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RE: C & C pressures, trusses[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: C & C pressures, trusses
- From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
- Date: Mon, 12 May 2008 20:31:13 +0000
I have not followed this from its beginning. But the definition for C&C contained in the ASCE 7-05 Commentary may shed a little light. Roof trusses are specifically cited as an example. |
"COMPONENTS AND CLADDING:Components receive
wind loads directly or from cladding and transfer the load to
the MWFRS. Cladding receives wind loads directly. Examples of
components include fasteners, purlins, girts, studs, roof decking,
and roof trusses. Components can be part of the MWFRS when
they act as shear walls or roof diaphragms, but they may also be
loaded as individual components. The engineer needs to use appropriate
loadings for design of components, which may require
certain components to be designed for more than one type of
loading, for example, long-span roof trusses should be designed
for loads associated with MWFRSs, and individual members of
trusses should also be designed for component and cladding loads
[Ref. C6-1]. Examples of cladding includewall coverings, curtain
walls, roof coverings, exterior windows (fixed and operable) and
doors, and overhead doors.
Effective wind area is the area of the building surface used
to determineGCp. This area does not necessarily correspond to
the area of the building surface contributing to the force being
considered. Two cases arise. In the usual case, the effective wind
area does correspond to the area tributary to the force component
being considered. For example, for a cladding panel, the effective
wind area may be equal to the total area of the panel. For a cladding
fastener, the effective wind area is the area of cladding secured
by a single fastener. A mullion may receive wind from several
cladding panels. In this case, the effective wind area is the area
associated with the wind load that is transferred to the mullion.
The second case arises where components such as roofing panels,
wall studs, or roof trusses are spaced closely together. The
area served by the component may become long and narrow. To
better approximate the actual load distribution in such cases, the
width of the effective wind area used to evaluateGCp need not be
taken as less than one third the length of the area. This increase
in effective wind area has the effect of reducing the average wind
pressure acting on the component. Note, however, that this effective
wind area should only be used in determining theGCp in
Figs. 6-5 through 6-8. The induced wind load should be applied
over the actual area tributary to the component being considered."
> Subject: RE: C & C pressures, trusses
> Date: Mon, 12 May 2008 16:02:23 -0400
> From: cbanbury(--nospam--at)arkengineering.net
> To: seaint(--nospam--at)seaint.org
> In most applications, wood trusses are part of the MWFRS since the
> structure has no stability under lateral loads without the trusses to
> brace the walls, support the diaphragm, and transfer uplift and lateral
> forces from the diaphragm to the wall.
> Strictly speaking then, trusses in this application would not be
> designed using C&C pressures since the ASCE-7 Standard makes MWFRS
> elements mutually exclusive from C&C elements by definition.
> However, in spite of the ASCE-7 Standard definitions, it is pretty clear
> that when the roof sheathing is nailed directly to the truss top chord
> or gable end webs that the cords/webs will experience something more
> like C&C pressures. For this reason, most truss design software is set
> up to design top chords and gable end webs for C&C pressures even if
> MWFRS pressures are used for the reactions and plate design.
> It is the responsibility and authority of the engineer using engineering
> judgment to determine if a structural element is operating as part of
> the MWFRS or C&C or both. It is a matter of function and no blanket list
> is authoritative. This is why the definitions in the ASCE-7 Standard are
> written in terms of function (MWFRS, C&C) not element type (i.e.
> trusses, joists, webs, connectors, etc).
> 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.
> Christopher Banbury, PE
> Ark Engineering, Inc.
> PO Box 10129, Brooksville, FL 34603
> 22 North Broad ST, Brooksville, FL 34601
> Phone: (352) 754-2424
> Fax: (352) 754-2412
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