Lacey V. Murrow Memorial Bridge
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Image:I-90 floating bridges looking east.JPG The Lacey V. Murrow Memorial Bridge is a floating bridge that carries the eastbound lanes of Interstate 90 across Lake Washington from Seattle to Mercer Island, Washington. It is the second longest floating bridge in the world, at 6,620 feet (2,019 meters); the longest, incidentally, is the Evergreen Point Floating Bridge just a few miles to the north.
The bridge was the brainchild of George W. Lightfoot, who came to be called the "father of the bridge". Lightfoot began campaigning for the bridge in 1930, enlisting the support of Miller Freeman. The original two-way, four-lane toll bridge was designed by the engineer Homer Hadley (1885-1967) and constructed of reinforced concrete in 1940. Tolls were removed in 1946. It sank in a storm on November 25, 1990, while it was undergoing refurbishing & repair. The current bridge was built in 1993.
Formerly known as the "Lake Washington Floating Bridge", the original bridge included a movable span that could be retracted into a pocket in the center of the fixed span to permit large boats to pass. This design resulted in a roadway "bulge" that required vehicles to swerve twice across polished steel joints as they passed the bulge. A "reversible lane" system, indicated by lighted overhead lane control signals with arrow and 'X' signs, compounded the hazard by putting one lane of traffic on the "wrong" side of the bulge at different times of day in an effort to alleviate rush-hour traffic into or out of Seattle. There were many serious collisions on the bridge. The problems grew worse as the traffic load increased over the years and far outstripped the designed capacity. Renovation or replacement were essential and a parallel bridge, the Homer M. Hadley Memorial Bridge, was completed in 1989.
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1990 disaster
The sinking of the original bridge was an example of human errors and decisions compounding to lead to disaster. The process started because the bridge needed resurfacing and was to be widened by means of cantilevered additions. The Washington State Department of Transportation decided to use hydrodemolition (high-pressure water) to remove unwanted material. Water from this hydrodemolition was considered contaminated under environmental law and could not be allowed to flow into Lake Washington. Engineers then analyzed the pontoons of the bridge, and realized that they were over-engineered and the water could be stored temporarily in the pontoons. The watertight doors for the pontoons were therefore removed. A large storm arose on November 22, 23, and 24, 1990 (the Thanksgiving holiday weekend). This filled some of the pontoons with rain and lake water. On November 24, workers noticed that the bridge was about to sink, and started pumping out some of the pontoons. However, on November 25, 850 meters of the bridge sank, ironically dumping the contaminated water into the lake along with tons of bridge material. The bridge sank when one pontoon filled and dragged the rest down because they were cabled together and there was no way to separate the sections under load. Fortunately, no one was hurt or killed, since the bridge was closed for renovation and the sinking took some time. All of the sinking was captured on film and shown on live TV.
Precedents and lessons for the future
Ironically, WSDOT lost another floating bridge, the Hood Canal Bridge, about a decade earlier under similar circumstances, and it is now known that another major floating bridge in Washington, the Evergreen Point Floating Bridge, is under-engineered for local environmental conditions[1].