Fragility Assessment of Roof-to-Wall Connection Failures for Wood-Frame Houses in High Winds

Abstract

The enhanced-Fujita scale (EF-scale) is used to identify tornado intensity. It uses several damage indicators (DIs), each of which has descriptions of the degrees-of-damage (DOD) along with associated wind speeds. Recent research has indicated that for wood-frame, one-family and two-family houses, differences in the structural details result in significant variations in the wind speeds estimated to cause specific levels of damage, particularly with respect to the performance of roofs. This suggests that a single damage indicator for this class of structure may be inadequate. In order to examine this point in detail, the paper focuses on failures of the roof-to-wall-connections (RTWCs) in wood-frame houses, which are frequently damaged in tornadic wind events. Fragility analyses were conducted using an extensive wind-tunnel-based dataset for the determination of the statistics of wind loads and full-scale house test data for the toe-nailed RTWC resistances. The wind load data came from wind tunnel simulations of the atmospheric boundary layer, which are likely to provide upper-bound failure wind speeds for tornadoes. The results indicate that the roof shape and the capacity of RTWCs (i.e., number/type of connections) are the primary factors affecting the failure winds for houses with dominant openings. Recommendations for modifications to the EF-scale to account for these are provided.