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Chapter 3

Wind loading

T.A.Wyatt

3.1 WIND GUST LOADING

3.1.1 Basic concepts

The real wind is generally turbulent. Particularly is this so in the extreme storm winds that are
the usual focus for conventional wind loads, because the passage of the wind over the
irregularities of the ground surface (terrain roughness) will create sufficient disturbance to
break down any stable stratification in the wind that may result from a temperature lapse rate
that is less than the adiabatic value (an ‘inversion’). On the other hand, additional
contributions to the turbulence created by thermal instability (convective gusts) are broken
down. The presumption is thus a neutral Atmospheric Boundary Layer (ABL) in which the
gust structure is dominated by the effect of ground roughness. A standard description is
postulated for the profile of mean wind speed with height, with a statistical description of the
turbulence superimposed on it, with primary dependence on the ground roughness parameter
z In practical terms z is inferred by relating the observed mean speed profile near the ground
0
0·
to a theoretical model. However, a temperature lapse rate lower than the adiabatic value can
lead to abnormally smooth flow, which may increase susceptibility to aerodynamic instability
in light or moderate winds, discussed later.
In practice, evaluation of gust action is commonly based on classification of z in three
0
steps: z =0.3 for very smooth surfaces typified by tundra or water (correction for sea surface
0
roughness is possible as a function of storm strength), z0=0.03 for typical UK inland
countryside, z0=0.3m for suburban housing and forests. Although higher values of z0 are
possible (e.g. for city centres), the basic presumption of a generalized statistical pattern is
unreliable (see buffeting, Section 3.2.6); an estimate of structural response based on the model
for suburban roughness will give a conservative estimate of the mean load and probably of the
peak gust response, but may underestimate the specifically dynamic action. An ad hoc wind
tunnel test will give valuable additional information in such cases, but interpretation is beyond
the scope of this chapter.
The basic measure of gust strength is the root mean square (r.m.s.) value of the