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RE: Components & Cladding for Wood Roof Trusses[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Components & Cladding for Wood Roof Trusses
- From: "Chris Banbury" <cbanbury(--nospam--at)nicholson-engineering.com>
- Date: Fri, 16 Apr 2004 08:11:30 -0400
Please allow me to make the following observations on this topic. Please take all of my comments in the most polite and conversational way. I haven't detected enough consensus in the engineering community to be dogmatic on this issue.
>ASCE 7-98 requires that wood roof trusses be designed for C&C loading
The Standard is silent as to what specific structural elements are C&C and which are part of the MWFRS. It is the responsibility of the engineer to determine what structural elements require C&C loading and which require MWRS pressures. For guidance in making this determination you can refer to the following publications.
ASCE 7 Commentary p243
Guide to the Use of the Wind Load Provisions of ASCE 7 (Mehta)
SBCCI Guide to Concrete Masonry Residential Construction in High Wind Areas, p1.5
AF&PA Wood Frame Construction Manual for One and Two Family Dwellings 1.3 & Commentary 1.1.2
AF&PA Considerations in Wind Design of Wood Structures (Douglas, Weeks)
WTCA Metal Plate Connected Wood Truss Handbook, p6-5
>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
C&C loads are not intended to be applied to a 2D planar roof truss for the purpose of calculating reactions. An assemblage of structural elements assigned to provide support and stability for the overall structure and that generally receives wind load from more than one direction is part of the MWFRS. In the case of a roof truss, the top chords are not parallel to the span used to calculate effective wind area. There is no clear method for determining and applying C&C pressures in this case. C&C pressures are not intended to be resolved into horizontal and vertical "components". C&C pressures are applied perpendicular to the component/cladding.
>evidently all trusses designed using software and plates made by Robbins Engineering are
designed as MWFRS.
From my research and experience, the truss industry policy is to apply C&C pressures to top chords, overhangs, gable end webs and other elements acting as C&C. The software can do this.
>The ASCE 7-98 commentary specifically mentions that roof trusses are to be
designed as C&C for wind (re: ASCE 7 -98, p 243). The ASCE 7-02 Commentary
is a bit different and breaks out "long span" roof trusses, but it still
lists roof trusses as C&C (RE: ASCE 7-02 p 271). Ultimately it is a
function of trib area.
C&C are elements of the building envelope that do not qualify as part of the main wind force resisting system. Effective wind area does not define what is or is not MWFRS. It just tells when a component or cladding is large enough to be designed with MWFRS pressures.
Christopher A. Banbury, PE
Nicholson Engineering Associates, Inc.
PO Box 12230, Brooksville, FL 34603
7468 Horse Lake RD, Brooksville, FL 34601
(352) 799-0170 (o) (352) 754-9167 (f)
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