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

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Another point which may help.

The MWFRS pressure coefficients are determined from the maximum coefficient
which occurs in a 45 degree sector either side of the wind direction
considered: transverse, longitudinal. These give an approximation of the
distribution of wind loading experienced by building as the wind blows in a
given direction. The C&C ignore direction.

ASCE7-05 Fig6-6 indicates that the side wall pressure coefficient is
Cp=-0.7, whilst AS1170.2 indicates the pressure is stepped (-0.65,
-0.5,-0.3,-0.2) in similar manner to that on roof for longitudinal. Note we
have one extra step of 3h.

As move away from windward edge the wall pressure coefficient and roof
coefficient for longitudinal loading are thus decreasing. Thus a point along
the length of the building is reached where the transverse loading is
controlling. Also the windward edge, end frames only have half the load
width of typical frames: so higher pressures there tend not to be critical.
Also for longitudinal loading the parapet has suction on both faces,
therefore not a critical condition: the transverse loading is.

Pressure coefficients for transfer loading of roofs are typically lower than
the longitudinal pressure coefficients, until reach a point along length of
roof where longitudinal have stepped lower. So in terms of AS1170.2 we could
be applying the local pressure factor to lower pressures (Use ASCE7 Fig6-6
for Cp). For example Cp=2*-0.9 at windward edge longitudinal loading versus
Cp=2*-0.6 leeward face of transverse loading, versus say Cp=2*-0.4 on
windward face for transverse loading (assuming 20 degree roof). But leeward
has suction on both faces of parapet so not critical in terms of maximum
load: just important with respect to location of compression flange. Taking
Cp=+0.8 for windward wall (AS1170.2 has +0.7), then get Cpn=2*(0.8+0.4) =
2.4, smaller than the extreme Cpn=3.2 previously estimated.

Ok! To ASCE7-05 C&C pressures apply irrespective of direction. However what
ever the prescriptive requirements in our codes, the wind pressures are
still distributed across the surfaces. And ASCE7 commentary does indicate
permitted to use any rational methods in the literate: so seek texts and
research papers which identify the distribution.

Good luck.

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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