Last edited 26 Apr 2021

State Route 520 Floating Bridge

In March 2016, Washington State in northwest America officially opened the world’s longest floating bridge. The ‘State Route 520 Bridge’ (SR520), as it is known, is a six-lane pontoon structure 2.4 km in length. It was constructed as part of the ‘Connecting Washington’ package of measures, Washington State’s largest and greenest transportation investment.

It replaced a four-lane bridge that was completed in the early-1960s. The bridge boasts bigger and stronger pontoons and anchors than the previous bridge, along with a stormwater collection and treatment system to help improve the water quality of Lake Washington. A bus and carpool lane flows in either direction, as well as a 14 ft-wide bicycle and pedestrian path.

Floating bridges are constructed on large water-tight concrete pontoons rigidly connected end-to-end, upon which the road deck is constructed. Individual bridge pontoons are built on the land before being floated and towed to the bridge site. Once there, they are connected at each end to grounded approach structures, starting at the edge of the floating structure and then pieced together towards the bridge’s centre. Enormous cables hundreds of feet long hold the pontoons in place, connected to anchors buried deep in the lake bed.

A more conventional suspension bridge was not suitable for the location as the deepest point of Lake Washington is 214 ft, meaning that the bridge’s support towers would have to be approximately 630 ft tall, nearly the same height as the Space Needle.

The use of floating bridges is typically limited by the wind and wave forces that are likely to be experienced. The ‘fetch’ is the unobstructed clear distance over the water that wind can travel to the bridge. The longer the fetch, the higher the wind and wave forces will be. Pontoons bend, heave and twist as a result of wind and wave forces, creating large stresses in the pontoons and anchor system. However, even in the event of a ‘100-year storm’ (a storm that statistically has 1% chance of occurring in any given year), the pontoons are designed so that they will not crack, and so the bridge will not sink.