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RE: Question on need to use "skip" live loads in 2-way slabs

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If you are having trouble with punching shear, it is worth remembering that it isn't possible to skip load both directions simultaneously in a two-way slab.  Skip loading in one direction implies that only part of the tributary area in the opposite direction is loaded.  I have had to remind myself of this a few times when I check punching shear for the maximum unbalanced LL moment in both direction at the same time, which isn't a case that can happen. 
 
Paul Crocker, PE

-----Original Message-----
From: GSKWY(--nospam--at)aol.com [mailto:GSKWY(--nospam--at)aol.com]
Sent: Thursday, October 24, 2002 7:11 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Question on need to use "skip" live loads in 2-way slabs

Cliff,

Out of curiosity,  what is the difference in the reinforcement required when you skip live loads versus when you don't skip live loads- i.e. what is the difference in the the post-tensioning required and what is the difference in the additional mild steel required for ultimate moment?  What is the difference in the shear calculations?

Another engineer once came to me in a panic  because at the end of designing a 24 story apartment building,  they realized they had forgot to check "yes" on the "skip live load" question in ADAPT.  

We went through and identified what were likely to be the critical runs and re-did them - the only difference was in a few places,  the mild steel required for ultimate moment increased slightly.  In no place, did it increase above what was already being provided (since bars come in finite sizes,  you are generally always using a little more than you need).

This may not be the case for your design but I would be interested in knowing the answer.

Gail S. Kelley, P.E.