Thursday, December 15, 2011

Silver Bridge Collapses

December 15, 1967:

The Silver Bridge was an eyebar-chain suspension bridge built in 1928 and named for the color of its aluminum paint. The bridge connected Point Pleasant, West Virginia and Kanauga, Ohio, over the Ohio River.
On December 15, 1967, the Silver Bridge collapsed while it was full of rush-hour traffic, resulting in the deaths of 46 people. Two of the victims were never found. Investigation of the wreckage pointed to the cause of the collapse being the failure of a single eyebar in a suspension chain, due to a small defect 0.1 inch (2.5 mm) deep. Analysis showed that the bridge was carrying much heavier loads than it had originally been designed for and was poorly maintained.
The collapsed bridge was replaced by the Silver Memorial Bridge, which was completed in 1969.
At the time of the Silver Bridge construction, eyebar bridges had been built for about 100 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. An example can be seen in the Clifton Suspension Bridge, designed by Isambard Kingdom Brunel. The chain eyebars are redundant in two dimensions. This is an early suspension bridge still in service. Other bridges of similar design include the earlier road bridge over the Menai Strait built by Thomas Telford in 1826; the Széchenyi Chain Bridge in Budapest, built in 1839-1849, destroyed in the closing days of World War II by retreating Germans in 1945, and rebuilt identically by 1949, with redundant chains and hangers; and the Three Sisters, suspension bridges of similar design in Pittsburgh.
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 the tensile strength as common mild steel), rather than a thick stack of thinner bars of modest material strength "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 — which would be unlikely if more bars were 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 six, 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. Wire-cables have extremely high levels of redundancy, with the failure of a single wire almost unnoticeable.
The towers were "rocker" towers, which 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. The towers required the chain on both sides for their support; 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.
The bridge failure was due to a defect in a single link, eyebar 330, 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 inch 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 then held together only by three eyebars, and another slipped off the pin at the center 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 took only about a minute for the whole bridge to fall.The collapse focused much needed attention on the condition of older bridges, leading to intensified inspection protocols and numerous eventual replacements. There were only two bridges built to a similar design, one upstream at St. Marys, West Virginia and the notably longer Hercilio Luz Bridge at Florianópolis, Brazil. The St. Marys bridge was immediately closed to traffic and was demolished by the state 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 Hercilio Luz Bridge remained in active service until 1991 and still stands at Florianópolis due to being built to a higher safety factor than the West Virginia bridge.
That's for all the engineers out there who are fascinated by failures.

No comments:

Post a Comment