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Re: lateral drift

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Mark & Jessica Pemberton wrote:
> 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.

I am unsure what a trellis is but I believe I can answer your question

The allowable lateral drift is a serviceability requirement to ensure
comfort of the users of the building. A similar criteria exists for
vertical deflection of building floors. 

The allowable lateral drift is not the maximum deflection for which the
theory of small deformation is valid. This limit would be a lot more
than .005H .

 One way to be convinced of that is the fact that deflection of floors
is evaluated only under live loads. If there were any worries that
limitations of small deformation theory could be exceeded, the total
deflection (dead load + live load) would have to be considered.

If the users of your structure could live with a larger deflection, than
the lateral drift requirement does not apply. Most likely, if the
structure you are designing is a building, you do not want to exceed the
specified drift.

I am not aware of California regulations for earthquake. My
understanding of serviceability requirements such as horizontal and
vertical deflections is that they should be evaluated under service
loads. I would not think that earthquake would fall into this category
unless your earthquake code is so elaborate that the requirements are in
a form similar to:

   a) small earthquake  	: small deflection
   b) medium earthquake		: no member failures
   c) major earthquake		: no collapse

Hope this is of any help.

Bruno Côté