Post load on slab 1

[ztengguy]

Im working a project where its desired to take out a load bearing stud wall (2x4) and put in a beam and posts to open up the wall. I am looking about about 2.2K DL and 5K LL on each post, supported on a baseplate 3.5"x6" (to keep within wall footprint thats remaining.

I have no info on the slab, other than thickness (6"). I checked punching shear and got about 65.3K which seems high to me. Anyone disagree with my number?

Also, as for cracking, what would you recommend doing?

Thanks

 

[MiketheEngineer]

It might work - but you are pushing it a bit. Typically - a square saw cut is made and a new footing poured - say 30" x 30" - but I have done it your way with affirmation of the owner that any problems down the road will require what I just suggested.

Any doubts - cut it out and pour a new footing!! Not that expensive and when you go to sell the property - everybody will be happy!!

 

[ztengguy]

I think you are right. A few cuts, a few bars, good to go.
Thanks

 

[MiketheEngineer]

Yep

 

[mijowe]

Does the load bearing wall no have a foundation? possible a thickened slab?

 

[ToadJones]

there was an excellent paper floating around on here a few weeks back that covered point loads on slabs.
I saved it somewhere. I'll have to look tomorrow.
Or, you could sear the forum for it.
It basically showed that our normal approach to checking the slabs is overly conservative....but in case like yours where a new footing is so small and cheap, I'd just pour new.

 

[shobroco]

I don't know what soil capacity you are assuming, but you've got to be looking at some bending in your floor as well as your punching shear. What have you assumed for reinforcing? It looks to me like a case of CYA like Mike said & tell the owner "these are the assumptions that allow this to work, and if any of them are wrong & you have trouble, don't call me".

 

[ztengguy]

@ mijowe - The best we can tell there isnt a footing or thickened slab. I have asked the contractor to do some exploration, or just plan on a small foundation.

@ toad, I searched, nothing came up, but of course I might be searching wrong.

@ Shobroco, bending is what I am concerned about. although the slab has been loaded for 30+ years, the soil should be good and consolidated, I am still worried about cracking. Shear looks ok, but long term its the cracks I dont want to be an issue. At the load magnitude I have, I dont see them been severe, so if they are going with carpet might be able to live with them. Its when they rip up the carpet and want exposed concrete or something. I assumed no reinforcing in the slab.

 

[fancypants]

This might be the paper that is referred to. "Load carrying capacity of slabs on grade subject to concentrated loads" You can find it at Structure Magazine. Here is the link.

http://www.structuremag.org/article.aspx?articleID=613

 

[shobroco]

Great article fancypants. I think I've both over- and under- designed this situation many times. Residential renovations seem to result in accepting the exisitng slab just for convenience, and industrial racking or mezzanine installations seem to result in a lot of concrete cutting just to CYA.