Live Load Reduction for multi-story columns with different floor live loads 3

[EDub24]

I'm trying to figure out how to use live load reduction (ASCE 7-10 section 4.7.2) in a multi-story column with different lives loads per floor. Specifically how to determine the tributary area. Would you consider the tributary area for all of the stories with the same live load above the story in question ignoring the other levels that have different values when determining the reduction? For instance, say you have a 10-story building. The top 2 stories are residential the next 4 levels are various (mechanical, storage etc) and the bottom 4 levels are residential with the ground floor being commercial let's say. Assuming the tributary area, At, for a specific column in question is the same at all levels would the 'effective' tributary area (for use in the LL reduction equation) for each of the residential levels be At and 2*At for the top two levels followed by 3*At, 4*At, 5*At and 6*At for the bottom 4 levels (basically ignoring the intermittent levels with differing live loads)? Thanks!

 

[KootK]

I would agree with this but only if the intent of ASCE 7's was to use the total tributary area and not break the tributary areas up for each occupancy load type.

I guess I don't understand what has led you to believe that ASCE intends for us to break up tributary areas by occupancy. Can you point to a particular statement? I see it as wannabe seems to: there are only two kinds of load in this context, reducible and unreducible.

Though, you could of course have live loads that peak at the same time. Classrooms and a Library for example so this is such a case-by-case basis that I would find it had to ever take advantage of.

Agreed. I wasn't suggesting that the temporal effect be taken advantage of so much as suggesting that, because of the temporal effect, I see no rationale for introducing additional conservatism in to the live load reduction calculation. You know, other than individual designer preference.

Wait, isn't this what I was saying about having tributary areas based on same-type occupancy loaded areas? My approach would be more conservative as it includes maximum force effects with a minimal live load reduction but otherwise is a similar concept.

It is similar which is why I tabled it. The snippet below, applicable to one and two family dwellings, conveys a similar sentiment I believe.

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.

 

[TehMightyEngineer]

It is similar which is why I tabled it. The snippet below, applicable to one and two family dwellings, conveys a similar sentiment I believe.

Interesting, what is that snipped from as I don't recognize it.

I guess I don't understand what has led you to believe that ASCE intends for us to break up tributary areas by occupancy. Can you point to a particular statement? I see it as wannabe seems to: there are only two kinds of load in this context, reducible and unreducible.

I initially based my concept off of a PPI example, the fact that it was most conservative, and the ASCE 7 definition of L[sub]o[/sub] (see my first post from March 13th above). Since then wannabeSE and JAE convinced me that my interpretation didn't match what the commentary implied. Ignoring the PPI example, the only thing I have is the fact that it's more conservative and I see nothing in ASCE 7 that says that we're not supposed to break it up by occupancy load. But, I've conceded that this probably is not the intent of ASCE 7-10 4.7.1 nor is it what would be expected on the SE exam.

Agreed. I wasn't suggesting that the temporal effect be taken advantage of so much as suggesting that, because of the temporal effect, I see no rationale for introducing additional conservatism in to the live load reduction calculation. You know, other than individual designer preference.

Ah, that makes sense. I could see that balancing out some other un-conservative area of the live load reduction process.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[EDub24]

Thanks for all the helps guys. It's been very informative reading these posts! If I ever get a response back from ASCE I'll be sure to pass it on but for now I'll move forward with the interpreation given originally by Agent666 and JAE.

 

[KootK]

Interesting, what is that snipped from as I don't recognize it.

That's actually the tail end of ASCE 7-10 4.7.2.

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.

 

[TehMightyEngineer]

Eh? Not in my edition. Third printing w/ supplement 1.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[EDub24]

That section was removed in the errata for the 2nd printing. With that said I think it shows what they were thinking for reduced live load on multiple floors.

 

[TehMightyEngineer]

Agreed! That does seem to settle it even if they removed it.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[jh50000]

A solution to an example in the book TehMightyEngineer mentioned addresses this specific topic. Although, he uses IBC and IBC states that a LL >100psf cannot be reduced.The exception in ASCE 4.7.3 says you can. What would you use on the SE exam?

 

[TehMightyEngineer]

Thanks for posting that! Saves me $65 and that's a good example of a lot of the topics discussed. I think we can finally put this to rest as I consider that example official enough for my purposes.

IBC 2012 has an exception for the live load reduction of heavy loads (Lo > 100 PSF) in 1607.10.1.2 similar to ASCE 7 where if more than 2 floors are supported the heavy load can be reduced 20%.

On the SE exam I would reduce the heavy live load by 20% because they may be trying to see if you know about this exception.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[EDub24]

Awesome! Thanks! I think we can finally put this to bed.