Curved Steel Plates

[blackers7]

Mr Robot,

I wondering your help and some advice regarding a project i am working on, i apologise if this is not the best place to post.

Attached i have file in which i am tring to model a curved steel stair case with steel stringers (250 x 20 flats). Having examined moments and deflections i am happy that the steel has the capacity to resist these forces but am worried about effects of buckling and torsion of the members since they are plates. I have tried to follow the steel design guide to examine tese forces but have had not such success, i have not come across a help topic which clearly explains how this could be done, if at all.

http://we.tl/TjaVCnEeXy 

Givent he unique shape i expect this i not something that may have been posted on the forum before but would really appreciate some advice with reagrds to signing off that the desin is acceptable, proving this okay and that the design has been produced to british standrds etc where applicable. Any advice and modelling would be really apreciated, even on how i ahve modelled it currently?

Mnay thanks