Tuesday, December 11, 2012

Foundations: Working Stress Design.

A common design method (based on working stress) used in the past was to determine the ultimate bearing capacity of the soil, then divide it by a factor of safety, commonly 3,  to determine the  safe bearing capacity. 

The safe bearing capacity is the maximum allowable design loading intensity on the soil. The  ultimate bearing capacity is exceeded when the loading intensity causes the soil to fail in shear.

Typical ultimate bearing capacities are 150 kN/m2 for soft clays, 300–600 kN/m2 for firm clays and loose sands/ gravels, and 1000–1500 kN/m2 for hard boulder clays and dense gravels.

Consider the following example for a column foundation.

The ultimate bearing capacity for a stiff clay is 750 kN/m2.

If the factor of safety equals 3, determine the area of a pad base to support a column load of 1000 kN (ignoring the weight of the base and any overburden).

The method has the attraction of simplicity and was generally adequate for traditional buildings in the past. However, it can be uneconomic and ignores other factors. A nuclear power station, complex chemical works housing expensive plant susceptible to foundation movement or similar buildings, can warrant a higher factor of safety than a supermarket warehouse storing tinned pet food. A crowded theatre may deserve a higher safety factor than an occasionally used cow-shed. The designer should exercise judgement in the choice of factor of safety.

In addition, while there must be precautions taken against foundation collapse limit state (i.e. total failure) there must be a check that the  serviceability limit state (i.e. movement under load which causes structural or building use distress) is not exceeded. Where settlement criteria dominate, the bearing pressure is restricted to a suitable value below that of the safe bearing capacity, known as the  allowable bearing pressure.

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