Slab on Grade? 1

[ejm166]

I am in the process of designing a slab and footings for basically a raised equipment pad for a 90,000 lb generator (about 700 lb/sq ft). It is a 21' by 60' pad supported on all four sides raised about 6' above grade. There will be compacted "structural fill" on the inside of the footings underneath the slab. The slab will sit on the footings and fill material. How would I go about finding a thickness for the slab? a one way slab design? slab on grade? where can i find info about slabs on grade?


thanks

 

[wa9lfg]

I would consider using 21'-0" span Precast Concrete Floor Planks with a concrete topping.

 

[jheidt2543]

ejm166:

Since this is a slab for a generator, won't there be some dynamic loading involved? Also, since the slab will be on fill, you will need to make some assesment of the potential for settlement, particularly due to dynamic loads. You note that the slab will be 6' above grade, this means that the four walls will be retaining walls too.

You are really designing a machine foundation on a raised platform so, you will need to get all the relavent dynamic load information from the generator's manufacturer.

 

[shemp]

If you were to design the slab as a structural slab (one or two way design), then what would be the purpose of the "structural" fill?
I believe I would design the slab as a "floating" slab on grade. At the perimeter walls, I would place the slab on expansion joint material with enough thickness so that when the soil settles, the exp. joint mat. will compress also. This would prevent the slab from spanning the 21' dimension and taking additional stresses. Vertical dowels into the walls with a greased end could prevent horizontal movement of the slab without restricting the vertical settlement.

As for the thickness of the slab, it's a good idea to design the slab to weigh at least 1.5 times the weight of the generator. Or if there are multiple generators being supported, use a factor of 2 (b/c of the possibility of greater torque reactions).
Hope this helps. If anyone disagrees with this, please let me know! Currently, I'm designing a generator foundation too (but not elevated).

 

[Zulak]

Shemp's ideas are good here. Look at the slab like a wide thin footing with the loads of the machinery acting as column loads (or line loads). Outer edges want to curl, and interior of slab wants to bow upwards between macinery supports. Turn model upside-down and design as one-way slab with cantilevers using calculated soil pressure per one-foot strip. This will give you top reinforcing steel between supports and bottom steel reinforcing at supports and edges.

 

[Ron]

I agree with shemp...

This slab is not heavily loaded, but will have a vibration component. This won't significantly affect the concrete, but it will affect the soil below. Isolate the slab from the walls and make sure the fill is compacted to 100 percent of the modified Proctor, maximum dry density, from bottom to top. This will reduce the tendency of the generator vibrations to cause additional consolidation of the fill, leading to settlement.

Assuming the load is well distributed, I would design the thickness based on an elastic layer method with overlapping stresses. Reinforcement is not likely going to be necessary. Control cracking with proper jointing, spaced at about 24 times the thickness, each way.