A practical modelling technique to assess the performance of wood-frame roofs under extreme wind loads

摘要

Wood-frame residential roof failures due to extreme wind events are among the most common and expensive types of damage. Following extreme wind events, forensic damage investigations observe the final state of failed structures, and research must work backwards to identify the likely points of failure initiation under the expected wind loads. Based on previous studies, hip roofs are commonly understood to be more resilient during extreme wind in relation to gable roofs. Two predominant modes of residential roof failure have been studied in the literature; sheathing loss and failure of the roof-to-wall connection (RTWC). Hip roofs have specifically been proven more resilient to damage through these two modes. However, inspection of damage survey data from several recent tornadoes has identified that failures may have occurred in hip roofs at the roof framing members and their connections, rather than in the RTWC. Under wind uplift, the vertical load path through typical light-frame wood structures is still not well-defined, especially considering the possibility of failure of the framing members and connections. To prove the concept of partial failures within the frame of a hip roof, while conserving experimental resources, a finite element modelling technique is required. In the present work, a modelling method is developed to analyze the internal load effects and strength behaviour of the components of a wood-frame roof under wind uplift. Due to the uncertainties in construction of connections, lack of experimental data required for validation, and variability of the parameters affecting joint stiffness, the current study proposes a method which takes an envelope of the extreme member and joint forces under uplift, and compares them with unfactored strength values to produce conservative demand-to-capacity (D/C) ratios. Two-dimensional truss models are developed and validated against data from the literature to prove the modelling method before the D/C analysis is done for a truss under uplift. This method allows for performance, in terms of relative D/C ratios, of the structural components to be compared and the "weak-links" to be identified. Then, comparing the results for the framing members and connections to the known wind resistance of RTWCs provides a point of comparison for the observed framing failures. The results show that toe-nailed RTWCs are likely to be the weak-link, however the D/C ratios of certain truss connections confirm that their failure may be possible in certain configurations, or when the RTWCs are strengthened using hurricane ties.