The Tacoma Narrows Suspension Bridge - Case Study Example

College: The Tacoma Narrows Suspension Bridge Question The Washington Toll Bridge ity following its establishment, decided to act upon a request placed, for a bridge providing a connection between Kitsap and Tacoma. The engineer consulted initially, recommended an ordinary bridge design, which would have deep girders supporting it. However, this was deemed too costly and a suspension bridge specialist, Leon Moisseiff was consulted. The engineer based his design on the deflection theory, positing that the force of wind would be principally be borne by the cables and not the stiffening bridge parts, thus inhibiting lateral deflection of the bridge. The design also integrated piers along the bridge’s span for support. Additionally, incorporated in the design were several checking cables, as well as, devices proposed to be installed along the bridge’s spans to hold the deck down, preventing it from turning in the wind. Evident from the planning phase, the Authority and suspension bridge specialist, Moisseiff had the sole intention of constructing an affordable and safe bridge to benefit residents of Kitsap and Tacoma. However, the phase was not without flaws and ethical missteps. First, the authority turned down a bridge design previously proven safe, for a narrow suspension bridge design, never constructed before just because it was cheaper. The other design flaw was failure to take into account the actual wind force to which the bridge would be exposed. Additionally, during the planning phase, emphasis was placed on the structural components of the bridge. Of particular interest were the recommended open girders, which were later replaced with solid ones by the local building engineer during design execution. The plan was also flawed in regard to location selection for the bridge construction. The Tacoma Narrows; topography is highly susceptible to winds of high intensity, making it an unsuitable location to put up a suspension bridge (Pinto 221). Question #2 Qualitative risk matrix Likelihood Insignificant 1 Minor 2 Moderate 3 Major 4 Catastrophic 5 A(Almost Certain) M H H E E B(Likely) M M H H E C(Possible) L M M H E D(Unlikely) L M M M H E(Rare) L L M M M Level of Risk: (E)-Extreme Risk (H)-High Risk (m)-Moderate Risk (L)-Low Risk According to the chart above, the level of risk, rather obvious for the Tacoma Narrows Bridge, was high. This is because, the level of risk posed by the bridge necessitated formulation of a detailed action of plan on the way forward, in dealing with the issues arising when the bridge was still under construction. For instance, during construction the attaching tie-down cables snapped and proved to be ineffective. The bridge was also swayed by the wind presenting critical danger, an aspect attributable to its light weight and its narrow nature. Therefore, the level of risk was undoubtedly high, even though it could not be termed as catastrophic at this early stage. However as time progressed there were indications that the bridge would collapse. This notion rose from the increased number of risk factors that were identifiable on the bridge. For instance, slight winds would make the bridge sway to great levels and even cause wave like oscillations, posing great danger to motorists. This was because; instead of the bridge allowing wind to pass, it was acting like a kite, trapping moving air with its flat sides. Another risk factor warranting the classification of TNB as a high risk structure was, the topography of the Tacoma Narrows which made the bridge weaker, since it lay directly across the path of the wind, thus exposing it to maximum impact. The engineers also changed some of the recommended design elements thus increasing the structure’s probability of collapsing. All these were major factors that heightened the TNB’s risk level. Even though it was hard for individuals to notice all the risk aspects when the bridge was being built, the engineers should have identified the risk in the design, sighting of the bridge or during construction (Pinto 222). In assessing the riskiness of the project, the first thing to consider would be the height of the bridge as well as the quality of materials being used. This would assist in determining the likelihood of the risk’s occurrence, as well as, in ensuring that mitigation measures are set in place. For instance, one of the most common types of risks in suspension bridges is the Aeolian risk. This is the kind of risk that is instigated by wind storms, in addition to other natural causes. Based on the past events of natural hazards causing bridges to collapse, it is clear that wind risk is the principal contributing factor to the collapse of a bridge. Therefore, I would assess the risk based on the events that have happened in the past. For example, in the last thirty years, 45% of fatalities were caused by natural hazards. Evidently, if only the losses insured get considered, then the rise of the losses caused by wind would increase to 70%. For these reasons, I would base the risks of the Tacoma Narrows Bridge on past events; principally catastrophes relating to suspension bridges that have occurred. This would be the mode of risk analysis and identification that would be most appropriate for the Tacoma Narrows Bridge project. Question #3 Clearly, the Tacoma Narrows Bridge’s collapse was as a result of a numerous factors, including design mistakes. For example, the bridge was constructed using shallow girders meant to reduce its weight rather than using deep but open constraining trusses, thus rendering the bridge unstable. Also evident, is the fact that the bridge was too long in comparison with its width. It is possible that a broader bridge could have withstood the wind force, but it was too constricted to endure other load stresses (Pinto 222). Basic risk mitigation measures that could have worked for this project include: Use of open rigidifying trusses that could not only provide extra support, but also allow free passage of wind thus preventing it from oscillating. Increasing the width to length or span ratio, to enable the bridge to withstand wind and other loads. Increasing the bridge’s weight to further prevent excessive swaying Dampening the structure to reduce amplitude of wave oscillations. Use of an un-tuned forceful constraint to limit the structure’s movements. Increasing the rigidity and deepness of the girders or trusses. Streamlining the structure’s deck. Since the TNB collapse, engineers have adopted mitigation measures suggested above, which have proven helpful in construction of contemporary suspension bridges, by helping prevent damaging bridge vibrations. For example, bridge engineers do not build suspension bridges, which lack ample stiffening supports. Another crucial lesson learnt is the importance of testing structures for wind resistance at different pressures. An imperative risk mitigation measure in consideration of this aspect, therefore, is focus on aerodynamics to establish potential risks early enough. Additionally, 3-dimensional models should be used in simulations, to determine the exact effect of natural forces on structures, especially suspension bridges, which are highly vulnerable. This would help formulate strategies to guarantee not just the long life of a bridge, but the safety and well being of members of the public as well. Work Cited Pinto, Jeffrey. Project Management: Achieving Competitive Advantage, (2nd Ed). Upper Saddle River, New Jersey: Prentice Hall. 2009. Print. Read More