While searching the web for truss related things, I found this article
(PDF). I was not aware of this problem, but it makes sense. These
trusses are used in most large barns these days. What is happening is
the moisture from animals, and ammonia from the manure is building up on
the truss plate connectors and causing them to corrode. This could
cause the building to collapse. I thought I'd share the article.

http://www.omafra.gov.on.ca/english/...cts/10-071.pdf