Tuesday, December 11, 2012

Principles of Foundation Design.

Foundation design could be thought of as analogous to a beam design. The designer of the beam will need to know the load to be carried, the load-carrying capacity of the beam, how much it will deflect and whether there are any long-term effects such as creep, moisture movement, etc. If the calculated beam section is, for some reason, not strong enough to support the load or is likely to deflect unduly, then the beam section is changed. Alternatively, the beam can either be substituted for another type of structural ele-
ment, or a stronger material be chosen for the beam.

Similarly the soil supporting the structure must have adequate load-carrying capacity (bearing capacity) and
not deflect (settle) unduly. The long-term effect of the soil’s bearing capacity and settlement must be considered. If the ground is not strong enough to bear the proposed initial design load then the structural contact load (bearing pressure) can be reduced by spreading the load over a greater area – by increasing the foundation size or other means – or by transferring the load to a lower stratum. For example,
rafts could replace isolated pad bases – or the load can be transferred to stronger soil at a lower depth beneath the surface by means of piles. Alternatively, the ground can be strengthened by compaction, stabilization, preconsolidation or other means. The structural materials in the superstructure are subject to stress, strain, movement, etc., and it can be helpful to consider the soil supporting  the superstructure as a structural material, also subject to stress, strain and movement.

Structural design has been described as using materials not fully understood, to make frames which cannot be accurately analysed, to resist forces which can only be estimated.

Foundation design is, at best, no better. ‘Accuracy’ is a chimera and the designer must exercise judgement.

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