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RE: Wood continuous beams

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Gerard,
 
We always brace these slender beams at 24" o.c. (at each rafter) in the
region of high negative moment.  What is frustrating is that using the
NDS equations in section 3.3, for the beam in question, the stability
factor (C sub L) still only comes out to 0.88, even though we're bracing
it at every rafter.  So I'm scratching my head, wondering whether to
consider it "full lateral support" and let C sub L = 1.0, or look to
some either means.  We had considered 24" oc bracing to be "full lateral
support" in the past, but I don't know if that conclusion complies with
NDS.  Add to this the fact that NDS Table 3.3.3 doesn't even address
continuous beams in determining effective length.  I use the formula for
uniformly distributed load on a cantilever.
 
Thus my question as to whether some degree of moment redistribution can
be rational for continuous wood beams.  I would theorize that for the
dead load portion some would occur over time, given creep.  But again,
if DL only consumes 25% of the maximum load, there might not be any DL
creep, since the stresses are to low.  I'm wondering if anybody has seen
any commentary or research on this.  I'll probably copy this inquiry to
the LVL manufacturer, Trus Joist.
 
This may all sound a little like overkill - you might be thinking - oh
just upsize the thing and forget it, but I actually do one of these
beams almost every day, and because of product availability and
construction practice there is a need to tweak the beam to its limit.
 
Ed
 
 
*	To: < seaint(--nospam--at)seaint.org <mailto:seaint(--nospam--at)seaint.org> > 

*	Subject: RE: Wood continuous beams 

*	From: "Gerard Madden, SE" < gmadden(--nospam--at)maddengine.com
<mailto:gmadden(--nospam--at)maddengine.com> >
 
For something extremely slender like that, it is critical that you brace
the bottom of the beam near the supports where negative moment occurs
(compression on the bottom). 

Otherwise, it's just a continuous beam design.

Hth
-gerard
Lodi, CA

-----Original Message-----
From: Ed Tornberg [ mailto:edt(--nospam--at)blazerind.com <mailto:edt(--nospam--at)blazerind.com>
] 
Sent: Thursday, October 09, 2003 9:43 AM
To: seaint(--nospam--at)seaint.org
Subject: Wood continuous beams

Being aware that it's standard practice to apply moment redistribution
to continuous concrete beams,  is there any information out there on the
possibility of applying moment redistribution to continuous wood beams?
I'm not pretending to think that wood forms plastic hinges like
concrete, but I'm wondering if the creep characteristics of wood can
produce a similar, although lesser, effect.  The member in question is a
24" x 1.5" LVL.


Ed Tornberg, PE 
Blazer Industries, Inc. 

Aumsville, OR
503-749-1900 

Ed Tornberg, PE 
Blazer Industries, Inc. 

Aumsville, OR
503-749-1900 

 

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