Aerodynamic forces on the roofs of low-, mid- and high-rise buildings subject to transient winds

Transient winds, such as thunderstorm downbursts, are the cause of design-load wind speeds in many countries. An understanding of the loading experienced by buildings during a downburst is therefore important to allow well designed and engineered buildings to be constructed. In contrast to boundary layer winds, the maximum wind speed in thunderstorm downbursts occurs as low as zm=30 m above the ground, within the range of heights of man-made structures, suggesting that the wind loading will be dependent on the building eaves height relative to zm. In a novel set of experiments, the University of Birmingham Transient Wind Simulator (a 1 m diameter impinging jet with aperture control) has been used to simulate a downburst striking buildings of different heights, ranging from below to above zm. Two forms of building have been used - a square-plan, flat-roofed structure, and a rectangular, portal-frame - at three angles (0°, 45° and 90°) relative to the radial wind direction. Pressure coefficients have been calculated (using eaves height velocity) over the roofs of these buildings, and are shown to be of greatest magnitude when the roof is above the region of maximum outflow velocity, with the exception of windward edges perpendicular to the flow, when they are generally greatest for the lowest building heights.