Investigation on the Effect of Warping on the Stability Behaviour of Cold Formed Steel Beam-Columns

Design of cold formed steel beam-column members has always been a subject of research due to its complex behavior and its propensity for undergoing interactive buckling under axial loads and varying moments. This paper adds further dimension to the ongoing research on beam-columns by investigating the important aspects the 'warping' and 'bi-moment'. The experimental studies reported in literature on the effect of warping on beam-column behavior is very scarce. This may be due to the complex nature of experimental procedures to create free warping of the cross section under compression and bending. In this study the 'warping free' boundary condition is simulated using a specially designed loading fixture which transmit the load through the zero sectorial coordinate. Controlled experiments are conducted on cold formed steel lipped channel cross section beam-column members under both axial and eccentric compression. Companion numerical models for the experimental specimens are created using ABAQUS and calibration studies are carried out. The numerical study is extended to various loading situations for predicting the strength of members under different warping boundary conditions. The results of the present study show that for axial compression, the use of effective length factor of Kt=0.5 to account for warping restraint, lead to overestimation of the axial resistance of the CFS member as per AISI S100-2016. With the present experimental studies, it is seen that the use of warping restraint factor Kt=0.8 results in acceptably close predictions of beam-column capacities based on the linear interaction framework. The present paper also brings out the need for integrating the 'warping' in the beam-column non-linear interaction framework.

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