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

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Cliff,

I _might_ know why, but haven't done the math to prove it.  Sit down and
do some calculations.  Do one continuous beam analysis with DL and
LL<=75%DL on each span.  Then do one continuous beam analysis with DL on
each span and LL<=75%DL on every other span.  Then do some other
combinations of LL patterns if you want.  My guess is that none of the
cases with LL patterns will create larger maximum moments or shears than
the case with LL on every span.  In otherwords, my guess is that when the
LL is 75% or less of the DL, it becomes impossible for the "skip" patterns
to "beat out" the case with LL on every span.  Thus, there would be no
reason to even spend the time analyzing the various cases since they will
not govern.

Since I have not sat down and proven this to myself with some
calculations, I could be wrong.

HTH,

Scott
Ypsilanti, MI


On Wed, 23 Oct 2002, Cliff Schwinger wrote:

>
> I would like to get some opinions on whether "skip" live loads always
> have to be used when designing reinforced two-way floor slabs. (By
> "skip" live loads, I'm referring to determining "worse case"
> arrangements of live loads for computation of moments and shears in
> structural systems with continuous span floor framing members.)
>
> Section 1607.10 in IBC 2000 says (to paraphrase) that when designing
> continuous floor framing members, worse case live load patterns have to
> be used for determination of design moments, shears, etc.
>
> Section 1607.2 in the 1997 UBC Code says essentially the same thing as
> is stated in IBC 2000 - arrange live loads in a pattern as necessary to
> determine maximum shears and moments.
>
> The 1996 BOCA Code and ASCE 7-98 (don't know which sections because I
> left my codes in work) have similar requirements for figuring pattern
> live loads to determine maximum shears and moments.
>
> Section 8.9.2 in ACI 318-02 says (essentially) the same thing as the
> above publications.
>
> All five publications are in agreement with each other on this point
> UNTIL..
>
> Section 13.7.6.2 in  ACI 318-02 says (to paraphrase) that if you are
> designing a two-way slab and the LL is less than 0.75 x DL, then you may
> assume that all bays are loaded simultaneously and ignore the various
> pattern load arrangements for determination of design moments and
> shears.  The ACI Commentary gives a logical explanation to justify the
> Section 13.7.6.2 allowance for ignoring "skip" live loads when live
> loads are less than 0.75 x DL; however this allowance for ignoring
> "skip" live loads seems to fly in the face of the "ironclad" rules in
> the various codes that require "skip" live loading to always be
> considered when designing continuous floor framing members.
>
> What am I missing?  There is no doubt that the many knowledgeable
> engineers on ACI 318 are aware of the "skip" live load requirements in
> the building codes - indeed they make reference to the need to review
> "skip" live loads in section 8.9.2.
>
> My understanding about two-way slabs is that (as far as flexural
> strength goes) they are a very redundant and ductile structural system.
> I am assuming that the "permission" granted in Section 13.7.6.2 was in
> recognition of this ductility.  The dilema however is that Section
> 13.7.6.2 flies in the face of the "thou shalt always checketh skip live
> loads" commandment in all the building codes.
>
> The reason I'm asking for opinions on this matter is that the "skip"
> live load requirement imposes a serious design penalty when designing
> two-way post-tensioned flat plate floor slabs for hotel structures.  75%
> of the dead load of a 7 1/2" floor slab (with 15 psf of partitions) is
> 82 psf.  My reduced live load on a guestroom floor is usually about 35
> psf - less than half the maximum permitted in order to ignore skipping
> the live load!  According to ACI 318-02 I'm fully justified ignoring
> "skipped" live loads - but how do I answer someone who points to the all
> the building codes and says to me, "SEE!  I told you you had to "skip"
> the live load!"
>
> What makes matters worse is that IBC Section 102.4 says that when there
> is disagreement between IBC and one of the referenced codes or standards
> (in this case ACI 318-02), IBC governs.
>
> Thank you to those out there who can share some wisdom on this topic.
>
> Clifford Schwinger P.E.
>
>
>
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