Wind loads on fasteners used to attach flexible porous siding on multi-layer wall systems

Abstract

Recent full-scale studies of wind loads on components of multi-layer wall systems have shown that maximum instantaneous pressure differences across individual layers can be a large fraction of the net load across the wall system, as noted in Cope et al. (2012) The results of these studies are considerably different from results obtained using dynamic pressure chambers where the entire section of the wall is exposed to the same quasi-steady pressure difference or time varying pressures. The largest differences in results between the full-scale wind tests at the Insurance Institute for Business & Home Safety (IBHS) Research Center and the pressure chamber tests occurred for the exterior flexible multi-part siding layer. In the pressure chamber tests, the pressures tend to equalize rapidly between the exterior and interior surfaces of the flexible siding resulting in very low net loads on the siding. In the full-scale wind tests, the response of the flexible siding to the temporal and spatial variations in wind loads imposed by the flow around the building could be observed as a wave moving along the wall surface.The study reported in this paper investigates the loads on the fasteners used to attach the siding to the wall. Specifically, the relationship between the instantaneous pressure differences across the siding and the loads on the fasteners is presented; with the goal of determining whether any systematic reduction or amplification of loads on the fasteners resulted from the very short duration peak instantaneous pressure differences applied to the siding. Specialty instrumentation was developed that allowed measurement of outward acting loads applied to the siding fasteners. This paper describes that specialty instrumentation and presents comparisons of pressure differences multiplied by the tributary area assigned to a fastener against the withdrawal loads measured using the instrumented fasteners.While there is significant scatter in the results, probably due to friction in the fastener load system among other things, results show a strong overall one to one correlation between the net outward loads calculated by applying the pressure load to the tributary area and the measured loads on the fasteners.