Historical Civil Engineering Failure - Essay Example

Case study on an Historical Civil Engineering Failure Introduction Civil engineering projects have one of the best presentations of evidence that human civilisation and sophistication can be put transformed from mere ideas to the most complex structures. Using very complicated scientific and mathematical principles, engineers design structures and enhance their stability like expected in theory. Engineers learn from each other to ensure continued quality of the structure designs formulated, which leaves no room for lack of creativity. According to Delatte (2009, p1), civil engineering projects fail because there is some level of deviation of the obtained results, from the expected results. The author also cites other definitions of failure in engineering projects to be occasioned by the lack of conformity between the design and the expectations. In these definitions of project failure in the civil engineering sector, the author comes up with a simplified and precise approach for civil engineering designs that takes care of two basic aspects. On one side of the aspects, the author states that everything likely to go wrong in the project is highlighted in the design while protection measures must be introduced on the other hand. It is therefore correct to state that engineering designs are prepared in contemplation of difficult and disturbing realities that perfect conditions of implementation are inexistent. Engineers proactively introduce certain measures to overcome the challenges of imperfect conditions at every stage of project implementation. However, certain factors not foreseen in design, foreseen but inadequately tackled in the design or totally uncontrollable factors present the most difficult challenge to an engineer. Failure in civil engineering sector has been a matter of debate since antiquity, yet how to completely avoid it still remains elusive. This case study enumerates circumstances and details of the Tacoma Narrows Bridge collapse as a classical example of how devastating it could be in case of a failure in the civil engineering sector. Background information is complemented by causes and responses thereon have been included in the essay. Tacoma Narrows Bridge Failure (1940) Background Information In Tacoma, Washington, the Tacoma Narrows Bridge that is over 850 meters of span was designed and commissioned at a cost of about six and a half million US dollars to connect Seattle and Tacoma to Puget Navy Yard. It was seen as a major transport solution for both economic and military purposes around the Olympic peninsula. The University of Washington reported that the bridge was celebrated as a triumph of man’s ingenuity due to the anticipated benefits that the bridge brought (Horenstein, 2009, p233). Engineer Moisseiff designed the bridge such that it was strong enough using steel under the roadway and construction had begun two years before its collapse. On the 7th of November, the bridge had just been four months old since official opening for public use in 1940. Strong winds were blowing at an average speed of 68km/hr caused such strong excitations that the bridge collapsed, so soon from launching. Facts on the Bridge According to Dellate (2009, p26) the following fats can be isolated from the condition of the bridge design before the collapse. The design of the bridge made it the third longest span in the 1940s. This means that it was among the very first suspended bridges in the world to stretch over 2,800 feet. Among the other bridges of its kind at the time, it was having the narrowest breadth. Its design therefore ensured that it was lighter by avoiding unnecessary space. It was also among the most flexible bridges at its time, when compared to all other famous bridges of its kind. It lacked stiffening trusses but had plate girders which were of a lighter weight by almost a half. Reasons of the Collapse While there was no doubt about the competence of the design engineer and his co-designer, there was an unavoidable attribution of the collapse to natural occurrence beyond human control; strong wind. Leon Moisseiff had an impeccable record in previous bridge designs and expansive experience for working as a consultant in several leading engineering firms. Competence of the technical staff was not an issue during the construction period. These facts erased possible negligence or ignorance of basic bridge engineering projects. The wind was blowing at a dangerously high speed that could bring down any bridge of a similar nature. When the wind blew, according to Carper and Feld (1997, p139), the collapse was began halfway the span resulting from the buckling of the support girders. The author reports that the suspender cables were thrown into the air whereas the deck fell off approaching the towers of the bridge. Side spans were not affected up to a length of about 335 meters but the towers were caused to bend following the occasioned pull. The material used and workmanship were exempted from any blame regarding the collapse. Besides the wind strength, another cause of the collapse can be attributable to the narrowness of the deck. A very narrow deck