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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 ) and the chain bars are redundant in two dimensions. This is a very early suspension bridge still in service. SILVER BRIDGE INNOVATIONS Low redundancy, 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 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, 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 an 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 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. 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 lb. REQUIREMENTS CREEP At the time of the collapse, a typical family automobile weighed about 4000 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. WRECKAGE ANALYSIS The bridge failure was found to due to a single manufacturing error in a single link. A minor casting 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." {Link without Title} AFTERMATH The collapse focused much needed attention on the condition of older bridges, leading to intensified inspection protocols and numerous eventual replacements. 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 additional benefit containing additional lanes. The new bridge that replaced the Silver Bridge was named the Silver Memorial Bridge . URBAN LEGENDS Odd events before the collapse, particularly the appearances of a " Mothman ," led to a 1975 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). Before the Mothman stories, the collapse was sometimes blamed on the supposed curse of Shawnee Chief Cornstalk , who was murdered nearby during the American Revolutionary War . SEE ALSO EXTERNAL LINKS
REFERENCES
''Beyond the Bridge'', by Jack Matthews. A novella about a man presumed dead in the Silver Bridge collapse who uses the tragedy as an attempt to discard his old life and begin a new one. Told in diary form. |
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