The metal skin on a modern post frame building is a quantifiable diaphragm and does contribute greatly to the building's lateral load resistance. The truss bracing is to prevent buckling of the deep, thin trusses. In testing a braced but unskinned 40' post framed building was laterally loaded to design load. Midspan of the building length the plate bowed out 6". Going from memory when they then applied the metal skin the deflection was under an inch. This is all part of design that Roger mentions above.
The upcoming frame builders expo, the post frame industry conference, will be in Louisville Feb 17-19. Dr Harvey Manbeck will be presenting his senior class lectures in post frame construction including the engineering behind the diaphragm models they use. Dr Bonhoff who modelled the ground/post interactions will also be presenting as will others. These guys wrote the book and they're offering a crash course.
http://www.nfba.org/i4a/pages/index.cfm?pageid=1http://www.nfba.org/files/conf/2010brochure.pdfDr Manbeck wrote a program that has been available on the NFBA website, DAFI, Diaphragm and Frame Interaction. You'll need to be an old school computer geek and engineer to use it. He has been working on a more user friendly version. I attended one of his presentations about a year ago and had plenty to think about afterward.
Do I sound bummed that its been a slow year, maybe next time.
