[mikeimp]

I am thinking of erecting a frame as a permanent site for my runner beans next year.How long could I grow runners in the same area? Would I need to trench and move the soil each year to grow them in.

[JayG]

Runners are subject to very few persistent pests and diseases which could get carried over from year to year - if you want to break the "rotation at all costs" rule then runners are probably the crop to do it with (I do!) 

Don't ignore them completely though - digging a new trench under your frame and filling it with plenty of compost, manure, or vegetable kitchen waste before planting to retain moisture will still be appreciated.