Variability of External Pressure Coefficients with Terrain Roughness on Low-Rise Buildings

Abstract eng:
Post-hurricane damage investigations of suburban houses provide evidence of wind damage to components and cladding of buildings, despite continuous improvements to building codes and construction practice. Recent investigations by the authors find damage frequently occurs at wind speeds well below the design wind speeds provided by model building codes. Reasons abound for such failures, including sub-standard construction and materials, localized wind field anomalies (mini-tornadoes) that create pressures exceeding design wind pressures and reliance on standardized tests for cladding systems that do not represent actual failure mechanisms. Recent discussions on wind forces in suburban areas have raised another possible reason; that design wind pressures determined by model building codes may not completely represent external pressures occurring on low-rise buildings located in suburban areas. Currently, model building codes use external pressure coefficients that are the same for all structures regardless of the terrain roughness or surroundings around that structure. ASCE 7, and most building codes, have historically determined the component and cladding loads for low-rise buildings assuming an “open country” wind exposure, for example, wind tunnel studies on Texas Tech experimental building. However, recent studies have provided evidence that terrain roughness and surroundings may affect the external pressure coefficients produced on components and cladding of buildings. These studies [Liu et. al.], corroborate findings from field data on prototype houses that showed external pressure coefficients for two suburban buildings exceeded ASCE 7 code values by factors of two or more in a number of critical areas on the roof. Given the significant increases in external pressure coefficient observed in field and wind tunnel studies over ASCE 7 values, there is a critical need to identify reasons for this increase and to characterize the wind load climate around suburban buildings. This paper provides preliminary results of wind tunnel studies to compare the external pressure coefficients for prototype buildings subjected to hurricane force winds during the 2004 North Atlantic hurricane season. The goal is to use pressure data collected on six prototype buildings instrumented by the Florida Coastal Monitoring Program to validate wind tunnelderived pressure data for suburban buildings. In this initial phase, the authors provide comparisons of the previously derived external pressure coefficients using tropical storm winds on the prototype structure with pressure coefficients derived for the same structure that experienced hurricane-force winds, during Hurricane Ivan. The authors provide preliminary recommendations based on these studies and discuss directions for further wind tunnel studies and potential modifications to ASCE 7’s external pressure coefficients for low-rise suburban buildings.

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