Direct Strength Design of Metal Building Wall and Roof Systems - Through-fastened Simple Span Girts and Purlins with Laterally Unbraced Compression Flanges
A Direct Strength Method (DSM) prediction approach is introduced and validated for metal building wall and roof systems constructed with steel panels through-fastened with screws to girts or purlins. The focus is capacity prediction for simple spans under wind uplift or suction, however the DSM framework is generally formulated to accommodate gravity loads, continuous spans, standing seam roofs, and insulated roof and wall systems in the future. System flexural capacity is calculated with the usual DSM approach – global buckling, local global buckling interaction, and distortional buckling strengths are determined with a finite strip eigen-buckling analysis including a rotational spring that simulates restraint provided by the through-fastened steel panel. The DSM flexural capacity is then reduced with a code-friendly equation consistent with existing Eurocode provisions to account for the additional stress at the intersection of the web and free flange that occurs as the girt or purlin rotates under a suction (uplift) load. A database of 62 simple span tests was assembled to evaluate strength prediction accuracy of the proposed DSM approach alongside existing Eurocode and American Iron and Steel Institute (AISI) provisions. The proposed DSM approach is confirmed to be viable and accurate for simple spans. Modifications to the Eurocode approach are proposed, and if they are made, the Eurocode is also an accurate and potentially general prediction method. The AISI R-factor prediction method is accurate for C-section simple spans, unconservative for Z-section simple spans, and overall lacks the generality of the DSM and Eurocode
