Wind Hazard Risk Assessment and Management for Structures

as:
Chapter 2. Structural Components and Wind Directionality 20
Venv(d) = Vt · exp − d
, (2.4)
RG
where RG, the length scale of the environmental-scale processes, is taken to be 500 km
(Jakobsen and Madsen, 2004). The tangential wind speed, Vtan, is calculated based on
the parametric tropical cyclone model developed by Jakobsen and Madsen (2004) and the
air pressure distribution model described by Holland (1980). The latter expresses the air
pressure as follows:
P (r) = Pc + ∆P exp
−
Rmax
r
B
, (2.5)
where r is distance from the storm center, Pc is the central pressure, and B is a dimension-
less scaling parameter (affecting the shape of the pressure profile). A simple empirical
equation is used to relate B to Pc, expressed in mbar (Hubbert et al., 1991):
B = 1.5 +
980 − Pc
. (2.6)
120
This expression, originally derived based on data from Australian tropical cyclones but
found to be generally applicable, is adopted herein mainly for its simplicity. The magnitude
of the tangential wind at the structure’s location is (Jakobsen and Madsen, 2004):
Vtan(d) =
λd
P
· ∂P
∂r
+ 1
4 f 2d2 − 1
2
fd, (2.7)
where β is a parameter relating tangential and radial wind speeds and f is the Coriolis
parameter. In the Northern hemisphere, the tangential wind is counter-clockwise, so the
tangential wind angle at the structure location is φ − π/2. The radial wind, Vrad, at the
structure’s location has a wind angle equal to φ + π. Its magnitude is based on that of the