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The Collapse of the Silver Bridge:
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Why Did the Eyebar Break?
The eyebar broke at a load below what was normally expected to occur. Examination of the brittle fracture surface that initiated the failure of the eyebar (and the ultimate collapse) revealed two small pre-existing cracks. These cracks were covered by an endcap that held the pin in place and could not be seen or detected during the regular inspections or while painting. (The last inspection was two years before the collapse.)
3. Photo of the actual eyebar, on display in the NIST Museum.
Extensive studies at NBS indicated that these small cracks started as corrosion pits. Under the action of the applied loads, rainwater combined with sulfur from air pollution to grow these pits into the cracks.
In addition, it was found that the high strength steel eyebars made in 1927 had a relatively low toughness - the property that makes steel resist fracture from cracks - and cold exacerbates this sensitivity to cracks. The longer the crack, the less stress is required to cause fracture and, in a sensitive steel, only a short crack is required to cause brittle fracture.
4. Close-up of the eyebar, shown in photo 3 above, on display in the NIST Museum.
Finally, on that cold evening in 1967, the crack length (between 3 mm and 4 mm), the load on the bridge, and the weight of the bridge itself combined to reach a critical condition. The fracture that led to the catastrophe began.
What Was Done As a Result of the Collapse?
The fact that the failure of one component led to the complete collapse of this bridge meant that other bridges might be at risk. A Presidential Commission was appointed by President Lyndon Johnson. They reviewed the design of similar bridges and made recommendations.
The sister bridge at St. Mary's, West Virginia was immediately closed. It was then torn down and replaced with one of modern design. Bridge design was improved to assure structural redundancy (that is, more than one part must fail before collapse becomes possible).
A program was started to identify bridge steels that might be particularly susceptible to slow flaw growth and avoid their use. Another program was started to develop a new generation of inspection equipment to detect flaws on heavy structures like bridges.
One year after the collapse, the U.S. Department of Transportation established standards for proper safety inspections of bridges and training programs for bridge inspectors. The eyebar suspension bridge at Florianopolis, Brazil was also closed to traffic, but still stands to this day.
Date Created: July 8, 2009