Eurocode reduction factors for U-profile sheet pile walls

The standard BS EN 1993: Part 5 Steel piling (Eurocode 3) requires that in-plane bending effects and reduced modulus action (RMA) be considered where appropriate in the ultimate limit state (ULS) design of steel sheet pile (SSP) walls. Appropriate reduction factors are defined on a country by country basis in a national annex to the Eurocode. However, there are contradictory experiences in practice both between countries and with respect to experimental and theoretical evidence for such effects. Without challenging the actual values suggested for use, in this paper a simplified approach is used to assess the impact of section modulus and inertia reduction factors on the available section resistance and bending moments likely to be mobilised in the wall system. This assessment has been undertaken by comparing moment demand curves, generated from a suite of numerical analyses, with structural curves derived from a set of generic SSP sections, to which section modulus and inertia reduction factors, recommended by the Eurocode, have been applied. It is concluded that the effects of moment reduction due to wall system flexibility effects may, in many instances, either completely or partially compensate for reductions in moment resistance due to RMA or in-plane bending, although this will lead to the safety margin with respect to structural ULSs becoming ill defined. It is considered inappropriate to apply the modulus reduction factors in isolation, due to the strong interaction between wall system flexibility and the bending moment mobilised in SSP retaining walls.