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Re: lateral drift

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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.