Analysis and Design of Small RCC Bridge

Small bridges and culverts form the largest number of bridge structures on the road network of India. They are built to carry rural roads, district roads and state highways across streams, canals and drainage channels, and although each structure is modest in size, their total number makes their economical and safe design important. This study presents the analysis and design of a small reinforced cement concrete slab bridge with a clear span of 8 m, intended to carry a two-lane rural road over a seasonal stream. The design follows IRC 6:2017 for loads and IRC 112:2011 for the design of concrete road bridges. The bridge is a simply supported solid slab deck resting on reinforced concrete abutments with open foundations, and the superstructure is supported on elastomeric bearings. The deck is 8.4 m wide, providing a carriageway of 7.5 m with crash barriers on both sides, and it carries a 65 mm wearing coat. The hydraulic design of the bridge is carried out first to determine the design discharge, the linear waterway, the high flood level and the scour depth, so that the deck level and foundation depth are fixed on a rational basis before the structural design is attempted. The deck slab is modelled in STAAD.Pro as a plate grillage and also analysed by the effective width method, which is the traditional approach for solid slab bridges. Dead load of the slab, superimposed dead load from the wearing coat and crash barriers, and live loads corresponding to IRC Class A and Class 70R vehicles are considered, along with the impact factor specified in IRC 6. The moving load facility of the software is used to place the vehicles at the positions that produce maximum bending moment and shear force in the slab. The slab is designed for the ultimate limit state of flexure and shear and checked for the serviceability limit states of stress, crack width and deflection according to IRC 112. The abutments are designed for the vertical reactions from the superstructure, the earth pressure from the backfill, the live load surcharge and the braking force, and their stability against overturning and sliding is verified. The base pressure under the abutment footing is kept below the safe bearing capacity of the foundation soil. The results show that the Class 70R wheeled vehicle governs the design bending moment in the slab for this span, and a slab thickness of 650 mm with 25 mm diameter main bars is found adequate. The moments obtained from the plate model and from the effective width method agree within about eight percent. The study demonstrates a complete and practical procedure for the design of small bridges and can be extended to box culverts, skew slab bridges and multi-span slab bridges.