Silver Bridge

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On December 15, 1967, the Silver Bridge, connecting Point Pleasant, West Virginia and Kanauga, Ohio (near Gallipolis, Ohio) over the Ohio River, collapsed while it was choked with rush hour traffic, resulting in the deaths of 46 people.

The bridge was an eyebar chain suspension bridge built in 1928 and was named for the color of its aluminum paint. Investigation of the wreckage pointed to the failure, due to a small defect, of a single eye-bar in a suspension chain as the cause of the collapse. It was also noted that the bridge was carrying much heavier loads than it was originally designed for and was poorly maintained.

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[edit] 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 (designed by Isambard Kingdom Brunel). The chain eyebars are redundant in two dimensions. This is a very early suspension bridge still in service. Other bridges of similar design include the earlier road bridge over the Menai Straits built by Thomas Telford in 1826, the Széchenyi Chain bridge in Budapest (built in 1839-1849, destroyed by retreating Nazis in 1945, rebuilt identically by 1949, features redundant chains and hangers) and several suspension bridges in Pittsburgh (the Three Sisters, all identical non-redundant designs).

[edit] Silver Bridge structure

[edit] Low redundancy, high strength

The eyebars in the Silver Bridge were not 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 strength combined 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 correct, high quality manufacturing and assembly.

In comparison, the Brooklyn Bridge, with wire cable suspension, was designed with an excess strength factor of 6, which proved fortunate owing to a contractor's substitution of wire weaker than that specified. (This was discovered before completion and additional strands were placed in the bundles.)

[edit] 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 or by stressing the towers in bending. 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 three chain spans would result in the complete failure of the entire bridge.

[edit] Design loads

At the time of its construction, a typical family automobile would be the Ford Model T, with a weight of about 1,500 pounds. The maximum permitted truck gross weight was about 20,000 lb.

[edit] Requirements creep

At the time of the collapse, a typical family automobile weighed about 4,000 pounds and the large truck limit was 60,000 lb. or more. Bumper-to-bumper traffic jams were also much more common - occurring several times a day, five days each week.

[edit] Wreckage analysis

The bridge failure was found to be due to a defect in a single link, eyebar 330. It was situated on the north of the Ohio subsidiary chain, the first link below the top of the Ohio tower. A small crack was formed through Fretting Wear at the bearing, and grew through internal corrosion, a problem known as stress corrosion cracking. The crack was only about 0.1 inches deep when it went critical, and it broke in a brittle fashion. Growth of the crack was probably exacerbated by residual stress in the eyebar created during manufacture. When the lower side of the eyebar failed, all the load was transferred to the other side of the eyebar, which then failed by ductile overload. The joint was now only held together by three eyebars, and another slipped off the pin at the centre of the bearing, so the chain was completely severed. Collapse of the entire structure was inevitable since all parts of a suspension bridge are in equilibrium with one another. Witnesses afterward estimated that it only took about a minute for the whole bridge to disappear.

[edit] 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]

[edit] Aftermath

The collapse focused much needed attention on the condition of older bridges, leading to intensified inspection protocols and numerous eventual replacements. There were only three other bridges built to a similar design, one upstream at St Mary's and a longer bridge at Florianopolis, Brazil. They were both closed immediately, and the St Mary's bridge demolished in 1971. Explosive charges were placed on the main chains, and fired to remove the structure, although a small truss bridge was kept to allow access to an island in the river.The Brazilian bridge remains, but is closed to traffic. It was built to a higher safety factor. Modern non-destructive testing methods allow some of the older bridges to remain in service where they are located on lightly traveled roads, while most heavily used bridges of this type have been replaced with modern bridges of various types, and as an extra benefit containing additional lanes.

The new bridge that replaced the Silver Bridge was named the Silver Memorial Bridge.

A scale model of the original Silver Bridge can be seen at the Point Pleasant Museum, and there is an archive of literature kept there for public inspection. The museum also has an eyebar assembly from the original bridge on display on the lower ground floor.

[edit] Urban legends

Odd events in the area over several months before the collapse, including appearances of a "Mothman," led to a 1975 book The Mothman Prophecies by John A. Keel. The 2002 "based on a true story" movie of the same name is not set in the 1960s, but in the present day. Point Pleasant has a Mothman Museum and holds an annual Mothman Festival.

Another theory blames the collapse on a legendary curse of the area by Shawnee Chief Cornstalk, who was murdered nearby during the American Revolutionary War.

[edit] See also

[edit] External links

[edit] References

Crossings of the Ohio River
Upstream
Point Pleasant Rail Bridge
 Silver Bridge
(collapsed)		 Downstream
Silver Memorial Bridge

Coordinates: 38°50′05″N, 82°08′52″W

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