Live load reduction in one way slab
Live load reduction in one way slab
(OP)
I am designing a one way slab with a steel deck spanning over several floor beams in a typical bay (35'x35'). Beams are spaced at 7'. I would like to take advantage of a live load reduction, but I am having difficulty trying to interpret what the tributary area should be for use in the reduction equation.
For slab design, we usually design for a 1' wide strip and assume it is continuous over multiple supports. My first thought is that the trib area would be 1' x 7' (assume 3-span condition).
But there is a note that says the trib area for one way slabs shall not exceed an area defined by the slab span times a width normal to the slab span of 1.5 times the slab span.
So for my case, would it simply be 7' * (1.5*7') = 73.5 sq.ft.
If that's the case, KLL * trib area < 400 sq.ft, and I cannot take advantage of the reduction.
Is my interpretation correct?
For slab design, we usually design for a 1' wide strip and assume it is continuous over multiple supports. My first thought is that the trib area would be 1' x 7' (assume 3-span condition).
But there is a note that says the trib area for one way slabs shall not exceed an area defined by the slab span times a width normal to the slab span of 1.5 times the slab span.
So for my case, would it simply be 7' * (1.5*7') = 73.5 sq.ft.
If that's the case, KLL * trib area < 400 sq.ft, and I cannot take advantage of the reduction.
Is my interpretation correct?






RE: Live load reduction in one way slab
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Live load reduction in one way slab
RE: Live load reduction in one way slab
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Live load reduction in one way slab
I like to think about it in terms of joists.... beams... columns... the likely hood that a joist will at some point have the full live load of its tributary area is pretty likely(small trib area). The likelihood that a beam carrying say 10joists would have all the tributary area live loaded is less likely.... and the likelihood that a column carrying four beams will the whole tributary live loaded isn't very likely which results in it getting the greatest reduction.
But anytime you are designing something on a per foot basis I would be very careful on reducing the live load.
RE: Live load reduction in one way slab
But to think this way is to deny one way slabs of their intrinsic ability to distribute loads laterally. I also do not often use live load reduction for slabs. My reasoning is simply that it rarely ever seems to amount to anything much.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Live load reduction in one way slab
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Live load reduction in one way slab
RE: Live load reduction in one way slab
@DETstru and Antnyt23, Honestly I have never used LL reduction for a one way slab either. I was attempting to reach into my bag of tricks to get around shear issues with the slab. I cannot increase the thickness so I also thought about using the deck to resist a portion of the shear. But after thinking about it, I would have to distribute the shear proportionally according to the stiffness of the slab and the deck and I don't think it would buy me that much.
RE: Live load reduction in one way slab
Not so. It's straight Vs + Vc just like concrete beams with stirrups.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Live load reduction in one way slab
RE: Live load reduction in one way slab
Is the deck galvanized? If so, you can consider the weight of the concrete to be permanently carried by the deck. That would draw a chunk of shear away from the concrete. I'm determined to find a way to be useful here damn it!
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Live load reduction in one way slab
As someone who has experienced the adverse effects of an unknown LL reduction used in the design of a floor slab, I would NEVER advocate taking a LL reduction for the design of a one-way slab.
I can justify a LL reduction for girders and columns and perhaps some beams, but never for a slab.
Ralph
Structures Consulting
Northeast USA
RE: Live load reduction in one way slab
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Live load reduction in one way slab
Is it really 100 psf? Might be 80 psf, might be 120 psf.
@KootK, no temporary shoring. Contractors cringe when they hear temporary shoring.
RE: Live load reduction in one way slab
MotorCity - I HAVE seen a floor slab loaded with 100% of its design loads. It occurs in multistory concrete construction at the worst possible time in the life of the structure - during construction. This happens all of the time in multistory concrete construction. The worst nightmare I have ever seen had to do with a 2-way flat plate (30' x 30' bays) where the EoR took a LL reduction on the slab (it was after all carrying a 900 SF area). The stated design LL of 100 PSF was actually 57 PSF after reduction. Consequently the shoring load from the floor above, combined with the reshoring installed below, was not adequate to carry the imposed construction load. The resulting floor deflections over time were in excess of 3".
Your situation may not be a multi-story structure involving shoring & reshoring. But I would submit that the current and future owners could easily impose a live load in excess of the reduced live load used in the design of the slab. It may not occur over the entire floor, but could easily occur over some portion of the floor (& 100% of the slab's span).
Just my NTBHO.
Ralph
Structures Consulting
Northeast USA
RE: Live load reduction in one way slab
RE: Live load reduction in one way slab
If you really want to sharpen your pencil, I imagine you can consider only the clear span between the beams. That saves you a flange width's worth of load. Then check the shear at d from the edge of the flange.
Maybe spec higher strength concrete?
RE: Live load reduction in one way slab
pmt
RE: Live load reduction in one way slab
RE: Live load reduction in one way slab