Commuters travel eastbound on SR 520. The Portage Bay Bridge is in the background.
This is a computer simulation of what could happen to the SR 520 bridge during a catastrophic windstorm. View the
video.
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.
Photo by Greg Barnes
White-capped waves from Lake Washington batter the southern wall of the floating bridge. Get real-time SR 520 bridge closure and traffic
updates.
This is a computer simulation of what could happen to the SR 520 Bridge during a catastrophic earthquake. View the
video.
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.
Waves batter the south side of the floating bridge during a storm in February 2007.
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
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 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. - an event experts expect has a one-in-twenty chance of happening in the Puget Sound region during the next 50 years.
Today's SR 520 bridge could fail during a severe windstorm
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.
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 withstand severe windstorms
The replacement Evergreen Point Floating Bridge pontoons, bridge deck and anchor cables will be designed to withstand wind storms up to 92 mph. Today’s advances in 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 real threat to our region. WSDOT recognizes this threat and is developing a response and recovery plan in case the bridge fails. We are also exploring early pontoon construction.
The response and recovery plan includes:
- Testing emergency response scenarios through guided simulations such as a tabletop exercise.
- Communications plan to keep people, emergency responders and media informed.
- Transportation plan to manage diverted bridge traffic with alternate routes .
- Action plans to replace the bridge during a partial or full closure of the SR 520 corridor.
The final draft of the SR 520 catastrophic failure plan will be available in summer 2008.