Science: Odd Bridge

Proud is Seattle of her position as “U. S. gateway to Alaska and the Orient.” And proud is Seattle of great, 20-mile-long Lake Washington, which Seattle considers hers although it stretches far beyond the city limits. Lake Washington is beautiful but sometimes a nuisance. Seattle’s main gateway to the east is North Bend on the Sunset Highway, until recently a 42-mile ride. The road could have been 14 miles shorter had not Lake Washington lain athwart the way.

Three years ago, the State of Washington pondered plans for spanning deep, blue Lake Washington. Because of the unstable mud bottom, State highway engineers figured that an orthodox bridge, without approaches, would cost $18,000,000. Lacey V. Murrow, Director of Highways, and Charles E. Andrew, consulting engineer, decided that a pontoon bridge, approaches and all, could be built for $8,854,400. PWA offered to chip in $3,794,400. Seattle’s City Council squeezed through a 5-to-4 endorsement.

Last July, Seattle opened its floating bridge, the longest, oddest pontoon bridge in the world. Its four-lane concrete highway, one and a quarter miles long, is the deck of 25 cement pontoons. The bridge actually floats, seven feet deep, in the water. As if the engineers had not had a hard enough job, they had also to include a draw-span, to take care of lake shipping. The draw-span section is made up of two pontoons. One forms a Y, the other floats between its arms, sliding out to close the bridge, slipping in to leave 200 feet of open water. The 25 concrete pontoons are honeycombed with watertight compartments. From both sides of each floating section a 65-ton reinforced concrete anchor juts into the clay bottom of the lake. Hydraulic adjustments on the anchor cables accommodate the bridge to the rise & fall of the lake level (it varies about three feet in a year).

By last week, no gloomy prophecies about the floating bridge had been fulfilled. No pontoons had sunk, none had been wrenched loose by storms, there was no sign of a leak. Amazed visitors still asked, “How do they float?” but always ventured across. Toll revenue for July and August on the bridge, from cars, busses, trucks, pedestrians, totaled $107,707.