Pontoons are the foundation of a floating bridge and can take several years to construct. They are large, hollow concrete structures designed to support the weight of the road and the thousands of cars, trucks and buses that use the bridge daily.
WSDOT is advancing pontoon construction to restore the floating section of the SR 520 floating bridge in the event of a catastrophic failure and to store those pontoons until needed. If the SR 520 bridge failed, it could take several years to construct pontoons and restore the bridge for drivers.
Project area map
Where will pontoons be built?
We are currently evaluating two sites to build a pontoon construction site. These sites are:
Anderson & Middleton Hoquiam
Ownership: Anderson & Middleton Company
Size: 90 acres
Aberdeen Log Yard
Ownership: Weyerhaeuser Company
Size: 55 acres
How will WSDOT choose a site to build?
In the draft EIS, we will screen each site for the following criteria:
- Physicial characteristics of the site, including size and shoreline features to make sure they are compatible with our distinct pontoon construction needs.
- Logistics, including towing feasibility.
- Compliance with environmental regulations.
- Property availability.
How are floating bridges built?
Building a floating bridge starts with pontoons:
Step 1. Form the box-like shape of the pontoons by assembling wood forms around a steel framework.
Step 2. Pour concrete into the forms to create the pontoon floors, walls and top slabs.
Step 3. Tow the pontoons to a moorage location for storing.
Step 4. Tow pontoons to the bridge site when needed for bridge recovery or replacement.
Step 5. Join pontoons together and secure in place by steel cables that are anchored to the seafloor or lakebed.
Step 6. Add the road, concrete columns or beams on top of the pontoons. These structures can be added either before or after the pontoons are transported to the bridge site.
Pontoons under construction for the existing SR 104 Hood Canal Bridge in 2006
The tug Peggy Foss helps turn the pontoon after it clears the casting basin.
How do pontoons float?
Pontoons are similar to a tanker ship or heavy barge. Despite being heavy structures, pontoons float because they are able to displace a volume of water that weighs the same as the pontoons and road (including traffic) they support. This allows the pontoons -- and the SR 520 bridge -- to float.
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