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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.”
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