Design Wind Loads for Air-Permeable Multilayer Cladding Systems
Abstract
Monetary losses from severe wind storms continue to rise despite improvements to building codes. This is because air-permeable multilayer cladding systems such as vinyl siding and roof pavers are a major component of the monetary losses in severe wind storms, but there are not legal requirements for these products. Some air-permeable multilayer products have manufacturing standards that deal with design loads by eliminating the cavity flow and applying a pressure equalization factor to account for the reduction in net load caused by this cavity flow. The objective of this thesis was to develop design guidelines for wind loads on air-permeable multilayer cladding systems.
A multi-chamber airbox system for testing air-permeable multi-layer cladding was developed and compared to a benchmark study performed by the Insurance Institute for Business & Home Safety by implementing a latex barrier system and examining the control strategy for the Pressure Loading Actuators. The results indicate that using multi-chamber, pressure-based, testing to obtain wind loads for air-permeable, multi-layer wall systems with flexible cladding is sufficiently accurate. It also showed that the pressure equalization factor (the ratio of the net pressure over the external pressure) for vinyl siding is inaccurate, which could be leading to the common failures seen in damage surveys.
Another issue with the codification of the wind loads on air-permeable multilayer systems is that design approaches to determine loads on different types of building cladding elements can vary significantly by product type, even though they may have similar geometries. Using the full-scale wind tunnel at the Insurance Institute for Business & Home Safety, external and net wind loads on two discontinuous metal roof systems were measured. The results indicate that cavity pressures in air-permeable multilayer cladding, such as discontinuous metal roofs are approximately uniform across individual panels and closely related across multiple panels which share the same cavity. The testing also showed that there is a short time lag between the peak net loads and the peak spatial gradient of the external pressure across the cladding elements. The two types of discontinuous metal roofing products, which have significantly different cavity geometries, have similar ratios between the net and external wind loads. This suggests that design values have the potential to be fairly simple for typical residential building products, in spite of fairly complex aerodynamics.
With that goal in mind, a unified approach to the pressure equalization factor for air-permeable multilayer cladding was created by enveloping the worst case pressure equalization factor values for each type and taking into account the effect that exposed edges and parapets have on the net loads. The design recommendations could be used to fill the current gap in knowledge in air-permeable multilayer cladding in building codes such as ASCE 7 or NBCC.