I know this isn't the answer you are looking for, how much force does it take to buckle sheet steel? Not much, it would depend on gauge, fastening schedule, scheme and the specfic product. Call a supplier's tech rep. Ask youself what does failure look like, how rapid and how well can you predict and avert failure during a storm? Lumber truss roof pole barns require bracing across the webbing and so should other forms.

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=1
http://www.nfba.org/files/conf/2010brochure.pdf

Dr 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.

Don, the bracing in the web addresses other critical concerns, gable wall and post wind loads are transfered to the side walls and the whole roof frame with purlins is structured into a skeletal unit. The use of steel cladding for structure is a brittle affair due to lack of stiffness, so take care...

This is a link to the referenced documents the post frame industry uses as the basis for diaphragm design;
http://bse.wisc.edu/bohnhoff/Publications/Copyrighted/EP484.2%20PDF%20File1.pdf
http://bse.wisc.edu/bohnhoff/Publications/Copyrighted/558.pdf

I found the graph of the test I described above

Don,

Thanks. That's exactly the kind of info I was looking for.

So, we can build a timber framed building without the bracing and fasten 2x4 horizontally to the post, screw on sheet metal and have a stabilized building?

Tim

... essentially, diaphragm construction is another way of bracing a building, you can use architectural steel panels to form a diaphragm.
Is that different than a sheet of ply or a sip?

Kevin,
I would like to take this opportunity to apologize for my previous post. Although I meant only to be constructive, it has come clear to me know that I was hasty. After reading some very positive responses from other Guild members I began to feel very ashamed of myself. Kevin, I'm sure you are very capable in what you are planing to do.
To all other Guild members, I hope that I did not cause disrespect to this forum.

Regards
Thane

Hi Thane,

I came into this thread somewhere in the middle with a question about the diaphragm effect of a metal roof. Is it possible that I'm being confused with someone else?

Best,

Yes, (face red) The appology was meant for Carlmill.