# Live Load Reduction



## MOOK (Jan 27, 2009)

I have a question about live load reduction in ROOFS (NOT FLOORS)

I understand from ASCE that if live load is more than 20 lb/ft2 on the ROOF, live load reduction is not applicable. Is that correct?

ASCE is ambiguous in this point.


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## McEngr (Jan 27, 2009)

MOOK said:


> I have a question about live load reduction in ROOFS (NOT FLOORS)I understand from ASCE that if live load is more than 20 lb/ft2 on the ROOF, live load reduction is not applicable. Is that correct?
> 
> ASCE is ambiguous in this point.


MOOK, Could you please reference the section? I'll have to check it at work tomorrow.


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## McEngr (Jan 28, 2009)

McEngr said:


> MOOK, Could you please reference the section? I'll have to check it at work tomorrow.


MOOK,

Now that I'm in my element, I can respond: The roof live load shall be between 12 and 20 psf except if there is special scaffolding or a greenhouse for your live load. Otherwise, you would simply just use 20psf.

Also, Roof Live Load and Roof Snow Load are never used simultaneously. Therefore, Lr or S will be the combined with dead, wind, and other loads if applicable. Determining balanced roof snow, unbalanced roof snow, and snow drift are much more tricky than roof live load.

I hope that helps.


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## MOOK (Jan 28, 2009)

McEngr said:


> MOOK,Now that I'm in my element, I can respond: The roof live load shall be between 12 and 20 psf except if there is special scaffolding or a greenhouse for your live load. Otherwise, you would simply just use 20psf.
> 
> Also, Roof Live Load and Roof Snow Load are never used simultaneously. Therefore, Lr or S will be the combined with dead, wind, and other loads if applicable. Determining balanced roof snow, unbalanced roof snow, and snow drift are much more tricky than roof live load.
> 
> I hope that helps.


Thanks McEngr for your answer.

Actually what made me confused that ASCE or IBC says that Reduced live load on roofs should not be more than 20 psf as you said in your reply. So, let us assume that we have a roof with 80 psf live load and after reduction it became 55 psf so according to ASCE or IBC I should take it 20 psf instead of 55psf which does not make sense at all.

What I understood from other answers that Roofs with live load higher than 20 psf are considered speical roofs and should be treated as FLOORS when it comes to live load reduction.

Please correct me if I were wrong.


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## ODB_PE (Jan 29, 2009)

MOOK said:


> What I understood from other answers that Roofs with live load higher than 20 psf are considered speical roofs and should be treated as FLOORS when it comes to live load reduction.Please correct me if I were wrong.


In my understanding that is a rational interpretation.


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## McEngr (Feb 3, 2009)

MOOK said:


> Thanks McEngr for your answer.
> Actually what made me confused that ASCE or IBC says that Reduced live load on roofs should not be more than 20 psf as you said in your reply. So, let us assume that we have a roof with 80 psf live load and after reduction it became 55 psf so according to ASCE or IBC I should take it 20 psf instead of 55psf which does not make sense at all.
> 
> What I understood from other answers that Roofs with live load higher than 20 psf are considered speical roofs and should be treated as FLOORS when it comes to live load reduction.
> ...


Mook, I think you're understanding is accurate. However, my conscience is telling me that I should explain further. The main thing that my professor in college pointed out to me in my Steel II class was (in regards to load reduction) that it's a probability analysis. If you ever took a stat/probability class in college, the one thing that speaks loud and clear is that you are likely to never get the kind of load you're designing for. It's the same reason we have recurrence intervals for seismic. No one can design for the worst possible load case known to man in a particular region. All of our buildings would be scrutinized by presumed "over-design".

If it's not in the range of 12 to 20, then you can't reduce it. It's just that simple. You are not likely to see a saw-tooth or arched roof on the test. Those are highly unusual, but aren't off the deep end when it comes to difficulty.

Good luck and don't be bashful with questions. I'm still mulling over stability analysis with steel. I've got a learning curve there in regards to the new methods, so I'm definitely still learning too.


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