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# RE: C &C wind load on parapets - really around 45 psf?

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: C &C wind load on parapets - really around 45 psf?
• Date: Wed, 14 Jan 2009 10:47:47 +1030

```Sharad,

Your approach seems reasonable. Also using AS1170.2, kl=1.25 for positive
pressures on windward wall, so my estimate of C&C drops lower Cp=
1.25*0.8+2*-0.4 = 1.8.

However I am aware that whilst AS1170.2 and ASCE7-05 are basically based on
research more than 30 years old, the C&C pressures in ASCE7-05 are
relatively current research and do suggest that AS1170.2 may be under
does indicate that Cp on a roof surface can get as high as Cp=-20, and that
is not in either code. Some complex statistics are used to account for the
duration of a spike in loading and its magnitude and frequency of
occurrence.

The C&C pressures are spikes in pressures on small areas, but the spikes do
not occur over the entire surface at once. Small vortices attach themselves
to the surface travel along, detach and get swept away in the main flow of
the fluid. So whilst the individual stud has to be assessed for the extreme
attaches them together. For example the noggins and plates connect studs,
battens connect rafters, joists connect bearers. Displacement of one leads
to displacement of others.

In Australia our timber framing code AS1684, and domestic metal framing code
AS3623 (now replaced by NASH specification), all contain formula for load
sharing between members. Though from memory they mostly relate to live
load, if the fascia board has a minimum stiffness. In effect the load is
reduced for the individual rafter.

Thus studs in parapet are probably attached by a plate at the top. So when
one stud experiences the spike in pressure the others don't and can help
resist total displacement of the highly loaded stud. The problem is ASCE7-05
doesn't mention the footprint or area of influence of the vortex causing the
spike: therefore don't know how many studs are influenced at once. So appear
to be stuck applying high load to individual stud with no load sharing.

Regards
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
South Australia

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