joezano wrote: - material (I assume pressure treated, but 2x4 or 4x4, how thick for the plywood
You don't really need pressure treated, paint it and keep it off the ground on a few bricks. Drill some holes in the floor so the water from the coal can drain out. That will dry out very fast in the summer and once you have roof on moisture won't be a problem. 2x4's and 3/4 plywood will work fine. the key is bracing it properly. A few strategically placed cables or ratchet straps inside the bin in the center of your plywood where the support is the least from one side to the other will go a long way.
- suggestions for 6 ton (e.g. 4x18x4 or 6x12x4)
You need approximately 40 cubic feet per ton, 4x8x8 will hold that, probably mounded up a little.
- suggestions for "roof" (i.e. metal roof) if and how to lay it
- suggestins on how to build to remove or prop up roof for access
They make that plastic roofing too, just make it easy to remove,. I have one customer that framed the roof as you normally would with 2x4's. He has two or three pieces of that plastic roofing that he built a very light frame for around the edges. Only weighs about 30 pounds at most. He just straps it down. The plastic roofing is laying directly on the 2x4's like it would be anyway.
- Any key things I may have forgotten
Access, many build access door using removable slats. You can visualize it as looking like a guillotine. You first remove the coal by the slats, you can then remove the slats when it gets low enough and walk into the bin to get the rest of it.