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SR 520 - Floating Bridge and Landings Project: Pontoon Tracking and Bridge Assembly

Updated: Nov. 24, 2015

Follow along with WSDOT as we continue to make progress on the new SR 520 floating bridge on Lake Washington. 


The SR 520 Floating Bridge and Landings project has been under construction since early 2012, and the new bridge is scheduled to open to traffic in spring 2016. Construction of the bridge is nearing completion, but a substantial amount of work remains to commission the bridge before traffic can be switched onto the new structure. Commissioning is the process of verifying that all systems in new construction are fully functional and meet contract requirements.

Bridge electrical progress, by pontoon
The entire bridge structure is kept afloat by 77 concrete pontoons. The pontoons are aligned in the configuration shown in the graph below and labeled alphabetically from west to east. The graph shows the progress of completion (in percentages) for the different electrical elements in each longitudinal pontoon and both cross pontoons.  

Upon completion, crews will have installed more than 300 miles of electrical wire and various electronic components and sensors along the bridge’s entire length -- all electronically linked to a new bridge maintenance facility in Medina and WSDOT's northwest regional office in Shoreline.

Bridge electrical progress
Bridge electrical progress by pontoon- click image above for a larger, printable graphic. (pdf 603 kb)


  • Electrical: Completion percentage of all electrical equipment in each pontoon.
  • Conduit: Piping that protects electrical wires through pontoons and under the roadway.
  • Under deck supports: Brackets and hardware that support electrical conduits between the top of the pontoons and below the roadway.
  • Medium voltage system: Main electrical system that carries electricity to the entire system.
  • Pontoon electrical components: Sensors, electrical wires, and other electrical components. 
  • Pontoon commissioning: Testing of all electrical components and monitoring systems.

Bridge construction element completion estimates

Other key work remains to be completed and requires extensive inspection and testing before the bridge opens to traffic next spring.

Bridge construction element progress

Bridge construction element completion estimates- c
lick image above for a larger, printable graphic. (pdf 603 kb)


  • Roadway deck: 776 of 776 pre-cast deck panels placed on the low-rise roadway deck, 23 of 23 cast-in-place deck spans placed on the high-rise roadway deck, and 4 of 4 spans placed on the transition spans between floating and fixed sections of the bridge.
  • Bridge barrier: 23,052 of 25,848 feet of barrier installed along the length of the bridge.
  • Noise walls: 500 of 1,600 feet of noise walls constructed along both sides of the bridge structure to minimize excessive traffic noise to nearby areas.
  • Ballast: 38,152 of 70,000 tons of ballast rock placed inside the pontoons to maintain floatation at the appropriate height.
  • Bridge maintenance facility and dock construction: Bridge maintenance facility and dock construction under the east approach.
  • Fire safety systems: 0 of 23 pipes and pumps installed for the fire safety system.
  • Static and active traffic management systems: 1 of 53 traffic signs and gantries installed.
  • Bridge control systems: Pontoon commissioning and security systems testing.
  • Stormwater drainage: 7,700 of 15,450 feet of piping and catch basins placed inside supplemental pontoons.
  • Architectural elements: 4 of 8 sentinel sections constructed and 30 of 8,434 feet of bicycle/pedestrian path railing placed.
  • Lighting: 0 of 45 luminaire light poles installed at each approach/merge point. (Note: There are no luminaires on the floating section of the bridge.) 
  • Roadway grinding: 7,897 of 34,580 feet of roadway prepared with the Next Generation Concrete Surface grind. 
  • Approaches and striping: 3 of 40 approaches constructed.
  • Viewpoint trail and south walls: Construction of the public viewpoint trail on the south side of the bridge. This work will not begin until traffic is moved onto the new floating bridge.
  • Existing bridge demolition: Demolition and removal of decommissioned bridge after new bridge is open. Expected to be complete at the end of 2016.

Pontoon Tracking and Bridge Assembly
All pontoons are now moored in their final location on Lake Washington. Crews poured the new floating bridge's final roadway deck section (on pontoon A) in mid-October. Learn more about the types of pontoons and pontoon assembly process below.



Floating Bridge Assembly

Diagram showing the construction status and approximate configuration of elements of the new floating bridge. Last updated October 26, 2015.

View the full, printable graphic. (pdf 1.5 mb).

Low-rise roadway deck includes pre-cast deck panels being constructed in Kenmore.
- 776 out of 776 pre-cast deck panels have been constructed. 776 of 776 have been set.

High-rise roadway deck includes girders and cast-in-place deck.

