SILVER BRIDGE COLLAPSE

GENERAL INFORMATION

The Silver Bridge, named because of its shiny aluminum paint, was constructed by the General Corporation and the American Bridge Company and was completed in May of 1928 .

The bridge connected Point Pleasant, WV across the Ohio River to Kanauga, Ohio.

First eye bar suspension bridge in U.S.The collapse of the bridge took place

after 39 years after construction.

STRUCTURAL FEATURES

Total length of bridge – 681.23m(2235 feet)

Central span-213.36 m (700 feet)Side spans- 115.82 m (380 feet)The bridge was considered to be an

engineering marvel at the time of its construction because of its use of twin flat steel plates with eyelets located at each end to form the suspension component of the bridge instead of woven steel cables.

These steel plates were joined together with joints containing a metal pin which held this suspension component together,  very much like a bicycle chain is held together by links.

  Unfortunate in this design is the fact that if even one of these joints were to fail the whole suspension system would also fail.

UNIQUE ENGINEERING FEATURES

The bridge was designed under the specifications set forth by the American Society of Civil Engineers.

'High Tension' eye-bar chains, a unique anchorage system, and 'Rocker" towers are the unique features of this bridge.

High tension eye bar chains

The bridge's eye-bars were linked together in pairs like a chain.

A huge pin passed through the eye and linked each piece to the next.

Each chain link consisted of a pair of 5.08x30.48cm(2" x 12") bars and was connected by an 27.94cm(11") pin.

The length of each chain varied depending upon its location on the bridge.

The bridge was designed for a load of 4 million pound.

This new heat treated carbon steel would allow the individual members of the bridge to handle more stress. Along with the two eye-bars sharing the load, the steel could easily handle the 4 million pound load.

The bridge was of suspension design with “eyebars” chained together instead of the conventional wire cables. These “eyebars” were linked together with massive pins.

The The American Bridge Company developed a new heat-treated carbon steel to use for the eye-bars in the construction of the Silver Bridge.

Unique anchorage system.

Because of the unique design of the structure, the anchorage design needed to be innovative.

Bedrock was only found at a considerable depth, making the ordinary gravity type anchorage impractical.

An unusual anchorage was designed consisting of a reinforced concrete trough 60.96m( 200 feet) long and 10.36 m(34 feet) wide filled with soil and reinforced concrete.  

The huge trough was supported on 405, 40.64cm (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 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.

Rocker towers

Another unique design technique used on the Silver Bridge was the 'Rocker' towers.

The innovative towers, which had a height of 39.9m(130 feet, 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 done by placing a curved fitting next to a flat one at the bottom of the piers. The rocker was then fitted with dowel rods to keep the structure from shifting horizontally. With this type of connection, the piers were not fixed to the bases.

COLLAPSE OF THE BRIDGE

On December 15, 1967 during a Christmas shopping rush hour the collapse occured around 5:04 pm on this Friday evening taking with it 64 people in 31 vehicles,  of which 18 survived and 46 lost their lives.

With the previous inspections, no one conceived that the structure might fall and collapse into the riverbed.

View of bridge after collapse from Ohio side

 

Fallen roadway on West virginia pier

Another aerial view from Pt. Pleasant

The removal process begins

Ohio shore the morning after collapse

Another view of collapsed bridge

THE SUSPECTED REASONS FOR COLLAPSE

Three of the reasons that were commonly heard were:

1.A supposed 'Sonic Boom' prior to the collapse.

2.The 'Curse' of Chief Cornstalk. 3.Structural failure of a bridge memberAfter investigations it was confirmed that

the reason for collapse was the stuctural failure of the bridge.

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.

Stress corrosion cracking is the formation of brittle cracks in a normally sound material through the simultaneous action of a tensile stress and a corrosive environment.

Combined with corrosion fatigue, which occurs as a result of the combined action of a cyclic stress and a corrosive environment, disaster was inevitable for the Silver Bridge.

The two contributing factors, over the years continued to weaken the eye-bar and unfortunately the entire structure.

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.

FORMULATION OF MAINTENANCE LAWS

After the Silver bridge collapse a number of maintenance laws were formulated to prevent major accidents due to improper inspection.

    The Federal Highway Act of 1968 created the National Bridge Inspection Program(NBI), which ordered state agencies to catalogue and track the condition of bridges on principal highways.

In 1971, as a result of our recommendations following the Silver Bridge tragedy, the National Bridge Inspection Standards came into being .

   The Federal Highway Act of 1970 used the information contained in the NBI as the basis for funding for the Special Bridge Replacement Program (SBRP). 

This program provided federal funding for states to replace bridges that were in the most danger of failure.

   The FHWA also published the Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges. 

The Coding Guide outlined the specific elements that are required to be inspected on each bridge and guidelines regarding the inspection procedures. 

The Coding Guide has been revised in 1972, 1979 and most recently in 1995. 

Why Did the Eyebar Break?

The eyebar broke at a load below what was normally expected to occur. Examination of the brittle fracture surface that initiated the failure of the eyebar (and the ultimate collapse) revealed two small pre-existing cracks. These cracks were covered by an endcap that held the pin in place and could not be seen or detected during the regular inspections or while painting. (The last inspection was two years before the collapse.)

Extensive studies at NBS indicated that these small cracks started as corrosion pits. Under the action of the applied loads, rainwater combined with sulfur from air pollution to grow these pits into the cracks.

In addition, it was found that the high strength steel eyebars made in 1927 had a relatively low toughness - the property that makes steel resist fracture from cracks - and cold exacerbates this sensitivity to cracks. The longer the crack, the less stress is required to cause fracture and, in a sensitive steel, only a short crack is required to cause brittle fracture.

Finally, on that cold evening in 1967, the crack length (between 3 mm and 4 mm), the load on the bridge, and the weight of the bridge itself combined to reach a critical condition. The fracture that led to the catastrophe began.

CONCLUSION

Even though the collapse of the Silver Bridge was a disaster, there were positive aspects to the failure.

Bridge inspections are now more routine and in-depth because of the Silver Bridge.

Engineers are now more knowledgeable about corrosion fatigue and stress corrosion, which allows better quality structures to be designed and built.

With today's technology, as well as better design techniques and materials, there is hope that a Silver Bridge disaster will never again take place.

The silver bridge was replaced by Silver Memorial bridge in 1969.