A bridge on the rocks

News November 06, 2002
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The shortest distance between these two points was a brand new lifeline

The shortest distance between these two points was a brand new lifeline.

"If that bridge went out, we'd be in a world of hurt," reported Eugene Calvert, director of transportation for Mendocino County. "It provides an absolutely critical transportation link between Mendocino County and Northern California. Without it, emergency services, interstate commerce and the traveling public would be severely limited. And a detour around the bridge would mean, at best, traveling over 30 miles of gravel road. And that would greatly affect our services to the public. To put it mildly, we're fully dependent on that bridge as a lifeline between Mendocino and Humboldt Counties."

Bridging a canyon is one thing. Bridging a steep, V-shaped chasm that is completely enclosed by forest--on the only north-south highway (U.S. 101) for more than 80 miles around--is quite another. And if that weren't enough of a challenge, that gorge sits in earthquake country, so any bridging solution must address serious seismic issues as well. Taking advantage of its "On Call" Phase II - Seismic Retrofit Bridge Engineering contract, the California Department of Transportation (Caltrans) called on DMJM+Harris to examine the bridge over Rock Creek in Mendocino County and devise a seismic retrofit solution. After looking at it, however, DMJM+Harris implemented a completely different approach.

"Beyond the seismic issues--and they were considerable--there were some very serious problems that no retrofit would fix," explained Neil Harris, project manager for DMJM+Harris. "The bridge deck was in pretty poor condition. Even if we repaired it, it still would not support current standard live loads. And because the bridge was a two-girder system, the existing deck could not be replaced while maintaining traffic flow, which was extremely important to Caltrans, the county and the state. Given the fact that there wasn't another road for almost 100 miles, this also was not a site where you could set up a practical detour. But the challenges didn't end there.

"The bedding rock under the bridge's tower footings was fractured. It could easily split if there were an earthquake. If that happened, no conceivable seismic retrofit measures to the bridge would prevent at least partial failure. And it was impossible to ascertain the precise fatigue cycles over the past 50 years for the steel girders on the bridge. So the overall life cycle of the existing structure was suspect. It became very clear to us that this wasn't a matter of retrofit, but replacement."

Calling in the replacement

One doesn't just decide to replace a bridge without hard facts. So the project team developed an approved seismic retrofit strategy that would allow the structure to withstand the Maximum Credible Earthquake (MCE). Demonstrating to Caltrans the best possible solution given the constraints, the project team also pointed out that no retrofit solution would address all of the issues: bridge fatigue, live-load deficiencies, maintaining traffic flow during retrofit, and the fact that the cost of the retrofit was estimated to be 60% of the cost of replacement. With those facts in hand, Caltrans elected to replace the bridge.

Several replacement bridge types were considered, from steel girder to precast/prestressed girder to cast-in-place concrete, but the steep canyon walls and fractured bedrock at the site made their conventional foundations problematic. The project team chose a slantleg foundation structure to take advantage of the high foundation-bearing capacities of the site. The slantleg scheme also provided a balanced superstructure span arrangement, along with an aesthetically pleasing and appropriate foundation.

Due to the site's remote location and the curvature of the alignment, steel and precast girder solutions were ruled out immediately. And because of the absolute need to sustain complete traffic operations during construction, it was imperative that staged or phased construction be used for the superstructure and substructure. The team chose two-column bents with a concrete box girder superstructure for the bridge. Since structure depth was not an issue and the span lengths were relatively short, a reinforced concrete box girder superstructure was chosen.

For more on the story, read the November issue of ROADS & BRIDGES.

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