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Fwd: Re: Lateral Stability of Wood Beams

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on 2/28 Roger Turk wrote:

<<<<<<<<  Is there a problem with the lateral stability requirements for
unbraced wood beams in the 1991 NDS?
A while back, I was designing a wood beam that had what I considered a
rather large unbraced length (it was a beam supporting an exterior
bearing wall at a stairwell) and was getting very small reductions in
allowable stresses using the provisions of Section 3.3.3, so I decided
to check R(B) (Eq.  3.3-5) to see what would an effective length be
for R(B) =3D 50.  Assuming a 2 X 12, it would have to be *42'-8"* long
before R(B) would be equal to 50!  >>>>>>>

For the case of the 2 X 12:  R(B) would be much higher than 50 for a =
42'-8" long, laterally unsupported member:  The effective length of the =
member should be used in equation 3-3-5 of the 91 NDS, which can be =
obtained using Table 3.3.3.  Assuming the 2x12 is uniformly loaded AND =
laterally unsupported throughout, l(e)=3D1.63*(512") + 3*(11.25") =3D =
868" and R(B) =3D 65.9 which exceeds the maximum slenderness ratio of =
50.  The maximum allowable effective length of a 2x12 is 500", which =
relates to a maximum laterally unsupported span of 286" or 23'-10" for =
uniform loads (and the reduction in strength would be severe for such a =
member since R(B)=3D50!!!).  The key is to use "effective lengths" when =
calculating R(B), which depend upon both the end conditions of the beam =
and the type of loading.  All said though, it is NOT suprising to see =
only minor effects due to lateral stability in wood beams (although I =
almost always check for them).  Breyer offers an interesting insight =
concerning lateral stability of wood beams compared to standard steel =
structural shapes:
	"In the case of laterally unsupported steel beams (W shapes), the
	problem of lateral stability is amplified because cross-sectional
	dimensions are such that relatively slender elements are stressed
	in the case of rectangular wood beams, the
	dimensions of the cross section are such that the depth-to-thickness
	ratios are relatively small.  Common framing conditions and cross-
	sectional dimensions cause large reductions in allowable bending
	stresses to be the exception rather than the rule."=20


Joe McCormick
Pillar Consulting Group, LLC
Corvallis, Oregon

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From: jmccormi(--nospam--at) (Joe McCormick)
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Subject: RE: Lateral Stability of Wood Beams
Date: Fri, 28 Feb 1997 21:13:03 -0800
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