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SR 520 - Floating Bridge and Landings Project: Construction Overview

The new SR 520 Floating Bridge is the longest floating bridge in the world. Building a bridge that sits on the water comes with several highly unusual construction challenges as well as a few advantages.

The following images describe how the new floating bridge was built.

Step 1: Build the necessary pontoons, anchors and roadway sections in Aberdeen, Tacoma and Kenmore.
Step 2: Begin constructing staging area near Medina with construction barges and cranes. Drive temporary piles.
Step 3: Install anchors for the floating bridge.
Step 4: Tow pontoons to Lake Washington.
Step 5: Install cofferdams to build bridge piers for East Approach structure. Move pontoon into staging area.
Step 6: Build bridge piers in cofferdams and begin pontoon assembly in staging area.
Step 7: Join supplemental pontoons to longitudinal pontoons at staging area. Begin installation of columns on cross pontoon.
Step 8: Move assembled pontoons into position. Begin installation of columns on longitudinal pontoon.
Step 9: Begin installation of superstructure on pontoons. Continue assembling pontoons in staging area.
Step 10: Complete roadway superstructure in final alignment.
Step 11: Connect new floating bridge to completed East Approach and west connection bridges.
Step 12: Shift traffic to new floating bridge. Decommission the existing bridge and remove from Lake Washington.
Note: Images for illustration purpose and are not to scale.

Types of pontoons

The new SR 520 floating bridge is supported by three types of concrete pontoons:

Longitudinal pontoons (21)

These are the largest pontoons at approximately 360 feet long. They form the backbone of the bridge and support the roadway superstructure.

  • Constructed in Aberdeen facility
  • Weight: 11,100 tons

Cross pontoons (2)

These mark the ends of the floating bridge section and the transition to the East and West Approach structures.

  • Constructed in Aberdeen facility
  • Weight: 10,100 - 10,550 tons

Supplemental stability pontoons (54)

These smaller pontoons help stabilize and support the weight of the new floating bridge.

  • Constructed in Aberdeen facility (10) and Tacoma facility (44)
  • Weight: 2,500 - 2,820 tons

Where will WSDOT connect the new floating bridge?

As shown below, crews will install the new floating bridge north of the existing bridge.

How does the floating bridge connect to the stationary bridge?

The moveable, floating bridge connects to the stationary, elevated bridge segment at the east approach with a transition span. The transition span is made up of girders, each 190 feet long and 45 tons. On either end of the span, hinges allow the transition span to move up to 24 inches up and down or side to side to accommodate varying lake water levels.

Illustration of how transition span fits between the floating and stationary bridges.
Click image for an aerial view of how the transition span fits into the new SR 520 (pdf 2.52 mb). 

What type of anchors are used to secure the new floating bridge?

Three different types of anchors secure the new bridge from shifting during wind and wave action. These types are: fluke anchors, gravity anchors, and drilled shaft anchors.

Fluke anchors

  • Dimensions: 35' x 26' x 17.5'
  • Weight: 100 tons
  • Quantity: 45
  • Locations: Deep, soft soils of the lakebed and flat areas
  • Manufactured: Kenmore


Gravity anchors

  • Dimensions: 40' x 40' x 23'
  • Weight: 420 tons as built, 587 tons fully loaded
  • Quantity: 8
  • Locations: Solid soils with sloped topography, typically near shore. Underwater grading and installation of gravel creates a level footing for anchor placement.
  • Manufactured: Kenmore

Drilled shaft anchors

  • Dimensions: 10' diameter drilled shaft, 79'-92' long
  • Quantity: 5
  • Locations: Solid soils near shore where gravity anchors may cause navigation hazard.
  • Manufactured: Concrete cast in place from a barge on Lake Washington.


Questions about floating bridge design?

  • View our common questions document (pdf 71 kb) for more information about public involvement in SR 520 floating bridge design and the purpose of key design elements.