Low-rise buildings - Wind Loading of Structures - page 202

As discussed in Section 8.2.1, Martin Jensen identified the Jensen number, h/z 0 , the
ratio of building height to the aerodynamic roughness length in the logarithmic law
(Sections 3.2.1 and 4.4.5), as the most critical parameter in determining mean pressure
coefficients on low-rise buildings. The Jensen number clearly directly influences the
mean pressure distributions on a building through the effect of the mean velocity profile
with height. However, in a fully developed boundary layer over a rough ground surface,
the turbulence quantities such as intensities (Section 3.3.1) and spectra (Section 3.3.4)
should also scale with the ratio z/z 0 near the ground. There is an indirect influence of the
turbulence properties on the mean pressure coefficients (Section 4.4.3), which would
have been responsible for some of the differences observed by Jensen (1958), and seen in
Figure 8.1. In wind-tunnel tests, the turbulence intensity similarity will be achieved only
with h/z 0 equality, if the turbulent inner surface layer in the atmospheric boundary layer
has been correctly simulated in the boundary layer in the wind tunnel. Many researchers
prefer to treat parameters such as turbulence intensities and ratios of turbulence length
scale to building dimension as independent non-dimensional quantities (see Section
4.2.3), but unfortunately it is difficult to independently vary these parameters in wind-
tunnel tests.
Fluctuating and peak external pressures on low-rise buildings, which are most relevant
to structural design, are highly dependent on the turbulence properties in the approach
flow, especially turbulence intensities. Consequently peak load effects, such as bending
moments in framing members, are also dependent on the upwind turbulence. For
'correctly' simulated boundary layers, in which turbulence quantities near the ground
scale as z/z 0 , as discussed previously, peak load effects can be reduced to a variation with
Jensen number (e.g. Holmes and Carpenter, 1990).
Finally, the question of the dependency of pressures and load effects on low-rise
buildings in wind storms of the downdraft type (Section 1.3.5) arises. As discussed in
Section 3.2.6, these winds have boundary layers which are not strongly dependent on the
surface roughness of the ground—hence the Jensen number may not be such an important
parameter. Further research is required to identify non-dimensional parameters in the
downdraft flow which are relevant to wind pressures on buildings in these types of
storms.
8.3.3 Flow patterns and mean pressure distributions
Figure 8.5 shows the main features of flow over a building with a low-pitched roof,
which has many of the features of flow around a two-dimensional bluff body described in
Section 4.1. The flow separates at the top of the windward wall and re-attaches at a
region further downwind on the roof, forming a separation zone or 'bubble'. However,
this bubble exists only as a time average. The separation zone is bounded by a free shear
layer, a region of high velocity gradients, and high turbulence. This layer rolls up
intermittently to form vortices; as these are shed downwind, they may produce high
negative pressure peaks on the roof surface. The effect of turbulence in the approaching
flow is to cause the vortices to roll up closer to the leading edge, and a shorter distance to
the re-attachment zone results.
The longitudinal intensities of turbulence at typical roof heights of low-rise buildings
are 20% or greater, and separation zone lengths are shorter, compared to those in smooth,