- 27 out of 27 cast-in-place deck spans have been constructed.
- 331 out of 331 girders have been constructed. 331 of 331 have been placed.

The number of columns shown in the graphic is a representation of the total number of columns.

- 771 out of 771 columns have been installed.

Pontoon count:
    -Current number of pontoons on Lake Washington: 77.
    -Total number of pontoons constructed to date: 77.
    -Pontoons currently under construction: 0.

Pontoon towing
On Thursday, April 9, 2015, the last three pontoons needed to construct the new floating bridge arrived on Lake Washington. No other pontoon arrivals are planned.

Pontoon transit route
Pontoons for the new floating bridge were constructed at two different sites in Washington state: Aberdeen and Tacoma. We completed pontoon construction in Tacoma in December 2014 and in Aberdeen in March 2015. After the pontoons were completed and ready for assembly, tugboats towed them to Seattle. The trip from Aberdeen to Seattle is nearly 260 nautical miles .

Pontoons were visible from multiple locations along the Ship Canal. Many of the photos on our Flickr set were taken from locations on this map of suggested viewing locations  (pdf 2.5 mb). 

Build your own pontoons for the floating bridge
The floating bridge pontoons represent a key milestone in a critical safety project. Using the printable pontoon templates linked below, you can build your own bridge.

  • Each pontoon is labeled according to its location within the final bridge structure.
  • For each pontoon, download and print the template file (each pdf is approximately 400 kb), cut along the indicated lines, and then fold your pontoon for final construction.
  • We will maintain the links for each pontoon template, so that you can catch up with any you might miss.

Aug. 11, 2012: Pontoon V
Aug. 14, 2012: Pontoon W
Aug. 18, 2012: Pontoon VNW
Aug. 19, 2012: Pontoon VSW
Aug. 22, 2012: Pontoon U
Sep. 22, 2012:  Pontoon VNE and Pontoon VSE
Sep. 30, 2012:  Pontoon UNE and Pontoon USE
Feb. 2, 2013:  Pontoon UNW and Pontoon USW
May 15, 2013: Pontoon T
Jun. 7, 2013: Pontoon A
Jul. 22, 2013: Pontoon TNW
Jul. 23, 2013: Pontoon TSW
Aug. 27, 2013: Pontoon TNE and Pontoon TSE
Aug. 30, 2013: Pontoon S
Oct. 17, 2013: Pontoon BSE
Oct. 20, 2013: Pontoon Q
Oct. 23, 2013: Pontoon R
Oct. 29, 2013: Pontoon BSW
Dec. 9, 2013: Pontoon SNE
Dec. 10, 2013: Pontoon SSE
Dec. 11, 2013: Pontoon SNW
Dec. 12, 2013: Pontoon SSW and Pontoon RSW
Dec. 13, 2013: Pontoon RNW and Pontoon RNE
Dec. 14, 2013: Pontoon RSE
March 13, 2014: Pontoon PSW and Pontoon PNW
April 13, 2014: Pontoon P
April 16, 2014: Pontoon O
May 4, 2014: Pontoon M  and Pontoon N
May 8, 2014: Pontoon QNW , Pontoon BNW  and Pontoon DSW
May 9, 2014: Pontoon QSW  and Pontoon OSW
May 10, 2014: Pontoon ONW
May 11, 2014: Pontoon NNW and Pontoon NSW
May 12, 2014: Pontoon MNW
May 13, 2014: Pontoon MSW
Aug. 11, 2014: Pontoon B  and  Pontoon C
Aug. 16, 2014: Pontoon D and Pontoon E
Sept. 17, 2014: Pontoon BNE
Sept. 18, 2014: Pontoon CNW and Pontoon CSW
Sept. 22, 2014: Pontoon DNW
Sept. 23, 2014:  Pontoon ESW
Sept. 24, 2014:  Pontoon ENW
Sept. 25, 2014: Pontoon KNW
Oct. 1, 2014: Pontoon LSW
Oct. 2, 2014: Pontoon KSW
Oct. 3, 2014: Pontoon LNW
Oct. 4, 2014: Pontoon I , Pontoon J , Pontoon GSW and Pontoon GNW
Oct. 10, 2014: Pontoon L  and Pontoon K
Dec. 5, 2014: Pontoon INW and Pontoon ISW
Dec. 6, 2014: Pontoon HNW , Pontoon HSW , Pontoon FNW and Pontoon FSW
Dec. 7, 2014: Pontoon JNW and Pontoon JSW
April 9, 2015: Pontoon G, Pontoon H, and Pontoon F

Types of pontoons
The new SR 520 floating bridge will be 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 by joining to larger longitudinal pontoons.

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