In the immediate aftermath of the disaster, it was vital to prevent any further collapses, especially on bridges of similar design. Two other bridges were built to a design similar to that of the Silver Bridge, one upstream at St Mary's, West Virginia and the other in Brazil at Florianopolis. The bridge upstream on the Ohio river, at St Mary's, was the focus of concern, and it was closed to traffic immediately after the disaster. The eye-bar design was actually quite widespread in other bridges, but frequently eye bars were provided not in pairs but in multiple connections, increasing the safety factor significantly. In the case of a single eye-bar failure, the others could support the load until repairs were made. This is certainly true of many British chain suspension bridges as well as US structures.
The inquiry recommended several key measures, which were enacted by President Johnson. They included:
- identification of safety-critical parts of existing bridges, and the materials of construction
- examination of possible failure mechanisms, including corrosion fatigue and stress corrosion fatigue
- development of new ways of inspecting critical parts of such bridges
- development of safeguards against future problems, including modified standards
- expansion of the knowledge of corrosion problems.
A nationwide inspection of existing bridges (about 1 million) was quickly undertaken, and many problems identified and corrected.
Now watch the video below on the ‘Silver Bridge’ disaster and then answer SAQ 8.
Transcript: Video 3
Transcript: Video 4
Transcript: Video 5
Describe the failure sequence of the Silver Bridge in December 1967, indicating the direct cause of the accident and any contributing factors that led to the failure. Include in your answer the evidence for the particular causes you mention.
The Silver Bridge accident occurred owing to stress corrosion cracking of a pin joint (no. 330) on the upper part of the subsidiary suspension chain on the north of the Ohio side of the structure. The critical crack occurred at the bottom of the northfacing lower eye bar of the joint. Each joint comprised two pairs of hardened steel eye bars fitted onto a steel pin with screwed caps to close the joint. The joint was the first one below the top of the Ohio tower. The accident happened about 39 years after construction, when the crack became sufficiently deep to grow catastrophically. The disaster happened for a combination of reasons, including the following:
- The joints of the bridge had been painted after construction, but the paint failed to protect the inner bearing surfaces.
- Inspection of the bridge joints could not reveal the corrosion problem caused by lack of a protective coating, since the bearing parts were totally inaccessible.
- Fretting corrosion occurred at the joints over a long period of time, encouraging initiation of stress corrosion cracks at the most highly stressed part of the joint at right angles to the axis of the main chain. The critical crack occurred at the lower part of the joint where water would naturally tend to collect.
- The crack was encouraged to grow by a very high level of tensile residual stress in the eye bar, itself probably caused during manufacture by casting followed by heat treatment and machining. The residual stress in the edge of the eye-bar hole was up to a third of the yield stress of the steel.
- The fracture toughness of the eye bar was low owing to the hardening process that increased the stiffness of the material. It was further lowered by the freezing conditions on the morning of the accident.
- Sulphur compounds were found in the corrosion cracks, suggesting that they exacerbated crack growth.
The evidence in support included direct examination of the failed eye-bar fracture and other surfaces, and tests on eye-bar steel including residual stress experiments by removal of material. X-ray analysis was used to examine the cracks in eye bar 330 for traces of impurities.
In terms of current knowledge of failure analysis, there are several gaps that could have been addressed at the time. In particular, the fact that the critical crack occurred in an eye bar below the top of one of the towers suggests very strongly that the level of residual stress varied between the eye bars. The origin of the eye bar analysed and discussed above is not stated in the official report, and it also appears that only this one eye bar was actually studied for residual stress. It would have been of great interest to have seen the variation of residual stress levels across the upper eye bars, because it is the only explanation of the formation of a critical crack in an eye bar exposed to lower imposed dead and live loads.
A second question arises about the source of the sulphur found in the critical crack initiation region. The official report points towards H2S, but this is a rare gas to have occurred in an open environment. Sulphur dioxide is a much more common pollutant, and could have been produced by a local power station using high-sulphur West Virginia coal. There was also a foundry in Point Pleasant, close to the east side of the bridge, which probably produced quantities of the gas during smelting.
A subsequent court case was brought by the injured victims and relatives of the deceased, alleging negligence on the part of the builders of the bridge. The case was rejected on the grounds that stress corrosion cracking of the kind found in the critical eye bar was not known at the time the bridge was designed. Although the plaintiffs received no compensation, the disaster had at least raised the importance of thorough inspection of an ageing infrastructure. However, bridge failures unfortunately continue to occur.