Compression Tests of High Strength Cold-Formed Cross-Shaped Steel Columns (No. R869)

Abstract

This report describes the experimental investigation of a series of compressive tests on a stiffened-cross shape section fabricated from cold-formed high strength steel of thickness 0.42 mm and nominal yield stress of 550 MPa. The complex shape has been chosen so that it has a local buckling mode, two distinct distortional buckling modes, and a flexural-torsional mode. The first distortional buckling mode has a shorter buckle half-wavelength and hence classified as a short half-wavelength distortional buckling mode, while the other distortional mode has a much longer buckle half-wavelength and is classified as a long half-wavelength distortional buckling mode. The compression tests were carried out in a 300kN capacity SINTEC testing machine over a range of lengths with fixed-ended conditions. The varying lengths were chosen so as to observe the buckling modes and the possibility of interaction existing design standards. The methods include the Direct Strength Method (DSM) recently incorporated in the North American Specification and the Australian Cold-Formed Steel Structures Standard AS/NZS 4600. When compared to the existing methods, the test results indicated that at intermediate and longer specimen lengths, the interaction of local distortional buckling modes has a significant effect on the strength of the sections. This report presents the procedures taken to design the complex shape and the experiments carried out to obtain the geometric imperfections and material properties of the specimens and the test results. New design methods are proposed for the local buckling Direct Strength Method curve as well as for the distortional buckling strength curves so as to account for the effects of interaction of local and distortional buckling modes.