Silver Bridge

The Silver Bridge was an eyebar chain suspension bridge that connected the cities of Point Pleasant, West Virginia and Gallipolis, Ohio over the Ohio River. It was built in 1928, named for its aluminum paint, and collapsed on December 15, 1967, killing 46 people; investigation of the wreckage pointed to the failure of a single eye-bar in the suspension cables due to a small flaw that was introduced at the time it was made.

Bridge type history

At the time of its construction bridges of this type had been constructed for about a hundred years. Such bridges had usually been constructed from redundant bar links, using rows of four to six bars, sometimes using several such chains in parallel. These can be seen in the Clifton Suspension Bridge, a bridge designed by Isambard Kingdom Brunel and the chain bars are redundant in two' dimensions. This is a very early suspension bridge still in service.

Silver bridge innovations

Low redunancy, high strength

The eyebars in the Silver Bridge were not very redundant, as links were composed of only two bars each, of high strength steel (more than twice as strong as common mild steel), rather than a thick stack of thinner bars of modest material stregth combed together as is usual for redundancy. With only two bars, the failure of one could impose excessive loading on the second, causing total failure, unlikely if more bars are used. While a low-redundancy chain can be engineered to the design requirements, the safety is completely dependent upon proper manufacturing and assembly.

In comparison, the Brooklyn Bridge, with wire cable suspension, was designed with a excess strength factor of 6, which proved fortuitous owing to a contractor's substitution of wire weaker than that specified.)

Rocker towers

Also, the towers were "rocker" towers. These allow the bridge to respond to various live loads by a slight tipping of the supporting towers which were parted at the deck level, rather than passing the suspension chain over a lubricated or tipping saddle. Thus the towers required the cable on both sides for their support, so failure of any one link on either side, in any of the chain three chain spans would result in the complete failure of the entire bridge.

Design loads

At the time of its construction a typical family automobile would be the Ford Model T, with a weight of about 1500 pounds. The maximum permitted truck gross weight was about 20,000 lbs.

Requirement creep

At the time of the collapse, a typical family automobile weighed about 4000 pounds and the large truck limit was 60,000 lbs. or more. Bumper-to-bumper traffic jams were also much more common - an almost every day occurance.

Wreckage analyisis

The bridge failure was found to due to a single manufacturing error in a single link. A minor flaw gradually expanded due to a combination of corrosion and repeated stress.

Inspection difficulties

"Inspection prior to construction would not have been able to notice the miniature crack. ...the only way to detect the fracture would have been to disassemble the eye-bar. The technology used for inspection at the time was not capable of detecting such cracks.[1]

Urban legends

Odd events before the collapse, particularly the appearances of a "Mothman," led to a 1975 nonfiction book named The Mothman Prophecies, which led in turn to a 2002 "based on a true story" movie of the same title (although the movie is not set in the 1960s, but in the present day).

Very specific information on the 46 victims and others who have died linked to the collapse of the Silver Bridge can be found in an ongoing list on this subject.

External links

References

Mothman and Other Curious Encounters by Loren Coleman (Paraview Press, 2002, ISBN 1931044341)