Civil Engineering Reference
In-Depth Information
than 18 m height in ASCE-7. Both documents give graphs of shape (and gust) factor as a
function of tributary area. There are numerical differences, however, such that the values
in ASCE-7 are 50-60% of those in ISO 4354. This is because, as shown in Table 15.1,
ASCE uses a 3-s gust wind speed rather than a 10-min mean. However, as is
0-35−0.40, it appears that ISO 4354 will give peak loads on low-rise buildings about
two-thirds of those specified in ASCE-7. However, ASCE-7 allows a further reduction of
up to 15% through the use of a 'wind directionality factor',
K
d
.
ISO 4354 does not
consider any variation of load with terrain for low-rise buildings designed by the
'Simplified Method'.
The tables in ISO 4354 and ASCE-7 for low-rise buildings do not allow for variation
with height—to width ratio. However, an alternative figure for
C
p
in ASCE-7 (for
buildings of all heights), which has been derived from equivalent tables in the
Australia/New Zealand Standard AS/NZS 1170.2, does allow for the variation with
height/width ratio. The ASCE-7 Standard and AS/NZS 1170.2 require alternative positive
roof pressure coefficients to be considered. These are important values for the design of
frames, especially for those in colder climates where dead loads are often high, as pointed
out by Kasperski (1993).
prEN 1991-1-4.6 gives tables of external pressure coefficients,
c
pe
, which are
comparable to those in ASCE-7 and AS/NZS 1170.2, as they are effectively applied to
gust dynamic pressure through the use of the exposure coefficient,
c
e
(z)
. The tables give
two values:
c
pe 1
,
intended for tributary areas less than 1m
2
, i.e. local cladding design; and
c
pe,10
intended for major structural members. It appears that the numerical values for flat
and gable ('duopitch') roofs in prEN 1991-1-4.6 are comparable to those in ASCE-7 and
AS/NZS 1170.2, and alternative (positive or lower negative) values are given for most
roof pitches. However, no variation with height/width ratio is given.
The factors incorporated into the shape factor in the Australia/New Zealand Standard
AS/NZS 1170.2 for flat and gable-roofed buildings have already been discussed.
However, it should also be mentioned that the effect of tributary area and correlation
effects is dealt with by the use of the three factors:
K
a
(area reduction factor),
K
c
(action
combination factor) and
K
ℓ
(local pressure factor). The action combination factor,
K
c
,
a
new feature in the Australian and New Zealand Standard, allows for a reduction when
wind pressures from more than one building surface, e.g. walls and roof, contribute
significantly to a load effect.
The AIJ Recommendations also separate the specification of loads on the structural
frames and on the 'components and cladding' of buildings. The specification of pressure
coefficients is separated from the specification of the gust factor. Unlike any of the other
documents, the gust factor,
G
R
, for the loads on the roofs of low-rise buildings has a
dependency on natural frequency. Buildings are classified as those with heights less than
or greater than 45 m, a somewhat greater height than used in the other documents.
The 'size effect factor',
C
a
, in the British Standard BS6399: Part 2, is specified in a
graph. It depends on the diagonal dimension of the 'load-sharing area', with a minimum
value of 5 m. When overall loads involving wind pressures on both windward and
leeward facing surfaces are being calculated, a reduction factor of 0.85, to allow for the
'nonsimultaneous' action between faces, is allowed.
