Civil Engineering Reference
In-Depth Information
height of the site above sea level and a 'seasonal factor',
S
s
.
The 'terrain and building
factor',
S
b
, includes an allowance for the distance of the site from the sea, as discussed
previously; it also incorporates a gust factor to convert the hourly mean wind speed to a
peak gust wind speed. A 'site wind speed' is calculated by multiplying the basic wind
speed,
V
b
, by factors for altitude
(S
a
)
, wind direction
(S
d
)
, season
(S
S
)
and probability
(S
p
)
.
The seasonal factor,
S
s
, may be used to reduce loads for temporary structures that are
exposed to wind loads for defined periods less than a year. The altitude factor
incorporates the aerodynamic effects of topography, as well as the increase of wind speed
with height above sea level.
Table 15.2 summarizes the formats for calculation of design wind velocities and
dynamic pressures in various documents. ISO 4354 is alone in calculating a basic
dynamic pressure from the basic (unfactored) wind velocity. Variation with height and
terrain, topography, etc. is incorporated at the stage of calculating building pressure.
15.5
Building external pressures
Table 15.2 also shows the general format for calculation of external pressures on wall or
roof surfaces of enclosed buildings.
The formulas (in the right-hand column) appear to be quite different from each other,
but they all contain quasi-steady or mean pressure coefficients
(C
fig
, c
pe
, C
p
, C
f
, C
pe
)
and
factors to adjust the resulting pressures to approximate peak values. In the case of ISO
4354 and AIJ, there are gust factors on pressure (
C
dyn
and
G
f
); in the case of the
Eurocode, the function is incorporated in the exposure coefficient,
c
e
(z)
, which also
includes terrain/height and topographic (orographic) effects through the relationship:
(15.2)
where
c
r
(z)
and
c
0
(z)
are roughness and orography factors, respectively.
I
υ
(z)
is the
turbulence intensity.
The term in square brackets can be regarded as a gust factor on pressure.
In ASCE-7, the quantities
G
and
C
p
are usually combined together as
(GC
p
)
in tables.
In AS/NZS 1170.2, the 'aerodynamic shape factor',
C
fig
, consists of pressure coefficients,
multiplied by factors for area reduction, combination of roof and wall surfaces, local
pressure effects and porous cladding. The local pressure factor,
K
ℓ,
is always greater than
1, and the area reduction factor,
K
a
,
which allows for correlation effects over large areas
in separated flow regions, is less than 1. AS/NZS 1170.2 is alone in having a factor
(K
p
)
for porous cladding.
The tables of shape factors and pressure coefficients of exterior surfaces of buildings
given in the various documents are also sources of significant differences. However, in
all cases, the nominal wind directions are normal to the walls of buildings of rectangular
plan. However, as previously discussed in Section 15.2.6, the British Standard has a
directional method, which incorporates pressure coefficients for 15° direction increments.
ISO 4354 gives graphs of
C
fig
C
dyn
for the cladding on walls and roofs and the frames
of low-rise buildings (widths>2×height, and height <15m) with flat and gabled roofs.
There are strong similarities between these figures and the ones for
GC
p
for buildings less
