An Examination of Static Pressure and Duration Effects on Tornado-Induced Peak Pressures on a Low-Rise Building

Abstract

This study investigated the role of duration and tornado-induced static pressure on peak pressures on a low-rise building. A tornado simulator was used to generate both translating and stationary vortices to measure pressure time series on a building model. Time-resolved velocity measurements were also made on the vortex to aid in the analysis. Past studies have suggested that peak pressures on buildings in tornadoes were up to 50% higher than straight-line atmospheric boundary layer values as provided by ASCE 7-10. This study showed that much but not all of this increase can be explained by the static pressure of the vortex. While subtracting the static pressure from pressure time series and normalizing by a local horizontal velocity brought peak pressures closer to what one would expect from straight-line flows, and these data showed that some peaks could still be significantly larger than ASCE 7-10 provisions. To consider duration effects, translating and stationary vortex data were used with varying exposure times. Results showed that peak pressure magnitudes could increase by factors of 1.1-1.4 depending on duration. Work like this could lead to factors to adjust tornado pressure coefficients for the effect of event duration. The largest pressure peaks were observed to occur in or near the vortex core, and profiles of vertical velocity and static pressure suggest that strong unsteady vertical gusting and strong static pressure fluctuations could play a role in creating these large peaks.

Tornado-Induced and Straight-Line Wind Loads on a Low-Rise Building With Consideration of Internal Pressure

“…Haan et al (2010) using a laboratory tornado simulator and a single gable building model with no openings, reported that vertical and lateral tornado-induced wind loads exceeded ASCE 7 wind loads by factors as high as 3 and 1.6, respectively, when atmospheric pressure was included. Haan (2017) using the same experimental data from Haan et al (2010), subsequently found that analytically removing the atmospheric pressure component brought tornado-induced wind loads closer to ASCE 7 wind loads, but the aerodynamic loading still differed significantly from that expected under straight-line winds, with local tornadoinduced wind pressures in some zones exceeding ASCE 7 pressures by as much as 89%. Yousef and Panneer Selvam (2018) compared wind pressures in tornado-like winds against those generated in straight-line winds on dome-shaped and prismatic buildings using large eddy simulation, finding that tornado forces exceeded the straight-line wind induced forces by factors of 1.5 laterally and 1.3 vertically, with tornado-induced wind pressures greater by factors as high as 2.…”

“…For example, the choice of reference velocity for tornadoes, the choice of gust factor for re-referencing the TPU pressure coefficients, and other experimental or analysis choices can contribute to the differences observed. But more importantly, the mechanics of tornado-induced wind loads, with significant vertical velocity components, accelerating flow, and more are fundamentally different than what is simulated by boundary layer wind tunnels, as evidenced in several recent studies (Haan, 2017;Kopp and Wu, 2017). As of yet however, it is unclear to what extent each of these factors contribute to potential differences in observed wind loads.…”