Web Crippling Resistance of Cold-Formed Steel Z-Sections under Different Flange Conditions

This paper presents an experimental investigation on web crippling behavior of cold-formed steel Z-sections with fastened and unfastened flanges. A total of 52 tests were conducted on Z-section specimens under end-two-flange (ETF) and interior-two-flange (ITF) load cases. The effects of flange fastening and flange stiffening on the web crippling resistance were examined. The accuracy of codified design rules for Z-sections in the North American Specification for the Design of Cold-formed Steel Structural Members AISI S100, which mainly employs design rules developed by C-section tests, was assessed by comparing all available test results with predicted values from AISI S100 provisions. It is revealed that AISI S100 predictions were generally scattered, especially for the tested unfastened Z-sections with unstiffened flanges under ETF loading. It is explicit that employing design rules of C-sections for Z-sections could yield unreliable predictions. In addition, the newly updated EN 1993-1-3, which incorporates the data pool of AISI S100 for Z-sections undergoing web crippling, was also evaluated. It is found that EN 1993-1-3 generally yielded conservative predictions with mean values of test-to-prediction ratios all above unity for the tested Z-sections. Furthermore, modified design rules were proposed for AISI S100 underpinned by the newly reported test data together with those collected from available literature. It is found that the modified design rules are able to provide safe-sided and reliable web crippling strength predictions for cold-formed steel Z-sections under the studied load cases.