Foundations: Limit State Design.

Attempts to apply limit state philosophy to foundation design have, so far, not been considered totally successful.

So a compromise between working stress and limit state has developed, where the designer determines an estimated allowable bearing pressure and checks for settlements and building serviceability. The actual bearing pressure is then factored up into an ultimate design pressure, for structural design of the foundation members.

The partial safety factors applied for ultimate design loads (i.e. typically 1.4 × dead, 1.6 × imposed, 1.4 × wind and 1.2 for dead + imposed + wind) are for superstructure design and should not be applied to foundation design for allowable bearing calculations.

For dead and imposed loads the actual working load, i.e. the unfactored characteristic load, should be used in most foundation designs. Where there are important isolated foundations and particularly when subject to significant eccentric loading (as in heavily loaded gantry columns, water towers, and the like), the engineer should exercise discretion in applying a partial safety factor to the imposed load. Similarly when the imposed load is very high in relation to the dead load (as in large cylindrical steel oil tanks), the engineer should apply a partial safety factor to the imposed load.

In fact when the foundation load due to wind load on  the superstructure is relatively small – i.e. less than 25% of (dead  + imposed) – it may be ignored. Where the occasional foundation load due to wind exceeds 25% of (dead + imposed), then the foundation area should be proportioned so that the pressure due to wind + dead + imposed loads does not exceed 1.25 × (allowable bearing pressure). When wind uplift on a foundation exceeds dead load, then this becomes a critical load case.