Page:America's Highways 1776–1976.djvu/447

 commentary, was developed and circulated by the American Iron and Steel Institute in 1968. This criteria for steel design was similar to the strength and serviceability criteria for USD in that the designs are based on yield conditions and load factors. Basically, these methods represent a departure from designing a structure for service loads with an overall safety factor. Under USD and load factor design, magnification of individual safety factors are applied to the various types of loads. The magnified loads are then totaled and used to proportion a member assumed to be in a state of incipent yielding. These methods, with additional controls for overload, excessive cracking, deflection, vibration, and permanent set and for fatigue of material at working loads, have been incorporated in the AASHTO Bridge Design Specification criteria.

Comparisons of costs have shown an initial economy in material costs for both steel and concrete members proportioned by these methods over those proportioned by working stress design, especially for long spans.

The events chronicled here, aside from a few setbacks, have been a series of successes. One failure should be recorded to alert the bridge engineer fraternity that these is more to be learned about materials and fatigue structures. On the evening of December 15, 1967, a 39-year old eyebar suspension bridge quivered slightly and then collapsed, carrying 46 persons to their deaths in the icy Ohio River. This catastrophic failure of the Silver Bridge at Point Pleasant, West Virginia, shocked the Nation.

An extensive investigation, including mechanical and chemical tests of the failed structure, showed that the failure was initiated by fracture of an eyebar in the suspension chain. It was further determined that a small corrosion pit on the pin hole face of the eyebar started a small crack and that the crack reached critical size under the joint action of stress-corrosion and fatigue.

The C&O Bridge over the Ohio River between Cincinnati, Ohio, and Covington, Ky., was removed by explosives after inspection revealed it was unsafe for highway traffic. The replacement bridge was opened to highway traffic in 1974.

The disturbing part about the Silver Bridge failure was the contributing causes listed as:

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 * In 1927 when the bridge was designed, the phenomena of stress corrosion and fatigue were not known to occur in the classes of bridge material used under conditions of exposure normally encountered in rural areas.
 * The location of the flaw was inaccessible to visual inspection.