A Study of the Ill-defined Parameters of the Internal Pressure Dynamics of a Low-rise Building

Abstract

Damage to low-rise buildings under the influence of wind storms are of much concern. It is known that internal pressure induced through an opening in the building envelope could become a major cause for such damage. Mathematical models used for the estimation of peak internal pressures and envelope loads on the building use two parameters, namely loss and inertia coefficients; the values of these coefficients depend upon the opening parameters such as area, thickness and location. The present work studies this dependency. Very limited data for these coefficients is available in the literature. Loss coefficient data that is available has a large range and could assume unrealistically large values of up to 45. Fluctuating external and internal pressure measurements were carried out on a 1:60 scale model of a low rise building in a simulated atmospheric boundary flow (roughness length = 0.005m) in the wind tunnel. External point and area pressures were measured on the windward wall over a range of areas representing typical windows. Internal pressure measurements were undertaken with openings on the wall, representing open or broken windows. Since external pressure drives internal pressure through the opening, it was important to study external pressure first. External pressure measurements were found to be in good agreement with past studies. High frequency attenuation was observed in both point and area pressure fluctuations. With respect to internal pressure, it has been established that in addition to the area, the thickness and location of the opening also have a profound influence, particularly on the Helmholtz resonance frequency, and the loss and inertia coefficients. Inertia coefficients were found to increase as the opening moved closer to the sidewall or the roof. This is attributed to the extension of the oscillating air slug at the opening, closer to these surfaces. The inertia coefficient was found to range between 0.77 and 1.34. Furthermore, the loss coefficients were found to have an opposing trend to inertia coefficients, decreasing as the opening moved closer to the sidewall or the roof. This is again attributed to the presence of surfaces near the opening, modifying the separated flow region. For the range of opening areas, thicknesses, and locations studied, it was found that the loss coefficients ranged between 1.50 and 7.80. Unrealistically high values of up to 45 reported in the literature could not be confirmed.