Civil Engineering Reference
In-Depth Information
Gaussian distribution. Clearly the measurements showed upward curvature, or positive
skewness (Figure C3). This can, in part, be explained by the square-law relationship
between pressure and velocity (see Equation (4.12); Holmes, 1981 and Section C3.3).
Negative skewness occurs for pressure fluctuations in separated flow regions of a
building.
The spatial structure of fluctuating pressures on low-rise buildings has been
investigated in detail by a number of researchers, using a technique known as
Proper
Orthogonal Decomposition
(e.g. Best and Holmes, 1983; Holmes, 1990a; Bienkiewicz
et
al.,
1993; Letchford and Mehta, 1993; Ho
et al.,
1995; Holmes
et al.,
1997; Baker, 1999).
The mathematics of this technique is beyond the scope of this topic, but the method
allows the complexity of the space-time structure of the pressure fluctuations on a
complete roof, building or tributary area to be simplified into a series of 'modes', each
with its own spatial form. Surprisingly few of these modes are required to describe the
complexity of the variations. Invariably, for low-rise buildings, the first, and strongest,
mode is 'driven' by the quasi-steady mechanism associated with upwind turbulence
fluctuations.
8.4
Buildings with pitched roofs
8.4.1
Cladding loads
Figures 8.11 and 8.12 show contours of the worst minimum pressure coefficients, for any
wind direction, measured in wind-tunnel tests on models of single storey houses with
gable roofs of various pitches (Holmes, 1994). The simulated approach terrain in the
Figure 8.11
Largest minimum pressure
coefficients, for houses with roofs of 10°
and 15° pitch (for any wind direction).
