Wind tunnel measurement of the parapet height role on roof gravel blow-off rate for two dimensional low rise buildings

A new experimental technique has been developed to measure the rate of removal of aggregate from built-up roofs in severe storms. The technique was used to quantify the rate of removal of aggregate from a two-dimensional building as a function of the building parapet height and the wind speed. The experimental results indicate that the mass flux varies over time. Initially the mass flux is quite high, and there is significant movement of aggregate within the roof cavity formed by the parapet. After this initial adjustment, the mass flux reduces by a factor of four and then remains steady. The adjustment time was found to be independent of the parapet height but strongly dependent on the wind speed. This suggests that the primary role of the parapet is to prevent removal of aggregate from the roof cavity, but that parapets are less effective at preventing aggregate motion within the roof cavity. Increasing the parapet height significantly reduces the mass flux. A parapet height equal to 15% of the building height reduces the blow-off rate by a factor of 100 compared to a roof without a parapet. Further, the addition of a parapet always reduced the mass flux. This stands in contrast to studies of the wind speeds required to initiate motion in which small parapets have been observed to reduce the critical wind speed required to initiate blow-off. Although the experiments were run at relatively small scale, the results were not significantly influenced by viscosity, indicating that small scale wind tunnel studies of mass blow-off rate may, with care, be scaled up to full-size buildings. Again this is in contrast to blow-off initiation studies, in which the wind speed required to start mass removal is strongly dependent on the Reynolds number, and only full scale experimental results should be used in design.