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Re: lateral drift[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: Re: lateral drift
- From: gngolde(--nospam--at)ibm.net
- Date: Sun, 22 Mar 1998 14:12:39 -0500
Mark, To the best of my knowledge 0.005h is a floor drift requirement from the code (if you are in CA then probably UBC). The requirement you are quoting comes from seismic portion of UBC (I am using UBC as an example) in 1994 edition it is 1628.8. By the way there are couple of other requirements in there. However it clearly excludes a structure as a trellis (I assume you designed it not for hospital beds) where life safety is not an issue. It would appear to me that you are free to make your own assumptions what lateral deflections (sway) you want to allow. I would establish them on the basis of wind loads and any long term live load lateral deflection you may get from unbalanced loads. Then I would include them in determining my secondary moments (P-Delta) and design the structure based on strength requirements. You may want to consider moment connections between your columns and horizontal members to see if that helps ( if nothing else it will shorten your column design length). Without knowing the geometry I don't want to guess. (I am also representing a program STRAP. It is the only program that I know that will allow you to specify your lateral deflection and would automatically redesign your trellis for minimum weight of material for the deflection you specified. You can specify the desired deflection in any or all directions which is helpful if you designed this as a 3D structure). Regards, Garrick Goldenberg, P.E. ATIR Engineering Software 24 Fairfax Road Needham, MA 02192 Tel. 1.800.644.6441 (for int'l calls 781.444.9944) Fax 781.444.3464 email: garrick(--nospam--at)atir.com For the latest news about STRAP, check our web site http://www.atir.com/whatnew.html -----Original Message----- From: Mark & Jessica Pemberton <jmpember(--nospam--at)softcom.net> To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org> Date: Saturday, March 21, 1998 12:48 AM Subject: lateral drift I recently designed a trellis using a cantilevered column lateral resisting system. I seems that deflection always controls on these type structures when trying to abide by allowable lateral drift. (0.005h) First, is this what should be used for wind as well as seismic lateral drift? Secondly, because the low stresses and the type of structure involved is it acceptable to allow much more than this? At what point does the amount of deflection render the predicted stresses in substantial error (with formulas using the assumption of small deflections)? I have seen steel building manufacturers use allowable drifts as much as L/100. Where do they get that? (by the way the trellis involved utilized tube steel - OK HSS to be precise - and the wall thickness would be 1/4" by stress/column analysis and nearly 1/2" to meet the 0.005h requirement) Mark Pemberton, P.E.
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