Bearing Strength of CFS Studs on Concrete Floor Slabs

[GordonInghamLesyk]

I am doing some preliminary cost sizing for a building with cold-formed steel studs and concrete floor slabs. I'm using axial load strength tables for the studs, and in trying to do a check for the bearing strength of the stud on the concrete, it seems that the bearing strength heavily governs (given that the cross sectional area of the stud is so small). Based off of an ASCE paper "Bearing Strength of Slabs on Grade Supporting a Cold-Formed Steel Wall in Low-Rise Building", I've increased the bearing area to account for some additional spreading of load through the bottom track, but the bearing strength is still half the strength of axial loads coming down.

Has anyone had a similar issue and found a more realistic way to consider bearing of these skinny studs?

Thank you,
Gord

 

[BSVBD]

I've much to learn.

How can you specify a CFS stud(s) or "column" that has only half the "bearing strength" of the axial loads coming down?

If you've designed the CFS member to adequately support the axial load, doesn't this satisfy the bearing strength at base of stud?

Also, how have you increased the bearing area? I've got my own thoughts, but, i'm curious what your idea(s) are. Do you have a detail to share?

 

[BAretired]

It has never occurred to me to check bearing of a cold formed steel stud on concrete. It is difficult to imagine a failure in bearing. I think you would be entitled to consider the spreading of load at a 45 degree angle through the bottom track. You could also consider an enlarged area if the concrete is confined.

BA

 

[jayrod12]

I would like to assume you could account for the bearing area being as if the stud were a full rectangle, i.e. 1 5/8"x3 5/8", as opposed to checking it with super thin line loads on the concrete. I refuse to believe a CFS stud could punch through a concrete slab.

 

[spats]

I don't have any magic formulas for you, but the same kind of thing happens with wood stud walls. Bearing perpendicular to grain on the sill plate can often control the wall design.

 

[GordonInghamLesyk]

@BSVBD
I apologize, I misspoke. What I meant to say is that the bearing strength is half of the axial strength of the studs, not half the strength of the axial loads... that would be poor design!

@BAretired
Thank you for the tip. What you recommended is actually exactly what I was referring to when I mentioned increasing the bearing area to account for spreading load through the track. I assumed the load spread at a 45 degrees angle through the track, so that the bearing area increased to be t_stud + 2*t_track wide. Unfortunately, the bearing walls are also end walls typically so they are the edge of a slab, I don't think I can count on confinement for much.

 

[XR250]

I would like to assume you could account for the bearing area being as if the stud were a full rectangle, i.e. 1 5/8"x3 5/8", as opposed to checking it with super thin line loads on the concrete. I refuse to believe a CFS stud could punch through a concrete slab.

I have never checked it or even thought about it. I would think a punching shear failure would occur first.

 

[jayrod12]

I have never checked it or even thought about it. I would think a punching shear failure would occur first.

Nor have I.

In regards to spats' comments, yes wood stud walls are more often then not governed by sill plate crushing when we talk about anything bigger than a single storey residential application. But I don't feel as though bearing failure of CFS studs has the same ramifications, I.E. if you exceed the bearing capacity of the stud track what are the actual ramifications, it's not like you have 38mm of material to crush through but rather a mm or 2 at most. I would expect there to be more issues with buckling of the actual stud at the bearing locations which would be cured with some bearing stiffeners or the like.

 

[KootK]

..and in trying to do a check for the bearing strength of the stud on the concrete..

The procedure that I've encountered in the literature is as shown below. Probably similar to that paper that you cited.

There are some options but their viability very much depends on how may studs need to be addressed:

1) For a single stud pack, throw a steel base plate in there.

2) For a single stud pack, transfer to a clip angle (or a pair).

3) Beef up the gauge of your track and/or stud.

4) Put some thicker gauge strap below or inside the track.

5) use confined concrete f'c if appropriate.

6) put an HSS below your track.

7) reduce the stud spacing.

I agree with jayrod on the probably ramifications.

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.

 

[DaveAtkins]

I think this is one of those occasions when "numbers DO lie."

Like many of the other posts, I have never checked this before. I just can't imagine a steel stud "slicing" through concrete like a hot knife through butter

I think the track spreads the load, allowing you to take full advantage of the stud axial capacity.

DaveAtkins

 

[KootK]

I feel compelled to point out that most of the arguments being made in favor of ignoring the bearing checks could also be made for a rolled steel column bearing on concrete sans base plate.

I feel like the flanges of the track will be engaged to spread the load out a bit. Tough to quantify that however, especially with local flange crippling/buckling being a possibility.

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.

 

[BSVBD]

Koot - I feel we all would agree on that.

The big difference, however, is the extreme difference in thickness of material and allowable load.

The rolled steel column, able to support FAR greater load, could easily punch, puncture, shear fail the concrete.

On the other hand, the cold-form would easily fail/buckle/cripple FAR before the concrete footer <<<(This one's for Ron, Lomarandil, XR250 among others.)

 

[structSU10]

The paper I saw on this issue was more concerned when the stud is located close to the edge of a slab, which is the paper the OP is referencing. I don't see that being a big issue many times, since there is usually some kind of foundation wall.

The bigger issue I get concerned about is bearing on hollowcore plank. A local plank manufacturer actually did some testing to check some of the issues, such as offset of studs between levels, the effect of reinforcing the grout keys, and the effect of deep leg tracks.

From their testing, adding rebar to the grout key reduced the capacity, and deep leg tracks greatly increased the capacity.

 

[Ron]

Web crippling near the track will occur long before you have a concrete bearing issue!

 

[BUGGAR]

I have a chunk of steel stud that I was going to put out on my concrete driveway under my car ramp and see what really happens. Next to my driveway is an exposed aggregate patio. I got to thinking. Do you suppose the course aggregate could have anything to do with this "weak" concrete supporting these concentrated loads?

 

[KootK]

Last night I dug up the calc procedure that accompanies the diagram that I posted above. According to that, you can rely on the track acting as a base plate over a distance considerably greater than 45 degrees (more like 4X track thickness).

There is also some research confirming what others have mentioned above: it's more of an issue at concrete edges. Link

Some additional information on the design method: Link

And a journal article: Link

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.

 

[GordonInghamLesyk]

@KootK

Great research, thank you for the information! I spent quite a bit of time reading through the journal article you posted when I was initially looking for solutions, that journal article actually recommends using something very similar to my initial assumptions for determining my bearing area, which lead to the bearing heavily governing the strengths of the studs still. Upon reading into the article more, it seems like the slab they are testing is failing more so in shear instead of crushing under bearing. The pictures they provide don't show any crushing occurring, which leads me to believe their conclusions aren't very relevant for my case as my slabs are not on grade and will not fail in shear. I believe the article leads to results which are too conservative for my case.

I think a good conclusion may be to rely on Trestain's method (the method you've pictured above) which after some number crunching appears to provide bearing strengths greater than the axial strength of the studs.... which is what we would expect!

Thank you everyone for your helpful comments.
Gord