The highway bridge

Silver Bridge

The Highway Bridge known as Silver Bridge was constructed by the General Corporation and the American Bridge Company in 1928. The design consisted of a two lane eye bar suspension which was 2235 feet in length including the approaches. The bridge connected Point Pleasant, Kanauga and West Virginia. It was designed by the set specifications provided by the American Society of Civil Engineers.

The Bridge was the first in country to be aluminium painted, thus it was called Silver Bridge. The design consisted of a twenty two foot roadway and a single five foot pavement. There were some unique engineering techniques performed on the bridge including High Tension eye bar chains, a unique securing and stabilizing system, and Rocker towers.

The collapse of the Silver Bridge occurred on 15th December 1957. The bridge suddenly collapsed into the Ohio River. This resulted in thirty one vehicles to fell in to the river along with many deaths and injuries. Moreover the link connecting Ohio and West Virginia was destroyed and interrupted the transportation roots.

Construction

In the United States, the Silver Bridge was the first ever eye bar suspension bridge to be constructed of its type. The eye bars were connected together in pairs forming a link type chain. An enormous pin passed through the eye and connected each piece to the next. Each bar had a dimension of 2" x 12" and was connected by an 11 inch pin in every chain link. However depending on location of each chain the length designed varied.

One of the challenges was that to design the two eye bars in order to share the 4 million pound load of the bridge equally. By choosing the suitable material for the eye bars could resolve this issue. New heat treated carbon steel was developed by the American Bridge Company to use on the construction of the Bridge, which allowed each individual members to handle more stress across the bridge. By considering the two eye bars sharing the load, the steel could easily resist a load of 4 million pound.

The unique design of the structure meant that the anchorage design had to be ground-breaking. Bedrock was only found at a great depth, making the ordinary gravity type anchorage impractical. This resulted in a unique anchorage, which was designed consisting of a reinforced concrete trough 200 feet long and 34 feet wide filled with soil and reinforced concrete. The massive trough was supported on 405 sixteen inch octagonal reinforced concrete piles in which the cable pull is resisted by the weight of the anchorage and by sharing the halves of the piles.

The Rocker tower was the other unique design technique for the Silver Bridge. The unique towers, which had a height of 130 feet and 10 1/4 inches, allowed the bridge to move due to shifting loads and changes in the chain lengths due to temperature variations. This was achieved by positioning a curved fitting next to a flat one at the bottom of the piers. To avoid the structure from shifting horizontally, the rocker was fitted with dowel rods. This meant that the piers are not fixed to the bases due to this type of connection.

Once the structure was completed in 1941, the bridge was bought for $1,040,000 by the state of West Virginia. This included a $70,000 contract for maintenance and other engineering services.

In 1951, the bridge underwent a thorough inspection and some suggestions were reported to the West State Maintenance Engineers. All the necessary improvements were made upon the letter for the specified bridge corrections.

Frequent inspections were carried out in 1955, 1961 and 1965. However due to lack of funding, the suggestion which was made to the West Virginia Bridge engineers was not considered to every detail. Even though some of the corrections were not carried out, from the inspections it seemed that the structures of the bridge are still safe. With the technology those days, the awaiting tragedy could not have been seen.

The collapse of the Bridge

In 1967, after thirty nine years of allowing passage across the Ohio River, tragedy struck within minutes and the whole bridge collapsed into the river, injuring and killing the lives of many. Upon the previous inspections, no one was aware of any faulty in the structure.

Eye witnesses described bridge collapse as a falling deck card which sounded like a shotgun. People did not have the chance to escape this disaster. If the structure had shown some any warnings, the chances of people to escape this tragedy would have been decreased.

There were some questions raised about how this quivering of the bridge could suddenly result in collapse of the bridge. Every possible reason was researched thoroughly until proven wrong.

After examining the broken structure members of the bridge, it was found out the main cause of failure was due to the unique eye bar design made from the newly innovated heat treated carbon steel. The weakest link is to be described as the strongest point of the link. In other words if that weak point breaks then everything will fail. By the failure of this type of eye bar design, resulted in placing undue stresses on the other members. Moreover the remaining steel frame in the bridge buckled because of this unexpected concentrated stresses.

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 was the cause which attributed to the failure. "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." The only way to detect the fracture over the life span of the bridge would have been to disassemble the eye bar. At the time the technology which was being used could not detect cracks in these cases.

A stress corrosion crack occurs when brittle cracks are formed in a sound material through the simultaneous action of tensile stress and a corrosive environment. Moreover when this is combined with corrosion fatigue, (combination of a cyclic stress and a corrosive environment) as a result the failure was impossible to prevent. These factors contributing together resulted in weakening the eye bar over time and collapsing the structure of the bridge.

Another main cause of failure was the over loading the structure, which helped the corrosion fatigue and stress corrosion in the failure of the bridge. Initially the bridge was designed to resist a load of model T Ford vehicles, which it weight approximately less than 1,500 pounds. However in 1967 the weight of the cars were increase up to tripled the old cars which came upto 60,800 pounds. Cleary this shows that the civil engineers did not consider the life span of the project and for that reason no one predicted that in 40 years the weight of the traffic would be more than triple, crossing the bridge.

In result of the collapse, the other bridges which were built in similar design were shut down for inspection to prevent any other disasters. "The president Lyndon B. Johnson ordered a nation-wide probe to determine the safety of the nation's bridges."

Conclusion

The collapse of the Silver Bridge was a total disaster, but there are some positive attributes to the failure. No when a bridge is built, bridge inspectors are now more routine and thorough because of the Silver Bridge. Moreover, engineers are much more knowledgeable about corrosion fatigue and stress corrosion. This allows an improved quality of structure to be designed and built. Due to massive advancements in technology and better designed technique and materials there are less chances of this disaster to happen again.

In the future, new bridges promise to be "smart structures" with electronic monitoring to detect changes in a bridge, indicating potential problems, before they become problems of catastrophic size. Although some bridges are already being so monitored, but not many.

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