By looking at the final resting position of the structure after failure it is sometimes possible to work backwards and identify the area of failure.
For example, if a failure at the bottom of the columns brought the bridge down then we would expect it to fall together, largely in one piece.
So the girder should land the same horizontal distance away from the column break as it was originally away from the break point vertically.
The following trajectory interactive feature applies this to the famous Tay Bridge Disaster.You need the Flash Player (version 4 or higher) to view this - download Flash. http://media.open2.net/forensic_enigneering/riddle/trajectory.swf