Explain about structural design of combined footing.

Structural Design of Combined Footing

The rectangular and trapezoidal combined footings are designed assuming uniform so11 pressure. You can observe that the footings are positioned relatively close to the edges of the footings. Hence the columns are assumed as supports and the footing is subjected to an upward uniformly distributed load caused by the soil reaction. Moments which create tension at the top of the footing will predominate in the longitudinal direction. So the principal longitudinal reinforcement will be placed at the top of the footing equally distributed across the footing width. Smaller moments in the transverse direction will cause compression at the top of the footing. So, transverse steel will be provided under each column at the bottom of the footing to distribute the column loads. The combined footing can be considered as a wide rectangular beam in the longitudinal direction.