Web Bearing Behaviour and Design of Hollow Flange Beams

Abstract

Thin-walled steel hollow flange channel beams are commonly used as joists and bearers in various flooring systems in buildings. They are mono-symmetric sections with two closed rectangular hollow flanges, which make them highly structurally efficient when compared with conventional cold-formed steel sections. The LiteSteel beam (LSB) section is a welded hollow flange channel beam with two rectangular hollow flanges. It is made from a single steel strip using a combined dual electric resistance welding and automated continuous rollforming process. Recent research studies made improvements to the LSB section, by eliminating the expensive welding and replacing them with a riveting process. This innovative hollow flange channel beams known as the rivet fastened rectangular hollow flange channel beam (RHFCB) section. The rivet fastened RHFCB is made by intermittently rivet fastening two torsionally rigid rectangular hollow flanges to a web plate element, which allows section optimisation by selecting appropriate combinations of web and flange widths and thicknesses. The LSB and rivet fastened RHFCB sections are thin-walled flexural members, hence becoming vulnerable to web bearing failures including web crippling, flange crushing and their combinations. However, no investigation has been conducted on the web crippling behaviour and capacity of LSB and the newly introduced rivet fastened RHFCB sections. To address this issue, an experimental study was conducted consisting of over 170 web crippling tests to investigate the web crippling behaviour and capacities of LSBs and rivet fastened RHFCBs, based on the new AISI S909 standard test method. This study included LSBs and rivet fastened RHFCBs with their flanges unfastened and fastened to supports under all the four standard load cases used in web crippling studies - End-Two-Flange (ETF) Loading, Interior-Two-Flange (ITF) Loading, End-One-Flange (EOF) Loading and Interior- One-Flange (IOF) Loading. It provided significantly improved knowledge and understanding of the web bearing failures including web crippling, flange crushing and their combinations, and associated web bearing capacities. Comparisons of experimental web crippling capacity results with predictions using the current AS/NZS 4600 and AISI S100 design standards showed that web crippling design equations are unconservative in some cases while being conservative in other cases for both LSB and rivet fastened RHFCB sections with flanges unfastened and fastened to supports under ETF, ITF, EOF and IOF load cases. Hence this research study has proposed new equations to determine the web crippling capacities of LSBs and rivet fastened RHFCBs based on experimental results. Flange crushing led to higher web crippling capacities and thus the same capacity equations are recommended as a conservative design approach. The web crippling tests with flanges fastened to support conditions showed that web crippling capacities increased by 10 to 90 % on average for both LSBs and rivet fastened RHFCBs under all four load cases. This has thus demonstrated the need to include the benefits of fastening to supports in the web crippling design. Experimental results from this study showed that both hollow flange channel beams (LSB and rivet fastened RHFCB) perform structurally better due to their unique geometric characteristics (effective flanges), when compared to conventional open cold-formed channel sections. The two additional flange lips in the rivet fastened RHFCBs effectively stiffen the web plate and thus provide higher resistance against web crippling than the welded hollow flange channel beams (LSBs) without flange lips. Tests showed that intermittent rivet fastening can be used to produce similar outcomes as welded LSBs as there was no excessive separation between the web and flange plates during the tests. Based on the results from this study, a 100 mm rivet spacing is recommended for RHFCBs. In summary, this thesis has presented the details of an extensive experimental study into the web crippling behaviour and capacities of an innovative group of welded and rivet fastened hollow flange beams known as LSBs and RHFCBs with their flanges unfastened and fastened to supports under ETF, ITF, EOF and IOF load cases, the results and the improved web crippling design equations developed within the guidelines of AS/NZS 4600 and AISI S100. Such enhanced knowledge, understanding and design rules are expected to advance the use of those innovative sections as joists and bearers in floor systems.