Effects of roof geometry on tornado-induced structural actions of a low-rise building

Abstract

Continued loss of property and lives in tornadoes including the 23-person death toll resulting from the Lee County AL tornado on March 3rd 2019 reflects the ever-present vulnerability of the community to tornadoes, and necessitates improvement in building standards for more accurately predicting such extreme loads. This article investigates effects of three roof geometries: flat, gable, and hip, on maximum overall and local wind-induced uplift, shear and moment on low-rise buildings by a translating tornado. To explore local structural actions, maximum of five frames along the longer horizontal dimension of each building were considered. Maximum structural actions, independent of building orientation and its distance to the center of the tornado-track were calculated, with results showing that a flat roof building experiences the largest amount of overall uplift, overall shear, local uplift, and moment. Only local shears on a hip roof building were equal to or larger than those of a flat roof building for same frame locations. This study also showed that gable and hip roof buildings experience moments of similar magnitudes, whereas a gable roof performs better against shear and a hip roof performs better against uplift. Experimentally-calculated structural actions were compared with predictions of ASCE7-16, with results showing that ASCE7-16 under-predicts overall and local uplifts, and moments, while over-predicts shear for all roof geometries. The maximum under-prediction of overall and local uplifts using ASCE7-16′s procedure were 41% and 55%, respectively, both for a gable roof building. Internal frames experienced maximum under-prediction of uplifts, which makes them a vulnerable location of low-rise buildings that are built in compliance with ASCE7-16 building standard. Finally, correction factors were introduced to modify and improve accuracy of ASCE7-16 in prediction of structural actions on low-rise buildings with different roof geometries.