"1) Additional load -- Book storage is quite heavy. One of the heaviest load patterns in structural engineering is for library loads, due to the book storage."

My father came up against this problem while I was in high school. When we moved into our newly-built house, he put most of his considerable library against the interior walls of a 2nd-floor room, and didn't think anything of it for three or four years.

Then one night, I'd just gotten home when I heard a loud squawk and the sound of his shoes pounding up and down the stairs. When I went to find out what was going on, I found him frantically stacking piles of books against exterior walls and bringing them downstairs. It turns out that he'd been rehanging the door on the 1st-floor bathroom for the 2nd or third time that year when it finally struck him that whole side of the house was sinking into the basement.

We learned after the fact that our builder may have cut a few corners securing joists, but that just accelerated the progress from a span of decades to a span of years.