Concrete Podium Design Live Load Reduction 2

[DavitBek]

Hi All

Long time follower of the forum, and first time poster, as usually I find the answer to the question I have from posts in the past

I was looking to understand the acceptable procedure for reducing the live load on a concrete podium from wooden stories above. Specifically, a 6 story mixed use structure, a two story two way concrete podium (one underground garage, one commercial above), and four wooden stories residential sitting on top of the commercial level.

I am using a two stage analysis, designing the wooden stories separately, and superimposing the loads on a concrete podium.

Using that procedure, summing the forces in four wooden stories I would have a total of 180 lb/sqft (4x40lb/sqft+20lb/sqft) sitting on the concrete podium. And specifically 160 lb/sqft floor live load.

As you likely know, the ASCE 7-10 (Chapter 4.7) prescribed live load reduction is

L=Lo(.25+15/((KLL*AT)^.5))

L=reduced live load
Lo=unreduced live load
KLL=live load element factor (KLL=1 in the case of a slab)
AT=tributary area

(Max reduction allowed is L=0.4Lo)

I'm looking to reduce this 160lb/sqft floor live load in my design of the concrete podium, as all four stories will never be packed to capacity.

Specifically, what I am not sure of is what AT value to use.

Both in my office and on the forum I've found a lot of conflicting answers ranging from using the entire footprint of the wooden stories(16,000 sqft in this case), the area between four columns supporting the slab multiplied by the number of stories (30ft*30ft*4=3,600 sqft), and the area between four columns supporting the slab (30ft*30ft=900 sqft).

Needless to say, those three values produce wildly different reduction factors

16,000 sqft: L=.37Lo=.4Lo
3,600 sqft: L=.50Lo
900 sqft: L=.75Lo

In all past projects of this nature I have used a 25% reduction to be on the safe side, however it would nice to get a firm answer to this question and to know whether I am over designing, and if I would be okay using a 40% reduction factor recommended by the owner of a company- a very experienced and respected engineer.

 

[CURVEB]

Yes - that was the issue. 4 floors, with a 60x60 bay per floor. Got it.

 

[BAretired]

I was not aware of the current definition of influence area, i.e. KLL*AT. It would appear that, for the design of a column, KLL = 4. Therefore, I accept KootK's conclusions with regard to design of a column.

However, the OP was concerned about the design of the podium slab. For a two way slab, KLL =1, so Influence Area = Tributary Area. My response to the OP's question remains as before, KLL*AT = 1*4*30*30 = 3600 SF.

BA

 

[BA55]

I do not have the latest National Building Code of Canada. Is treatment of Live Load Reduction similar to that of ASCE?

 

[calvinandhobbes10]

Good question. It sounds like you may already have plenty of experience with podiums, but i love talking two-way slabs, so heres a few of my thoughts:

I agree uniform load method as a start point. Different LL reduction factors for different failure modes:

Slab positive bending, ~0.61L0. span=28.5’(span minus column size), width=30’ KLL=2. Use same factor for column and middle strip, since they both draw from M0 per ACI. This would also apply to slab one-way shear which is almost certainly non-controlling.

Slab negative bending, 0.50L0. 30’x30’, KLL=4. All four bays contribute to the slab panel negative bending. Also use same factor of CS/MS.

Slab punching, same as negative bending, 0.50L0.

Some may argue that 0.50L0 should apply to all failure modes, but pattern live load allows a single span to receive all live while adjacent receive none, which intensifies positive bending, so i think this is a reasonable (and code-intended) approach. You cant get this same thing with negative bending.

If you have widely spaced wood bearing walls you might consider modeling line loads on your transfer slab to cross-check moment/shear demands against the uniform-load assumption. This is REALLY important if you are thinning your rebar to the brink, which i strongly discourage for podium/transfer slabs designed only with uniform load method, no wall line loads.

Of course, dont forget to check longterm deflection in your transfer slab too. Often controls thickness in my experience. And watch out for pipe sleeves around your columns.

