Explicit and interaction direct strength methods for combined web crippling and bending moment failure of first-generation trapezoidal steel sheeting

For trapezoidal steel sheeting, design codes so far calculate the web crippling strength via curve fitted rules; bending moment failure is predicted by the effective width method or the Direct Strength Method (DSM); and their combination is handled by a curve fitted interaction rule. As the DSM is easy to use, potentially more accurate, and closer to a mechanical description than curve fitted rules, this paper investigates whether it can be used for trapezoidal sheeting, limited here to the first-generation, i.e. without stiffeners. First, an existing set of experiments is presented, in which first-generation sheeting has been subjected to combined web crippling and bending moment via three-point bending tests. In these experiments, web-crippling deformation, beam deflection, and support rotations have been recorded, and failure behaviour has been studied well beyond the ultimate load. These experiments are simulated by the finite element method, and after verification, the simulations are used to record the first Eigenvalue, first yield, and the ultimate load of the experiments. Additionally, each experiment is also modelled as an Interior One Flange (IOF) web crippling test and a pure bending moment test. Finally, the DSM is developed based on the simulation data (not the experimental data), using two approaches. (i) For the interaction approach, only the web crippling strength and the bending moment capacity are predicted by the DSM, using the simulation data from the IOF web crippling tests and pure bending moment tests. Interaction is described by an interaction rule, calibrated to the simulations of the three-point bending tests. (ii) For the explicit approach, the DSM is used to predict the combined load capacity, based on the simulation data from the three-point bending tests. It is concluded that both approaches work, but the interaction approach performs better than the explicit approach. Using Interior Two Flange (ITF) Eigenvalues in combination with IOF yield loads and IOF ultimate loads increased the performance of the IOF web crippling DSM equations, however, it not leads to a further improvement of the interaction approach.