One of the problems with our allotment is that it is very exposed and gets quite alot of strong winds especially in Autumn and Winter. We unfortunately were unable to concrete the base of the greenhouse in (allotment rules) but because the soil is clay it hardened quite significantly. However we are still trying to re-inforce and brace the glass greenhouse for these strong winds.
One of the best resources I found was this blog http://hfgh10x12.blogspot.se/2007/08/this-is-greenhouse-we-bought-link-it.html in regards to bracing a greenhouse, he has some great and very detailed instructions and photos of how he modified his greenhouse. Hopefully it will help you with your modification needs.