What is a pad foundation?
A pad foundation carries a building’s loads at points: on individual heavy blocks of reinforced concrete — the pads — instead of on a continuous strip or a full slab. Ground beams between and over the pads take wall loads to them.
As long as the pad bears directly on a competent stratum, it is a form of shallow foundation. If a pile sits beneath it, the block over the pile or pile group is called a pile cap, and it is then a piled foundation.
When to choose one
The pad foundation is the obvious choice as soon as the load is naturally point-shaped:
- Framed and portal-frame buildings: each column gets its own pad.
- Extensions, verandas, carports and canopies on timber or steel posts.
- Freestanding structures such as a shed, garden room or pergola.
- Situations where a continuous strip would be too expensive, for instance when the bearing stratum lies deep: you then only excavate where it is needed.
With a strip foundation the load is spread over the whole length; with a pad it concentrates on a few square metres. The bearing pressure under the pad is therefore much higher, and that makes the ground conditions decisive.
Sizing and points to watch
- Ground investigation first. The pad size follows from the load divided by the allowable bearing pressure. Without a site investigation any dimension is a guess — and with a point load a wrong guess shows up immediately as differential settlement.
- Found below frost depth. In north-western Europe the underside of the foundation is normally taken at least 600 to 800 mm below ground level to avoid frost action in the soil beneath.
- Differential settlement. This is the weak point of the type. Pads with different loads or on different strata settle differently. Stiff ground beams between the pads spread those differences out.
- Reinforcement. A pad must be reinforced to resist punching shear from the column and bending at its underside; a plain concrete block only works for very light structures.
- Connection to the column. Base plate, holding-down bolts or starter bars must be cast in at the correct position. Drilling into a green pad afterwards weakens it.
Pad foundation: checks before and during casting
- Ground investigation done: Pad size follows from load divided by allowable bearing pressure; without it any dimension is a guess.
- Underside below frost depth: In north-western Europe normally at least 600 to 800 mm below ground level.
- Stiff ground beams between pads: They spread out differential settlement between pads.
- Pad reinforced: Resists punching shear from the column and bending at its underside; plain concrete only for very light structures.
- Column connection cast in: Base plate, holding-down bolts or starter bars at the correct position; do not drill into a green pad afterwards.
Related terms
- Foundation
- Concrete pad
- Shallow foundation
- Strip foundation
- Ground beam
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