Civil Engineering Reference
In-Depth Information
of the roof. The British Standard specifies a 'size effect factor',
C
a
, which depends upon
the diagonal dimension of the tributary area and also on the terrain and height.
The comparisons will be separated into loads in major structural members and loads
on small elements of cladding. In these comparisons, external pressures only will be
considered for the enclosed buildings. Internal pressures are an important part of the net
pressure, but are much less dependent on the building shape, being mainly affected by the
number and size of openings in the building envelope.
Case (a)—Square plan enclosed building with flat roof
The effective pressure coefficients for the structural loads on the roof are given in Figure
15.1 and for areas of cladding of the order of 1m
2
are given in Figure 15.2. In Figure
15.1,
K
a
in the Australia/New Zealand Standard is taken as 0.8, and
C
a
in the British
Standard is 0.85. In the case of the American (ASCE) Standard, Figures 6.10 and 6.11b
have been used to obtain values of
GC
p
.
Figure 15.1 shows that the largest magnitude negative pressure coefficients, which
occur at the windward end of the roof, are reasonably similar in magnitude in all the
codes; however, the zoning systems are quite different from each other. The American
(ASCE-7) and International (ISO 4354) Standards give very similar pressure coefficients
to each other for structural loads. The British Standard and the Eurocode also give similar
values to each other. Only AS/NZS1170.2, prEN 1991-1-4.6 and BS6399.2 allow for the
possibility of positive pressures occurring on the leeward half of the roof, and only
AS/NZS1170.2 allows for alternative negative pressures on the windward end. As
discussed in earlier chapters, the nature of the fluctuating and turbulent flow over large
roofs can produce large fluctuations in the instantaneous pressures.
There are bigger differences in both the zonal system and the specified pressures for
small areas of cladding between the various codes, as shown in Figure 15.2. All the codes
specify higher pressures along the edge regions of the roof, i.e. the regions mainly
affected by the separated flow from the walls. All except the Australian Standard
(AS/NZS1170.2) give higher pressures at the corners, with the largest values being
specified by the Eurocode and the American Standard.
Case (b)—Square plan canopy with flat free roof
Five documents of the group specify pressures for canopies or free roofs. In all these
cases,
net
pressure difference coefficients are given. Figures 15.3 and 15.4 give the
pressure coefficients for structural and cladding loads, respectively. The Eurocode gives
the same pressure coefficients for main structural loads and cladding and applies them
over the whole roof—obviously very much a simplification. AS/NZS 1170.2 specifies
pressures for zones based on distance from the windward edge as for the enclosed
building. ASCE-7
