Basement slab on Grade design 1

[mkrei]

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

 

[gvcstructural]

Perhaps a grade-beam system could be used to provide designed load paths for force collection w/ the slab acting as a diaphragm.

 

[msquared48]

Or if your 16'wakll is on piling - how about adding batter piles to ake the lateral soil kick?

Mike McCann
MMC Engineering

 

[haynewp]

Are there perpendicular walls to the one you are trying to restrain? If so how far are they apart?

 

[SteelPE]

How about using a lopsided footing to engage some of the 16’-0” backfill on your footing. Maybe this in combination with a key below the footing to engage passive pressure will work.

Don’t forget to add in any floor dead load into your calculation.

 

[lkjh345]

mrkei:

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.

 

[mkrei]

The equivalent fluid pressure is 60 psf per the geotech. I think that is high also but.......

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???

 

[DaveAtkins]

It's hard to make large walkout basements work. I generally end up using a cantilever retaining wall.

DaveAtkins

 

[hokie66]

I would agree with ljkh345 about the basement slab. I can't see it displacing upward unless subject to hydrostatic pressure. I would omit any isolation joint material and just turn the polyethylene up against the wall face.

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.

 

[mkrei]

It is being tied into a concrete composite slab 6 1/2" thick normal weight concrete. From my experience the 5 kip/ft I need in the 45' return wall will be working that slab hard. I may need to reinforce that zone. I don't think designing as a cantilever is an option because at some point it is going to think the top of the wall is supported etc. Plus adds a bunch more piles to the footings.

 

[akastud]

The slab as a compression member is obviously braced against buckling downward, so the question is how do you treat buckling upwards, right? So in several standards it recommends bracing be capable of taking 2% of the axial load in the member (I like 5% but thats just the CYA in me). SO if you have a slab that has 5000# compression, the buckling restraint has to account for 100# you have a 6" slab and therefore your slab weight is 75 plf. This leads me to say that your slab is unbraced from upward buckling at 16" O.C. (100/75*12"/'). So now you have a concrete column 1' wide 6" thick and 1'-4" tall with fixed fixed ends. Easy to analyze. Remember too that that 5000# will be reduced throughout the length of that slab with friction wether you want to account for it or not.

akastud

 

[Lion06]

I don't think that 60 pcf is high. That's an at-rest coefficient of 0.5 if the unit weight of the soil is 120 pcf. That is reasonable. If it were an active pressure, I would think that is high, but not for at-rest.
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?

 

[lkjh345]

mrkei:

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?

 

[bylar]

The IBC Table 1610.1 calls for an at rest pressure of 60 psf

 

[civilperson]

The 6" deck will experience 70 psi from lateral load transfer. Seems nominal if included with compression due to flexure.

 

[Riggly]

What type of soil is in your foundation? Is it all excavated?
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?

 

[haynewp]

Is the vapor barrier directly beneath the slab?

 

[mkrei]

Yes a vapor retarder is present so friction between slab and subgrade is minimal.

 

[Riggly]

If the friction is negligible, then the load has to be transferred to the piles, preferrably through battered piles. FYI: There are some HDPE barriers with roughened surfaces that can build up enough friction force at the interface.

 

[tngolfer]

I can't recall the study off the top of my head, but the resisting friction force is huge, even with the vapor barrier. The research was studying the effects of friction from a slab on ground and how it contributes to shrinkage cracking. Basically, the study used two vapor barriers with oil in between and the slab still cracked. There was negligible movement as measured at the bottom of the slab (i.e. the friction force was very great in order to resist the shrinkage strain)