Sept. 22, 2008
News from King County Department of Transportation
Release date: Sept. 22, 2008
Ambitious bridge retrofit program wraps up
115 county bridges strengthened to withstand major earthquake
Crews are hammering home the last braces and bolts of the final bridge in a 14-year effort to retrofit 115 King County bridges at risk for damage or destruction during an earthquake. The $22 million project is a major investment in public safety and mobility in the event of a major earthquake in King County.
The Green River Gorge Bridge east of Black Diamond is the last of 115 seismic projects, marking the completion of a program that will keep key lifeline routes open all across the county when the next earthquake strikes.
“Hardening county bridges to better withstand an earthquake is an investment that will one day save lives and keep vital roadways open,” said King County Executive Ron Sims. “Basic infrastructure investments like this may be expensive, but they are well worth the cost.”
In 1994, King County kicked of an ambitious plan to improve the seismic safety of county bridges. Sims said given what is known about the seismic vulnerability of the region, it was a critically important – and timely – investment.
“It served us well when the Nisqually Earthquake hit in 2001 – not a single county bridge was lost during that big quake,” he said.
Sims said the $22 million seismic retrofit program ensures that key lifeline routes in unincorporated King County will likely remain open and functional in the event of an earthquake. The King County Roads Division maintains almost 200 bridges located throughout the unincorporated areas. These bridges range in age from brand new to more than 90 years old, but all are inspected frequently and undergo regular maintenance to ensure they are safe for the public.
Over the years, Sims has made basic infrastructure investment a centerpiece of his administration, calling such work “a fundamental responsibility of government.” In addition to continued funding for the seismic retrofits, in 2007 Sims budgeted money for the Road Services Division to begin a new $16 million program to accelerate the replacement of 57 aging short span bridges. High priority bridges will be re-constructed within 10 years, rather than 20.
The seismic retrofit program begun in 1994 targeted 115 county bridges considered to be most vulnerable to collapse or major damage during an earthquake. The strengthened bridges are located all across the county; some in cities that incorporated or annexed since the program began.
Bridges scheduled for replacement or rehabilitation within 10 years were excluded from the program when it was developed.
The King County Road Services Division’s project currently underway on the Green River Gorge Bridge is the last bridge in the program to be retrofitted. It began in 1994, when the division completed a comprehensive Bridge Seismic Study. Each bridge was evaluated for: its structural vulnerability; its location in relation to known seismic activity; the significance of the bridge for the traveling public; and the possibility of severe injury or loss of life in the event of an earthquake. A cost analysis for each bridge was prepared, and money was set aside each year for the work.
“This was a complex program to manage because the bridges are so diverse in location, design, age, condition, and the amount of traffic they carry,” said Division Director Linda Dougherty. “We put together a team of in-house staff and consultants who were able to develop a program that approached each bridge as a unique project, yet took the knowledge learned from each previous project to develop engineering efficiencies to save time and money. In the end, we completed all of the projects originally identified – something we didn’t dream could be possible 14 years ago.”
Each seismic retrofit project is accomplished in three phases. The first phase involves analyzing the bridge to evaluate its vulnerabilities and potential retrofit concepts. Computer models and the most current seismic design code are used to predict the motion of the bridge during a major earthquake. In Phase II, the best retrofit approach is fully developed and construction drawings are produced. In Phase-III, a construction contract is awarded, and the physical improvements are made to the bridge.
Seismically retrofitting these bridges had other benefits, as well. For example, the extra piles installed for better resistance to earthquake forces along five bridges on the Woodinville-Duvall Road also gave the bridges better stability during river flooding. And, the projects have also upgraded many load-restricted bridges, eliminating load limit signs.
The state also supported King County’s efforts with by administering federal grants that helped pay for some of the retrofit work.
“Public safety is the primary goal with all of the projects,” said Dougherty. “We are always striving to preserve and improve each bridge to maintain it as a safe and reliable component of King County’s transportation network.”