Civil Engineering Reference
In-Depth Information
The speed-up effects are greatest near the surface and reduce with height above the
ground. This can have the effect of producing mean velocity profiles, near the crest of a
topographic feature, that are nearly constant or have a peak (see Figure 3.10).
The above discussion relates to topographic features, which are two-dimensional in
nature, i.e. they extend for an infinite distance normal to the wind direction. This may be
a sufficient approximation for many long ridges and escarpments. Three-dimensional
effects occur when air flow can occur around the ends of a hill or through gaps or passes.
These alternative air paths reduce the air speeds over the top of the feature and generally
reduce the speed-up effects. For structural design purposes, it is often convenient, and
usually conservative, to ignore the three-dimensional effects and to calculate wind loads
only for the speed-up effects of the upwind and downwind slopes parallel to the wind
direction of interest.
3.4.2
Topographic multipliers
The definition of topographic multiplier used in this topic is as follows:
(3.32)
This definition applies to mean, peak gust and standard deviation wind speeds, and these
will be denoted by and respectively.
Topographic multipliers measured in full scale or in wind tunnels or calculated by
computer programs can be greater or less than one. However, in the cases of most interest
for structural design, we are concerned with speed-up effects for which the topographic
multiplier for mean or gust wind speeds will exceed unity.
3.4.3
Shallow hills
The analysis by Jackson and Hunt (1975) of the mean boundary-layer wind flow over a
shallow hill produced the following form for the mean topographic multiplier:
(3.33)
where is the upwind slope of the topographic feature;
k
a constant for a given shape of
topography; and
s
a position factor.
Equation (3.33) has been used in various forms for specifying topographic effects in
several codes and standards. It indicates that the 'fractional speed-up', equal to is
directly proportional to the upwind slope, . The latter is defined as
H
/2
L
u
,
where
H
is
the height of the crest above level ground upwind and
L
u
the horizontal distance from the
crest to where the ground elevation drops to
H
/2.
