Column on Pad Foundation: Specification and Requirements

Where a steel column in a goalpost frame or a standalone post carries a concentrated load to ground, it bears onto a concrete pad foundation. Here is what the pad must provide.

Standard Pad Specification

A typical column pad foundation on a PorthouseDean project is specified as:

– Concrete grade: C35 (minimum compressive strength 35 N/mm² at 28 days)
– Reinforcement: Two layers of A393 mesh — one layer top, one layer bottom
– Cover: 50 mm to all reinforcement faces from the outer concrete surface
– Column position: Centrally located on the pad in both directions

The plan dimensions and depth of the pad are calculated in the structural documents and will be stated on your drawings — typically expressed like “1000 × 1000 × 450 mm pad” as a minimum size.

Bearing Capacity Requirement

Before the pad can be poured, Building Control must inspect the formation and confirm that the bearing capacity at that depth is acceptable. PorthouseDean designs to a minimum bearing capacity of 100 kN/m², which represents medium-dense gravel or sand, or well-consolidated clay. If the BCO inspects and finds softer material, deeper excavation is required.

If you have a ground investigation report showing lower bearing capacity, let your structural engineer know — the pad may need to be increased in size.

Column Positioning

The column base plate must be positioned so that the column is centrally on the pad. Eccentric loading (where the column sits off-centre) creates bending moment in the pad that the standard design does not account for. If the column genuinely cannot be centred — for example due to a boundary constraint — an eccentric pad design must be requested as a change.

The Picture Frame Alternative

If the pad location is close to a party wall and the required pad size would cross the boundary, the picture frame detail (column on steel ground beam) is used instead. In this arrangement, a concrete-encased steel ground beam cantileveres out from a pad that sits fully within the homeowner’s property, and the column bears onto the end of the beam rather than directly onto the pad.

This avoids the need to excavate or pour concrete beneath the neighbouring property’s foundations.

Ready to Order Steelwork?

Your structural drawings will include the full pad specification. If foundation conditions differ from the drawings, contact PorthouseDean before proceeding — revised calculations can be produced quickly.