đź•‘ Reading time: 1 minute
Point Pleasant Bridge, also known as Silver Bridge, was constructed over the Ohio River to provide the crossing for highway number-35 of the United States of America. The bridge connected two major cities of Ohio and West Virginia, and provided direct access to their respective state capitals.
On 15th December 1967, the Point Pleasant Bridge collapsed due to heavy traffic on the bridge. It was the holiday season and people were shuttling back and forth between the two cities via the bridge. The traffic on the bridge was crawling. When around three big trucks and 72 cars came to a halt, the bridge began to sway wildly and ultimately collapsed.
A total of 46 people lost their lives due to this accident. Following this incident, the U.S. government brought several new reforms to check the functionality and safety of bridges across the nation.
The construction of the Point Pleasant Bridge was undertaken with the adoption of the latest technologies and new construction materials of the time. It was considered to be the most unique and cost-effective bridge. However, the check for the safety of the bridge could not be verified due to the limitation on advancements in bridge engineering. Also, the bridge was not designed with a sufficient number of redundancies. Further, no testing methods were developed to measure the strength of large eyebar chains.
When the Point Pleasant Bridge was being constructed, the knowledge of long-term fatigue combined with the crevice corrosion was limited. Also, the designers did not consider the effect of unsymmetrical loading on fatigue. No one thought that a small, exposed surface flaw included in the design could have caused the fatigue failure and, ultimately, the collapse of the bridge.
Contents:
1. Design and Construction
The construction of the bridge started in 1920 and completed in 1928. There were two main contractors who were awarded the work. The works related to the pier foundation were carried out by the general contracting company of Pittsburgh, and the superstructure was constructed by the American bridge company of Pittsburgh.
The design of the bridge was unique because it had high tension eyebar chains, rocker towers, and a unique anchorage system. The system of the eyebar consisted of two force bar members with rounded eyes at each end; the anchorage system consisted of reinforced concrete troughs filled with concrete; the towers were constructed on octagon-shaped piles of reinforced concrete.
A similar design was used earlier in the St. Mary Bridge in West Virginia, which is also located on the Ohio river. It was the second eyebar suspension bridge consisting of heat-treated eyebars, and a rocking base was constructed on the Ohio river. Key features of the Point Pleasant Bridge are described below:
- The bridge was constructed as a continuous truss between anchorages.
- The suspending eyebar chain acted as the top chord of a variable-depth stiffening truss.
- Eyebar chain greatly improved the bridge’s stiffness with less steel.
- Rocker towers were hinged at the bases to accommodate thermal deformations.
- The bridge was designed using the deflection theory.
2. Collapse of the Point Pleasant Bridge
On a cold winter evening, the bridge suddenly collapsed due to the failure of the connection between the eyebar to the first joint on the north side of the bridge. This failure caused the Ohio tower to fall and resulted in the collapse of the West Virginia tower. The center span broke and flipped over into the water below, carrying 64 people from the roadway.
A few hours after the collapse, the St. Mary’s Bridge was closed for safety as its design was similar to that of the Point Pleasant Bridge. Once all the survivors had been rescued, and vehicles had been recovered, the operations shifted towards the removal of the collapsed structure. This response was important because the Ohio River played a major role in the transportation of freight, and the wreckage prevented ships from navigating the waterway.
After the collapse of the Point Pleasant bridge, the President of the United States created a task force on bridge safety. Its objectives were to identify the cause of the failure of the bridge, to figure out how to fund a new bridge in an efficient manner, and to define a system of standards that would ensure the safety of the bridges in the country.
3. Reasons behind Failure of the Bridge
The experts investigated many possible causes of the collapse. Some of them are discussed below:
3.1 Failure Due to Explosions and Impact
The U.S. Army Ordnance Group ruled out the possibilities of explosions and impacts. It first verified with the military that no airplanes had passed through the vicinity at the time of the collapse. The vehicles removed from the wreckage were checked for explosives, and structural members were inspected for signs of damage due to explosives. Design features also indicated that the towers were protected from possible contact with a vehicle.
3.2 Substructure of the Bridge
One feature considered to be potentially faulty was the substructure of the bridge. If the piers shifted, the towers would be out of alignment, inducing a sag in the cables. This sag would change the load distribution throughout the structure. This shift could have been induced by sediments around the piers being washed away (known as scour), a deficiency in the foundation that would cause the anchors and piers to move, or an impact to the piers from a water vessel.
