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Re: Heavy line loads on thin slabs on grade

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Its in the middle of a very large slab area. Seismic forces are small, and the unit is inherently stable wrt OTM at the seismic loads for the area. The soil capacity is more than double the line load through a 4" slab, so even a full load on one side isn't a problem. My thought is that if the entire assembly is mounted on a pad (8"real reinf. concrete) which is isolated from the transfer of vertical loads across the new/old interface (though a bond break joint), then if the slab can support the weight the system should perform as intended even if it settles (as long as differential settlement isn't excessive). The gypcrete comment was concerning when the tenant leaves and a new tenant doesn't want the (potential) 1/4" tripping hazard in the middle of the floor. I have no idea how the unit will be fastened to the slab, if it will at all - the specs are vague and the vendor has not been forthcoming with actual interface drawings, but the new thickened slab will be heavy enough to ballast one side under maximum seismic loads.

I appreciate everyones has certainly reinforced my misgivings about the existing slab capacity and guided the design towards a more forgiving, if not inexpensive, solution.

Neil Moore wrote:

It's more than the 750 lbs vertical load, it's also the tipping load where the load goes to one side of the rack as it travels or experiences a seismic event. Does the track have to be connected to the slab?

I've analyzed a number of these units but never got involved in the foundation as that was by someone else. If these units a stored at one end of the room, then it becomes a fairly large loaded area.

If the slab cracks and settles, then the system may not work and an attorney may be contacting you about that.

Neil Moore, SE, SECB

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