Seattle's Floating Bridges: Engineering on Water

The bridge you are crossing, or about to cross, is a floating bridge. Not metaphorically floating. The roadway sits on concrete pontoons, each one sealed and hollow, displacing enough water to support the weight of the deck and the traffic above it.

Lake Washington is two hundred fourteen feet deep in places, too deep for conventional bridge supports to reach bottom economically. The solution, first attempted here in nineteen forty, was to float the bridge rather than anchor it. Interstate Ninety's Homer M.

Hadley Memorial Bridge was the world's widest floating bridge until the Evergreen Point Floating Bridge opened in two thousand sixteen, and it remains one of the longest. It sits alongside the older Lacey V. Murrow Memorial Bridge, which opened in nineteen forty and was the first floating concrete bridge in the world.

The Murrow bridge sank in nineteen ninety during a windstorm while it was being repaired and its waterproof hatches were open. The workers escaped. The bridge did not.

It was rebuilt and reopened in nineteen ninety-three. Floating bridges have a specific failure mode that conventional bridges do not: if water gets in and the pontoons lose buoyancy, they go down quickly. The Lake Washington crossings are engineered against this, but the Murrow sinking demonstrated that the engineering can be overcome by human error.

The lake itself was lowered nine feet in nineteen seventeen when the Lake Washington Ship Canal was dug connecting the lake to Puget Sound. The lowering exposed lakeshore that became some of the most valuable real estate in the region and destroyed the habitat of the Duwamish people, who had fished and lived along the original shoreline for thousands of years. The Duwamish Tribe does not have federal recognition.