Scour was eliminated because of the observations of the divers at the scene. They reported no holes around the piers or any signs of damage to the piers. Further verification showed that the piers were in proper orientation. The design was also evaluated, and the bridge did not provide any redundancy in the structural members that connected the suspension chain to the eyebars. Each joint contained two eyebars that connected to the suspension chains.
3.3 Consideration of Dynamic Loading
The stress calculations that had been prepared for the design were verified for assurance that the bridge met the engineering criteria that were used at the time of construction. This analysis took into consideration static and dynamic loads. The stresses that occurred during the collapse were less than the maximum stresses identified in the design. However, there was no evidence that calculations were made for the correct dynamic loading.
This stress increase was considered in terms of an increased percentage of the original live loads. To verify the fact that dynamic effects were not a factor leading to the collapse, vibration response, and dynamic stress amplification tests were performed on the St. Mary’s Bridge. The tests sought to establish the natural frequencies of the bridge and to determine the excitation created by heavy vehicle use.
These tests concluded that the live loads were not excessive and that the loading caused some vibrations in the individual members of the structure.
3.4 Evidence of Fracture
The reassembly of the bridge showed evidence of a cleavage fracture in the eyebar on the Ohio side of the truss. However, different opinions surfaced to justify the cause of this fracture. One opinion cited evidence for fatigue stress, whereas another one explained the cause as a combination of stress corrosion and corrosion fatigue.
The U.S. Bureau of Standards noted the fracture pattern on the eyebar. The cross-sections of the eyebar revealed a number of small cracks, about the size of pinholes. One of these, it appeared, had grown into the 33 mm flaw that had initiated the fracture.
The brittleness of the steel connection, and thus its susceptibility to sudden fracture, was enhanced by two factors. The first factor was the use of higher-strength steel, which is more brittle than the mild steel typically used for structural steel and reinforced concrete construction. The other factor was the low temperature that on the day of the collapse, was near to freezing temperature.
3.5 Corrosion
The growth of the crack was most likely due to stress corrosion. The two predominant factors were the application of tensile stress and the attack of pollutants on the steel. The tensile stress came from traffic loads and thermal effects. The coal-burning factories and locomotives, and automotive pollution provided many airborne chemicals that could attack steel.
The narrow gap between the eyebar and the connecting pin made the problem worse. It was wide enough to provide an air space for pollutants to attack the steel. However, it was not large enough to allow engineers to inspect the connection.
3.6 Fatigue Stress
The evidence supporting failure by fatigue stress was based on changes in the live loads of the structure, which changed the orientation of the eyebars. As the loads redistributed in the eyebar chain, the eyebar heads and the connecting pins would rotate. This adjustment would cause a sliding friction to occur rather than a rolling friction.
Live loads produced by a heavy vehicle would cause the hole in the eyebar to be placed in tension. After the vehicle passed, the connection would respond in compression to return to its original static dead load.
This oscillation caused fatigue stress in the member and could, over time, lead to failure. The eyebar would still be vulnerable to failure even if the resulting stress did not exceed the yield strength of the material. It was determined that the stress reversal was unique to a few eyebars because the vehicle lanes were not centered on the roadway.
3.7 Lack of Symmetry of the Bridge
A sidewalk and a vehicle lane on the south side, instead of two vehicle lanes on the north end, caused this lack of symmetry. This change of symmetry caused uneven loads across the suspension spans.
The combination of uneven loading and changes in stress levels caused the pins to shift. This theory implied that the pin cap would become responsible for resisting the lateral forces within the connection. It was possible for these forces to exceed the available resistance. This exceedance would cause the eyebar to detach from the pin, allowing the connection to rotate and thus cause the collapse.
3.8 Manufacturing Defects
A different theory of the collapse found support in the metallurgical aspects of the eyebar chains and the consequence of both loading and the environment on the steel. Tests of different eyebars showed a consistent composition between the specimens, which indicated that the fractured eyebar was not unique. However, certain characteristics suggested the cause of the failure of the material.
The hardness was not uniform throughout the thickness of the material. This lack of consistency indicated that the eyebar had a more severe susceptibility to stress corrosion and hydrogen embrittlement in the harder section of the material. Further examination of the eyebar showed a crack that had occurred during manufacturing.
