What forms of risk mitigation would you consider appropriate for this project? Classic Case: Tacoma Narrows Suspension Bridge The dramatic collapse of the Tacoma Narrows suspension bridge in 1940, barely four months after completion, was a severe blow to the design and construction of large-span bridges. It serves as a landmark failure in engineering history and is, indeed, a featured lesson in most civil engineering programs. The story of the collapse serves as a fascinating account of one important aspect of project failure: engineering’s misunderstanding of the effect that a variety of natural forces can have on projects, particularly in the construction industry.

Opening in July 1941, the Tacoma Narrows Bridge was built at a cost of $6.4 million and was largely funded by the federal government’s Public Works Administration.

The purpose of the bridge was essentially viewed as a defense measure to connect Seattle and Tacoma with the Puget Sound Navy Yard at Bremerton.19 As the third-largest single suspension bridge in the world, it had a center span of 2,800 feet and 1,000-foot approaches at each end.

Even before its inauguration and opening, the bridge began exhibiting strange characteristics that were immediately noticeable. For example, the slightest wind could cause the bridge to develop a pronounced longitudinal roll. The bridge would begin to lift at one end and, a wave action would “roll” the length of the bridge.

Depending upon the severity of the wind, cameras could detect 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 weaving motion of the bridge become that they nicknamed the it “Galloping Gertie.”

That the bridge was experiencing increasing and unexpected difficulties was clear to all involved in the project. In fact, the weaving motion of Galloping Gertie became so bad as summer moved into fall that heavy steel cables were installed externally to the span in an attempt to reduce it. The 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.

On November 7, 1940 a mere four months after the bridge’s opening and with winds of 42 miles per hour blowing steadily, the 280-foot main span that had already begun exhibiting a marked flex went into a series of violent vertical and torsional oscillations.

Alarmingly, the amplitudes steadily increased, suspensions came loose, the support structures buckled, and the span began to break up. The bridge seemed to have come alive, struggling like a bound animal, and was shaking itself apart. Motorists caught on the bridge had to abandon their cars and crawl off of it, as the sideto-

side roll had become so pronounced—45 degrees in either direction, causing the sides of the bridge to rise and fall more than 30 feet—that it was impossible to traverse the bridge on foot.