Low-rise buildings - Wind Loading of Structures - page 205

Figure 8.7 Conical vortices for oblique wind
directions.
Similar flow separation and re-attachment, as described for roofs, occur on the side
walls of low-rise buildings, although the magnitude of the mean pressure coefficients is
generally lower. The mean pressures on windward walls are positive with respect to the
free-stream static pressure. Leeward walls are influenced by the recirculating wake and
generally experience negative pressures of lower magnitude; however, the values depend
on the building dimensions, including the roof pitch angle.
When the wind blows obliquely on to the corner of a roof, a more complex flow
pattern emerges as shown in Figure 8.7. Conical vortices similar to those found on delta-
wings of aircraft occur. Figure 8.8 shows these vortices visualized by smoke—their axes
are inclined slightly to the adjacent walls forming the corner. The pressures underneath
these are the largest to occur on the low-pitched roofs, square or rectangular in planform,
although the areas over which they act are usually quite small, and are more significant
for pressures on small areas of cladding than for the loads in major structural members.
In the following sections, the effects of building geometries on design loads will be
discussed in more detail.
8.3.4 Fluctuating pressures
The root-mean-squared fluctuating, or standard deviation, pressure coefficient, defined in
Sections 4.6.4 and 8.3.1, is a measure of the general level of pressure fluctuations at a
point on a building. As discussed in Section 8.3.2, the values obtained on a particular
building are generally dependent on the turbulence intensities in the approaching flow,
which in turn are dependent on the Jensen number. In boundary-layer winds over open
country terrain, for which longitudinal turbulence intensities are typically around 20%, at
heights typical of eaves heights on low-rise buildings, the values of rms pressure
coefficients (based on a dynamic pressure calculated from the mean wind speed at eaves
height) on windward walls are typically in the range of 0.3-0.4. In separated-reattaching