I think it is unreasonable to be applying C&C pressures to structural
primary elements. I believe that is a misinterpretation of the experimental
data.
From the top of my head using AS1170.2. Maximum Roof Cp=-0.9 behind parapet,
windward wall Cp=+0.7. Give Cpn= 1.6 net across parapet.
ASCE7-05 Clause 6.5.12.2.4 GCpn = +1.5, GCpn=-1
AS1170.2 doesn't have C&C but a local pressure factor Kl on areas 'a x 'a
and 'a/2 x a/2' for cladding and immediate supporting members only. Kl=2
maximum for low rise buildings, on 'a/2 x a/2': applied to external pressure
coefficients only.
If apply then Cpn = 1.6*2 = 3.2 net across parapet wall. But we would only
apply to small areas on cladding and say supporting girts, but not to say a
column extending from ground to top of parapet. If there are short infill
studs the height of the parapet then would apply the higher pressure to
those.
Sharp edges generate high turbulence: eaves, ridge, wall corners and edges
of walls (top & ends).
Read the commentary to ASCE7-05 those C&C pressures are the extremes,
irrespective of direction of airflow: they do not peak at the same time
across the entire tributary area of an element unless that element is small.
The importance of which is that: in a grid structure, when one point
experiences the peak, it can share and distribute the load to other members
not yet experiencing the peak. So my understanding is that C&C like
pressures are to stiffen the cladding and immediate support so that such
structure is strong enough locally to distribute to the larger structural
elements. And otherwise prevent breach of the fabric by direct action of the
wind which would increase internal pressure coefficient to that experienced
on the surface where the hole is formed.
The pressure coefficients given in the codes are mainly obtained from model
tests on shapes, not buildings. So they provide a guide to expectations from
different shapes. So for real buildings, obtain conservative estimate by
reviewing all the figures tested. For example airflow over a sawtooth roof
towards wall.
AS1170.2 basically uses the same pressure coefficients as given in ASCE7-05
Fig6-6: there are no C&C pressure coefficients in AS1170.2 just the
magnification factors for localised peaks. A diagram like Fig6-11C I
consider to be misleading. Zone 2 on the left hand eaves does not experience
the GCp along the entire length of the roof at the same time, nor does the
right hand eaves experience such peak pressure at the same time as the left
hand eaves.
To design an economical structure, it is necessary to take advantage of the
nature of the loading, and investigate pattern loading and/or load sharing
within a grid.
Because as you say the pressures are high, and therefore need to be
accommodated in a reasonable manner.
Feedback on other threads so far is ASCE7-05 is highly prescriptive, and few
want to think about wind loading, just want a simple single pressure to
apply to projected areas.
The other issue is I also apparently have more reasonable building
officials/engineers reviewing my work, and more willing to accept rational
application of Australian codes.
It is necessary to have rational interpretation of the code, because as you
say, as the parapet wall increases in height it moves further from the roof
and the airflow over the roof is now changed. Thus there will still likely
be turbulence and suction behind the top edge of the wall, but it is
unlikely to match the pressure coefficient on the roof. And for that matter
what is the pressure coefficient on the roof under such conditions: unlikely
to be that given in the code. Just one of the day to day problems of using
the wind loading code for real buildings.
Wind loading from the code is your best guess, and risky. And wind tunnel
tests are no use if the data cannot be transformed into useful guidance for
design.
The load is high, but have to find a justifiable/defendable approach which
permits reducing the effect. And common view maybe that C&C pressures have
to be used. But how are they used?
Regards
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic@bigpond.com
Adelaide
South Australia
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