foot way- This is used by pedestrians and animals and given about 4-5 Kn/m2

2. WIND LOAD

Wind load (P) is directly proportional to the V2
P=KV2
 P wind load -N/m2
 V- velocity kmph K- constant(0.051-0.095)
-Wind load is assumed to act at a height of 1.5m above the base of road on the moving vehicle.
-Wind load consideration is neglected in case of span of bridge less than 18 m, but conditioned that
lateral bracings should be provided.

3. SEISMIC LOAD

Its assumed in earthquake susceptible zones(zone- 3,4,5) where its considered horizontal force equal
to the certain % of the weight of the vehicle.
S= x W x – seismic coefficient , S- earthquake force, W-weight of bridge under consideration.
 Its considered as a horizontal force to act in any horizontal direction through the C.G. of
structure. vertical seismic component if to be accounted then considered as 1/2 of the
horizontal coefficient.
 For superstructure its assumed to act in only perpendicular direction of traffic.
For sub structure it will be acting separately in both direction. (in traffic as well as in flow
direction)
 In design assumption is made that annual flood and earthquake will not occur at same time.
4. EARTH PRESSURE
 The bridge component which do require to retain earth strata should be designed for suitable
earth pressure, it means that should be considered for ABUTMENTS only.
 Use of coulombs theory with slight modification is adopted by IRC.
 This modification is centre of pressure will act at 0.42 H from base rather than that of 0.33 H

5. IMPACT LOAD

 Stresses are developed due to fast moving vehicle over uneven surface causing the impact.
 Provision is made such that its represented by fraction of a live load stress, called impact
factor.
 When the span exceeds 45m,then the impact fraction are taken as
0.088 (Rcc bridge) and 0.154 (steel bridge)
 Impact allowance should be made for design of bearings.
 In sub structure a reduction factor according to depth is applied to the impact factor.
 If a filling of about 60 cm is provided to bridge floor, then Impact load is reduced by 50%
IMPACT FACTOR = 4.5/( 6+ span) <3m [RCC]
IMPACT FACTOR = 9/( 13.5+ span) span <3m [Steel].

6. DEAD LOAD

The dead load of a structure is assumed by a reference to a suitable empirical formula.
Basic dead load = volume of member X density of material.

7. CENTRIFUGAL FORCE

This is generated due to curvature of a bridge super structure.
Road bridge and rail bridge = WV2
/12.95 R
 R- radius of curvature
 W- live load in KN
 V – vehicle velocity
*The horizontal load due to the CF on roadway will act at a height of 1.2 m above carriage way.
*The horizontal load due to the CF on rail line will act-
-at a height of 1.83 m (Broad gauge) above rail line.
-at a height of 1.45 m (Meter gauge) above rail line.

8. DEFORMATION STRESS

 Only taken into consideration for steel bridges. These stresses are taken as not less than 16 % of
live load and dead load stresses. These deformation stresses are ignored in case of prestressed steel
girder.

9. LONGITUDINAL FORCE

It results from one or more of the following reasons.
1) application of breaks by vehicles
2) Frictional resistance offered by movement of free bearing due to variation in temperature.
3) tractive effort caused through acceleration of vehicles.
4) Force due to breaking is assumed to act along a line parallel to the road @ 1.2m above roadline.