In principles of engineering I have learned that certain forces act on members of a bridge. These member forces are tension and compression. When a member is under compression, the members internal forces are being pushed towards its center. When a member is under tension, the members internal forces are being pushed out towards the ends. Sherri g forces of course come into play, this force is the cause of when bridges begin to twist uncontrollably and exponentially until the bridge ripes itself apart.