I'm going to attach it to the floor joists

So it won't fall over and kill someone. Good idea. If you don't think the floor's strong enough, what exactly are you planning to pile on those shelves?

I'm guessing that he means that the shelves will be suspended from the joists and won't reach the floor of the basement.

It all depends on how much weight you plan on putting on these shelves and how they are attached to the joists.