Bridges are an excellent area to catch crappie. I have found that most all bridges hold crappie at some point in time. Some bridges are alot better than others, and then some barely hold fish at all but will generally have atleast a few crappie if they are in the waterway.
I have caught crappie on things such as worms and crickets before, however the go to thing is minnows and jigs. There are so many different variables to catching them around bridges. I generally cast without a float. Sometimes I use a float with a jig but mostly I do not. If I do use minnows, and that is a big if, I will use a float. Some people tightline a minnow at night.
Spend some time studying it and you should be able to catch some crappie there.