Stress corrosion cracking is the formation of brittle cracks in a normally sound material through the simultaneous action of a tensile stress and a corrosive environment.(20) Combined with corrosion fatigue, which occurs as a result of the combined action of a cyclic stress and a corrosive environment, disaster was inevitable for the Silver Bridge. The two contributing factors, over the years continued to weaken the eye-bar and unfortunately the entire structure.
Another major factor that helped corrosion fatigue and stress corrosion in bringing down the bridge was the weight of new cars and trucks. When the bridge was designed, the design vehicle used was the model-T Ford, which had an approximate weight of less than 1,500 pounds. In 1967, the average family car weighed 4,000 pounds or more.(21) In 1928, West Virginia law prohibited the operation of any vehicle whose gross weight, including its load, was more than 20,000 pounds. In 1967, the weight limit almost tripled to 60,800 pounds gross, and up to 70,000 with special permits.(22) Civil engineers must use a projected life span for nearly all projects, but no one could see that 40 years after the construction of the Silver Bridge that traffic loads would more than triple.
Although the collapse of the Silver Bridge was a major disaster in the West Virginia and Ohio areas, it also frightened the entire nation. The St. Mary's bridge, located upstream and similar in design to the Silver Bridge, was shut down for inspection after the collapse.(23) President Lyndon B. Johnson ordered a nation-wide probe to determine the safety of the nation's bridges. In 1967 there were 1,800 bridges in the United States which were 40 years old including 1,100 highway bridges designed for Model-T traffic. Many federal officials feared that other structures, built around the some time to handle Model-T traffic, could face the same fate as the Silver Bridge.(24)
Even though the collapse of the Silver Bridge was a disaster, there were positive aspects to the failure. Bridge inspections are now more routine and in-depth because of the Silver Bridge. Engineers are now more knowledgeable about corrosion fatigue and stress corrosion, which allows better quality structures to be designed and built. With today's technology, as well as better design techniques and materials, there is hope that a Silver Bridge disaster will never again take place.
Foot Notes
1. National Transportation Safety Board, A Highway Accident Report, collapse of U. S. 35 Highway Bridge, (Washington: GPO, 1971).
2. Wilson Ballard, "An Eye-bar Suspension span for the Ohio River," Engineering News-Record, June, 1929; 997-1001.
3. Ballard, 997-998.
4. Carl Shermer, "Eye-Bar Bridges and the Silver Bridge Disaster," Engineer, Jan- Feb 1968, 20.
5. Ballard, 999.
6. Alden Armangnaz, "Our Worst Bridge Disaster: Why Did it Happen?" Popular Science Magazine, March 1968, 104.
7. Shermer, 21.
8. Shermer, 21.
9. Letter of L. L. Jemison to H. K. Griffith, 21 December 1951, WV Department of Transportation, State Archives, Charleston.
10. National Transportation Safety Board, Highway Accident Report
11. "Bridge Fell Like a Card Deck," Charleston Daily Mail, 17 Dec 1967.
12. "Truck Driver Survives Tragedy," Charleston Daily Mail, 16, Dec 1967.
13. Ibid.
14. NTSB Highway Accident Report
15. Ibid.
16. Otis K. Rice and Stephen W. Brown, West Virginia, A History, Lexington: University Press of Kentucky, 1993.
17. "The Curse of Cornstalk?" Charleston Daily Mail, 17 Dec 1967.
18. Shermer, 20.
19. NTSB High Way Accident Report
20. Corrosion source, "Stress Corrosion Cracking," Compuserve 7 Oct 2000,
http://64.224.111.143/handbook/testing/scc.htm.
21. "Model-T Bridges Common," Charleston Gazette, 20 Dec 1967.
22. "Model-T Bridges Inviting Tragedy," Charleston Gazette, 19 Dec 1967.
23. "All Possible Help Pushed by Governor," Charleston Gazette, 16 Dec 1967.
24. "Model-T Bridges Inviting Tragedy," Charleston Gazette, 19 Dec 1967.
