Briefly summarize the situation in the case. In what ways were the project\'s pl

Question

Briefly summarize the situation in the case.

In what ways were the project's planning and scope management appropriate?

At what point in the project did the planners begin taking risks?

Explain how project constraints were or were not taken into consideration.

Conduct either a qualitative or quantitative risk assessment on this project. Identify the risk factors that you consider most important for the suspension bridge construction.

Develop a qualitative risk matrix to classify the various risk elements. Why was so little risk analysis done at the time?

What forms of risk mitigation would be appropriate?

Select two risk mitigation strategies. Explain how the strategy might be used, with whom it would be used, and how effective the strategy might be.

Case Study 7.3 249 CASE STUDY 7.3 Classic Case: Tacoma Narrows Suspension Bridge The dramatic collapse of the Tacoma Narrows suspen- a series of violent vertical and torsional oscillations. sion bridge in 1940, barely four months after comple- Alarmingly, the amplitudes steadily increased, suspen tion, was a severe blow to the design and construction sions came loose, the support structures buckled, and of large span bridges. It serves as a landmark failure in the span began to break up. In effect, the bridge seemed engineering history and is, indeed, a featured lesson in to have come alive, struggling like a bound animal, most civil engineering programs. The story of the col- and was literally shaking itself apart. Motorists caught lapse serves as a fascinating account of one important on the bridge had to abandon their cars and crawl off aspect of project failure: engineering's misunderstand- the bridge, as the side-to-side roll had become so pro- ing of the effect that a variety of natural forces can have nounced (by now, the roll had reached 45 degrees in on projects, particularly in the construction industry. either direction, causing the sides of the bridge to rise Opening in July 1941, the Tacoma Narrows and fall more than 30 feet) that it was impossible to tra Bridge was built at a cost of $6.4 million and was verse the bridge on foot largely funded by the federal government's Public After a fairly short period of time in which the was wave oscillations became incredibly violent, the sus- ks Administration. The purpose of the bridge essentially viewed as a defense measure to connect pension bridge simply could not resist the pounding Seattle and Tacoma with the Puget Sound Navy Yard and broke apart. Observers stood in shock on either at Bremerton. As the third-largest single suspension side of the bridge and watched as first large pieces of bridge in the world, it had a center span of 2,800 feet the roadway and then entire lengths of the span rained and 1,000-foot approaches at each er down into the Tacoma Narrows below. Fortunately, no human lives were lost, since traffic had been closed in Even before its inauguration and op ening, the bridge began exhibiting strange characteristics that the nick of time were immediately noticeable. For example, the slightest wind could cause the bridge to develop a pronounced supported by massive 130-meter-high steel towers longitudinal roll. The bridge would quite literally begin comprised of 335-foot-long spans. These spans man to lift at one end and, in a wave action, the lift would aged to remain intact despite the collapse of the main "roll" the length of the bridge. Depending upon the span. The second bridge (TNB II) would end up mak severity of the wind, cameras were able to detect any ing use of these spans when it was rebuilt shortly there- where up to eight separate vertical nodes in its rolling after, by a new span stiffened with a web truss ction. Many motorists crossing the bridge complair of acute seasickness brought on by the bridge's rising person committee was immediately convened to and falling. So well-known to the locals did the strange determine the causes of the Tacoma Narrows Bridge weaving motion of the bridge become that they nick- collapse. The board consisted of some of the top named the bridge Galloping Gertie The slender 12-meter-wide main deck had been Following the catastrophic failure, a three- scientists and engineers in the world at that time: Othmar Ammann, Theodore von Karman, and Glenn That the bridge was experiencing increasing and unexpected difficulties was clear to all involved in the Woodruff. While satisfied that the basic design was project. In fact, the weaving motion of Galloping Gertie sound and the suspension bridge had been con became so bad as the summer moved into fall that structed competently, these experts nevertheless were heavy steel cables were installed externally to the span able to quickly uncover the underlying contributing in an attempt to reduce the wind-induced motion. The causes to the bridge collapse first attempt resulted in cables that snapped as they were being put into place. The second attempt, later in the fall, seemed to calm the swaying and oscillating motion of the bridge initially. Unfortunately, the cables would prove to be incapable of forestalling the effects of the dynamic forces (wind) playing on the bridge; they snapped just before the final critical torsional oscillations that led to the bridge's collapse. . Design features -The physical constru of the bridge contributed directly to its failure d was a source of continual concern from the time of its completion. Unlike other suspension bridges, one distinguishing feature of the Tacom Narrows Bridge was its small width-to-length ratio-smaller than any other suspension bridge of its type in the world (although almost one mile in length, the bridge was only constructed to carry a single traffic lane in each direction) That ratio means quite simply that the bridge was On November 7, 1940, a bare four months after opening of the bridge, with winds of 42 miles per hour blowing steadily, the 280-foot main span that had already begun exhibiting a marked flex went into

Explanation / Answer

The case is about understanding the reasons behind the collapse of the Tacoma Narrows Suspension Bridge. Inappropriate design, not sticking to the plan, and some natural factors led to the collapse of the bridge just 4 months after its inauguration. In terms of design, the designers had kept the width to length ratio too small for such a long bridge. They also failed to take the dynamic factors of wind into account in their design. The plan initially called for use of open girders but a local contractor substituted them for flat solid girders. This may have contributed to the collapse by not allowing the wind to pass through. Finally, the location itself was such that it acted like a wind tunnel which doubled the velocity of the wind and thus other alternatives should have been taken into account.

At the point in time that we are speaking, i.e 1930s, it was widely held that static design features of the bridge would be sufficient to tackle the dynamics of nature (wind in this case). Thus, planning was appropriate in that it took all the factors into account that were prevalent in the construction industry at that point in time. Probably, the location of the bridge was fixed by some other non technical reasons and thus the project did not have the scope to change it. Thus, the scope of the project was to build a suspension bridge in a given location. In that sense, the scope management of the project is appropriate.

The planners began to take risks at a point during the design phase itself when the width to length ratio was kept one of the smallest in the world. Further, they took a risk at a point in execution when they substituted the girders.

Project constraints with respect to budget and timelines were taken into consideration. But the project barely identified nature as a constraint which ultimately led to the collapse of the bridge.

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