What Is Bearing Capacity?

Some designers, when in a hurry, tend to want simple ‘rules of thumb’ (based on local experience) for values of bearing capacity. But like most rules of thumb, while safe for typical structures on normal soils, their use can produce uneconomic solutions, restrict the development of improved methods of foundation design, and lead to expensive mistakes when the structure is not typical.

For typical buildings:

(1) The dead and imposed loads are built up gradually and relatively slowly.
(2) Actual imposed loads (as distinct from those assumed for design purposes) are often only a third of the dead load.
(3) The building has a height/width ratio of between 1/3 and 3.
(4) The building has regularly distributed columns or load bearing walls, most of them fairly evenly loaded.

Typical buildings have changed dramatically since the Second World War. The use of higher design stresses, lower factors of safety, the removal of robust non-load-bearing partitioning, etc., has resulted in buildings of half their previous weight, more susceptible to the effects of settlement, and built for use by clients who are less tolerant in accepting relatively minor cracking of finishes, etc.

Because of these changes, practical experience gained in the past is not always applicable to present construction. For non-typical structures:

(1) The imposed load may be applied rapidly, as in tanks and silos, resulting in possible settlement problems.
(2) There may be a high ratio of imposed to dead load. Unbalanced imposed-loading cases – imposed load over part of the structure – can be critical, resulting in differential settlement or bearing capacity failures, if not allowed for in design.
(3) The requirement may be for a tall, slender building which may be susceptible to tilting or overturning and have more critical wind loads.
(4) The requirement may be for a non-regular column/ wall layout, subjected to widely varying loadings, which may require special consideration to prevent excessive differential settlement and bearing capacity failure.

There is also the danger of going to the other extreme by doing complicated calculations based on numbers from unrepresentative soil tests alone, and ignoring the important evidence of the soil profile and local experience. Structural design and materials are not, as previously stated, mathematically precise; foundation design and materials are even less precise.

Determining the bearing capacity solely from a 100 mm thick small-diameter sample and applying it to predict the behaviour of a 10 m deep stratum, is obviously not sensible – particularly when many structures could fail, in serviceability, by settlement at bearing pressures well below the soil’s ultimate bearing capacity.

Bearing capacity
Probably the happy medium is to follow the sound advice given by experienced engineers in the British Standard Institution’s Code of practice for foundations, BS 8004. There they define ultimate bearing capacity as ‘the value of the gross loading intensity for a particular foundation at which the resistance of the soil to displacement of the foundation is fully mobilized.’ (Ultimate in this instance does not refer to ultimate limit state.)

The net loading intensity (net bearing pressure) is the additional intensity of vertical loading at the base of a foundation due to the weight of the new structure and its loading, including any earthworks. The ultimate bearing capacity divided by a suitable factor of safety – typically 3 – is referred to as the safe bearing capacity.

It has not been found possible, yet, to apply limit state design fully to foundations, since bearing capacity and settlement are so intertwined and influence both foundation and superstructure design. Furthermore, the superstructure itself can be altered in design to accommodate, or reduce, the effects of settlement. A reasonable compromise has been devised by engineers in the past and is given below.

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