I'm not an engineer, nor do I play with one
, but 3 200lb. people moving about on a 2 post bench in soft soil would bring that thing down in a day or less. Captured in concrete, the base would have a much better chance, which is your condition.
Hey, you're the one leasing to the county. If it were mine, I wouldn't even give them a bench because of the liability. If I did, they would submit the plans for approval and give them to me. I would then get a contractor that has the proper insurance to do work in a public environment. I would also use a consulting engineer.
Call me careful, or whatever. I don't hardly do anything but some handy work anymore. I gave up my license and insurance. It's really too scary and too expensive to continue on. And my competition all stands at the Home Depot for any kind of job, so why would I stoop? I have assets, they have nothing.
Well, for structural, you would need a deputy inspector for foundation bolts, etc. The bench is something else, but you might need a signed engineer's report on the fasteners to be used.