By Don Karol
Rumor has it that, just before the December 15, 1967, collapse of the US Highway 35 Bridge in Point Pleasant, West Virginia, a 7-foot-tall monster with large, piercing red eyes and huge, mothlike wings was seen lurking nearby, warning of the impending catastrophe. This “Mothman” was soon blamed for the tragedy in which 46 people died and 9 were injured. Of the 37 vehicles on the bridge at the time of the collapse, 31 fell with it, many plunging into the Ohio River. Fifty years after the collapse of what was then known as the Silver Bridge, paranormal speculation still swirls around the event, perpetuated by movies (like the Mothman Prophecies), legends, and myths. As a civil engineer, though, I put my trust in the laws of physics, materials science, and the findings of the NTSB investigation completed five decades ago, which proved without a doubt that the Mothman wasn’t to blame.
The Silver Bridge collapse was the first significant highway accident investigation in NTSB history. Working with experts from the Federal Highway Administration, the states of West Virginia and Ohio, and leading engineering consulting firms, we determined conclusively that the cause of the collapse was an eyebar fracture in one of the bridge’s suspension chains. The fracture resulted from stress corrosion and corrosion fatigue that had developed over the bridge’s 40-year lifespan. Not surprisingly, no evidence was ever found connecting the Mothman to the failure.
This catastrophic event prompted national concern about the safety of bridges across the United States. President Lyndon B. Johnson ordered all US bridges to undergo safety inspections. Congressional hearings resulted in mandates requiring the US Department of Transportation to develop and implement National Bridge Inspection Standards. In December 1970, landmark legislation was enacted that established national requirements for bridge inspection and evaluation. One would think that these rigorous new inspection standards would take care of bridge failures forever. Unfortunately, during the past half century, that’s not been the case.
On June 28, 1983, a 100-foot-long section of Interstate 95 (Mianus River Bridge) collapsed near Greenwich, Connecticut. Two tractor-semitrailers and two passenger vehicles went down with it, resulting in three fatalities and three serious injuries. We determined that corrosion-induced forces led to lateral displacement of the suspension assembly, which went undetected by the state’s bridge inspection and maintenance programs and ultimately led to the collapse.
On April 5, 1987, tragedy struck again when two spans of the New York State Thruway (Interstate 90) fell about 80 feet into the rain-swollen Schoharie Creek. Four passenger cars and one tractor-semitrailer plunged into the creek, and 10 people were killed. We determined that the New York State Thruway Authority failed to maintain adequate support around the bridge piers, leading to severe erosion in the soil beneath the bridge footings. We also determined that the state’s bridge inspection program was inadequate. Not surprisingly, neither the Mothman nor the Loch Ness Monster was seen in advance of this collapse to forewarn of the impending catastrophe.
Other notable bridge failures we investigated in the late 1980s involved localized flooding and water scouring. One collapse occurred on April 1, 1989, near Covington, Tennessee, when two columns supporting three bridge spans collapsed, sending an 85‑foot section of the US Route 51 bridge 20 feet into the Hatchie River. Five vehicles fell with it, killing eight occupants. Again, our investigation identified deficiencies in the state authority’s bridge oversight. In response to our investigations of these events, additional requirements were developed for periodic underwater inspection of bridges.
Probably the most memorable bridge collapse we investigated occurred 10 years ago in Minneapolis, Minnesota, when a catastrophic failure occurred in the main span of the deck truss in the Interstate 35W highway bridge. As a result, 1,000 feet of the deck truss collapsed during rush hour, with about 456 feet of the main span falling into the river. A total of 111 vehicles were on the portion of the bridge that collapsed; 13 people died and 145 were injured. We determined that a design error in the gusset plates compromised the bridge’s load capacity, causing it to fail under substantial weight increases. Our investigation prompted the development of additional bridge quality assurance and improved bridge inspection requirements.
On December 15, as we mark the 50th anniversary of the Silver Bridge collapse, let’s focus on the infrastructure improvements we need still need to make five decades later rather than try to place the blame on mythical creatures like the Mothman. Throughout the NTSB’s history, we have investigated catastrophic bridge collapses with one goal in mind: preventing future tragedies. Despite efforts to continually enhance the quality of bridge inspections, unforeseen disasters continue to occur, highlighting the need to thoroughly inspect and replace bridges before they collapse. Supernatural forces do not bring down bridges; neglect does.
Don Karol is a Senior Highway Accident Investigator and National Resource Specialist in the NTSB Office of Highway Safety.