John,
I have a 10ft x 8ft "Rhino" greenhouse. When erecting it I had half glazed the panels when a storm blew up. The wind hit 70MPH and blew the greenhouse off its plinth. Ddespite this, not a single pane of glass broke! The glass is toughened and the frame is strong & well braced. I have had this greenhouse bolted down to a concrete plinth for almost ten years now and the only problem I had was one winter when I had forgotten to remove the auto-openers on the windows and bolt them down for the winter storms (I live in the far north of Scotland). A full near-hurricane force wind got in underneath the window. It ripped one of the windows up and over the ridge, bending the ridge and shattering the glass in the window. I had to replace the ridge and window but spares are available and within a week it was up and running once again. Some cheaper greenhouses are flimsy and you cannot buy spare parts if they get damaged. I wholeheartedly recommend RHINO as a greenhouse make. They are not cheap but as they rightly say: "You get what you pay for."