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RE: C & C pressures, trusses

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Thanks Scott

Later in the evening, after making my statement, I realised I had focused on
the length of cladding rather than the tributary area of the fastener. So I
retract what I said about ASCE7-05 clause

As you say there is no magical designation as to whether an element is MWFRS
or C&C: the air flows over the building surfaces and there are pressure
fluctuations, as move from the small tributary area of the cladding fastener
to structural elements supporting larger and larger areas of the building
surface, the C&C pressures become less relevant and the MWFRS pressures
become increasingly relevant. Averaging of pressure fluctuations over small
areas gives way to more reasonable averaging over larger areas.

However, there doesn't appear to be any reference as to what area the C&C
pressures were averaged over to help determine their relevance and the
influence they have on structural elements of higher tributary areas: but
ASCE7-05 clause seems to suggest the change from C&C to MWFRS
pressures is a tributary area of 700 sq.ft (65 sq.m): which in the scheme of
things is not a small area: it is large compared to dimension 'a' having a
minimum value of 3ft.

The ASCE7 C&C pressures on small tributary areas are a different entity to
the localized pressure effects I am familiar with from the use of AS1170.2.
Using AS1170.2 I can drown out the influence of the localised effects as I
increase the tributary area experiencing "ordinary" pressures. There is thus
less issue as to when a given magnitude of load applies, and directionality
is always present, with the designer being responsible for addressing the
issue of wind changing direction. (eg. cladding on both roof faces designed
for the worst load case.)

As you suggest I need a different mentality and perspective to understand
when the ASCE7-05 C&C pressures cease to be relevant and the MWFRS pressures
become relevant.

To gain that alternative perspective seems to require researching some
background information, because the code and commentary do not appear to be
clear on the issue.

For whilst individual elements of a truss may need assessing for C&C
pressures, and they may influence the tie-down requirements: I don't believe
that means applying a symmetrical C&C loading to the whole truss. I still
believe the C&C pressures are for localised detailing of the truss, which
becomes increasingly the case as the truss increases in size. So for example
C&C pressures may be the critical case for localised bending of the chord
between webs, and for the cantilever overhang at the eaves: but the overall
design of the truss comes from MWFRS pressures. But without knowing more
about the basis of the C&C pressures I have no evidence to support my view.

So I will follow the prevailing view until I can show otherwise.

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

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