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Re: RE Plywood Roof Diaphragms/ Swedge Anchors

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Part of the roofs I have looked at over the years were houses caught up in the December 2000 tornado that hit Tuscaloosa, AL. The houses immediately next to those directly hit by the tornado still appeared to be intact although they did have mostly projectile damage and some isolated areas where roof sections got lifted off.  Garages were most noticeable because the garage doors buckled and typically over garages have the least amount of rafter bracing.  

I saw one truss roof I wished could have gotten a picture of.  Just happen to drive past it and someone I knew asked me to stop and explain why it had not fallen.  The steep trusses over the garage did not have any wall support on one side for about 20'.  The rest of the garage (about 12') still had the wall.  The roof was drooped down about 2' but had not fallen.  It appeared the horizontal bridging on the bottom chord (one 2x4 at third points) was acting as a compression strut and the plywood deck was the tension tie of a "deep cantilever beam".
The top plates of the wall were still attached to the trusses also although the studs were gone for the 20' section.  

I agree those that buildings in my area rarely experience their full design wind or gravity loads.   I am alway curious what would happen in this area if full design loads ever occured.  I bet lot of things would have problems.

Ron Martin

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