Resistant performance of cold-formed Q1100 ultra-high strength steel angle- and channel-section stub columns

This study presents a comprehensive investigation of cold-formed (CF) Q1100 ultra-high-strength steel (UHSS) angle- and channel-section stub columns (ASSCs and CSSCs), encompassing both tests and numerical simulation. The aim was to demonstrate the resistance behaviour, thus laying the crucial foundation for extending current design methods regarding the ultimate resistance of CF UHSS columns. First, mechanical property and stub column tests were conducted on 6 tensile coupon specimens and 16 stub columns. The cold-forming process (CFP) induced negative impacts on the ductility of the Q1100 UHSS, while the strength properties were barely influenced. Apparent local buckling emerged within the CF Q1100 UHSS ASSCs and CSSCs. The load-end shortening curves of the CF Q1100 UHSS ASSCs and CSSCs were revealed. The finite element (FE) models, whose validity was confirmed by the test results, were established, followed by a parametric study including 128 models. The assessment of the applicability of the design methods in EN 1993-1 - 12, AISI S100, AS/NZS 4600, and the direct strength method (DSM) was conducted based on the test and FE results. It is revealed that the EN 1993-1-12 Class 3 slenderness limit is appropriate for CF Q1100 UHSS angle sections but unsuitable for channel sections. The design method in EN 1993-1-12 is considered appropriate for CF Q1100 UHSS ASSCs. However, for CF Q1100 UHSS CSSCs, this design method yields some unsafe predictions. The effective width method in AISI S100 is safe for CF Q1100 UHSS ASSCs. Nevertheless, when the cross-section becomes slender, the ultimate resistance prediction for CF Q1100 UHSS ASSCs is overly conservative. In the case of CF Q1100 UHSS CSSCs, there are some unsafe predictions from this method. The predicted results from DSM are similar to those from AISI S100, where the predictions for slender angle sections are over-conservative and some predictions for channel sections are overestimated. Modification suggestions for each design method were proposed. The evaluations of the revised design methods were conducted to state the effective prediction of compression strengths of the CF Q1100 UHSS ASSCs and CSSCs.