SLAB CULVERT
Dr. Hassan Irtaza, Professor
Department of Civil Engineering, A.M.U.
Bridge
A bridge is a structure built to span physical obstacles without closing the way underneath such as a body of water, valley, or road, for the purpose of providing passage over the obstacle.
There are many different designs that each serve a particular purpose and apply to different situations.
Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed and anchored, the material used to make it, and the funds available to build it.
A bridge is a structure built to span physical obstacles without closing the way underneath such as a body of water, valley, or road, for the purpose of providing passage over the obstacle.
There are many different designs that each serve a particular purpose and apply to different situations.
Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed and anchored, the material used to make it, and the funds available to build it.
All road bridges in India are designed with Indian Road Congress specifications [IRC: 5-1998, IRC: 6-2000 and IRC: 21-2000].
Culvert: A culvert is a small bridge having a span less than 6 m.
Types of culvert:
Slab culvert
Box Culvert
All road bridges in India are designed with Indian Road Congress specifications [IRC: 5-1998, IRC: 6-2000 and IRC: 21-2000].
Culvert: A culvert is a small bridge having a span less than 6 m.
Types of culvert:
Slab culvert
Box Culvert
Slab Culvert
A slab culvert is supported on two opposite edges. The other two opposite edges are free.
Box Culvert
3-D view of the Box culvert
3-D view of the Box culvert
Analysis of Slab Culvert
The analysis of a slab culvert carries concentrated loads due to vehicles is highly statically indeterminate problem.
It is not possible to determine the accurate solution for the boundary conditions encountered in the slab culvert by the generally available tools of analysis.
An accurate analysis is possible using a three dimensional finite element method using a plate bending element.
However, a FEA requires a deep understanding of the method, modeling technique as well as versatile, efficient, and user friendly computer software.
The analysis of a slab culvert carries concentrated loads due to vehicles is highly statically indeterminate problem.
It is not possible to determine the accurate solution for the boundary conditions encountered in the slab culvert by the generally available tools of analysis.
An accurate analysis is possible using a three dimensional finite element method using a plate bending element.
However, a FEA requires a deep understanding of the method, modeling technique as well as versatile, efficient, and user friendly computer software.
It is possible to carry out a simplified analysis by modifying the available results of elastic analysis of slabs.
It is referred to as effective width method of analysis for solid slabs resting on two opposite edges.
If a solid slab supported on two opposite edges, carries concentrated loads, the maximum bending moment caused by the concentrated loads is assumed to be resisted by an effective width of slab (measuring parallel to the supporting edges) as follows in accordance with clause 24.3.2 of IS: 456-2000.
It is possible to carry out a simplified analysis by modifying the available results of elastic analysis of slabs.
It is referred to as effective width method of analysis for solid slabs resting on two opposite edges.
If a solid slab supported on two opposite edges, carries concentrated loads, the maximum bending moment caused by the concentrated loads is assumed to be resisted by an effective width of slab (measuring parallel to the supporting edges) as follows in accordance with clause 24.3.2 of IS: 456-2000.
Slab Carrying Concentrated Loads
1. For a single concentrated load. the effective width shall be calculated in accordance with the following equation provided that it shall not exceed the actual width of the slab:
where, beff = effective width of slab
K = a constant having values depending upon the ratio of the width of the slab (l’) to the effective width of the slab.
x = distance of the centroid of the concentrated load from nearer support
leff = effective width
a0 = width of the contact area of the concentrated load measured parallel to the supporting edge.
1 0 eff
eff
b Kx x a
l
1. For a single concentrated load. the effective width shall be calculated in accordance with the following equation provided that it shall not exceed the actual width of the slab:
where, beff = effective width of slab
K = a constant having values depending upon the ratio of the width of the slab (l’) to the effective width of the slab.
x = distance of the centroid of the concentrated load from nearer support
leff = effective width
a0 = width of the contact area of the concentrated load measured parallel to the supporting edge.
In case of a load near the unsupported edge of a slab, the effective is least of b1 and b2
where, b1 = effective span
b2 = b1 /2 + distance of the load measured parallel to the supporting edge.
Note:
(a) For two or more concentrated loads placed in a line in the direction of the span, the bending moment per metre width of slab shall be calculated separately for each load according to its appropriate effective width of slab calculated as in (1) above and added together for design calculations.
In case of a load near the unsupported edge of a slab, the effective is least of b1 and b2
where, b1 = effective span
b2 = b1 /2 + distance of the load measured parallel to the supporting edge.
Note:
(a) For two or more concentrated loads placed in a line in the direction of the span, the bending moment per metre width of slab shall be calculated separately for each load according to its appropriate effective width of slab calculated as in (1) above and added together for design calculations.
(b) For two or more loads not in a line in the direction of the span, if the effective width of a slab for one load does not overlap the effective width of slab for another loads calculated as in (1) above, then the slab for each load can be designed separately. If the effective width of slab for one load overlaps the effective width of slab for an adjacent load, the overlapping portion of slab should be designed for the combined effect of the two loads.
(b) For two or more loads not in a line in the direction of the span, if the effective width of a slab for one load does not overlap the effective width of slab for another loads calculated as in (1) above, then the slab for each load can be designed separately. If the effective width of slab for one load overlaps the effective width of slab for an adjacent load, the overlapping portion of slab should be designed for the combined effect of the two loads.
IRC Loading
Live load due to vehicles may be classified in three categories:
(1) IRC Class AA loading – Tracked vehicle or wheeled vehicle.
(2) IRC Class A loading (3) IRC Class B loading
A bridge may be designed for one or more of the above loadings depending upon its location and importance.
A vehicle may occupy any position which will produce maximum forces subject to the restrictions of minimum clearance between two passing or crossing vehicles and minimum clearance from the kerb or foot path.
Live load due to vehicles may be classified in three categories:
(1) IRC Class AA loading – Tracked vehicle or wheeled vehicle.
(2) IRC Class A loading (3) IRC Class B loading
A bridge may be designed for one or more of the above loadings depending upon its location and importance.
A vehicle may occupy any position which will produce maximum forces subject to the restrictions of minimum clearance between two passing or crossing vehicles and minimum clearance from the kerb or foot path.
Trailers attached to a vehicle are treated as undetachable.
The space on the bridge left uncovered by the standard vehicle can not be occupied by any other live load.
IRC Class AA Loading
Ground Contact Area
The vehicle load is transferred to the bridge through axle and wheels. The contact area of a wheel depends upon its load carrying capacity. The various wheels have been standardized by the Indian Road Congress. The width of a wheel, that is its tyre with the ground are standard values as shown in the figures above for various wheels. It is assumed that the wheeled load is dispersed at an angle of 45 degree through the wearing coat or solid slab.
The vehicle load is transferred to the bridge through axle and wheels. The contact area of a wheel depends upon its load carrying capacity. The various wheels have been standardized by the Indian Road Congress. The width of a wheel, that is its tyre with the ground are standard values as shown in the figures above for various wheels. It is assumed that the wheeled load is dispersed at an angle of 45 degree through the wearing coat or solid slab.
Impact Factor
The vehicle loads are dynamic loads. Allowance is made for the dynamic actions through impact factors expressed as percentage of the live load or vehicle load. For Class AA loading, the increase in forces due to impact is 25%. For Class-A loading, the impact factor is equal to
The vehicle loads are dynamic loads. Allowance is made for the dynamic actions through impact factors expressed as percentage of the live load or vehicle load. For Class AA loading, the increase in forces due to impact is 25%. For Class-A loading, the impact factor is equal to
4.5 , L is span in meters 6
0.088 0.50 I L
