Raised plywood false floor for heavy equipment usages

Question

I don't know if this is the correct stack, since it isn't in a home, but I think it still fits.

I have a electronics assembly company, and we run two assembly lines. They consist of several machines, with the heaviest being around 4000lbs, on 6 metal casters. We currently have the line on a concrete slab, inside a large converted warehouse.

The concrete slab is badly spalled, and apparently didn't have a vapor barrier when originally poured. If we leave anything heavy on the floor (cardboard box), it will be dripping with water the next day. We maintain a humidity level of 45%, which probably brings more water up also. We are located in Florida, so have a high water table.

The landlord is willing to work with us to fix this issue, and a contractor recommended we do a vapor barrier on the slab, then a grid of 2x4's, then tongue and groove plywood. On the plywood we would install ESD safe vinyl tiles. They said this is common in gyms and dance studio's. They use rubber strips under the 2x4's to help dampen noise and vibration.

It's hard to find information on this method, is this suitable for this amount of weight, in an industrial environment? I really don't see of many other options if this isn't acceptable to do. I would assume we would do the 2x4's either 6" or 12" on center.

Answer 1

My first guess was, with at least 667 pounds on a caster, there's no way 1/2" plywood is going to hold up. I'd want this plan stamped by a PE and a certificate of insurance for their E&O policy.

There's information available for uniform loads on plywood for general construction but not for concentrated loads. I would not put much faith in something used for dance studios holding up to industrial use with heavy machinery on casters.

I stumbled across this thread:

https://www.eng-tips.com/viewthread.cfm?qid=229324

There's a linked PDF in that thread with a photocopied page from a report on concentrated loading of plywood. It shows you might be OK with 5/8" plywood supported 16" OC or 3/4" plywood supported 24" OC. The APA (American Plywood Association) might have better info available. Note the difference in the dry and wet values in that PDF. Obviously you're going to have to use plywood that's strong and will stay strong exposed to a lot of moisture.

If your layout is not going to change, you might get better support if you space the joists closer in the aisles and under the casters. I have seen this done in very old industrial buildings built before structural steel with wood columns, beams, and floors.

Is there a plan for ventilation / dehumidification under the floor? I would think that's going to be very important.

Answer 2

If the slab is at grade I would use 2 part epoxy to seal the floor.

I have done this on quite a few homes and even basements with huge success. To protect the epoxy paint I would use at least 1/2” plugged and sanded plywood, why plugged and sanded because it has smaller gaps in the core and probably can handle the equipment load , the only other type of plywood I would use is hdf , this stuff is hand laid and way expensive. I used to work in a plywood mill , if you go with the less expensive plugged and sanded.

inspect each sheet and if any stripes even as short as a couple of inches (red, blue or black) those are problem areas , my plant used red That disappeared kind of but if you see stripes that is a weak point that may be a void, or a blow different issues but both identify a weak spot that may not hold up well with that kind of weight.

I would etch the floor first and paint then use a self leveling compound on top to fill the voids. Self leveling compound is not that strong and may crack if the holes are filled then painted. With damp floors like you have I wait until late summer and test the slab with a pice of plastic tapped down for 24 hours. If wet the epoxy paint I use won’t work.

A outside temporary sump may be needed to pump the water out if a spring late summer and a sump outside has made it dry enough to paint. Hope this info helps, as it is a commercial facility I would use the stuff for a garage it is tougher but higher in VOC’s so it takes a couple of days to totally outgas. If you have wave soldering or other soldering your vent system will take care of the VOC’s .