Existing Slab on Ground & Equipment Loads

[Skibur]

I have an existing 6" slab with #5 @ 12" OC. It is unknown whether or no the reinforcing extends through the joints. It was built in the 90's so its possible they used the old detail of extending 50% of the reinforcing through the joints. I am wondering how others are dealing with placing new equipment onto existing slabs. The "Designing Slab on Ground" guide takes into account flexural stresses in the slab where as the tables provided by PCA (i believe this is correct) does not. The results from these methods are VERY different. I have vessels that weigh 40,000 lbs in which i am planning to cut out slab and replace in these areas. Another piece of equipment weight 8000 lbs and has 6 small leveling feet. Another piece of equipment weight about the same, 8000 lbs, however is skid mounted. I also have storage racking (8'x4'x16' tall). The 6" slab is capable of handling these loads ignoring joints. What methods are others using?

1. Housekeeping pad on top of existing pad, ignore the joints.
2. Saw cut existing slab then install housekeeping pad on top of existing pad to match the perimeter of the saw cut
3. Ignore the joints and accept the flexural cracking that may occur and sit directly on the existing slab

As for the racking, I have not calculated loading, however, the feet are directly on or withing 12" of the control joints.

Any help or guidance would be appreciated. I did come across a document that someone posted ARMY TM 5-809, Concrete Floor Slabs on Grade Subjected to Heavy Loads, but is a very old document, 1987.

Thank you in advance

 

[skeletron]

How crucial are cracks in your environment?
How big is your slab versus the footprint of equipment?
What's the condition of the existing building/slab?

Option #1 would solve your exposed cracking problem (maybe cause an internal one) but you would have to be okay with the elevation/footprint effect. Also you would have to think about whether you tie it in.
Option #2 would solve your slab footprint problem but you would have to be okay with the equipment footprint effect. How do you feel about the information regarding the subgrade?
Option #3 if your slab can take it, your environment can deal with cracks, and you have the capacity but not the budget, then maybe this is the option.


...but I can't recall if I have ever solved that problem yet.

 

[Tomfh]

One option we have used when we needed to justify capacity but were happy to see cracking was to assess the point load yield line capacity of the slab. As you note this can give substantially higher capacity than when you ignore slab flexural capacity.

 

[Skibur]

The slab area is really large in comparison to the equipment. The slab condition is really good. The geotech came back yesterday with site class b subgrade modulus of 250 pci in the existing slab area.