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- Subject: Fwd: Re: Lateral Stability of Wood Beams
- From: rlewis(--nospam--at)techteam.org (Richard Lewis)
- Date: 30 Jul 1997 14:48:40 GMT
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 compression....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 Regards,=20 Joe McCormick Pillar Consulting Group, LLC Corvallis, Oregon jmccormi(--nospam--at)dnc.net --- Internet Message Header Follows --- Received: from server1.seaoc.org (bqe.com [184.108.40.206]) by host1.texramp.net (8.8.5/8.8.5) with SMTP id AAA09649 for <rlewis(--nospam--at)techteam.org>; Sat, 1 Mar 1997 00:50:28 -0600 (CST) Received: from peabody.dnc.net by server1.seaoc.org (NTList 3.02.10) id va005013; Fri, 28 Feb 1997 22:38:40 -0800 Received: from e-z.net.e-z.net ([220.127.116.11]) by peabody.dnc.net (Netscape Mail Server v1.1) with SMTP id AAA237 for <seaoc(--nospam--at)seaoc.org>; Fri, 28 Feb 1997 21:04:25 -0800 Received: by e-z.net.e-z.net with Microsoft Mail id <01BC25BC.3519E4A0(--nospam--at)e-z.net.e-z.net>; Fri, 28 Feb 1997 21:13:13 -0800 Message-ID: <01BC25BC.3519E4A0(--nospam--at)e-z.net.e-z.net> From: jmccormi(--nospam--at)dnc.net (Joe McCormick) To: "'seaoc(--nospam--at)seaoc.org'" <seaoc(--nospam--at)seaoc.org> Subject: RE: Lateral Stability of Wood Beams Date: Fri, 28 Feb 1997 21:13:03 -0800 MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Reply-To: seaoc(--nospam--at)seaoc.org Error-To: seaoc-ad(--nospam--at)seaoc.org X-Loop: seaoc(--nospam--at)seaoc.org X-Info: [SEAOC] Owner: seaoc-ad(--nospam--at)seaoc.org X-POP3-Rcpt: seaoc-ad(--nospam--at)seaoc.org X-Sender: seaoc-ad(--nospam--at)seaoc.org Precedence: list X-ListMember: rlewis(--nospam--at)techteam.org [seaoc(--nospam--at)seaoc.org] __________________________________________________ Richard Lewis, P.E. Missionary TECH Team rlewis(--nospam--at)techteam.org The service mission like-minded Christian organizations may turn to for technical assistance and know-how.
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