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SR 520 Bridge Program - Safety and Vulnerability

Waves batter the SR 520 bridge

Waves batter the south side of the floating bridge during a February 2006 storm.

Safety is our top priority. SR 520’s Evergreen Point Floating Bridge, Portage Bay Bridge, and bridge approaches are vulnerable to failure during severe windstorms and earthquakes. Improving and replacing the SR 520 bridge and corridor is critical to maintaining public safety and the safety of Washington state’s transportation infrastructure.

Today's SR 520 bridge could fail during an earthquake

Thumbnail: Damaged SR 520 hollow support column
The inside of a damaged SR 520 hollow support column. Tests show that the hollow columns of SR 520 could implode and collapse during a major earthquake.


Thumbnail: Simulated image of catastrophic earthquake
This is a computer simulation of what could happen to the SR 520 Bridge during a catastrophic earthquake. View the video.

The west approach of the floating bridge and connecting Portage Bay and Union Bay bridges of SR 520 are supported by hollow columns. During a major earthquake, the hollow columns could implode and collapse. Engineers designed the SR 520 bridge and its approaches during the 1960s, before modern earthquake design standards existed.

We developed an earthquake catastrophic failure simulation video to show how and why the west approach of the SR 520 floating bridge could fail during a major earthquake. This video is only a simulation. The actual amounts of shaking will depend on the location, intensity and length of the earthquake.

The new bridge will be able to better withstand an earthquake

We are designing the new west approach of the floating bridge and the connecting Portage Bay and Union Bay bridges to withstand a 1,000-year earthquake event.

Today's SR 520 bridge could fail during a severe windstorm

Thumbnail: Simulated image of catastrophic windstorm
This is a computer simulation of what could happen to the SR 520 bridge during a catastrophic windstorm. View the video.

Anyone who has driven over the SR 520 floating bridge on a stormy day has likely seen the white-capped waves of Lake Washington pound against the southern wall of the bridge deck.

When sustained windstorms gust over 50 mph, the drawspan, anchor cables, and the pontoons could all break or crack, which could cause the bridge to sink. Currently, crews close the bridge to traffic and open the drawspan when there are 50 mph gusts of wind sustained for 15 minutes.

We developed a windstorm catastrophic failure simulation video to show how and why the floating span of the Evergreen Point Floating Bridge could fail during a 20-year windstorm; or a storm with sustained winds exceeding 75 mph. This video is only a simulation. Actual damage to the floating span will depend on the speed of the wind gusts and duration of the windstorm.

The new bridge will be able to better withstand severe windstorms

Thumbnail: Alignment hook
A five-ton alignment hook was torn off the drawspan during the "Hannukah Eve Storm," December 2006. crews were forced to close the bridge to traffic three times in 2006 in response to severe weather conditions.

The replacement Evergreen Point Floating Bridge pontoons, bridge deck and anchor cables will be designed to withstand wind storms up to 89 mph. Building the bridge to updated design standards will improve safety for travelers during winter windstorms and year round.

WSDOT is prepared if the bridge collapses or sinks during a catastrophic failure

A catastrophic failure of the SR 520 floating bridge and structures poses a threat to our region. WSDOT recognizes this threat and has developed a response and recovery plan in case the bridge fails.

This Catastrophic Failure Plan (pdf 567 kb) includes:

  • Testing emergency response scenarios through guided simulations such as a tabletop exercise.
  • A communications plan to keep people, emergency responders and media informed.
  • A transportation plan to manage diverted bridge traffic through alternate routes.
  • Action plans to replace the bridge during a partial or full closure of the SR 520 corridor.
How does WSDOT keep the current bridge safe?

Our bridge crews closely monitor the forces of wind and water to keep the bridge in good condition. Our incident response teams and traffic engineers monitor the road condition to keep traffic flowing. Sometimes these incidents are large enough to require a full bridge closure, such as the Dec. 14, 2006 windstorm. Due to the age of the bridge, we cannot rule out future storm closures until a new bridge is built.

In order to keep the bridge safe for drivers, WSDOT crews close the bridge once every month to conduct routine maintenance. During monthly maintenance closures, crews test the working order of the draw span, conduct other routine safety checks and do preventive maintenance.

More information is available on the SR 520 Bridge Update Web page.

Past closures and inspections

Closure date Results/actions
August 21-23, 2010 WSDOT bridge crews closed the SR 520 Evergreen Point Floating Bridge from Aug. 21-23, 2010 to perform annual inspections and maintenance work.
June 19 - 22, 2009 WSDOT bridge crews closed the SR 520 Evergreen Point Floating Bridge from June 19-22, 2009 to perform annual inspections and maintenance work.
July 11 - 14, 2008
WSDOT bridge crews closed the SR 520 Evergreen Point Floating Bridge from Jul. 11-14, 2008 for annual maintenance and inspection work. Crews found small cracks, but bridge in overall good shape.
July 13 - 16, 2007
WSDOT bridge crews closed the SR 520 Evergreen Point Floating Bridge from Jul. 13-16, 2007 to perform annual inspections and maintenance work.
Feb. 22, 2007 The bridge was closed to traffic for up to 30 minutes at a time for emergency maintenance. Crews repaired a damaged expansion joint on the bridge drawspan.
Dec. 14 - 15, 2006 The bridge was closed due to high winds. When crews opened the drawspan, three of the hooks that connect the spans were sheared off by wave action. This is not unusual and is easily repaired. When winds decreased crews started repair work and scanning the bridge for damage.
July 16 - 19, 2006
Bridge crews conducted ultrasound scans to examine vulnerable parts in the drawspan and make sure they all remain in working order during the annual summer maintenance closure. WSDOT maintenance and signals crews also completed a range of maintenance activities throughout the corridor.
March 8 - 9, 2006
Crews closed the bridge after on-site inspectors heard unusual noises. Those noises prompted a closer look inside the mechanical parts of the draw pontoons. They found one bolt sheared off, several loose bolts, and flaking paint which is an indication of weakening steel. They immediately closed the bridge to conduct the safety check and make repairs.
Feb. 22, 2006 Bridge closed to vehicle traffic for more inspections and repairs from the Feb. 4 windstorm.
Feb. 16, 2006 Bridge closed to vehicle traffic for further inspections and repairs from the Feb. 4 windstorm.
Feb. 9, 2006 Bridge closed for monthly maintenance. Private contractors brought more sophisticated ultrasonic equipment and confirmed the damage to the trunnion beam detected after the Feb. 4 windstorm.
Feb. 4 - 5, 2006 Emergency closure due to wind gusts of up to 67 mph. Crews repaired three drawspan hooks that were torn off during the storm. Bridge crews performed a preliminary ultrasonic test during the closure and detected a small crack in the mechanical equipment for the drawspan (pdf 458kb). The last time the bridge was closed for high winds was March 1999.