The Silver Bridge was an eyebar-chain suspension bridge, the bridge was built in 1928 and it was named The Silver Bridge due to its’ modern aluminum paint. The Silver Bridge was designed by J.E. Greiner Company and was constructed by The General Contracting Company of Pittsburgh, Pennsylvania constructed the piers, The American Bridge Company of Pittsburgh and its’ subcontractors built the bridge. The bridge was 2,235 feet long and was supported by two 380-foot pillars that were driven into the bedrock underneath. The bridge carried U.S. Route35 and run over the Ohio River, the bridge connects point pleasant, West Virginia and Gallipolis, Ohio. Although the original design had called for conventional wire cables, an Eyebar-chain design bid as an alternate with a lower cost. The bridge was described in the June 20, 1929 Engineering News Record as “the first of its type in the United States”. On December 15, 1967 approximately 5pm the silver bridge suddenly collapsed without any warning, the bridge was fully loaded at the time of the collapse resulting in 46 casualties.
A lot of unique design ideas were imbedded in the design of the bridge such as High-Tension eye-bars a unique anchorage system and “Rocker” towers. An eye-bar is a tension only member that is connected by pin, in the case of the silver bridge a huge pin was used to connect tow following bars, the bars were 2in X 12in and the pin had 11in diameter. The length of each chain varied based on the requirements and the location of the bridge. Questions were asked by the original designers of the bridge before accepting the newer deign offered by the American Bridge company. Some of these questions were “what if the two Eye-bars did not share the 4-million-pound load coming from the bridge equally? Would the eye-bars resist the overload stresses?” these questions were answered by the designers of the American bridge company which stated that they had developed a new heat-treated carbon steel to use in the construction of the silver bridge, this new steel would allow the members of the bridge to handle more stress than what the members can usually handle, Along with the two eye-bars sharing the load, the steel could easily handle the 4 million pound load. The new heat-treated chain steel eye-bars had an ultimate strength of 105,000 psi, an elastic limit of 75,000 psi, a maximum working stress of 50,000 psi. The eye-bars embedded into the unique anchorage were also heat treated for an ultimate strength of 75,000 psi, an elastic limit of 50,000 psi and a maximum unit stress of 30 psi.
Bedrock was only found at a considerable depth which makes it very difficult for a normal gravity anchorage system not practical, hence an innovative design had to be made. The engineers designed an unusual anchorage system which consists of reinforced concrete trough which its’ dimensions are 200 feet long and 34 feet wide which was filled with soil and reinforced concrete. This huge trough of reinforced concrete and soil was supported by 405 piles the piles were 16in octagonal piles in which the cable pull is resisted by the weight of the anchorage and by sharing halves of the piles.
Another innovative and unique design that was used in the design of the silver bridge was the ‘Rocker’ towers. These towers which a height of almost 131 feet, allowed the bridge to move due to shifting loads and also due to the changes in the chain lengths that are made by the temperature changes. This process is done by placing a curved fitting next to a flat one at the bottom end of the piers then the Rocker is fitted with dowel rods to keep the structure from shifting horizontally, hence with this connection the piers were not fixed to bases.
After extensive studies of the broken structure members, the cause of failure was determined. The answer was the unique eye-bar design made from the newly innovated heat treated-carbon steel. The old saying, “A chain is only as strong as its weakest link,” turned out to be a fact in the failure of the Silver Bridge.(18) The heat-treated carbon steel eye-bar broke, placing undue stress on the other members of the bridge. The remaining steel frame buckled and fell due to the newly concentrated stresses.
The cause of failure was attributed to a cleavage fracture in the lower limb of eye-bar 330 at joint C13N of the north eye-bar suspension chain in the Ohio side span.” The fracture was caused from a minute crack formed during the casting of the steel eye-bar. Over the years, stress corrosion and corrosion fatigue allowed the crack to grow, causing the failure of the entire structure. At the time of construction, the steel used was not known for subduing to corrosion fatigue and stress corrosion. Inspection prior to construction would not have been able to notice the miniature crack. Over the life span of the bridge, 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.
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. 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. 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.