July 31, 2003
The reality of the Tacoma Narrows Bridge project is starting to sink in-literally. In late July, the longest suspension bridge to be built in the United States in almost 40 years began to take shape on Puget Sound south of Seattle, Washington, as the first of two massive tower foundation structures was towed to the construction site.
Tacoma Narrows Project Reaches First Milestone
"We are pleased and proud that the complete operation went so well and according to plans," said Project Manager Manuel Rondón of Bechtel, which is building the bridge in a joint venture with Kiewit Pacific. “Our success is due to the diligent effort of our team and to a formidable amount of detailed preparation and planning. Achievements like this are rewarding and encouraging.”
The new bridge is definitely needed. Right now, traffic is funneled across a 52-year-old bridge that carries 85,000 vehicles each day-25,000 more than the maximum for which it is intended. The new bridge, paralleling the existing one, will ease the congestion by handling eastbound traffic.
For just a few hours on July 21, however, the project had the opposite effect. Cars slowed down, pedestrians on the bridge walkway gawked, and pleasure boats filled the water below as tugboats brought the 12,700-tonne, concrete and steel caisson from the Port of Tacoma and maneuvered it into position.
If it looked like an enormous undertaking, it's because it was. Bechtel and Kiewit engineers had been planning the day for two months, determining the best strategy for bringing big structure-essentially a seven-story high box-18 kilometers along a stretch of water noted for large tidal swings, fast currents, and pesky winds.
In theory, the plan was simple: Pick a time when tidal exchanges would be low, creating slack tides and slow currents. Then use the ingoing tide to bring the Caisson in from the port, and the outgoing tide for final maneuvering. Nothing is simple, however, when the structure you are towing is 40 meters long, 24 meters wide, 24 meters tall, and almost 15 meters of it is submerged, making it easy prey to changing currents.
As it turned out, the narrows cooperated, no unforeseen problems developed, and the move turned out to be easier than anticipated. The three tugboats towed the caisson out of port shortly after 3 a.m., maintaining a leisurely pace of just under 3 kilometers per hour. At one point, they cut that to less than 2 kilometer per hour to take advantage of the slackening tide. They delivered the caisson to its new home adjacent to the Tacoma-side caisson of the existing bridge at about 1 p.m.
The days following, divers secured the caisson to prepositioned anchors on the floor of the narrows. Over the next few months, the structure-which is mostly hollow and floats on 15 steel domes-will sink slowly under the weight of concrete and steel during construction of the tower above it, which eventually will rise some 50 meters above the water. The second caisson-at a shallower spot-will be brought to the site in August. By December, both will be embedded firmly on the seabed, a solid foundation for the future of Tacoma Narrows.