• Design of stainless steel cross-sections with outstand elements under stress gradients

      Gkantou, Michaela; Bock, Marina; Theofanous, Marios (Elsevier, 2021-01-09)
      A significant amount of research has been reported on stainless steel tubular sections, while studies on I- and C-sections remain relatively limited. This paper presents a comprehensive numerical study on the response of stainless steel I- and C-sections subjected to minor axis bending, with outstand flanges subjected to stress gradients. Numerical models are developed and validated against reported test data on austenitic stainless steel sections under minor axis bending. Subsequently, parametric studies using standardised material properties on austenitic, duplex and ferritic stainless steel grades, covering a wide variety of cross-section slendernesses, are carried out to expand the structural performance data. The results are used to assess the applicability of the Eurocode slenderness limits, revealing that the Class limit 3 for outstand flanges under stress gradient is overly conservative. Moreover, Eurocode underestimates the predicted bending strengths, whereas the level of accuracy and consistency improves for stocky sections, when the Continuous Strength Method is used. Aiming to address the lack of accuracy and consistency in the design predictions of slender sections, particular focus is placed on their performance. It is demonstrated that outstand elements under stress gradients exhibit significant inelastic behaviour after the compression flanges have locally buckled. Inelastic buckling behaviour is not considered in current design guidance, thus resulting in overly conservative and fundamentally incorrect strength predictions. An alternative design method based on the plastic effective width concept is proposed for slender stainless steel I- and C-sections in minor axis bending, which leads to more favourable and less scattered strength predictions.
    • Structural response of cold-formed lipped Z purlins – Part 2 numerical modelling and optimisation of lip size

      Almatrafi, Meshal; Theofanous, Marios; Dirar, Samir; Bock, Marina (Elsevier, 2021-01-23)
      This paper reports a numerical study on the optimisation of the lip size of Z-sections under gravity loads. Numerical models of cold-formed steel Z purlins restrained by cladding and angle struts and subjected to sagging moment were developed and validated against a total of 8 experimental results on Z-sections that failed in local or/and distortional buckling reported in the companion paper. Models of varying levels of complexity were generated and the key parameters affecting the structural response were determined by means of a sensitivity analysis. The investigated parameters included the magnitude, shape and combination of initial geometric imperfections pertinent to local and distortional buckling and the simplified or explicit modelling of test details such as struts and sheeting. Having determined the appropriate modelling strategy that leads to the best balance between accuracy and computational cost, parametric studies were conducted to investigate the effect of decreasing or increasing the lip depth on the sections’ moment resistance and corresponding failure mode. Based on the parametric study results, the optimal lip size which maximizes the moment to weight ratio for each section was determined. Finally, all generated FE results are utilized to evaluate the accuracy of the moment resistance prediction of EN 1993-1-3 and the Direct Strength Method.