Steel Piles - Design Basis.

1 Design basis
The basis of design of any bearing pile is its ultimate axial capacity in the particular soil conditions at the site where the structure is to be built.The design resistance is determined from the ultimate capacity, and the designer verifies that this is adequate to carry the required loads from the structure.

Until recently, design resistance of foundations has been evaluated on an allowable stress basis. However, structural design is now using a limit state design (LSD) basis, whereby partial factors are applied to various elements of the design according to the reliability of the method and of the soil properties. However, the application of limit state design philosophy to geotechnical design is causing difficulty in a discipline where the allowable stress approach and terms such as the ‘allowable bearing pressure’, ‘permissible steel stress’, and ‘allowable pile capacity’ are widely accepted and understood. Nevertheless, the LSD approach is being progressively adopted in the British Standards as they are revised, and is the basis for all the Eurocodes, including that for foundation design.

2 Design standards
The common design standard used for the design of bearing piles is the offshore industry’s recommended practice for steel tubular piles, based on US and UK North Sea experience, which is contained in the American Petroleum Institute Code RP2A (API RP2A) that will be adopted in the ISO Code 13819-2.

A draft of the new Eurocode 7: Part 16 has recently been produced after several years of effort by an eminent team, and the UK pre-standard DD-ENV 1997-17 with the National Application Document (NAD), was published in July 1995. It presents a more rigorous treatment of limit state design (LSD) than any of the British Standards relating to foundations so far.Allowable stress design (ASD) is still, however, permitted in BS 80028 and BS 6349, in order to be compatible with the approach taken in BS 449.

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