If you have heavy book shelves sizing 2-3 meters hight or you like to have library in your house you should realize that the flooring system for normal houses are designed to support load 150-200 kg/sq.m. So if the floor has to support your book shelve, weight 500-1,000 kg , it might crack and settle down. For solving this unwanted problem you might quit reading books and sale all books to a recycle man but it will make you unhappy. There are two methods to solve the problem.
1. You design (or have an architecture to design) the room or floor part that you plan to be your library to be strong enough to support 500-1,000 kg. weight. But this way might not be practical because you don't think about it at the time you design or you just buy the house from housing project or the second hand one.
2. Consider your floor that which part is near or supported directly by beam. (That part is normally at the corner of the room) The floor at the corner is stronger than at the mid part because it can directly and quickly transfer load to beam and column. But it's not that you can put as much load as you want to the floor because overload could lead to totally collapse.
