Concrete slab on grade

[SAIL3]

I am designing some support stl for equipment that is going into an old warehouse.
This area covers about 30'x30' of this large warehouse.
Customer indicates that there is an existing 5" concrete slab on grade about 30 yrs old.
Within this are there will be about 12 conc loads from col's some ranging up to 7 kips load...Seismic, Cat B....max seismic uplift say 1.5 kips.
Obviously they do not want to put in new footings...
I am planning to use opost installed AB's..
I have the following concerns:
1 there is no guarantee that there stll is a good support for the slab due to soil settlement over time.
2 with no info on the reinforcement, I would expect cracks to develop in the slab..
3. post installed AB's..max effective depth 3 to 3 1/2" with 1" of grout..

I plan to make the base pl's as large as possible but I do not think this will
effectively address the introduction of all these conc loads over a defined area.
any suggestions would be welcome...thanks in advance

 

[dik]

A 5" slab is a little light to do very much with unless it is founded on very good stuff...

YOu will have difficulty anchoring anything to it if there is any uplift.

What type of loading? Static? Rotational? Vibratory?

How critical is his equipment to movement?

The spacing of loads can have an overall impact on the flexural stresses developed in the concrete.

What is the condition of the existing concrete?

Dik

 

[Ron]

I would verify the slab thickness by coring.

I also would not worry much about settlement below the slab, but can be checked with sounding if you think there are voids.

If you think you need footings, then recommend them. Just because the owner doesn't want them, doesn't mean they are not needed.

Don't count the grout in your AB embedment.

Check shear at the baseplate perimeter.

 

[JAE]

The ACI code limits footings to a minimum of 6" thick. See chapter 15 of ACI 318.

 

[ToadJones]

I know they dont want to hear it, but footings for this sound like they would be very small, cheap and also very cheap insurance.

 

[BAretired]

Do you have a soil report for the existing building? If not, get one now. Small drilled piles or screw piles can be used in a clay soil instead of footings and may be more economical.

BA

 

[SAIL3]

Thanks for your valuable insights and advice...
a core sample to determine the depth certainly makes sense...
no major vibration issues and to some extend the equipment can handle differential settlement...
not very reassuring for uplift, even though seismic uplift is small..say 1 kip then add in the overstrength factor of 2...
I can make the base pl's rediculously large but it still does not address the potential flexural problem of the slab..
reading your replies, it seems individual footings would be the sensible way to go and I aggree...
the problem with using individual footings is by the time you pock-mark the slab with each footing and isolate them from the slab you are left with a compromised slab...which would lead one to a new slab in that area designed to take the individual conc loads...

 

[Ron]

It sounds like an industrial application in an old warehouse...why are you so concerned about the look of individual footings to support equipment? No, you do not have to place another slab. Yes, the new footings will be apparent. So what?

 

[dik]

Cut a couple of strips out... excavate to provide a 12" strip footing or whatever... compact backfill add isolation joint and fill in balance of holes with concrete...

It's only 30'x30'... may be consider re-constructing the area.

Dik