Two major structural fractures, including the snapping of a south side chain, along with the weight of extra traffic on the Silver Bridge during Christmas shopping season caused the bridge to collapse.

With these damages, it only took about one minute for major parts of the bridge to fall downward into the Ohio River below. This disaster killed 46 people and injured 9 others.

The bridge was erected in 1928, and planners in the year preceding construction of the bridge decided to use carbon steel (tends to crack) for the bridge. In 1927, planners were inexperienced in and unfamilliar with the full effects of conditions known as stress corrosion and stress fatigue - however - the types of structural corrosion which cause the cracks in the Silver Bridge would not have been detected even today.

The problems of this bridge collapse and tragedy have been closely studied and experts feel that the accident was, indeed, a non-preventable one. For the cracks of the type which caused this tragedy, even today, the bridge would have to be pulled apart and examined to find those corrosion/stress related flaws in the structure.

Here is a link to a video about how the Silver Bridge collapsed:

Silver Bridge Disaster: The Collapse

This is actually a 3rd part in a series which examines the problem of the Silver Bridge structure and the disaster that occurred in December 1967.