Silver Bridge

The Silver Bridge was an eyebar-chain suspension bridge built in 1928 and named for the color of its aluminum paint. The bridge carried U.S. Route 35 over the Ohio River, connecting Point Pleasant, West Virginia, and Gallipolis, Ohio.

On December 15, 1967, the Silver Bridge collapsed under the weight 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 inches (2.5 mm) deep. Analysis showed that the bridge was carrying much heavier loads than it had been designed for and had been 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 having chain eyebars that are redundant in two dimensions; this early suspension bridge is 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, self-anchored suspension bridges of similar design and construction period (from 1924 to 1928) in Pittsburgh.

Silver Bridge structure

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 of common mild steel), rather than a thick stack of thinner bars of lower material strength, 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, safety is completely dependent upon correct, high-quality manufacturing, assembly, and maintenance.

In comparison, the Brooklyn Bridge, with wire-cable suspension, was designed with an excess strength factor of six. Wire cables have extremely high levels of redundancy, with the failure of a single wire strand almost unnoticeable. During the Brooklyn Bridge's construction, it was discovered that some rejected steel wire had been used. To compensate for the rejected steel wire already in the cables, 150 more good steel wires per cable were added to bring the total up to 5,434 redundant strands. The designer's son, Washington Roebling, decided the safety factor may have been reduced, but was far more than sufficient.

Rocker towers

The towers were "rocker" towers (a design common in Europe), which allowed 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.

Design loads

At the time of the bridge construction, a typical family automobile was the Ford Model T, with a weight of about 1,500 pounds (680 kg). The maximum permitted truck gross weight was about 20,000 pounds (9 tons). By contrast, at the time of the collapse, a typical family automobile weighed about 4,000 pounds (1,800 kg) and the large truck limit was 60,000 pounds (27 t) or more. Bumper-to-bumper traffic jams on the bridge were also much more common, occurring several times a day, five days each week, thus causing more stress to the bridge elements.

Wreckage analysis

The bridge failure was due to a defect in a single link, known as 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 inches (2.5 mm) deep when the link failed, breaking in a brittle fashion.

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. A collapse of the entire structure was inevitable since all parts of a suspension bridge are in equilibrium with one another. The damage to the link would have been difficult to see during inspection of the bridge:

Inspection prior to construction would not have been able to notice the tiny 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.