3.9 Atmospheric Conditions
It was also found that the eyebar was affected by the atmospheric conditions. These factors made the high-strength steel vulnerable to the propagation of cracks and reduced the time for these cracks to occur. Testing showed that as the environmental temperatures decreased, the material was more susceptible to fracture. This finding had significance, considering the extreme temperature changes in the region.
The composition of the atmosphere also played a critical role in the state of the steel. The linkage of the eyebar chains to the suspension cables by a pin created a section of the material that could not be protected by paint. The exposed surface was thus left open to natural elements and created a built-in weakness.
Inspection of the wreckage showed corrosion in many sections of the bridge. Studies were conducted to identify the influence of the environment on the material. Tests showed that the eyebar pin connection was susceptible to pitting corrosion due to water, hydrogen sulfide, and salt. The reaction most prevalent to the cause of the collapse was found in the eyebar chains and pins. Calculations showed a 3% loss of steel due to this degradation of material.
3.10 Conclusion
The study of high-strength steel in the environment determined that the combination of stress corrosion (due to tensile stress in a corrosive environment) and fatigue corrosion (induced by the oscillation of loading in the corrosive environment) led to the propagation of the crack in the eyebar and caused the failure of the eyebar and ultimately caused the collapse of the bridge.
4. Standards
When the bridge was designed, the loading was based on the American Association of State Highway and Transportation Officials (AASHTO) H-15 specifications for a 15-ton truck with 106 kN on the rear axle and 27 kN on the front axle. Since then, the design load has been increased to the heavier AASHTO HS-20.
Reduced factors of safety, 2 against ultimate strength and 1.5 against yielding, were used in the original bridge design. This limit contrasted with higher factors of safety, 2.75 and 1.75 respectively, for an alternate conventional suspension design. However, the steel used was of unusually high strength, and its properties were not well known. A higher, not lower, factor of safety should have been used for this bridge.
5. Changes to Bridge Inspection
The National Bridge Inspection Standards were created in response to the collapse. In 1968, the U.S. federal government enacted a procedure for national bridge inspection. Before this law, bridges were not consistently inspected. The new procedure provided exact protocols to be followed, and inspectors were required to be evaluated and certified by the Federal Highway Administration (FHA). Each bridge was to be inspected every two years.
The system of inspection is broken down into a standard procedure. The format categorizes the superstructure, the substructure, and the deck of the bridge into a set range of values. The importance of this standardization is to allow the accurate rating and comparison of the bridges in the inventory.
Other improvements to bridge management include nondestructive evaluation methods. In 1996, the FHA created the nondestructive evaluation center in McLean, Virginia. This center works to identify inspection methods through the use of technology that includes laser measurement, monitoring systems, and ultrasonic tests.
6. Lessons Learned
The following points describe the key lessons from the failure of the Point Pleasant Bridge:
- Stress concentration and fatigue can be a dangerous combination to cause the failure of a bridge.
- The built-in flaw in the eyebar led to the sudden propagation of the crack when the fatigue stress exceeded its limit.
- The cause of the fatigue stress was the cycling loading applied on the bridge over the four decades.
- The substructure of the bridge was lacking the structural redundancy. The connection at the eyebar was not redundant because it consisted of only two pairs of eyebars. When the first eyebar failed, the entire connection was rotated and got separated from the eyebar.
- Also, the overall redundancy of the structure was poor. The loss of one element would lead to the collapse of the bridge.
- Towers of the bridge were supported on the rockers. Thus, as soon as the tower toppled over, all the spans of the bridge collapsed because it was constructed as a continuous truss between anchorages.
- The American bridge company didn’t make its calculations and test results public. Also, the company lacked technical justification for the use of low safety factors against ultimate and yielding strength.
FAQs
A total of 46 people died due to the collapse of the Point Pleasant Bridge.
The Point Pleasant Bridge was a suspension bridge.
The length of the Point Pleasant Bridge was 681 m.
The Point Pleasant Bridge was constructed at the cost of USD 1.2 million.
Read More
Golden Gate Bridge: Construction of One of the Longest Suspension Bridges in the World
Chenab Bridge: Construction of the World’s Highest Rail Bridge
Austin Dam Failure: One of the Biggest Disasters in US History
