Failure of wood-framed low-rise buildings under tornado wind loads

Buildings in the “tornado alley” of the United States, are built to withstand 3-s wind speeds of 90 mph (40.2 m/s), whereas 90% of the tornados are of F2 or lesser intensity that generate anywhere from 40 to 157 mph (17.9–70.2 m/s). At the same time, these codes are based mostly on studying the effects of straight line winds and not on tornado type winds, especially on low-rise, wood framed buildings which make up majority of structures in the United States. Previous research at Iowa State University (ISU) includes extensive testing on a scaled down low-rise gable roof building model (1:100) to understand tornado induced loads as the tornado sweeps past the building. In the present work, Finite Element (FE) models were developed using ANSYS for full-scale numerical simulation of the gable roof buildings with three different roof angles (13.4°, 25.5° and 35.1°). The nail is modeled as a non-linear element but the wood is assumed to be linear. The tornado-induced wind loads recorded in the laboratory were scaled up and applied to the models to determine the detailed stress distribution in the structure. This numerical study was performed using the same parameter as in the laboratory experiments such as those listed earlier. The deterministic FE model incorporated the damage criteria to assess the damage potential due to tornadic forces. The stress distribution, pattern of failure, the order of failure and the type of failure have been studied as the tornado sweeps past the building at different angles to the building centerline.