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

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I am just in the process of putting the final touches on a two-way conventially reinforced 8-story structure. I did a finite element anlysis for slab and beam design and did use 75% of the live load for pattern loads. Note the ACI code allows the 75% under two-way slabs and under the section called equivalent frame method. ACI does not address under direct design method or a finite element analysis. Any comments?

It appears the 75% of live load is only for two-way slab construction. Could this reduction be because of the structural characteristics of this type of construction. Is it not correct to, take advatage of this reduction of live load with two-way slabs, and always check pattern loading even if the live load is less than 75% of the dead load since most buildings have unusual conditions such as unequal spans, varing live load, etc.

Consider the safety factor 1.275 live load and 1.4 dead load. Even if pattern loads were 100% live load the design would still work.

IBC 1901.2 states that we are to design per ACI except as modified in 1908. 1908 does not modify the two-way live load factor allowed in ACI.
I do not see a discrepency as far as two-way slab constuction goes.

James





From: Cliff Schwinger <clifford234(--nospam--at)comcast.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Question on need to use "skip" live loads in 2-way slabs
Date: Wed, 23 Oct 2002 23:13:35 -0400


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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