Surcharge transfer from SOG to footing

[KCPEngr]

I have a floating slab on the interior of a large commercial building. The slab is 5 1/2" and supports storage racks weighing as much as 3500 lbs and forklift traffic. The location of the storage racks is to be determined after the project has started. The slab bears approximately 1'-0" above the column footing and is not attached to the column footing (no dowels).

Does the slab live load or dead load need to be applied to the interior column footing as a surcharge load? I cannot find anything that gives recommendations for this condition. Everything I find applies to retaining walls. Any recommendations or code references would be appreciated. Thanks in advance.

 

[BadgerPE]

While I don't have any code references that require the load to be transmitted to the footing, just thinking about the mechanics of the slab load being applied to the bearing strata below makes me believe that the load will be transmitted to the footing through the soil between the footing and the slab. Even if the load transmits out at a 45 degree angle from the slab, some addtional stress will be placed on the footing. As a general rule, I always analyze footings below slabs with the slab dead and live applied to them.

 

[Ron]

Yes, but the influence area is sometimes difficult to assess. Unless the load is close to the column, it won't have much effect for a slab on grade, since most of the load, unless truly uniform across the slab, goes through the slab in shear and the shear plane is very near the load.

 

[msquared48]

Once you get down to the bottom of the footing, depending on the thickness of the footing, the additional load from any rack leg will only be in the range of 200 to 300 psf considering the spreading Ron mentioned. This should not be a deal breaker for the soil structure.

As for the forklift load, this is intermittent, and may not even be present depending on how close the forklift can get, or needs to get to the column.

Just use your judgement here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[KCPEngr]

Thanks for your help.