Analysis and Design of Bridge Substructure and Foundation

Abstract

Bridge is consisting of mainly two components like superstructure and substructure. Till nowadays lot many investigations and research work is done on Bridge superstructure’s type and shape. But the substructure topic is not being in light till now. Economy of bridge depends on the cost of substructure and superstructure and therefore the bridge should be designed in such a manner that the cost of substructure and superstructure should be almost equal.

Substructure and Foundation design consist of: • Pier cap design • Pier design • Pile cap design • Pile foundation design for central span • Well foundation design for end span

Pier cap is designed and reinforced to take care of loads of superstructure dispersing in pier. According to positions of bearings and top section of pier, when bearings are placed centrally over the pier, the load from bearings is directly transferred to pier and the pier cap need not to be designed for flexure but when bearings are not placed centrally over the pier, the part of pier cap have to be designed as cantilever portion.

The forces considered for pier design are Dead load, Buoyancy Force Live load (70 R Wheeled Vehicle), Impact Load, Braking force, Seismic Force, Wind Force, Water Current Force etc. The various load combinations are done for construction condition as well as for service condition. The pier behaves as an ordinary R.C.C. column and is designed as a R.C.C. column subjected to biaxial bending.

A rigid pile cap in reinforced concrete should be provided to transfer the load from the pier to the piles as uniformly as possible under normal vertical loads. Pile cap can be designed either by Truss analogy method or by bending theory. Here the pile cap is designed by Truss analogy method. In truss analogy method pile cap area is divided into various strips in both the directions considering number of pile and pile diameter. The truss is in triangular form with a node at the centre of loaded area. The lower node of the truss lies at the intersection of the centre line of the piles. Strips in both directions are designed as beam elements while remaining portion are designed as slab element.

For the analysis of pile, the forces considered are Dead load, Buoyancy Force Live load (70 R Wheeled Vehicle), Impact Load, Braking force, Seismic Force, Wind Force, Water Current Force. The various load combinations are done for construction condition as well as for service condition. Load distributed on individual pile is compared with load carrying capacity of individual pile which should be higher than the load on individual pile. The pile behaves as an ordinary column and is designed as a column subjected to biaxial bending.

The forces considered for analysis of Abutment wall are Dead Load, Live Load, Braking Force, Seismic force and Earth Pressure. For Earth pressure any rational theory shall be accepted subjected to modification that the centre of pressure exerted by the backfill is located at an elevation of 0.42 of the height of the wall above the base instead of 0.33 of that height (As per IRC Provisions). The various load combinations are done for construction condition as well as for service condition. Abutment wall is also considered as column, so it is designed as a column with biaxial bending.

Abutment cap should be suitably designed and reinforced to take care of concentrated point loads dispersing in abutment. Some part of the Abutment cap is designed as corbel portion. A dirt wall is provided to prevent the earth from approaches spilling on the bearings. Dirt wall is designed for self weight, Earth Pressure, Live load acting directly on dirt wall and braking force due to Live Load.

The main components of well foundation are Well Cap, Steining, Well Curb, and Bottom Plug. The forces considered for analysis of Well foundation are Dead Load, Live Load, Impact Force, Seismic Force and Earth pressure. A well cap is needed to transfer the loads and moments from the pier or abutment to the well or wells below. Well cap is designed by bending theory. Lateral stability of well is most important check for well foundations. Lateral Stability of well is checked by Elastic approach and Ultimate resistance approach.

The parametric study for “Loads and moments on Pier and Pile” is carried out for span of superstructure ranging between 15m to 40m is carried out. For parametric study the program is prepared in Visual Basic which gives design loads and moments for design of pier and pile.