Study of Load Effects on Structures Induced by Gust-Fronts

University Of Notre Dame, Notre Dame IN

Investigators

Abstract

The built environment experiences forces of nature generated by a host of damaging atmospheric events, including gust-fronts induced by downdrafts associated with thunderstorms. An accurate prediction of load effects that result from the interaction of structures with these extreme events is a critical part of designing and constructing the built environment and protecting its occupants. The primary goal of this project is to better understand and quantify wind loads and their effects on buildings under transient wind events, e.g., gust-fronts. To accomplish this goal, a systematic approach is proposed that focuses on capturing salient characteristics of transient flows associated with gust-fronts and attendant load effects on geometrically-scaled models of generic building shapes in a transient flow field simulator. The significance of transient wind events and their load effects can be readily surmised from an analysis of thunderstorm databases both in the U.S. and around the world, which suggest that these winds represent the design wind speed for many locations. As a result, these wind speeds become the input to codified design procedures validated in traditional boundary layer wind tunnels. However, the mechanics of gusts associated with convective gust-fronts differ significantly from conventional turbulence both in its kinematics and dynamics, e.g. their contrasting velocity profile and their transient nature. Accordingly, one should question the appropriateness of using design based on conventional analysis frameworks in codes and standards, which generically treat these fundamentally different phenomena in the same manner. Further, extreme loads on structures are potentially sensitive to the influence of transient flows, i.e., the load coefficients may be enhanced based on the gust form and the resulting localized, rapid changes in the surrounding flow. In addition, the characteristics of these flow structures suggest that the resultant load effects are likely to be correlated over larger areas than in conventional flows. These aerodynamic consequences clearly point at the prospect of higher loads on structures than would be predicted by the present codes and standards, calling for a careful examination of these procedures. The issues highlighted above serve as the primary motivation to study gust-fronts in this proposal. These investigations will initiate by gleaning information from surface wind data available during thunderstorm outflows to complement the existing knowledge base. The identified salient features of transient flow fields, e.g., uniformly accelerating flows, will then be simulated in a transient flow simulator comprised of individually controlled fan banks. The analysis of experimental data garnered from scaled building models exposed to these flows will enhance understanding and facilitate quantification of the resulting aerodynamic modifications. Upon combining numerically simulated structural load effects with the time-frequency analysis and modeling of gust-fronts and the measurements of pressure distributions and loads, the study will be capable of examining the efficacy of widely used design codes and standards, highlighting the possible need for modifications. Further, the collaboration with two distinguished aerodynamic/wind engineering centers in Japan will make available an impressive array of experience and provide access to unique experimental facilities to validate flow simulations and attendant loads resulting from this study. Gust-fronts associated with downbursts and tornadoes in the U.S. result in estimated 80 deaths, 1500 injuries, and property damage in the neighborhood of a billion dollars each year . a statistic that is rapidly escalating. Improved codes that are based on better understanding of the impact of gust-fronts on structures certainly promise to mitigate this hazard, thus benefiting society at large. Therefore, the broader impacts of this work will be measured in the reduction of these losses, both in property and, more importantly, in lives. Concurrently other major initiatives of this project, i.e., professional development, educational initiatives, etc., promise to have lasting impacts in raising public awareness and understanding of a phenomenon commonly experienced in the daily lives of Americans throughout this country. The flow simulator will be an attractive demonstration tool for public education in this regard. Further, the students involved in the proposed research will continue to embody the ethnically and gender diversified profile of the past research teams, enthusiastically maintaining the established traditions of pre-college outreach programs, targeted at primary and secondary level students, and involving underrepresented groups. Ultimately, the proposed study promises to provide a strong intellectual foundation, supported by well-trained students at graduate, undergraduate and pre-college levels for wind resistant design of buildings and civil infrastructure.