The Impact of Extreme Wind Events on Structures

Severe wind storm events, categorized as events which inflict a significant amount of damage as a result of wind, are difficult to both simulate and quantify with an acceptable degree of confidence. This experimental work focuses on the development of multiple simulation frameworks to simulate salient features of severe wind events and to assess their load effects on prismatic scale models. Thunderstorm generated gust fronts, and associated downbursts, constitute various extreme wind events which cause significant damage to life and property. Particularly vulnerable are low to high-rise buildings, transmission lines, industrial structures, wind turbines and possibly long span bridges. Similarly, larger scale severe wind events (i.e. hurricanes) have a more pronounced direct impact on urban environments as well, producing damaging flow environments in special cases as a result of various situational factors. Understanding the interaction between the built environment and these extreme events is critical to the design of structures, ensuring their resilience to these hazards for post-event functionality and the safety of occupants. In order to assess the impact these severe events have on structures, novel experimental approaches are developed to simulate the various features of these severe events. Downburst outflows and gust fronts are simulated using three techniques: (i) a flat plate which can be oriented to a high incidence to the oncoming flow in a wind tunnel, (ii) a computational model of a rotating flat plate within a numerical wind tunnel and (iii) a novel facility that uses multiple small fans to tailor the oncoming flow within a wind tunnel (through a collaborative effort with Miyazaki University and the Center of Excellence on Wind Effects in Urban Areas at Tokyo Polytechnic University). A severe wind flow environment within an urban setting is developed utilizing a computational domain, with both flow visualization and wind tunnel modeling, to assess attendant damage patterns caused by the flow field on the built environment, particularly to cladding and facades. Prismatic models are subjected to the simulated flow fields, capturing the resulting surface pressures. These pressure results are then analyzed and compared with characteristics developed in response to synoptic boundary layer flow fields. The data is examined using a host of time, frequency and time-frequency based analysis techniques, delineating salient features in the transient flow fields that may depart from those observed in the typical synoptic boundary layer winds. The results presented herein demonstrate that simple, prismatic structures experience significant changes in the resulting surface pressures when impacted by a gust front-like flow. Surface pressures show a marked increase/decrease in coefficient values, beyond those developed in boundary layer flow fields. The spectral description of the local pressure fluctuations and integral loads demonstrate differing behaviors based on the length of the storm event. Time-frequency wavelet scalograms highlight temporal variations in the frequency content in response to the oncoming flow field. The computational model of an urban environment subjected to a severe wind event reveals various features within the developed flow field, resulting from multiple factors, that enhance surface pressures and facilitate debris impacts which yield detrimental effects on the building facade and exterior glass. The results of these investigations serve to examine and delineate whether the effects of gust front outflows and other severe wind events are indeed captured by the simulation techniques provided herein, to assess the impacts of severe events on structures based on the experimental results in this study, and to fundamentally contribute to the general body of transient aerodynamics knowledge base.