Influence of Geometric Imperfections on Cold Formed Steel Columns

In the field of building construction, usage of cold formed steel is increasing every day because of its light weight and easy construction. However, the design provisions for cold formed steel members are limited. This study is a part of filling this gap of limited design technologies. Because of its thinness the member can deviate easily from its perfect geometry, these deviations are called Geometric imperfections. These imperfections can significantly affect the load carrying capacity of the member, however in the current code, North American Cold-formed Steel Specification for the Design of Cold-formed Steel Structural Members (S100-2012), the design equation was very conservative and AISI S240: North American Standard for Cold-Formed Steel Structural Framing gave only one equation for any type of imperfection. In the present study these geometric imperfections are included and modelled in ANSYS (13.0). Numerical analysis was done by considering imperfections, it shows that there is a significant change in the strength of the member. After carrying out the numerical analysis for all the lengths of the considered sections, the failure modes of the different columns are found out as Local buckling in stub column region, distortional buckling in intermediate column zone and in slender column zone the failure mode is purely Global. In stub column region the strength curve is not following any trend, it means buckling strength is increasing and decreasing in high amount at a very small change in length, so for stub columns once again numerical analysis was carried out without considering imperfections. Without imperfections, the strength of the columns in stub column region is decreasing gradually, it means that for very short columns imperfections will not affect the member as much as in slender columns