Wind pressure on low-rise buildings, an air infiltration analysis.

The distribution of wind pressure on a building envelope is governed by the size and shape of the structure and the turbulence characteristics of the wind. Observation of the mean wind pressures shows that surfaces are divided into pronounced zones of positive and negative pressure. The turbulence gives rise to fluctuating pressure components of appreciable magnitude. This fact changes the prerequisites of the ventilation for a given volume. The pressure in the cavity behind the facade materials depends on the external pressures over the facade. A levelling, or smoothing, of the pressure fluctuations can take place, depending on the cavity design and permeability distribution. The wind pressure spectrum shows that the greater part of the turbulent energy is concentrated at frequencies below 1 Hz. Full-scale measurements have shown that the fluctuations, which are quantified by the calculated variance of the spectrum, are of the same order of magnitude as the mean pressure. The degree of correlation between fluctuating components of the wind pressure on different parts of the building envelope is low. Because of this, the interchange of air is at all times based on a varying pressure distribution, and the participation of different leakages is variable. In the case when air infiltration can be expected to account for a considerable part of the total air exchange rate, the methods of calculation used should take into account the real wind characteristics and the response of the building to wind fluctuations.