Tuesday, December 11, 2012

Structures: Bearing capacity.

1 Introduction
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  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. Becauseof 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.

2 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.

3 Presumed bearing value
The pressure within the soil will depend on the net loading intensity, which in turn depends on the structural loads and the foundation type. This pressure is then compared with the ultimate bearing capacity to determine a factor  of safety. This appears reasonable and straightforward – but there is a catch-22 snag. It is not possible to determine the net loading intensity without first knowing the foundation type and size, but the foundation type and size cannot be designed without knowing the acceptable bearing pressure.

The deadlock has been broken by BS 8004, which gives presumed allowable bearing values (estimated bearing pressures) for different types of ground. This enables a preliminary foundation design to be carried out which can be adjusted, up or down, on further analysis. The presumed bearing value is defined as: ‘the net loading intensity considered appropriate to the particular type of ground for preliminary design purposes’.

The value is based on either local experience or on calculation from laboratory strength tests or field loading tests using a factor of safety against bearing capacity failure.

Foundation design, like superstructure design, is a trial-and-error method – a preliminary design is made, then checked and, if necessary, amended. Amendments would be necessary, for example, to restrict settlement or overloading; in consideration of economic and construction implications, or designing the superstructure to resist  or accommodate settlements. The Code’s presumed bear- ing values are given in Table 1.1 and experience shows  that these are valuable and reasonable in preliminary design.

4 Allowable bearing pressure
Knowing the structural loads, the preliminary foundation design and the ultimate bearing capacity, a check can be made on the  allowable bearing pressure. The allowable net bearing pressure is defined in the Code as ‘the maximum allowable net loading intensity at the base of the foundation’ taking into account:

(1) The ultimate bearing capacity.
(2) The amount and kind of settlement expected.
(3) The ability of the given structure to accommodate this settlement.

This practical definition shows that the allowable bearing pressure is a combination of three functions; the strength and settlement characteristics of the ground, the foundation type, and the settlement characteristics of the structure.

5 Non-vertical loading
When horizontal foundations are subject to inclined forces (portal frames, cantilever structures, etc.) the passive resistance of the ground must be checked for its capacity to resist the horizontal component of the inclined load. This could result in reducing the value of the allowable bearing pressure to carry the vertical component of the inclined load.  BS 8004 (Code of practice for foundations) suggests a simple rule for design of foundations subject to non-vertical loads as follows:

However, like all simple rules which are on the safe side, there are exceptions. A more conservative value can be  necessary when the horizontal component is relatively high and is acting on shallow foundations (where their depth/ breadth ratio is less than 1/4) founded on non-cohesive soils.

In the same way that allowable bearing pressure is reduced to prevent excessive settlement, so too may allowable passive resistance, to prevent unacceptable horizontal movement.

If the requirements of this rule cannot be met, provision should be made for the horizontal component to be taken by some other part of the structure or by raking piles, by tying back to a line of sheet piling or by some other means.

Table 1.1 Presumed bearing values (BS 8004, Table 1) (1)

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