Pull-through capacities of cold-formed steel roof battens considering loading rate sensitivity

Static and fatigue pull-through failures of thin steel roof battens in the vicinity of batten to rafter screw connections are part of the major premature connection failures which lead to severe roof failures during high wind events. As wind speed and thereby wind uplift loading rate on a building roof varies over a wide range, the loading rate which should be used to conduct prototype roof tests is a moot point. Current wind loading standards and guidelines recommend a wider range of loading rates/frequencies in simulating static and cyclic wind loads. This could adversely affect the test results of loading/strain rate sensitive materials. Since cold-formed steels appear to be sensitive to loading rate, it is necessary to investigate the effect of loading rate on both static and fatigue pull-through capacities of cold-formed steel roof battens. Therefore, a series of static and cyclic pull-through tests was conducted at various loading rates on roof battens made of two grades (G300 and G550) and two thicknesses (0.75/0.80 and 0.95/1.00 mm). Test results showed that both static and fatigue pull-through capacities increase with increasing loading rate. To understand the loading rate sensitivity of roof battens, the effect of loading rate on the ultimate tensile strength of cold-formed steels was also investigated through a series of tensile coupon tests. Based on both pull-through and tensile coupon test results, suitable modifications and recommendations have been made to the current pull-through capacity equations and the current Low-High-Low (LHL) cyclic test loading frequency. A suitable material model is also proposed to determine the dynamic mechanical properties of cold-formed steels.