Some bridge collapses are mysteries when they first happen. It isn't until a detailed investigation is completed that the true cause is revealed. Combing over the wreckage, engineers and accident investigators piece together the bridge's history, looking at inspection reports and witness accounts of the collapse. At times, the simple failure of a small piece of the bridge caused the entire collapse. Sometimes low-grade or faulty materials were used, rendering the entire bridge too weak to withstand the rigors of time.
The 1967 collapse of the Silver Bridge over the Ohio River at Point Pleasant, W. Va. has become infamous for its connections to Mothman, a strange creature supposedly sighted near Point Pleasant in the months prior to the collapse (The 2002 Richard Gere film "The Mothman Prophecies," chronicled the story). In truth, the collapse was due to a manufacturing defect in one of the steel eyebars that held the bridge up. Years of corrosion worsened the defect until it eventually failed, resulting in the deaths of 46 people [source: LeRose].
The 1994 collapse of the Seongsu Bridge in Korea was due to poor quality steel in some parts of the bridge and improper welding techniques in the bridge's construction. 32 people were killed in the collapse [source: Korea National Emergency Management Agency]. The De la Concorde overpass in Laval, Quebec, Canada collapsed in 2006, killing five. The investigation revealed that some aspects of the bridge's construction were done incorrectly and not according to the design, and that inferior quality concrete became too weak to support the structure.