Column (or wall) Live Load Reductions

[JJProczka]

thread167-142628

As pointed out in the referenced thread, the live load reduction for a column is based on its tributary area of all of the floors it supports.

It seems to me something is missing from this interpretation if for instance you have a column that services a 300 square foot area supporting 100psf and on the next level supports 1000 square foot of 50 psf. Wouldn't this type of situation if taken to the extremes actually allow an overall reduction of an otherwise unreducible first floor load to the column?

Thank you for any thoughts,
John-Jozef Proczka (Structural EI)

 

[JoshPlumSE]

Some live load is considered reducible and some is considered un-reducible. I would normally look at the 100psf load as an un-reducible load. But, the portion of 50psf live load could still be reduced when you design that column.

 

[JJProczka]

JoshPlum,

I agree some load is unreducible, but in this case 100 is reducible as only over 100 isn't reducible. I am more concerned with the problem posed by reducing based only on tributary area of columns without reference to what the various live loads might be.

 

[JoshPlumSE]

I understand better now.

In your case, the live load reduction factor for this column would be the following (per ASCE-7

(0.25+15/sqrt (4*1300) = 0.67. So, you're getting a maximum of 33%.

Total unreduced load in column = (300*100+1000*50) = (30 + 50) = 80kips
Total reduced load at bottom of column = 80*0.67 = 53.3 kips...

The 53.3 kips is still almost twice the total load from the 100psf area. So, you'd have to be at your code event live load on that floor + 50% of the code event level live load on the 50psf area for the column to be at it's design load. And, that's before we talk about LRFD load factors or ASD safety factors.

Given the probability of these two un-related areas both experiencing code level load events at nearly the same time, I think the reduction is reasonable. Granted there has to be some engineering judgment involved here. So, if I were more concerned about the behavior, then I would probably treat the 100psf as an un-reducible live load.

 

[IJR]

GuyFace

If you think of design loads as probabilistic, you will be less concerned. They are not extreme loads. Design for extreme loads. The concerns for extreme conditions are different from the concerns for design loads. Most extreme live loads are short lived and structures survive short term loads better than they do long term loads. The question then is how much is long-term.

My two- cents worth.

respects
IJR

 

[JJProczka]

Thanks for the replies JoshPlum and IJR,

I think I was taking this to an academic extreme that can't take place because floor live load values rarely dip below 50, which in turn means that the live load reduction change with area won't ever make the "heavy" floor's load to the column reduce enough to reduce the overall live load it experiences.