Comparison of wind uplift load sharing for Australian truss- and pitch-framed roof structures

Abstract

Windstorm damage surveys highlight that the discontinuity in load resistance and transfer of connections in timber-framed housing can lead to total failure of the roof structure. Field surveys of timber-framed houses under construction in Australia also indicated that, for a typical roof configuration, at least three of its six or more roof trusses had common construction defects (i.e. missing nails) in roof to wall connections (RWCs). It is expected that these construction defects will reduce the capacity and stiffness of the connections, creating variations in the wind load sharing, load transfer and the forces at the roof to wall connection of timber-framed houses. This study develops finite element models coupled with wind tunnel model tests of timber-framed houses to evaluate the load sharing and the hold-down force when there are construction defects and stiffness reduction on the RWCs. Both contemporary nail plate truss roofs and traditional (pre-1980s) pitch-framed hip roofs commonly used in Australia were modelled and analysed. The results show that when subjected to windstorms the contemporary houses possess higher load sharing capacity and lead to significantly improved performance than the pitch-framed hip roof houses. Construction defects and stiffness reduction on the RWC of a single truss/rafter can create significant variation in load sharing, and failure of a truss/rafter results in a 55% increase of the hold-down forces of adjacent trusses, leading to a housing structure vulnerable to windstorms. The outcomes of this research provide better understanding of load sharing and can be used as a basis for developing vulnerability models of contemporary and traditional houses to windstorms.