Live Load Reduction Factor of a Lightgage Stud

[Atomic25]

IBC design.

Would you assume a Kll factor of 4 or 1 for an interior bearing wall stud member?

I suppose the question would be if you consider a lightgage stud as a column or not.

 

[lkjh345]

See Steel Stud Manufacturers Association's (SSMA) Product Technical Information publication for Combined Axial and Lateral Load Tables for various stud sizes and heights.

Assume appropriate minimum lateral loading on an interior wall to get to the proper part of the chart.

This publication can be downloaded for free from SSMA at:

http://www.ssma.com

 

[lkjh345]

P.S. I would treat a load bearing wall same as a beam for live laod reduction and use a Kll of 2. Seems logical. Seems to treat the influence area the same.

 

[csd72]

lkjh345,

I cant see your logic. A beam is one continuous member whereas the wall is constructed of discrete studs (that can fail individually).

 

[lkjh345]

csd72:

If the studs are 16" O.C for example, the Area of Influence would be 16" x the tributary width onto the wall x Kll of 2. This would seem to capture the floor area that can influence the loading on an individual stud.

I would not use a Kll of 4, as this would capture floor area that is tributary to the next stud.

This appears to be a grey area of the code, where this situation was not contemplated. I could definately see an agruement for using a Kll of 1 as well.

On my own project(s), in reality, being very conservative, I would probably just use Kll = 1 and not go out on a limb.

 

[Atomic25]

Well, that brings up another interesting thought. Do you take the tributary area of the wall as a collective supporting unit or of the single stud?

If you just look at the stud, you'll never get enough tributary area to ever reduce the load.

 

[csd72]

I never reduce loads for studs.

 

[lkjh345]

Atomic25:

IMHO, I would take each idividual stud's influence area as seperate from each other as the wall would not have a way (except for the top plate) of transfering loads to adjacent studs along the length of the wall.

This would make it highly doubtful you would ever get over the 400 SF influence area minimum.

However, I still think 'in theory' Kll would equal 2 for a wall, even though in real world situations you would not likely get Ai > 400 SF to ever apply it.

JMHO.

 

[Atomic25]

Interesting. Is that a rule of thumb or have you seen anything regulating this? There's nothing in SSMA or the Design Manual mandating either way.

So if you were designing a multistory building, you would assume the full live load on all floors is present at once? I'm not sure that's the intent of the code.

 

[msquared48]

Neither do I.

I just go by the tabulsated, rated values of the specific manufacturer, and they all vary per their specific product.

In fact, I seldom use the LL reduction for beams, maybe columns and spread footings. I have run into too many instances where undocumented changes, construction errors, and/or omissions were made where the original exclusion of the LLR in the calculations provided enough capacity, that when later included, no changes had to be made. To me, it's a safety factor - an oops qoutient so to speak.

Mike McCann
McCann Engineering

 

[csd72]

Atomic25,

but you could have localised live loads in 2 areas directly above each other, fully stressing one stud.

 

[msquared48]

The difference here is that there is not enough rigidity in the steel stud track to span the load out to the next stud or two, unlike a beam that has much more rigidity along its length to distribute the effect of point loads.

Mike McCann
McCann Engineering

 

[Atomic25]

Thanks for the replies guys. Gives some good points of view to ponder. I fully understand your logic and if it were a 2 story situation there's no question Live load reductions wouldn't be used....because yea there's a questionable chance you could fully live load a certain area for 2 stories.

At the same time I think the code's loading probability equation breaks down for situations over 2 stories. There's a greater disconnect in the tributary area that IBC didn't account for. I think it's just as probable a certain strip of floor tributary to a stud could be fully live loaded for say 4 stories as would for a beam.