First, a correction. According to section 2.2.5 of the Game Manual ("The Arena chapter), the bridges, when level, are 12 inches off the ground, not 10.5.
As for force, check out the video FIRST posted about the bridges here:
http://www.youtube.com/watch?v=-AMaq...eature=related
The use standard FRC batteries in a great demonstration of the tipping point, and you should be able to use that to figure out how much you need to push on it with your manipulator to get it to go down.