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The continuous foundation to 48" below grade in Mass acts in two ways. One, it supports the structure and keeps it from moving. Your suggestion, with piers to 48" (min) below grade and 6x6 (PT) "rim joists" will also do that. But, the second thing the foundation does is to maintain the temperature of the earth below the structure at a minimum (usually about 55 degrees) preventing the infiltration of frost and frozen ground into the space under the structure. That frozen ground and frost can now move (heave) the earth under the structure. If your 6x6 rim joists are sitting in contact with grade, they may be susceptible to such movement, too. And, that would pretty much negate any savings you might gain by avoiding fouondation installation, when you have to repair the greenhouse structure above as it moves, and the glass cracks.
Other than that, it sounds like it might work.
[This message has been edited by treebeard (edited May 30, 2003).]