chrisscross,
we were cautioned against replacing only the lower panes with polycarbonate and leaving the upper ones in glass, apparently glass is so much heavier and stronger and unless the greenhouse frame is really solid, the strength of the glass has a role to play in making the greenhouse more solid. This could of course have been good salesmanship selling us more poly than we needed, but on the other hand ...... just thought I'd mention it.
I hope someone with real understanding of structural strength issues with greenhouses will come along and give proper advice.
Ahh... my old Robinsons frame is completely self supporting and doesn't rely on the glass for anything... plus on the side I did with twinwall I did the roof as well, only thing that's happened to it so far is that in a very high wind it pushed in one twinwall panel (the side with the twinwall faces south, the glass side is sheltered by a fence) it only took a few moments to put back...