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RE: C & C pressures, trusses

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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 determine GCp. 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 evaluate GCp 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 the GCp in

Figs. 6-5 through 6-8. The induced wind load should be applied

over the actual area tributary to the component being considered."


Regards,
Harold Sprague

> 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
> President
>
> Ark Engineering, Inc.
> PO Box 10129, Brooksville, FL 34603
> 22 North Broad ST, Brooksville, FL 34601
> Phone: (352) 754-2424
> Fax: (352) 754-2412
> www.arkengineering.net
>
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