NO additional $15k   

You do realize we have a progressive tax rate system, right?

Let's say you would have been $1 shy of the next tax bracket at the end of the year. You now get a $15,000 bonus. At the end of the year, you are now $14,999 over into the next tax bracket. You only pay the higher rate on the $14,999. You don't pay the higher rate on ALL of your earnins for the year. You aren't getting screwed by getting  bonus.

Speaking of bonues, here is a bonus tip for you. You know when you get that bonus they tax it at a high rate, right? At the end of the year, it is all just regular earlinings on your W-2. So you may have paid more when you got it, but it all works out in the end with whatever tax bracket you are in.