Wall Support in Basement

[Lion06]

I wanted to get an opinion on something. Say you have a basement with two posts supporting the main girder (W8). There is a wall oriented perpindicular to the girder that is currently not supporting it and is approximately 3.5' from one of the posts. Do you see any potential issues with the basement slab if the wall were made to be a load-bearing wall and the post closest to it was removed? Obviously any necessary reinforcemtne to the beam would be required, but I am asking more about any potential issues with the wall.
I am rationalizing it this way: it is an approximately 7.5' high wall. Even assuming a 4 kip reaction, this would provide a load to the SOG of about 400plf over the length of the 10' long wall. Does this seem like it could be a problem, possibly causing cracking of the slab?

 

[lkjh345]

How thick is the slab?

A old nomograph I have says 400 plf is right on the edge for a 4", non-reinforced slab with f'c = 3,000 concrete and a modulus of subgrade reaction (k) of 100 pci.

If the wall weight is in addition to the reaction load, you may be over.

But greater concrete strength or great k will increase the capacity.

 

[hokie66]

What kind of wall? Will the wall distribute the load over its full length? If so, I wouldn't think there is a problem.

 

[concretemasonry]

The posts should be sitting on cast in places footings and covered with the floor slab.

What is the wall that is 3.5' from the post constructed of?

If it is 8" concrete/masonry, you probably have a footing under it. In residential construction you have to be careful what you assume when going from non-bearing to bearing.

If it is wood frame, you cannot assume anything.

A basement slab is not intended to carry any structural loads. If you have a typical slab on grade in a residence, it is really not a slab on grade. In most parts of the country, the basement slab is about 3.5" thick and is cast on the footing supporting the foundation wall. The slab thickness can vary dratically and there was probably no control over the compaction or thickness, so you cannot assume any slab support from the soil.

Basement slabs frequently have as little as 2" thickness covering a settlemet void and the collection of construction scraps (wood, drywall, loose soil).

This will take some on-site investigation.

 

[Lion06]

The wall is currently wood, but I wouldn't have issues trading it out to make it work.
The idea that the basement slab could be 2" definitely scares me. Maybe I'll steer clear of that!
Why in the world would you build a nice, huge basement with the only obstruction being a couple of posts? Why not just size the girder to take the loads and make it a much better basement? Seems like common sense to me - would it really add that much cost to the house to do it at the beginning?

 

[civilperson]

4 kips of load is less then 100 square feet of floor. I think an analysis of the tributary area along with dead loads might exceed the 4 kip estimate.

 

[KootK]

You could just install a new steel column within the wood wall to open up the span. It would require demolishing part of the existing slab and casting in a new isolated footing. It's work but it's nothing that hasn't been done before.

If your wall is wood, runs parallel to your floor joists, and appears to bear directly on the slab, I'm willing to bet that wall was never intended to be bearing. As such, there probably isn't a thickened slab below it nor a slab designed to recieve a line load from the wall.

 

[Einsteim]

The wood wall will not distribute the point load from above over it's length.

 

[COEngineeer]

since it is a steel beam I am pretty sure the answer is no unless that wall is a load bearing wall and has continous footing underneath it.

Never, but never question engineer's judgement

 

[msquared48]

Being a wood wall, there would still have to be a new wood post at the end of the wall to support the existing W8 when the existing post is removed. This new post will transfer the reaction to the slab over the area of the post at the top and splayed out at 45 degrees on all sides to the bottom of the slab and the soil. You need to make sure the punching shear is OK. Otherwise you will need a larger wood post, a steel bearing plate, or a new footing cut into the slab.

The load from the W8 will not span out over the length of the wall as you suggest. It will concentrate at the post.

Mike McCann
McCann Engineering

 

[Lion06]

I was considering adding a steel beam as the top plate of the wall to help the load spread out. I wouldn't want to break up the slab to pour a new footing.

 

[msquared48]

I still think that due to basic statics, even with the steel beam, the vast majority of the load (80 to 90%) will be at the end of the wall at the wood column.

Mike McCann
McCann Engineering

 

[Lion06]

I apologize if I was unclear about the orientation, but the wall is perpendicular to the beam and the beam is approximately centered on the wall.

 

[msquared48]

AAH!!! That changes things then... Just look at the load to the slab as the deflection diagram for the steel beam. Itt will still not be uniform, the max under the W8, but a more workable solution. Good idea!

Mike McCann
McCann Engineering