STRUCTURAL DESIGN OF BOX CULVERTSThe box culverts were modelled as rigid frames and a detailed analysis of the frames was done using the moment distribution method in order to get the bending moments, shear forces and thrusts acting on the box culverts. The following load cases were considered:1. A uniformly distributed load on the top slab and an equal reaction from the ground below the slab.2. A concentrated imposed load on the top slab and an equal reaction from the ground below the bottom slab.3. An upward pressure on the bottom slab due to the weight of the culvert walls.4. A triangularly distributed horizontal pressure on each side wall due to increase in earth pressure owing to culvert depth.5. A uniformly distributed horizontal pressure on each side wall due to pressure from the earth and any surcharge above the level of the culvert top slab.6. The internal horizontal and possibly vertical pressures from the water in the culvert.Computation of loads acting on the box culvertThis comprised the dead load from the fill material on top of the culvert, the wearing course and the culvert self weight. The live load on the box culvert is composed of the vehicular traffic expected on the road and as such BS 5400 was used to calculate the imposed loads.The earth pressure on the vertical side walls of the box culvert was computed according to Coulombs earth pressure theory with the assumption that the culvert is installed in a relatively narrow ditch dug in a passive or undisturbed soil and which is then covered with an earth backfill material.Load CombinationsThe box culverts have been designed for three major load combinations:1. When the box culvert is empty while it is loaded with dead and live load.2. When the top slab of the culvert supports dead and live load but it is running full.3. When the sides of the culvert are not carrying any live load while the box culvert is running full.