The Direct Strength Method for Combined Bending and Web Crippling of Second-Generation Trapezoidal Steel Sheeting

Abstract

Second-generation trapezoidal sheeting, characterised by longitudinal stiffeners in webs and flanges, is loaded near a support by a concentrated force and a bending moment. Currently, design codes predict related failure by: (a) determining the ultimate bending moment via the effective width approach or the Direct Strength Method (DSM); (b) finding the web crippling load via a curve-fitted formula; and (c) using an interaction rule to take into account the load combination. However, the effective width approach is quite complex to use for many longitudinal stiffeners, and the accuracy of the design code approach is subjected to improvement. Moreover, nowadays the DSM provides a consistent and well-established method to predict ultimate loads for cold-formed steel structures. Therefore, in this paper the application of DSM for combined bending and web crippling of second-generation sheeting is investigated. First a set of internationally representative secondgeneration trapezoidal sheeting types is used to create a set of numerical experiments, where sheet-sections are subjected to a three-point bending test. Then finite element models are developed and verified, and used to predict the buckling, yield and ultimate loads for the set of numerical experiments. With the results from the numerical experiments, an explicit DSM approach is developed, which predicts the ultimate load for combined actions directly. Hereafter, also an interaction DSM approach is studied, which first predicts the ultimate bending moment by the DSM, then the web crippling load by the DSM and then uses a classic interaction rule for the load combination. The explicit and implicit DSM approaches perform equally well, with a coefficient of variation equal to 0.13. The interaction DSM approach resembles the current design rules most and is therefore the preferred approach, although the explicit DSM approach is more direct and certainly deserves consideration too.