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This approach has been used in Denmark since the early 1960s and was first codified in the 1965 Danish Code of practice for foundation engineering.8 It has since been widely adopted in European practice, for example in BS 8002.9

6.4.3 Load and resistance factor design (LRFD)

In northern America, an alternative approach to geotechnical design based on limit state principles has become popular, recently being incorporated into AASHTO's bridge design specifications,10 the American Petroleum Institute's recommended practice for offshore structures,11 and the Canadian Foundation Engineering Manual.12

The philosophy of the 'load and resistance factor design' (LRFD) method is to apply partial factors to the outcomes of the design calculations, i.e. to action effects and resistance.

The fundamental equation that must be satisfied in the LRFD method (using AASHTO's formulation) is:

^nYrQr = Rr i where n = a load modifier (which is numerically between 0.95 and 1.0) accounting for ductility, redundancy, and operational importance; Yi = a load factor, typically > 1.0; Qi = a force effect; 9 = a resistance factor < 1.0; Rn = the nominal resistance; and Rr = the factored resistance.

Since the LRFD resistance factor 9 is typically less than one, its reciprocal is directly analogous to the Structural Eurocode's resistance factor yr. Indeed, the equation above may be re-written with Eurocode-like symbols as:

Although the LRFD method has many features in common with Eurocode 7's Design Approach 2, the resistance factors it employs are different, as can be seen by comparing the table below with the values given in Section 6.3.2.

Structure

Partial factor on...

Value

Reciprocal

Overall stability

Earth slope resistance (9)

0 0