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Re: lateral drift

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Roger Turk wrote:
> 
> Bruno Côté wrote:
> 
> . > 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.
> 
> The deflection criteria for floors and roofs has nothing to do with large or
> small deformation theory.  The limit of L/360 for live load is the amount of
> deflection such that plaster (or other brittle finishes) probably will not
> crack.  The limit of L/240 for dead + live loads is the amount of deflection
> that probably will not be visually discernable.
> 
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
> 


Thanks for your comment Roger A. Turk. Actually I agree with you.

I am afraid that you misunderstood my post, then took one paragraph out
of context and pasted it on to your e-mail. What I really said was:

"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."


We both agree that deflection criteria has nothing to do with small or
large deflection theory.

I mentioned that the serviceability criteria was for the comfort of
users of the building. I am unsure if you agree with that. Believe it or
not, the engineering consultant I used to work for designed their own
building (before I was hired of course) and it had a springing floor.
This was slightly unpleasant. I also wouldn't want to work in a building
which has to much lateral drift.

You mentioned that the deflection criteria was also to ensure that
plaster finishes do not crack. I agree with you. As a matter of fact,
this is the most important reason why the deflection criteria should not
be exceeded. I overlooked that when writing my post.

You also mentioned the total load deflection criteria. I can see
perhaps, this is the source of the confusion. Where I come from, there
is no total load deflection criteria (unless they changed the building
code recently - after all, what do I know?, I'm just a bridge designer).

Another thing I overlooked when writing my original post in response to
Mark Pemberton's inquiry, was that whenever I exceeded the .005 H
deflection criteria, I considered the P-delta effect on the column. On
some miscellaneous structures, I did allow lateral drift up to .01 H. 

Bruno Côté
BOCTE(--nospam--at)ibm.net