Footings

A footing spreads a column or wall load over enough soil area that the soil is not overstressed and settlement stays within limits. Isolated pad footings serve single columns, combined footings serve two columns close together or near a boundary, strip footings carry walls, and rafts cover the whole building when loads are heavy or soil is weak.

Design follows a fixed order. First, size the plan area from service loads and the allowable bearing pressure, which comes from the geotechnical investigation. Second, find the net upward soil pressure from factored loads and use it to design the concrete. The footing behaves like an inverted cantilever slab projecting from the column face. Check bending at the column face, one-way shear at a distance d from the face, and two-way (punching) shear on a perimeter at d/2 from the face. Punching usually decides thickness. Then check bearing at the column-footing interface and anchor the column bars into the footing.

Eccentric loads or moments make the soil pressure non-uniform; the resultant should stay within the middle third of the base (the kern) so that the whole footing stays in contact with the soil, with no uplift at one edge. Footings near property lines may need strap beams. Minimum depth, cover in contact with soil and a blinding layer under the footing are standard requirements.