eyebar-chain

Over time, architecture has taken many turns. Some buildings last for thousands of years, and others struggle to last a hundred years. Also, some structures go through more things that can degrade them, such as the elements and usage. Such as the case with the Silver Bridge. The Silver Bridge was built in 1928 and connected Point Pleasant, West Virginia with Gallipolis, Ohio via US Route 35, taking it over the Ohio River. It was an eyebar-chain suspension bridge. The bridge got its name from the color of its aluminum paint.

The bridge was less that 40 years old when disaster struck on December 15, 1967. It is said that the bridge collapsed under the weight of rush-hour traffic, but that seems extreme to me. How many cars would it take to break a bridge, and how would they all fit on the bridge to make it break? When the bridge came down, 46 people lost their lives. Search and rescue operations were immediately put in place, and 44 people were found, but two of the victims were never found. When the bridge was rebuilt in 1969, it would be both a bridge and a memorial to those who lost their lives that fateful day.

No such disaster can take place without an investigation following it, to determine the cause. The causes of these disasters actually help to correct the flaws so that things like this don’t happen in the future. The investigation into the cause of the Silver Bridge collapse came down to the failure of just one eyebar. I suppose that once that one gave way, the load was too much for the rest of the eyebar links. It was further determined that the eyebar that failed had a small defect 0.1 inches deep. The analysis showed that the bridge was carrying much heavier loads than it was designed for. Cars were heavier too and there were more of them, putting more strain on the small defect in the eyebar. In addition, the bridge had been poorly maintained, further weakening the structure. These days, bridges can still fail, but they maybe don’t quite as often, and most last longer than 40 years.

The Silver Bridge, over the Ohio River, was an eyebar-chain suspension bridge built in 1928 and named for the color of its aluminum paint. In structural engineering and construction, an eyebar is a straight bar, usually made of metal, with a hole or “eye” at each end for fixing to other components on the structure. Eyebars are used in structures such as bridges, in settings in which only tension, and never compression, is applied. Also referred to as “pin – and eyebar construction” in instances where pins are being used. The Silver Bridge carried US Route 35 over the Ohio River, connecting Point Pleasant, West Virginia, and Gallipolis, Ohio.

The problem with the original form of this structural engineering is that as it rusted or corroded, the eyebar-chain was weakened. It wasn’t an obvious weakening, and often presented in the form of a small crack. 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. In the ensuing investigation of the wreckage, it appeared that the cause of the collapse was the failure of a single eyebar in a suspension chain, due to a small defect that was just 0.1 inches deep. Analysis showed that the bridge was carrying much heavier loads than it had originally been designed for and had been poorly maintained.

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 modest material strength “combed” together, 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, the safety is completely dependent upon correct, high-quality manufacturing, assembly, and maintenance. The bridge failure was due to a defect in a single link, eye-bar 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 towers were “rocker” towers…a common European design, 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. A 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. So many components that can so easily bring down a bridge…and no one knew.

At the time the Silver Bridge was built, a typical family automobile was the Ford Model T…weighing in at about 1,500 pounds. The maximum permitted truck gross weight was about 20,000 pounds. By contrast, at the time of the collapse, a typical family automobile weighed about 4,000 pounds and the large truck limit was 60,000 pounds or more. Bumper-to-bumper traffic jams on the bridge were also much more common, meaning that during rush hour…several times a day, five days each week…there was significantly more stress on the bridge elements. The whole situation was a recipe for disaster, and that is exactly what happened on December 15, 1967. After the collapse, and the subsequent replacement of the Silver Bridge in 1969, a scale model of the original bridge was on display at the Point Pleasant River Museum. An archive of literature about the bridge was also kept there for public inspection. On the lower ground floor, the museum displayed an eyebar assembly from the original bridge. The museum closed July 1, 2018 due to significant fire damage. The future of the Silver Bridge exhibit is not known.

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