Introduction
In General Bridges may be defined as a structure, which maintains the communication over an obstacle such as a channel, road, railway or a valley. Our main purpose is to design the bridge in such a manner so that both the safety and the economy can be ensured. In that context, a comparison between the RCC T-Beam Girder Bridge as published by MORTH and the RCC Integral slab bridge is discussed below.
In RCC T-Beam Girder Bridge the main load of the superstructure is taken by the girders which are simply supported over the wall type piers and abutments and the design of piers and abutments is mainly guided directly by the horrizontal loads (earth pressure, horizontal force from the superstructure, braking etc) and vertical loads ( mainly the reaction from the superstructure). But in RCC Integral slab bridge, being a portal frame structure, all the members are designed by the obtained forces and moments from moment distribution by Matrix Method of Analysis.
The advantages of the Integral Bridge are:
- As the pier thickness is much less for integral bridge compared to T-Beam Bridge, the corresponding discharge velocity will be less, leading to a relatively lower scour depth for the Integral Slab Bridge. Hence, the overhang depth in the pile cap will be comparatively less for the Integral slab Bridge and this is known to us that less overhang depth means more lateral capacity in soil and less moment per per unit load for the horizontal load in the individual pile. Hence this will be indirectly responsible for the reduced pile number and percentage of steel in the pile for Integral Slab Bridge.
- On the other hand it can be said that more discharge can be allowed through the Integral Bridges than the T-Beam bridges.
- As no-Bearing is required for the Integral Slab Bridge; it will add to cost saving.
- The earth pressures acting on both the abutments are same in magnitude and opposite in direction, the resulting horizontal forces acting on the piles (Via pier ) will be very less in case of Integral Slab Bridge. This will reduce the abutment thickness and pile number.
- Again in case of T-beam bridge, due to metallic bearing ( high seismic zone ) a huge horizontal force will come into the pier, resulting a thicker pier section.But this is not required for the Integral slab bridge.
- There will also be benefit in the Integral slab Bridge due to the pile length as the structure is lighter than the T-Beam Bridge.
- Less pile number in the Integral Bridge may also allow us to reduce the dimension of the pile cap.
- Integral bridges are more resistant to seismic forces and latest revision on seismic code recommends use of integral bridges for their high horizontal load resisting capacity.
- Because of lower span moment, thickness of beam/slab is lower. Leading to cost reduction.
- Again as the Finished Road Level of the Itegral Slab Bridge is lower than that of the T-beam Bridge (about 1m in this case), there is a fair chance of reduction of cost due to the approach road
Cost Comparison:
- We have done the detailed quantity calculation for both the bridges. The rate of the items in the cost estimation is taken from the latest (May’11) SOR published by Road Construction Department. The cost details are given below:
Type of Bridge (m) Per metre Cost (Rs. Lac)
T-Beam 8.529
Integral 6.384
Net reduction due to the Integral Slab Bridge = 25.14 %
Conclusion:
- From the above discussion it is assured that more than 25 % of cost can be reduced if Integral Slab Bridge is used in place of T-Beam Bridge.
- These can be used safely up to 180ft (58m). Beyond that some bearings are required to take care of thermal expansion.
- Considering the fact that about 85% of bridges fall within that category(length < 180 ft), we strongly recommend the use of integral slab bridge. Each 5th bridge in that case will become free compared to T-Beam Bridge.