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RE: Components & Cladding for Wood Roof Trusses

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This is not a new debate as you will be able to tell from the dates in the
literature contained in the link below.  The tradition from the 1980's and
1990's has been to design trusses as MWFRS not C&C. 
http://www.arnoldlumber.com/windy.html

Then along came Hurricane Andrew.  
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. 

The "We've always done it that way" folks need to be reprogrammed.  

Regards,
Harold O. Sprague

-----Original Message-----
From: John C. Jones [mailto:john(--nospam--at)struct-engr.com] 
Sent: Wednesday, April 14, 2004 10:58 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Components & Cladding for Wood Roof Trusses

Wow, I had to do a double take because I typed a message with almost this
exact title and then didn't send it to the group.

That's how I specify.

Here was what I started to post...I came across some shop drawings that said
the trusses were designed as MWFRS.  I questioned the supplier and he said
that he could get a bunch of PE's to say that MWFRS is the way they should
be designed.  I asked if these people work for the truss industry.  The
answer was yes.  This is nuts, evidently all trusses designed using software
and plates made by Robbins Engineering are designed as MWFRS.  Be on the
lookout for this.

John C. Jones, PE
Barnett Associates
Pell City, AL
205-884-5334
205-884-0099 (fax)


-----Original Message-----
From: Kestner, James W. [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Wednesday, April 14, 2004 9:33 AM
To: SEAINT
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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