Pad foundation design

Under load the ground pushes up on the pad and it tries to bend, cantilevering out from the column face in both directions, so it needs bottom reinforcement in two directions to Eurocode 2. The critical check on thickness is usually punching shear, where the column tries to push a truncated cone straight through the pad. We check the shear on the perimeter at the column face and at the code control perimeter around it, and it is often this, rather than bending, that fixes the depth of the pad.

Getting the depth and reinforcement balanced keeps the pad economic. A thicker pad may avoid shear reinforcement and reduce the main steel, while a thinner one uses less concrete but more bars and sometimes links. We size the pad so it is thick enough to spread load at a sensible angle from the column, detail the reinforcement with correct cover and anchorage past the critical sections, and provide starter bars or a holding-down bolt arrangement to tie the column or base plate into the foundation.

The ground still governs depth and type. A pad has to found below topsoil, made ground and, on clay, below the seasonal movement zone, and where columns sit close to a boundary or another pad we check that their stress bulbs do not overlap and overload the soil. Where two columns are close together, or one sits hard against a boundary and cannot be centred, we combine them on a single rectangular or balanced pad so the pressure stays uniform and the eccentric column does not tip its base.

On poor ground, or where uplift and moment make a spread pad uneconomic, a pile cap on two or more piles does the same job and we design it as such. Whichever route suits, we set out the pads clear of drains and services, coordinate holding-down bolts with the steelwork fabricator so the base plates land true, and issue calculations and drawings to Eurocode 2 and 7 that a groundworker can build from anywhere in the UK or Europe.