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RE: C & C pressures, trusses

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Christopher Banbury

With regard to using C&C pressures to calculate reactions on a common
wood truss I believe that MWFRS pressures are more appropriate. C&C
pressures are a function of effective wind area on a single flat surface
and are not suitable for vector addition or for summing moments about a
point in order to resolve reactions on multi-faceted trusses.

<end quote>

That is was what I was trying to get at. The MWFRS pressures on windward and
leeward faces of a roof are generally different, and have a net horizontal
reaction. The C&C pressures are the same on each face of the roof, and the
horizontal components cancel out. The C&C pressures put symmetrical loading
on the truss whilst the MWFRS pressures do not.

So it is matter of designing all structural elements for the MWFRS pressures
first, when assessing whole building, or large portions off such as the
roof. Then tracing the C&C pressures from building surface to as much of the
rest of the structural components as the designer considers appropriate.

So that the C&C pressures may have a significant influence on the size of
the truss top chord especially if the top chord cantilevers beyond the
bottom chord to form the eaves overhang. But is somewhat dependent on the
relative size of the structural component.

It is somewhat like the local assessment of the web in a beam and provision
of web stiffeners: if the beam section is heavy enough then it doesn't need
the web stiffeners. The MWFRS pressures are like sizing the beam for the
point load, and C&C pressures are the localised check on the web. Somewhat:
because the beam doesn't have to be sized for the C&C pressures only
strengthened locally.

So for the eaves overhang, to a roof truss, the sharp wedge at the eaves is
causing the high turbulence and the highly localised pressures, and the
cantilever is relatively small and encompassed by (a x a) pressure zone. The
pressure transferred from cladding, to batten to rafter, may place a high
point load on the overhang, and result in larger top chord being required.
Elsewhere these intermittent, highly localised peaks in pressure have an
influence on connection design, but little influence on member sizing. Each
individual element however needs to be appropriately checked for the
influence of both MWFRS and C&C pressures.

I don't agree with ASCE7-05 clause, which allows MWFRS pressures
to be used for C&C with large tributary areas. A large area heavy concrete
panel, or other rigid plate, may experience a high localised pressure with
no significant effect transferred elsewhere. But continuous span steel
sheeting 100 metres long over arches and the likes is flexible between
supports, and high localised pressures can fail the fasteners. That then
sets two spans of the cladding oscillating, which in turn fails more
fasteners, increasing the length of cladding in motion, until the whole
length of cladding is eventually detached.

Of course need to comply with requirements of code: but the designer has the
option of determining what clauses reflect a realistic model of the
phenomenon: which ultimately leads to changes in codes.

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

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