Engineering Disaster: The Silver Bridge Collapse - Essay Example

Engineering Disaster: The Silver Bridge Collapse Table of Contents Table of Contents 2 2 Introduction 4 The Unique Design of the Silver Bridge 4 The Silver Bridge Collapse 6 Investigations into the Collapse of the Bridge 7 Immediate Effects of the Collapse of Silver Bridge 9 Impact on Engineering Practices 10 Conclusion 10 References 12 Abstract The responsibility of engineers is to design systems that not only work but are also safe. The silver bridge is one system that was designed to work but ended up failing. The bridge was quite innovative in design and relied on newly developed materials. However, the design proved to have been associated with a low factor of safety. The incident has woken up engineers to the fact that they have to assign higher factor of safeties in their designs especially when dealing with systems that put lives at risk. The collapse, while quite unfortunate has seen several changes to the way engineers work and to the level of attention that bridges are given. Engineering Disaster: The Silver Bridge Collapse Introduction Engineers are charged with the responsibility of designing and producing systems that meet the purpose for which they are designed. While this is the case, they also have to ensure that their designs and creations are safe when and when not loaded. The Silver Bridge collapse that occurred in 1967 is one incident that brings to light the importance of ensuring a high factor of safety and choosing right materials in the design of engineering systems. This paper discusses the Silver Bridge collapse and its effects on the way engineers design and choose materials for their systems. The paper will further discuss how engineering disasters influences engineering practice and regulations. The Unique Design of the Silver Bridge The Silver Bridge was clearly the first eyebar suspension bridge of its ilk to be built in the U.S as noted by Ballard (1969). Apart from this, it had a number of special features that made it particularly unique. This section of the paper will discuss the special features that characterized the silver bridge and some of the concerns that experts had with respect to the innovative design and structure. The Silver Bridge was built in 1928 and was designed to last at least at least a hundred years. The eyebar-chain suspension bridge was built over River Ohio to connect Gallipolis, Ohio and Point Pleasant, West Virginia. The Silver Bridge was designed in line with the specifications prescribed by the American Society of Civil Engineers (Ballard 1969). It was designed as an H-15 load demand which means that the bridge had a limit in terms of the weight it could carry. The bridge was designed to have two lanes and measured 2,235 feet in terms of total length. It had a 22-foot roadway and a five-foot sidewalk. Fig. 1: The Silver Bridge before its collapse Courtesy of http://www.transportation.wv.gov For one, the silver bridge was special in its design considering the innovative material that was used to construct the chains. The eye-bar chains were made of high tension steel and rocker towers (Ballard, 1969). The eye-bars formed chain-like links as they were joined in pairs. The link between the two inch by 12 inch eye-bars was formed using a huge pin (11 inches in size) that passed through the eyes of each piece. Each of the chains had a different length depending on its position on the bridge. Being the first of its kind, several questions were raised over the structure of the bridge and it safety when loaded. Some engineers wondered what would happen in case the two eye-bars were not equally loaded to the extent that one bore more of the (approximately) four million pounds that was the weight of the bridge. They also wondered whether or not the eye-bars would fail when subjected to extreme stress. The designers of the bridge had an answer to these concerns. Instead of using ordinary steel, they would use a new heat –treated carbon steel. This innovative material was would squarely handle the concerns raised given that it would allow individual elements of the bridge to bear more stress. When applied together with the two eye-bars, the new material could comfortably handle a load of more than four million pounds. According to design, the chain steel eye-bars were heat treated and enjoyed an ultimate strength of 105,000psi and an elastic limit of 75,000 psi as noted by LeRose (2001, par. 5). In addition, the bars had a working stress of 50,000 psi. Similarly, the eye bars embedded into the unique anchorage were heat treated and enjoyed an elastic limit of 50,000 psi, an ultimate strength of 75,000psi and 30 psi in unit working strength (LeRose 2001, par. 5). Another unique feature that characterized the bridge was the use of innovative rocker towers. The towers were 130 feet in height and slightly more than 10 inches in width. These allowed the bridge to adjust itself to bear its load and respond to temperature changes that ultimately affected the lengths of the chains. The towers were installed such that a curved fitting was positioned next to a flat one at the base of the pliers and dowel rods used to fit the rockers so that the structure would not shift horizontally as noted by LeRose (2001, par. 5). Yet another unique feature of the bridge was its anchorage. Because of its design and the shear depth of bedrock in the area in which the bridge was to be constructed, it proved to be impractical to use ordinary anchorage. In response to this situation, a concrete trough made of reinforced concrete measuring 200ft x 34 ft was constructed and filled with reinforced concrete and soil. The trough was supported by reinforced concrete piles made of reinforced concrete such that the weight of the anchorage resisted cable pull as noted by LeRose (2001, par. 6). The Silver Bridge Collapse This section of the paper is dedicated to discussing the events that occurred during the collapse of the silver bridge. The Silver Bridge served its purpose well for 39 years after its completion. However, on the evening of December 15, 1967, the bridge collapsed as noted by The Herald-Dispatch (2013). This incident happened at a time when no one was expecting it to given that it had been subjected to frequent inspections the last of which was conducted on April 8, 1961. Although it was noted during some of these inspections that the bridge needed some improvements, engineers always noted that it was structurally sound (LeRose, 2001). On the fateful day, members of the public were going on with their activities as usual until they heard a deafening sound that marked the collapse of the structure. The bridge suddenly fell to the riverbed taking with it several vehicles and people. Fig:2 The collapsed Silver Bridge Courtesy of http://failures.wikispaces.com Investigations into the Collapse of the Bridge Soon after the Silver Bridge incident, investigations began in earnest to establish the cause of the