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RE: Unbraced Length of Cantilever
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaint(--nospam--at)seaint.org
- Subject: RE: Unbraced Length of Cantilever
- From: "Effland, Greg" <geeffland(--nospam--at)butlermfg.com>
- Date: Fri, 8 Dec 2000 11:15:31 -0600
Another good place for information regarding this issue is "Steel Structures: Design and Behavior" by Salmon and Johnson... My version is the 3rd Edition but I would assume others, particularly the earlier editions would have a good commentary on the ASD procedures. The 3rd or newer editions should also have LRFD commentary... Assuming that you are using ASD here is my interpretation (LRFD should be similar): For the cantilever beam Kx should be 2.10 per table C-C2.1 in the Chapter C Commentary ASD 9th Ed. (Recommended design value when ideal conditions are approximated)... Assume free end is free to rotate. For Ky use 1.2... this depends on the restraint of the beam and assumes that it is not free to rotate at the free end (laterally)... could possibly be 2.1 or 1.0 also (depends on bracing conditions, etc.) Kx and Ky only affect the axial allowables and since you mentioned no axial load then these won't matter. As far as flexural bending or lateral-torsional buckling, members whose unbraced length exceed Lc are those subject to lateral-torsional buckling problems. ASD Section F1.3 considers the lateral-torsional buckling when determining the bending allowables from equations F1-6, F1-7, and F1-8. Something else that might be of interest is that if the cantilever section (or any flexural section) is doubly symmetric then you are allowed to use rt(equiv) versus rt. Refer to ASD Commentary Section F1 page 5-146 for further information regarding rt(equiv). For your case, for gravity loads I would use a Lb = maximum distance of torsional restraint on the bottom flange; for uplift I would use Lb = maximum distance between torsional restraints of the top flange (purlins, etc.). Dr. Joseph Yura, Dr. James Fisher, and Dr. LeRoy Lutz each have several good papers on what is a good restraint for these conditions. For Ly I would also conside only points which restrain both flanges or at least a large part of the beam as a whole, but again with no axial this won't make a difference. Hope this helps, Greg Effland, P.E. -----Original Message----- From: Rick.Drake(--nospam--at)Fluor.com [mailto:Rick.Drake(--nospam--at)Fluor.com] Sent: Thursday, December 07, 2000 8:42 AM To: seaint(--nospam--at)seaint.org Subject: Re: Unbraced Length of Cantilever For a good discussion on effective-length factors for cantilever steel beams see the SSRC's Stability Design Criteria For Metal Structures, 5th Edition, Section 5.2.4 and Figure 5.11. The effective length factor for cantilevers is a function of the lateral bracing at the support, the lateral bracing at the end, and which flange is loaded. Most of the effective length factors are greater than one. For the condition you described, full lateral bracing at support, top flange bracing at the end, and top flange loading, the effective length is 1.4 times the cantilever length. Rick Drake, SE Fluor Daniel, Aliso Viejo ********************** Ricky Leblanc <Rick.Leblanc(--nospam--at)Halliburton.com> on 12/07/2000 05:36:43 AM Please respond to seaint(--nospam--at)seaint.org To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org> cc: Subject: Unbraced Length of Cantilever Here is the situation. Cantilever beam (W36 shape) laterally braced at two points on the top flange (free end and midspan). These two points offer bracing against lateral displacement only (no torsional restraint). At the fixed end there is full restraint about all bending axes (including torsional). Full shear restraint as well. Under gravity load the top flange is the tension flange and bottom flange is compression flange (i.e, the lateral restraint is at tension flange only). Major axis bending only, no axial load. Is the unbraced length of the compression flange the full cantilevered span, or is it something more? Thanks, Rick LeBlanc, PE Kellog, Brown & Root Houston, TX ---------------------------------------------------------------------------- ------------------------- The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential and/or privileged material. If you are not the intended recipient of this message you are hereby notified that any use, review, retransmission, dissemination, distribution, reproduction or any action taken in reliance upon this message is prohibited. If you received this in error, please contact the sender and delete the material from any computer. Any views expressed in this message are those of the individual sender and may not necessarily reflect the views of the company. ---------------------------------------------------------------------------- ---------------------------
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