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RE: Live Load Reduction for One-Way Slabs

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I was just discussing this with another engineer recently, and we both
agreed that one-way reduction is not done (or at least very very rarely).  I
have not known anyone to do it and have generally heard people opine that it
is forbidden.  As with you, I have not found anything in the UBC
specifically stating this, but I suspect it is more on the assumption that
no one would do it rather than a desire to imply it is acceptable.  Taking a
point at midspan, I suppose that you could assume the load can distribute
itself out at a 45 degree angle as the stress goes towards the supports.  In
this manner, you could "activate" an area as wide as 1/2 the span length to
resist your load.  Of course, this mainly works for reinforcements at the
supports, and is somewhat harder to conveniently rationalize for
reinforcement at midspan (though it can be done).  Also, this load
distribution is dependent on the point load being at midspan, and doesn't
work if the point is close to the support (very little distance for the load
to spread out.  Further, as spans get longer (which is necessary to get a
decent trib to be reducible), the basic load path assumptions get a little
more questionable.  You need at least a 17'-4" span to get to 150 ft^2 (the
threshhold for reduction) based on a span * 1/2 span dimension approach.
So, even if you can convince yourself that you can activate more than a
narrow strip of the slab, it wouldn't be worth it in most cases (a 20' span
gets you 4% reduction by that logic).  I will be interested to see if anyone
writes back saying they do reduce in that manner, as I don't know of anyone
who does.  

Paul Crocker


-----Original Message-----
From: Brian Harris [mailto:BrianH(--nospam--at)cplinc.com]
Sent: Tuesday, September 19, 2000 5:20 PM
To: SEAINT List Server (E-mail)
Subject: Live Load Reduction for One-Way Slabs


Does anyone have a methodology for determining tributary or influence area
under 1997 UBC in order to compute live load reduction for the design of a
continuous one-way concrete slab?  I have never reduced live load for
one-way slab design in the past, but am told that some engineers do.  I know
the BOCA building code and ASCE 7 specifically say that live load shall not
be reduced for one-way slabs, but the UBC and 2000 IBC do not address it
specifically (although they do reference ASCE 7.)

In any case, has anyone done this sort of reduction, and if so how?

Brian J. Harris
brianh(--nospam--at)cplinc.com


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