collapse. Three main reasons were postulated as to have been the reason behind the collapse of the super structure as noted by LeRose (2001). Some people noted that the collapse was as a result of a sonic boom. Others thought that it was an effect of the curse by Chief Cornstalk. Yet another school of thought believed that the collapse was a result of a failure of one or more members of the bridge. Investigation into the sonic boom hypothesis was quickly discarded after it was revealed that nearby military bases had no aircrafts that were able to produce sonic booms at the time of the incident. This decision was further reinforced by the revelation that surrounding buildings never suffered damage as would commonly be the case in the event of a sonic boom. The hypothesis that the bridge collapsed as a result of the curse by Chief Cornstalk was also discarded after broken structure members of the bridge had been inspected (LeRose, 2001). Investigations revealed that the eye-bar design was the main cause of the failure. It was revealed that the heat-treated carbon steel eye-bar broke thereby leading other members of the bridge to be subjected to extreme stress. As a result, the steel frame buckled causing the bridge An inspection of the eye-bar 330 revealed that the member had a cleavage fracture on its lower limb. The fracture occurred at joint C13N of the suspension chain at the Ohio side of the span LeRose (2001, par. 19). It was noticed during the investigations that the fracture had been formed as a result of the extension of a small crack that came into being as the steel eye-bar was being cast. The crack grew over time as a consequence of corrosion fatigue and stress corrosion. Back then, it was not known that steel could be subject to stress corrosion and corrosion fatigue. Inspections on the bridge during its lifetime could not have revealed the crack as it was hidden. It would have been necessary that the bridge be disassembled for the crack to be noticed. Fig. 3: The eye that fractured Courtesy of http://museum.nist.gov Another factor that aggravated the corrosion and fatigue that led to the collapse of the bridge was the load that it had to bear. The bridge was based on the model-T Ford and considered the average weight of a vehicle to be 15000 pounds as noted by LeRose (2001). However, at the time of the event, the weight limit for vehicles was 70,000 pounds - far much more than the design weight. Immediate Effects of the Collapse of Silver Bridge The Collapse of Silver Bridge occurred at a time when there was heavy traffic on the bridge and saw the deaths of 46 people and the loss of at several vehicles (The Herald-Dispatch, 2013). Nine people got injured as a direct result of the collapse. A result of the losses, the nation was gripped with emotional and physical pain. Following the collapse of the Silver Bridge, a lot of attention was directed toward the conditions of older bridges not only in the U.S. but in other parts of the world as well. In a bid to avoid similar events, bridge inspection protocols were intensified and several bridges or their parts replaced. Many bridges that were designed similar to the Silver Bridge got a lot of attention for their level of safety. One of the bridges, located at St. Marys, West Virginia, was immediately closed to traffic only later to be demolished by the government (LeRose, 2001). The Hercilio Luz Bridge in Brazil however stands to-date owing to the fact that it was designed with a higher factor of safety. One of the recommendations of the posted by the Safety Board to the US Secretary of Transportation was the expansion of research programs aimed at identifying susceptible bridge construction materials (National Transportation Safety Board, 1971). Another recommendation was the development of new inspection materials for testing bridges and their construction materials. The collapse of the bridge woke up engineers to find other ways of testing the structural conditions of bridges. In this respect, various non-destructive methods were developed and are in use to-date in testing not only bridges but other engineering materials as well. With the collapse of the bridge, the state and engineers got inspired to see to it that older bridges got inspected and maintained regularly (Armangnaz, 1968). In memory of the event, a scale model of the silver bridge was built and kept at the Point Pleasant River Museum together with literature about the bridge. A memorial has also been constructed at Point Pleasant in commemoration of the victims of the incident. Fig. 4: Scale model of the Silver Bridge Courtesy of http://museum.nist.gov Impact on Engineering Practices Following the incident, engineers in the U.S. have to inspect the country’s bridges every 24 months (Armangnaz, 1968). Engineers have also had to rely on new, more thorough inspection protocols when dealing with bridges and similar engineering systems. Yet again, engineers have learnt to incorporate a higher factor of safety when dealing with high risk systems such as bridges. Yet again, engineers now apply more advanced testing methods that involve non-destruction of materials as they inspect bridges and similar structures. Conclusion The Silver Bridge was an engineering system that was designed to solve a great need. The design of the structure was innovative and included the use of a material that was new in engineering circles. The bridge, however, failed as a result of corrosion and stress. While the incident is associated with a lot of loss, it has served as a wakening call to engineers and government authorities across the world when it comes to dealing with bridges. Bridges in the U.S., for example, are more regularly inspected and the inspection done on the bridges is a lot more thorough than it was done before the silver bridge incident. Engineers also put a lot of emphasis on a higher factor of safety in their designs owing to the lesson learnt from the incident. Engineering materials are developed over time to meet certain needs. While this is the case, engineers have learnt that there is need to test the materials that they develop thoroughly before using them as product parts so as to avoid risking the lives of the public. References National Transportation Safety Board (1971). A Highway Accident Report, collapse of U. S. 35 Highway Bridge. Washington: GPO: National Transportation Safety Board. Armangnaz, A. (March, 1968). "Our Worst Bridge Disaster: Why Did it Happen?" Popular Science Magazine. Ballard, W. (June, 1969). "An Eye-bar Suspension span for the Ohio River," Engineering News-Record, 997-1001. LeRose, C. (October, 2001). The Collapse of the Silver Bridge. West Virginia Historical Society Quarterly, XV(4) http://www.wvculture.org/history/wvhs1504.html The Herald-Dispatch (Dec. 15, 2013) Gallery: Historical photos of the 1967 collapse of the Silver Bridge http://www.herald-dispatch.com/specialsections/100years/x2054232791/Gallery-The-collapse-of-the-Silver-Bridge Read More