You might want to check out ShelterLogic. They make a broad line of steel, tube-frame structures, some with dimensions in the hundreds of feet. I set up a "temporary" greenhouse on a patio every fall to shelter plants that don't need the heat/comfort of the "permanent" greenhouse. (I use a small, circulating electric heater to keep frost off.) The fabric on this greenhouse easily handled a 17" snowfall two years ago. I've bolted the structure to 4x4 sills which in turn are secured to the 6x6 margins of the patio.