It's amazing how much things can deflect before they fail. That is why most designs are constrained by deflection limits, rather than chance of failure. You wouldn't want your floor to go bouncy-bouncy? And high deflection has some serious seismic implications. And so I come across this article.
Seems they might have forgotten that roofs with steel sprinkler systems shouldn't deflect too much, or the sprinkler heads will pop off, and interior ceiling systems collapse. Oh, no! When they called back the original engineer, he said the roof was nowhere near collapsing, and it was fine to bring back all the students. Taking this to the extreme, you could design the roof to never collapse at all. It just sags until everything inside is squooshed, and then springs back again after the snow melts!