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RE: Components & Cladding for Wood Roof Trusses

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UBC 97 (CBC 2001 for us Californian's who are lost in the gray-out davis
code for plumber pac cash quagmire) Chapter 23 Division V- Section 2321
states that design and fabrication of metal plate connected wood trusses be
in accordance with ANSI/TPI 1-1995.  ANSI/TPI 1-1995 requires that design of
metal plate connected wood trusses be in accordance with ANSI/ASCE 7-93.
Wood trusses are a component and should be designed as such.  Wood trusses
do not fit the definition of MFWRS until the diaphragm and all permanent
bracing is completely installed.  ANSI/TPI 1-1995 assigns the responsibility
of designing all permanent bracing to the EOR.  As I understand it, two sets
of calcs are required from the truss designers.  MWFRS for checking
reactions on supporting beams and such and C&C for verifying the
appropriateness of the truss design AND designing the permanent bracing
scheme.  Some engineers are comfortable with accepting MWFRS designs for
wood trusses and using those forces in designing the bracing scheme.  The
question is, "Is it appropriate to design the truss bracing scheme using
MWFRS loads"?  It is often the case that the C&C design produces buckling in
members requiring bracing that would have otherwise been overlooked and not
designed for bracing.

We require C&C loading for truss designs, as do most of the jurisdictions we
practice in.  It is our experience that most of the truss designers we deal
with at fabrication plants have never even heard of Component & Cladding
wind design.  If you are using a fabricator who uses Mitek's truss design
software and hardware, they have come out with a new version 5.2 that
produces C&C designs for the trusses and MWFRS reactions all in one easy to
read output.  This upgrade is free to the truss fabricator.

I've spent significant time educating the truss designers and fabricators we
work with on this very particular issue.  I have to give Charlie Truax many
thanks for educating me on this issue.

Keith De Lapp, P.E.

-----Original Message-----
From: Kestner, James W. [mailto:jkestner(--nospam--at)]
Sent: Wednesday, April 14, 2004 7:33 AM
Subject: Components & Cladding for Wood Roof Trusses

ASCE 7-98 requires that wood roof trusses be designed for C&C loading with
Zones 1, 2 and 3 and overhangs. It seems impractical for the EOR to list all
the loads for the truss designers because of the various effective areas for
different trusses. About the best that we can do is to reference that the
truss designer needs to comply with the C&C requirements of ASCE 7-98.
Additionally, our drawings must list: Basic Wind Speed, Importance Factor,
Exposure Factor and Internal Pressure Coefficient and minimum dead load (to
determine net uplift).

Does anyone else specify C&C loads differently?

Does the typical truss designer know how to handle this loading or is it
done automatically for him? Does the typical truss program take these C&C
loads into account?

Jim K.

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