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RE: C & C pressures, trusses

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Where you seem to "lose" me is when you start talking about the 'a'
distances.  C&C have nothing to do with 'a' distances in ASCE 7 (i.e.
localized higher pressures are edges and such)...at least not directly.
There are different C&C pressures for different areas of the surface...i.e.
there are C&C pressures for when an item is in the main central portion away
from edges, but also different C&C pressures for areas near corners or edges
(this is also true for MWFRS pressures).  The main difference between C&C
pressures and MWFRS pressures in ASCE 7 is wind trib area.

As to a truss, yes, C&C pressures would be prevalent typically for localized
detailing & design, but C&C pressures can also be prevalent for overall
design of a truss.  Take uplift on a truss (i.e. load reversals on ALL
members).  A truss should be checked for C&C pressures for uplift.  The C&C
pressures will be greated than MWFRS pressures since for all things being
equal, C&C pressures will be greater than the MWFRS pressures.  Thus, for a
pure uplift case (call it a flat truss to keep it simple), C&C pressures
will produce a more overall severe load reversal on a truss then MWFRS
pressures.  Thus, the overall truss needs to be checked for C&C pressures.
It must also be checked for MWFRS pressures if it has a peak with dual
slopes and thus different MWFRS pressures on each side (resulting in a net
horizontal force on it as well as some sort of uplift).

The key point is that JUST because an element is part of the MWFRS system
does NOT mean that it does not need to be checked/designed for C&C
pressures.  It ultimately goes back to why C&C pressures are higher than
MWFRS.  Since they have a "smaller area" there is a higher probability of
higher localized pressures occurring over that smaller area than higher
localized pressures occuring over a larger area of say the MWFRS (i.e. whole
building).

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Conrad Harrison [mailto:sch.tectonic(--nospam--at)bigpond.com] 
Sent: Thursday, May 15, 2008 12:08 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: C & C pressures, trusses


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 6.5.12.1.3.

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 6.5.12.1.3 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.


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



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