Numerical and experimental study of a new system for retrofitting roofs subjected to extreme wind loading

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The vast majority of structures in North America are residential. Light-framed wood construction is preferred in this region due to the low cost, the availability of materials and the ease of construction. Typical light-framed wood structures follow the requirements of governing building codes to determine the member sizes and connection details; consequently, structural analysis and design are not needed for these structures. Despite this fact, past high speed wind events have exposed vulnerabilities in existing residential light-framed wood structures, with the resulting damage being a major source of economic loss. For example, the damage to light-framed wood structures which represented a large portion of the US$20-25 billion of economic losses caused by Hurricane Andrew in 1992, with approximately 95% of those losses resulting from failure of components of the roof system. During these events, wind flowing over a roof creates lift similar to that on airplane wings. The resultant increase in the uplift forces on the roof causes failure of the roof anchorage and once the roof has failed, walls become laterally unsupported at the top and have less capacity to resist wind loads. This in turn causes the walls to fail and the whole building to collapse. As a result, such failures are often the cause of loss of life during wind storms, which requires an improvement in the behaviour of light-framed wood structures under uplift loading. The main objective of this research is to assess the technical and economical viabilities of a developed prototype for enhancing the uplift capacity of roofs subjected to strong wind. This system is in the form of a net that can be cast over a house's roof and anchored in the ground preventing roofs from failing at the critical joints during strong wind events. The proposed system provides the uplift forces an alternate load path to the ground, reducing the demand placed on the weak, nailed connections within the structure. This research project describes the numerical and experimental work required to assess the ability of the proposed retrofit system in increasing the uplift capacity of a house's roof and to optimize the system components to transform the prototype into a commercial system that can be successfully introduced to the target market and easily applied by homeowners.