I tie to the bridges because even my Minn Kota on low power will spook them at times. After I cruise the area with my depth finder on, I'll usually turn it off when I anchor and fish, occasionally turning it on to check the depth of fish or schools of shad. Some bridges may require a bar-clamp attached to the concrete piling ... then tie to that. Crappie aren't always right beside pilings either, so don't be afraid to fish jigs in open water between them.