Also, my general flat slab advice (would be curious who disagrees):
Thickness/drops=expensive
Shearheads=Expensive
Rebar tonnage=Cheap
Unless this fine-tuned reduction in live load shaves slab thickness (unlikely due to punching/LT) or omits/avoids shearheads (it well may), I think the savings is just some rebar.
Slabs are tension-controlled by a longshot. If you try to save every rebar you can, be prepared for a construction change or a new floor opening or something else unforeseen to push a demand over the limit.

Good luck!

 

[KootK]

I do not have the latest National Building Code of Canada. Is treatment of Live Load Reduction similar to that of ASCE?

It is similar in many respects but not the one being debated here. NBCC live load reduction IS indexed to tributary area rather than influence area which, admittedly, does make it a fair bit easier to apply. For kicks, I set up a comparison of the methods, shown below. The Canadian method seems to be much more conservative and would effectively align with the US method for KLL = 0.35. I've frankly forgotten what I'd intended to prove with this but, since it's already a bird in the bush, I figure that it wouldn't hurt to share. Maybe it's flawed and someone can tell me how. I probably coulda just some to the same conclusion via algebra.

At KLL = 1.0, NBCC/ASCE ~1.32
At KLL = 2.0, NBCC/ASCE ~1.53
At KLL = 4.0, NBCC/ASCE ~1.70

The scatter is greater at higher KLL values.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[KootK]

lab negative bending, 0.50L0. 30’x30’, KLL=4. All four bays contribute to the slab panel negative bending. Also use same factor of CS/MS.

Now that is interesting. I'd fully expected to be the most liberal voice in this conversation but now you've gone and out done me. I see the logic in KLL = 4 for negative bending however. Moreover, that could be used to real practical advantage as it is often negative moment steel that creates congestion issues.

The ASCE commentary defines the influence area as that over which applied load will appreciably affect the demand parameter being considered (paraphrase). And I think that the "appreciably" is important. In a continuous system, does a square foot of load applied at the far end of a neighboring span appreciably impact negative moment in the design span? I really don't know but I can certainly see some diminishing returns entering into the calculus. That said, nearly the same can be said of how load applied at the end of a simple span would affect moment at mid-span. In this, reduction based on tributary rather than influence area certainly has a leg up in terms of simplicity of application.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[BAretired]

Thanks KootK. You're one step closer to a steak dinner.

What we are talking about is not really structural engineering. It is more a question of probability which I claim no particular expertise in, but it seems to me that probability of a live load occurring in a member could be expressed using either tributary area or influence area.

In the case of residential loading such as we are discussing in this thread, LL = 40 psf. In single family residences, second floor live load is sometimes taken as 30 psf, so there is already a reduction in design live load for certain rooms. In the case of apartment loading, 40 psf starts out as being conservative because it does not take into consideration lightly loaded areas such as bathrooms and bedrooms.

If we were talking about a multi story office building, some engineering judgment might be necessary. If all office spaces are configured in a similar way, heavy areas such as filing would tend to occur in the same region on each floor and would tend to load some members more than others. It seems to me that some conservatism is warranted when calculating cumulative live load on a member.

BA

 

[KootK]

You're one step closer to a steak dinner.

Yessss! I agree on the rest. A bit of an aside but I've always wanted to know the reliability basis for live load reduction so that I could more confidently mess with it. I don't care what anybody says, there IS a way to rationally do live load reduction on wood / CFM stud walls. And if I knew what I was doing, I'd be the just man to develop it.

I'll trow one additional thing out for consideration. Another of my partially developed thoughts.

I wonder about those SQRT terms. They must mean something physically. I speculate that they are akin to a radius of gyration on the loaded area. Load applied further away having less effect on the member considered and, therefore, the diminishing return per unit area that you get with increasing influence area.

That's all I've got...

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[EDub24]

Haven't read through all the responses yet but I posted a similar question a few years back when I was studying for the SE exam. See link below:

https://www.eng-tips.com/viewthread.cfm?qid=405000

 

[BAretired]

EDub24:

Was there a question about Live Load reduction on the exam?

BA

 

[EDub24]

Was there a question about Live Load reduction on the exam?

I took the exam back in 2016 and was lucky enough to pass both portions on my first try so my memory is bit hazy but I don't remember that being on the exam. If it was it was probably a simple question in the multiple choice portion of the exam.