Basement slab on Grade design
Basement slab on Grade design
(OP)
I have a basement for a hospital facility with 16' tall walls. Assuming a restrained retaining wall with pinned supports at the first floor slab and basement slab I am getting about 5000 lb of load per foot in the basement slab.
The opposite walls is over 150' away to provide the balancing load. I am currently showing a 6" slab reinforced with #5 @ 12" oc ew. I am not sure how to analyze this slab. I am sure in the past I have somewhat blindly assumed the slab can tranfer the load as an axial member but would like to hear some oppinions. The soil is not great so the slab is a slab on grade but the remainder of the building is pile supported. So passive pressure at the base is not an option.
Any opinions would be appreciated.
Thanks
The opposite walls is over 150' away to provide the balancing load. I am currently showing a 6" slab reinforced with #5 @ 12" oc ew. I am not sure how to analyze this slab. I am sure in the past I have somewhat blindly assumed the slab can tranfer the load as an axial member but would like to hear some oppinions. The soil is not great so the slab is a slab on grade but the remainder of the building is pile supported. So passive pressure at the base is not an option.
Any opinions would be appreciated.
Thanks






RE: Basement slab on Grade design
RE: Basement slab on Grade design
Mike McCann
MMC Engineering
RE: Basement slab on Grade design
RE: Basement slab on Grade design
Don't forget to add in any floor dead load into your calculation.
RE: Basement slab on Grade design
Just out of curosity, what equivalent fluid pressure are you using? The base reaction seems pretty high for a 16' tall baement wall.
JMHO: Unless there is something unusual about the situation, your slab should be fine. I've done basements this wide with 5" slabs reinforced with WWF or fiber mesh, and have not had any trouble with the slabs not being able to take the thrust from the bottom of the walls.
The stress in the slab only ammounts to 70 psi. Not really that much.
RE: Basement slab on Grade design
Also in response to the other posts. No perpendicular walls.
The batter piles might work but are costly and dont engage unless vertical load is applied to the pile.
Thanks so much for all the advice. I still am not sure about if the slab will have a buckling problem even with the low axial load of 70 psi factored.
Another issue I have is a portion of the opposite wall is a walkout so I need to tie the first floor diaphragm into the return walls for lateral stability. With 170' of wall and only a 45' return life in the world of the first floor slab could get interesting for that kind of diaphragm shear. Any help with this one???
RE: Basement slab on Grade design
DaveAtkins
RE: Basement slab on Grade design
As to the diaphragm, if the first floor is a concrete floor, there should be no problem. Anything else, I would agree with Dave, use a cantilevered retaining wall.
RE: Basement slab on Grade design
RE: Basement slab on Grade design
akastud
RE: Basement slab on Grade design
I might check it as a compression member, but if there are any other walls between the two you are talking about, I would use them to your advantage. Consider them brace points and cut down on the "length" of your compression member.
Is this new construction? If so, can you provide something from the footing back into the soil to take the load instead of throwing it into the S-O-G?
RE: Basement slab on Grade design
Vulcraft gives allowable diaphragm shear values for deck with concrete fill in their deck catalog. Your instinct appears to be correct in that 5 kips/foot is going to be problematic.
Do you have a mechanism to take the horizontal reaction out of the bottom of the return wall once you get it into the top? Battered piles?
RE: Basement slab on Grade design
RE: Basement slab on Grade design
RE: Basement slab on Grade design
Why is this such a big problem? Unless I misunderstood something, why is the friction between the foundation and slab ignored in resisting the lateral load? (You are using slab-on-grade so you are assuming contact between the soil and slab) With 1'wide by 150' long strip of footing, we a looking at about 33psf for friction, which is a small load. This may further be reduced by the piles carrying some of the lateral load. And you are looking at about 70psi within the concrete from this load, a fairly small load indeed to cause buckling in the slab. However, if you are concern about buckling, why not thicking the outer sections of the slab?
RE: Basement slab on Grade design
RE: Basement slab on Grade design
RE: Basement slab on Grade design
RE: Basement slab on Grade design