If you watch the field tour videos on FIRST's youtube channel, you will see that the bridges return to a level state if they are not influenced externally. Further they give you guidelines to ensure that your bridges have similar dynamics to the actual bridges.
Because of the amount of FIRST!!!!!! titled videos that show up in a simple search, I have included the link to the channel below, and below that the specific bridge video.
http://www.youtube.com/user/FRCTeamsGlobal
http://www.youtube.com/watch?v=gkxeP...2&feature=plcp
Good Luck!
As an aside, can anyone tell me why on earth someone would take a video, that is already on youtube, then upload it again under their username and not at least give a link to the rest?