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Mighty Mac Comparison To Tacoma Narrows Bridge

12 Feb

A good comparison can be made between the engineering successes and failures witnessed in the construction of two bridges; Mackinac Straits Suspension Bridge (also known as the “Mighty Mac”), and the Tacoma Narrows Bridge (also known as “Galloping Gertie” or TNB).

At the time of its construction in 1940, TNB was the third longest suspension bridge in the world [1, pg xviii]. The construction of TNB was to alleviate the use of ferry fleet to cross the Puyallup River at the mouth of Pugent Sound. The bridge reduced the commute time to cross the river. The sound crossed by TNB often witnessed heavy winds, in some cases in the upper 70 mph range. Record temperatures range from 107ºF to -2ºF. On the day of its collapse, winds at Tacoma Narrows were recorded at 42 mph, [2, pg. 69]. The largest span between pillars on TNB is 2,800 feet.

The Mighty Mac spans a distance of five miles and uses only two support pillars. At the time of its construction, 1957, it was the largest of its kind in the world — nearly 60 years later it is still the largest in the Western hemisphere. When considering the enormity of this undertaking at Mackinac Straits, we must also be aware of the environment. In that area, record temperatures range from 115ºF to -35ºF, winds reach 78 mph, and as much as six feet of ice accumulates on the water beneath it. The two support pillars for the Mighty Mac are 3,800 feet apart. In the planning of he y Mac, engineers reviewed and took lessons-learned from similar construction, such as TNB, San Francisco-Oakland Bay bridge, and Golden Gate bridge.

The engineers’ attention to detail was a key factor in the success or failure of these grand projects. In the case of TNB, engineers witnessed unusual motion during construction. Quoting [Wake, pp. 46–47], “during the final stages of work, an unusual rhythmic vertical motion began to grip the main span in only moderate winds….” This torsional oscillation causing a positive feedback loop.

TNB construction placed stiffening girders to a shallower depth than its predecessors, only eight feet deep [2, pg. 69]. This reduced the dampening effect of its weight on aerodynamic lift. The reduced weight combined with the aerodynamic effects of the wind, setup an unexpected change. “… the extreme flexibility, slenderness, and lightness of the TNB allowed these oscillations to grow until they destroyed it. [3, pg 901].” [3] goes on to demonstrate mathematically how the resonance occurred and eventually collapsed TNB. In part, this was also attributed to the depth of the girders.

In contrast, construction of the Mighty Mac gave greater attention to natural forces including the rock underneath and the atmospheric effects. Steinman’s [4] explains how they investigated the geology beneath the piers. The rock in some areas of Mackinac strait is composed of Mackinac Breccia, a breccia formed by collapsed caverns [5, pg 246]. This potential for collapse necessitated thorough research of the rock. After diligent inspection and scientific work, including 51 borings and extensive data analysis [4, pp. 4-5], the geologists determined the rock had no caverns and significant enough strength to support the bridge in its environment. This exhaustive investigation ensured proper placement and depth of the foundations for the pillars.

Steinman also took into consideration aerodynamics. Prior to work on Mighty Mac, he had suggested “critical wind velocity” as a contributing factor that induced oscillations in TNB [6, 760]. The design of Mighty Mac allowed air to pass through a grate in the center two lanes. This eliminated the aerodynamic lift.

The final contrast is evident in human witness. Tacoma Narrows Bridge succumbed to natural forces after only four months. Mackinac Straits Bridge has withstood fifty-eight years of high-winds, subzero temperatures with ice flows pounding at its pillars, and, as of 2009, 150 million vehicles has crossed the Mighty Mac.

BIBLIOGRAPHY

[1] Scott, Richard. “In the Wake of Tacoma.” ASCE Press, Reston, VA, 2001.

[2] Steinman, David B. “Bridges” in Scientific American, vol. 191, no. 5, pp 60-71, Nov, 1954.

[3] Arioli, Gianni, and Filippo Gazzola. “Applied Mathematical Modelling: A New Mathematical Explanation of what Triggered the Catastrophic Torsional Mode of the Tacoma Narrows Bridge,” Elsevier, vol. 39, pp 901-915, Jan. 2015.

[4] D. B. Steinman, “Mackinac Bridge Final Geological Report,” St. Ignace, MI, Geology Report, Apr, 1956.

[5] J. Monroe, and R. Wicander, (2014). “Engineering and Geology,” in The Changing Earth: Exploring Geology and Evolution, 7th ed. Stanford: Cengage, 2014, ch 10,  sec. 10.1, pp 246-247.

[6] Robert C. Post, “The Bridge At Mackinac Straits: Another Fiftieth Anniversary”. Technology and Culture, vol. 49, no. 3, July, 2008, pp. 752-763.

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Posted by on February 12, 2015 in Class Writing, Critical Inquiry

 

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