Basis of design

Eurocode 7 requires spread foundations to be designed using one of the following methods: [EN 1997-1 ยง6.4(5)P]

Method Description Constraints

Direct Carry out separate analyses (ULS) Model envisaged for each limit state, both failure mechanism ultimate (ULS) and serviceability (SLS) (SLS) Use a serviceability calculation

Indirect Use comparable experience Choose SLS loads to with results of field and satisfy requirements of laboratory measurements all limit states and observations

Prescriptive Use conventional and Use presumed bearing conservative design rules resistance and specify control of construction

The indirect method is used predominantly for Geotechnical Category 1 structures, where there is good local experience, ground conditions are well known and uncomplicated, and the risks associated with potential failure or excessive deformation of the structure are low. Indirect methods may also be applied to higher risk structures where it is difficult to predict the structural behaviour with sufficient accuracy from analytical solutions. In these cases, reliance is placed on the observational method and identification of a range of potential behaviour. Depending on the observed behaviour, the final design of the foundation can be decided. This approach ensures that the serviceability condition is met but does not explicitly provide sufficient reserve against ultimate conditions. It is therefore important that the limiting design criteria for serviceability are suitably conservative.

The prescriptive method may be used for Geotechnical Category 1 structures, where ground conditions are well known. Unlike British standard BS 8004 - which gives allowable bearing pressures for rocks, non-cohesive soils, cohesive soils, peat and organic soils, made ground, fill, high porosity chalk, and Keuper Marl (now called the Mercia Mudstone)1 - Eurocode 7 only provides values of presumed bearing resistance for rock (via a series of charts1 in Annex G).

The direct method is discussed in some detail in the remainder of this chapter.

This book does not attempt to provide complete guidance on the design of spread foundations, for which the reader should refer to any well-established text on the subject.2

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