Live load reduction in one way slab 1

[MotorCity]

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?

 

[KootK]

I would interpret it the same as you have with K = 2.0. KLL x TA = 147 sf.

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.

 

[MotorCity]

Not that it matters in my case since I cannot reduce the live load, but why use KLL = 2.0? The table I am looking lists KLL = 1.0 for one way slabs.

 

[KootK]

I may have led you astray originally MotorCity. Originally, I was thinking that the difference between the influence area and the tributary area ought to be 2.0. That's true but seems to have already been incorporated into ASCE7's table-o-K values.

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.

 

[Antnyt23]

I wouldn't use a live load reduction on the one way slab. If you think about the reasoning for a live load reduction - reducing the load due to the whole tributary area not being loaded at once thus why the elements carry greater tributary areas get a great reduction. The one way slab which you are designing on a by foot basis should be subject to the full live load because it is likely that that live load could actually occur over the area you are considering. Plus your tributary area does not meet the requirements to reduce it.

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.

 

[KootK]

I like to think about it in terms of joists.... beams... columns... t

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.

 

[KootK]

@MotorCity: scrap all of my previous comments. I got turned around and was thinking of something altogether different from the situation at hand. My bad.

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.

 

[DETstru]

@MotorCity: They're essentially precluding you from using live load reduction in one way slab design and as Antnyt23 says, I wouldn't want to anyway. By the code, you need a +/- 16' span before you hit the 400 SF minimum, which is rare unless you're using hollow core plank (which ASCE 7 considers as a one-way slab for LLR purposes).

 

[MotorCity]

@KootK No problem, thanks for the input

@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.

 

[KootK]

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.

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.

 

[MotorCity]

@KootK, I would agree if it were composite deck, then the Vs + Vc makes sense as the two materials would act compositely together like concrete and stirrups. But in my case, its just plain deck and there is no guarantee that the two materials will act in unison to resist the shear. Its like placing a concrete beam on top of a "metal deck beam". I am thinking that the slab would have to deflect or crack in order for the deck to "see" any shear. I would like to use the deck as you suggest, as it would help may case. Maybe I just need a little more convincing.

 

[KootK]

Nah, I was thinking composite deck. While I suspect that the deck would help much the same for non-composite, I wouldn't risk that without some kind of credible reference to point to.

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.

 

[RHTPE]

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

 

[KootK]

Although, at 7', it might be a temporarily shored system which would change things.

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.

 

[MotorCity]

@RHTPE, I agree that its generally not a good idea for a slab. But....in keeping with the idea of LL reduction, have you ever seen 100% of the floors with 100% of the slabs loaded with 100% of the LL at the same time? If it worked out, I may be inclined to take a small reduction given the inherent uncertainty of the live load. For example, say the LL = 100 psf.
Is it really 100 psf? Might be 80 psf, might be 120 psf.

@KootK, no temporary shoring. Contractors cringe when they hear temporary shoring.

 

[RHTPE]

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

 

[MotorCity]

@RHTPE, Thanks for sharing your experience....scary. With the flat plate, there is certainly less redundancy compared to my beam and slab system, but even so I would not be comfortable cutting the LL almost in half.

 

[DETstru]

@MotorCity: how close are you to getting the shear to work?

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?

 

[pmtrevisan1]

per NBCC 2010 commentaries, section F LL reduction not permitted for one-way slabs due to the uncertainty of the degree of lateral distribution of loads.

pmt

 

[MotorCity]

I am within about 15%, usually if I am within 5% or less I will let it slide. I think if I spec a higher concrete AND use the edge of flange idea it will work.