CASE STUDY

The Tacoma Narrows Suspension Bridge (the third largest in the world after the Golden Gate and George Washington bridges) is a legendary example of a project that failed through a combination of poor planning, unforeseen technological effects, and blinkered optimism on the part of the bridge’s developers. Though it fell over 60 years ago, less than four months after being opened for use, the Tacoma Narrows case illustrates a number of important lessons for proper project scope management.

Opening in July 1940, the bridge was built at a cost of $6.4 million and was largely funded by the federal government’s Public Works Administration. The bridge was intended to connect Seattle and Tacoma with the Puget Sound Navy Yard at Bremerton, Washington. It had a centre span of 2,800 feet and 1,000-foot approaches at each end. Interestingly, the bridge was designed for only one lane traffic in each direction, making it not only very long but also very narrow.

Even before its inauguration and opening, the bridge exhibited strange characteristics that were immediately noticeable. For example, the slightest wind could cause the bridge to develop a pronounced longitudinal roll. The bridge would quite literally begin to lift at one end and in a wave action, and the lift would “roll” the length of the bridge. Depending upon the severity of the wind, cameras were able to detect anywhere up to eight separate vertical nodes in its rolling action. Many motorists crossing the bridge complained of acute seasickness brought on by the bridge’s rising and falling! So well-known to the locals did the strange motion of the bridge become that they nicknamed the bridge “Galloping Gertie.”

On November 7, 1940, a bare four months after the bridge was opened, with steady winds of 42 miles per hour, the 2,800-foot main span, which had already begun exhibiting a marked flex, went into a series of violent vertical and torsional oscillations. The amplitudes steadily increased, suspensions came loose, the support structures buckled, and the span began to break up. In effect, the bridge had seemed to come alive, struggling like a bound animal, and was literally shaking itself apart. Motorists caught on the bridge abandoned their cars and crawled off the bridge as the side-to-side roll had become so pronounced (by now, the roll had reached 45 degrees in either direction, causing the sides of the bridge to rise and fall over 30 feet) that it was impossible to walk. After a fairly short period in which the wave oscillations became incredibly violent, the suspension bridge simply could not resist the pounding and broke apart. Observers stood in shock near the bridge and watched as first large pieces of the roadway and then entire lengths of the span rained down into the Tacoma Narrows. Fortunately, no lives were lost, since traffic had been closed just in time.

A three-person committee of scientists was immediately convened to determine the cause of the Tacoma Narrows collapse. The board consisted of some of the top scientists and engineers in the world at that time: Othmar Ammann, Theodore von Karman, and Glenn Woodruff. While satisfied that the basic design was sound and the suspension bridge had been constructed competently, they nevertheless were able to quickly uncover the under-lying contributing causes of the bridge collapse.

First, the physical construction of the bridge contributed directly to its failure and was a source of continual concern from the time of its completion. Unlike other suspension bridges, one distinguishing feature of the Tacoma Narrows Bridge was its small width-to-length ratio – smaller than any other suspension bridge of its type in the world. That ratio means that the bridge was incredibly narrow for its long length, a fact that contributed hugely to its distinctive oscillating behavior. Although almost one mile long, the bridge carried only a single traffic lane in each direction.

Another feature of the construction that was to play an important role in its collapse was the substitution of key structural components. The chief engineer in charge of construction, Charles Andrews, noted that the original plans called for the use of open girders in the bridge’s sides. At some point, a local construction engineer substituted flat, solid girders, which deflected the wind rather than allowing it to pass. The result, Andrews noted, was that the bridge caught the wind “like a kite” and adopted a permanent sway. In engineering terms, the flat sides simply would not allow wind to pass through the sides of the bridge, which would have reduced its wind drag. Instead, the solid, flat sides caught the wind, which pushed the bridge sideways until it swayed enough to “spill” the wind from the vertical plane, much as a sailboat catches and spills wind in its sails.

A final problem with the initial plan lay in the location selected for the bridge’s construction. The topography of the Tacoma Narrows is particularly prone to high winds due to the narrowing of the valley along the waterway. As a local engineer suggested, the unique characteristics of the land on which the bridge was built virtually doubled the wind velocity and acted as a sort of wind tunnel.

Before this collapse, not much was known about the effects of dynamic loads on structures. Until then, it had always been taken for granted in bridge building that static (vertical) load and the sheer bulk and mass of large structures were enough to protect them against wind effects. It took this disaster to firmly establish in the minds of design engineers that dynamic and not static loads are really the critical factor in designing such structures.

Source: Kharbanda, O. P. and Pinto, J. K. (1996), What Made Gertie Gallup? New York: Van Nostrand Reinhold.

Discuss the following based on the information on the case.

When did the planners begin taking unknowing or unnecessary risks? Support your answer with relevant statements from the case. (5 marks)

Discuss any four project constraints and three other unique aspects of the bridge in the risk management process. Were these issues taken into consideration in the execution of the project? Why or why not? (15 marks)

What forms of risk mitigation would you consider appropriate for this project? Explain (10 marks)