characterising the bridge subjected it to poor torsional stiffness. The bridge had weaknesses in both its horizontal and vertical planes. In addition, the collapse could possibly have been caused by its great flexibility. Aerodynamic forces of the blowing wind subjected the bridge to excessive torsional sensitivity, having been designed such that it could withstand lateral rigidity tests. Investigations reports released thirty years after the collapse found out the actual cause was related to an eyebar failure (National Transportation Safety Board). The report mentioned that the eyebar had a crack which got worse under the torsional stretch caused by the excessively strong winds. Despite initial challenges and problems with the eyebar of the bridge, corrections were not made. Enquiry These findings were held both at a public enquiry and research levels. The University of Washington tested a modified bridge with improvements introduced in various ways in which the bridge was flawed (University of Washington, 2009). It was after the research on the improvements that the bridge was rebuilt, ten years later from the collapse. The Public Works Administration (PWA) constituted a panel of engineers to investigate the failure and the subsequent collapse. The findings of the PWA gave revelations to the effect that design flaws were minimal. It found out that there were deliberate measures put in place to counter and control oscillation amplitude and that it was out of imagination that wild oscillations would overcome its design. No structural damage took place according to the findings (Delatte, 2009, p30). Further to these revelations, it was found out that the cable band twisted causing the motion of the bridge that caused its collapse. Rigidity to counter static and dynamic forces would be hard to determine if the same formulae were used. Finally, the PWA proposed that further and extensive research was the only way to establish the actual aerodynamics of a suspended bridge. Contradiction Contrary to the findings of the PWA, Karman applied his aeronautical engineering prowess to propose a different position. The engineer attributed the bridge’s excessive motion under the effect of the wind to intermittent shedding of vortices of the air. Following the shedding, the wind created a wake which was in tandem with the structural oscillations which synergistically collapsed the bridge. Billah and Scanlan (1991, pp121) proposed that although the wake created by air vortices shedding was experienced, it could not have assisted the structural oscillations in collapsing the bridge. Instead, the authors proposed that there was resonance that caused the bridge and the vortices to propagate motion enough to cause a collapse. Research and Contributions to Engineering From the failure of the bridge, several research initiatives in contemplation of the Tacoma bridge collapse have been carried out. The circumstances under which the collapsing the Tacoma Narrows Bridge are still difficult to comprehend, having been marred by controversy regarding the theory to apply between the prominent theories produced on its effects. The above three propositions could perhaps be explained by the adoption of the University of Washington improvements which finally worked in the reconstruction design. Tunnel testing for bridges was however discovered as a reliable bridge integrity option only after the failure at Tacoma. Suspension bridges consider the flaws that Tacoma could have been overlooked are considered closely, including; deck design and mass, trusses integrity and stays. The structural oscillation could have overcome excessive motion if some of the flexibility flaws were not overemphasised. Stiffness and mass determine the resistance that a suspension bridge makes against excessive motion. Mass moment of inertia is usually targeted when implementing rotation resistance to suspension bridges. Mass moment of inertia (I) is given by; I= ∫m r2dm Four lanes of the bridge doubled mass m and moment r, while trusses plate girders had an excess of excess between eight to six metres. Resistance to rotational movement was calculated to give eight times of a reliable resistance level. However, stiffness was not very promising to counter serious forces such as the strong wind (Delatte, 2009, p36). Images of the collapsed Tacoma Narrows Bridge (University of Washington, 2009) References Billah, Y. & Scanlan, R. (1991) “Resonance, Tacoma Narrows Bridge Failure and Undergraduate Physics Textbooks,” American Journal of Physics, vol. 59 no. 2 pp.118-124 [online] Available from: http://www.scribd.com/doc/22244624/Resonance-Tacoma-Narrows-Bridge-Failure-And-Undergraduate-Physics-Textbooks [Accessed February 2011] Carper, K. L. & Feld, J. (1997) Construction failure. New York, NY: John Wiley and Sons Delatte, N. J., (2009) Beyond failure: forensic case studies for civil engineers. Reston, VA: ASCE Publications History of the Tacoma Narrows Bridge. [online] University of Washington. Available from: http://www.lib.washington.edu/specialcoll/exhibits/tnb/ [Accessed February 8 2011] Horestein, M. N., (2009) Design concepts for engineers. Upper Saddle River, NJ: Prentice Hall Read More