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Re: C & C pressures, trusses

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I agree with ADK.  I would design a roof truss for C&C loads, particularly for wind pressures wanting to blow a roof off.  The wind pressures act on the roof sheathing, then the roof truss top chord, and finally the truss bearing.  C&C loads are about 25% (<--- guess) greater than those pressures due to main system forces...

 
On Mon, May 12, 2008 at 1:02 PM, Christopher Banbury <cbanbury(--nospam--at)arkengineering.net> wrote:
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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David Topete, SE