A Black Diamond-area span is the capper in a 14-year, $22 million effort to retrofit 115 King County bridges and help them withstand earthquakes.

Crews were hammering home the last braces and bolts Sept. 22 in the Green River Gorge Bridge, marking the completion of a program that officials said will keep key routes open countywide after the next major earthquake strikes.

The combined projects are “an investment that will one day save lives and keep vital roadways open,” said County Executive Ron Sims. “Basic infrastructure investments like this may be expensive, but they are well worth the cost.”

The work, which started in 1994, “served us well” when the Nisqually earthquake hit in 2001, Sims said. “Not a single county bridge was lost during that big quake.”

The county maintains almost 200 bridges. They range in age from brand new to more than 90 years old, but all undergo regular maintenance and inspections, officials said.

Bridges that received seismic retrofitting were considered to be most likely to collapse or be heavily damage during an earthquake. Ones scheduled for replacement or rehabilitation within 10 years were excluded from the program.

The Green River Gorge Bridge, located east of Black Diamond, is the last bridge to be retrofitted.

Each bridge targeted for such work was scrutinized for structural vulnerability, proximity to known seismic activity, the significance of the bridge for travel, and the possibility of severe injury or loss of life in the event of an earthquake.

“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 Linda Dougherty, director of the county’s roads division. “We 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 – something we didn’t dream could be possible 14 years ago.”

Computer models and current seismic design codes were used to predict the motion of bridges during a major earthquake.

Seismically retrofitting these bridges had other benefits. For example, many load-restricted bridges no longer have load-limit signs as a result of being beefed up.

The state administered federal grants that helped pay for some of the retrofit work.

“Public safety is the primary goal with all of the projects,” said Dougherty.