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SR 520 - Safety and Vulnerability

Waves batter the SR 520 bridge
Waves batter the south side of the old, 1960s-era floating bridge during a February 2006 storm.

Safety is our top priority. SR 520's old Evergreen Point Floating Bridge was vulnerable to failure during severe windstorms and the highway's fixed-column bridge approaches could fail in a strong earthquake. Replacing these bridges with structures that meet today's safety standards helps maintain public safety, protects Washington state's transportation infrastructure and ensures that traffic continues to flow on a key, urban highway.


SR 520's old bridges vulnerable to failure during an earthquake or windstorm

Hollow columns are susceptible to earthquakes

The old SR 520 west approach bridge and the Portage Bay Bridge were designed and built in the early 1960s before modern earthquake standards existed. The bridges' hollow supporting columns could break and collapse during a major earthquake.

This simulation video demonstrates how a major earthquake could cause a catastrophic bridge failure.

Old floating bridge vulnerable to high winds

The storms that sent waves pounding into and over the old floating bridge's southern wall demonstrated the bridge's vulnerability. The drawspan, anchor cables and pontoons all could have broken or cracked when stressed by the sustained winds of a severe storm. 

This simulation video demonstrates how and why the old floating bridge could have failed during a storm with winds exceeding 75 mph.


New bridges designed to withstand once-in-a-century events

Modern design to withstand earthquakes

The new West Approach Bridge North and the funded Portage Bay Bridge and West Approach Bridge South are designed to withstand a 1,000-year earthquake. (A 1,000-year earthquake refers to a magnitude of earthquake expected to happen only once in 1,000 years.)

Stronger cables and bridge to resist the waves

The new Evergreen Point Floating Bridge pontoons, bridge deck and anchor cables are designed to withstand storms with sustained winds of up to 89 mph. The new bridge's updated design standards will protect travelers while extending the bridge's lifespan.


WSDOT is prepared if the bridge collapses or sinks

Response plan to deal with bridge failure

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

This Catastrophic Failure Plan (pdf 562 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.