Thursday, November 25, 2010

Transport: The Loss of the Lacey V. Murrow

TORONTO, ONTARIO - Today is Thanksgiving Day in the United States, but twenty years ago, 25-November fell on a Sunday, and it was quite an interesting day. From my 25-November-1990 Highlight report:
When it was opened in 1940, the Lacey V. Murrow floating bridge was the longest floating bridge in the world. It served well as US 10 and temporary I-90 until the new, third Lake Washington floating bridge opened parallel to the old bridge last year. The classic old span began to be renovated, with some of the work, including the eastern approach and part of the decking completed by the beginning of the Thanksgiving work break.

However, when workers arrived to check on the bridge this morning, they found one of the middle pontoons rapidly taking on water and sinking. They immediately went to shore for safety, and as they called in help, a loud cracking noise was heard. The bridge broke in two, with a middle pontoon immediately sinking. The western section broke away from the approach and began drifting southward. The eastern section remained attached to shore, but continued to take on water.

By this time, the local media had arrived on the scene, and live coverage of the disaster soon reached the airwaves. Another pontoon began to lean downward and bubble. Water come over the roadway, and then the section tipped downward, causing a construction crane to topple into Lake Washington. The pontoon soon followed in turning over and sinking, sending up circular bubbles on the way down.

A few more pontoons went down in such dramatic fashion before the remainder of the bridge stabilized. A tugboat was called to drag the drifting section and hold it against the shore. At the end of the eastern section, the last pontoon hung facing downward, threatening to break off and sink.

Meanwhile, the new bridge was shut down as a precautionary measure and it will remain closed tomorrow morning which will certainly cause a traffic jam the likes of which have never been seen in the Seattle area. Once the new bridge is confirmed safe, the investigation into what happened to the old bridge this morning can begin.
It turned out that the sinking pontoons had snapped some of the cables attaching the new bridge to its southern anchors, meaning that there was stress on the bridge from the still-tensioned northern anchors, threatening to break that bridge apart as well, though it was not in danger of sinking. A fleet of tugboats and other heavy watercraft were recruited to pull on the old bridge until new anchors could be attached.

The old bridge had been undergoing hydro-demolition, the process of using pressurized water to break apart the concrete surfaces that were to be removed. If that doesn't sound ill-advised enough on a floating bridge, the waste water from the process was being stored inside the pontoons, and some of the water-tight barriers had been removed to facilitate this practice. The significant storm that hit around the Thanksgiving holiday was enough to combine with these factors to sink the fifty year-old bridge.

As the parallel Homer M. Hadley Bridge had just been recently designed and completed, it was a comparatively simple process to adapt the plans for that five-lane bridge to the replacement three-lane Lacey V. Murrow Bridge, which was constructed on a somewhat expedited schedule and opened in 1993--with the insurance company of the contractor on the original bridge's refurbishing paying a good portion of the bill.

Now, the 1990's-era bridges are a fixture between Seattle and Mercer Island, and the only controversy surrounds whether to install light rail tracks on the Homer M. Hadley Bridge.

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