The Governor Albert D. Rosellini Bridge, commonly known as the Evergreen Point Floating Bridge on Washington State Route 520, was an engineering marvel built over Lake Washington. Because the lakebed is composed of soft silt hundreds of feet deep, traditional bridge piers were economically unfeasible.
Engineers instead opted for a floating bridge design utilizing massive, hollow concrete pontoons bolted together and anchored to the lake floor with heavy steel cables. However, floating on water created a unique engineering challenge: accommodating maritime traffic without building a sky-high fixed arch. The original 1963 bridge solved this with an ingenious drawspan mechanism.
Unlike conventional drawbridges that lift upward on hinges, the SR 520 drawspan opened horizontally. To allow ship passage, giant hydraulic cylinders lifted draw pontoon deck sections vertically by several feet. Once elevated, these road sections were retracted longitudinally beneath the adjacent fixed roadway, creating a 200-foot-wide gap in the middle of the floating bridge.
When closed, the pontoons locked back into alignment using massive hydraulic steel pins. Operating this floating drawspan was a delicate dance of buoyancy and weight distribution. Opening the bridge changed the center of gravity and shifted ballast water within internal pontoon chambers to maintain structural equilibrium against wind and waves.
Although the original bridge was replaced in 2016 by a modern floating bridge with a fixed high-rise navigation span, the mechanical choreography of the original SR 520 drawbridge remains a classic lesson in floating civil engineering.