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Stumper #47.






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A Very Low Bridge

The bridge took three years to construct and opened in August 1961. One-and-one-fifth miles long it has approach spans and truss spans built on piers, and a center span built on 23 floating hollow concrete sections, each weighing about 5,000 tons. Floating?   Yes, this is a floating bridge crossing the Hood Canal, a fjord in Washington State, to connect the Kitsap and Olympic peninsulas

The constraints to be addressed, and the techniques used in building a floating bridge are unique. Water is 1,000 times more dense than air and although it generally flows more slowly than a strong wind it generates very strong forces. The saltwater Hood Canal is subject to heavy currents, tides, and giant waves;   the water level rises and falls as much as 18 feet. Clearly this bridge faces different challenges from those of a more typical bridge.

On February 13, 1979 the western half of the Bridge, battered with 85 mph winds that gusted to 120 mph producing huge swells, sank. A redesigned replacement section, with much heavier anchors, enabled the bridge to re-open to traffic in October 1982.  

Other factors besides physics and engineering challenges can be involved in construction projects like this one. In 2003, a replacement for the aging eastern section of the bridge was begun. However, as workers were constructing a graving dock near Port Angeles they came across a midden -- the refuse dump of an ancient Native American village, as well as many human remains and artifacts. Work stopped and a new location for the dock was found. This was only a temporary problem for the project and simple to deal with.

Some other problematic aspects of the influence of this bridge are not so easy to resolve.   Construction always impacts the environment. Marine creatures and plants have, for many years now, been dying off in the Hood Canal fjord. This is the result of a complex of factors but amongst them is the fact that the bridge restricts tidal currents and blocks surface water circulation between the Strait of Juan de Fuca and Pacific Ocean thus contributing to an accumulation of stagnant water.

Stumper Questions:
1. What is another name for a floating bridge?
2. Would it make any difference in the design if the bridge were across a freshwater lake?
3. What life-essential gas becomes deficient in stagnant water?   Why?
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