Along Highway 101, one of the most scenic routes in Northern California, the obsolete, high-maintenance bridge over Rock Creek was due for an upgrade. Rather than repair and seismically revamp the existing steel bridge, Caltrans decided to construct a new bridge. But to do so required maintaining traffic flow, building in a remote location, accommodating the curvature and very high elevation, and producing a structure appropriate to the site's natural beauty.

“Reinforced concrete is economical and versatile and was the material of choice for this bridge,” said Carl Huang, Caltrans senior bridge engineer and project manager.

Engineers used their considerable experience with cast-in-place concrete box girder bridges to meet the challenges. Staged construction solved the traffic maintenance issues. Using locally available ready-mixed concrete, steel reinforcing bars, and form-work reduced transportation problems. Site-cast work allowed for field adjustments when necessary, and combining a closed box superstructure with slanted, tapered columns resulted in a stunning, safe bridge.

The bridge's supporting columns are unique. Slanted supports are especially suited to bridges over deep canyons because shorter, slanted columns can be founded in the canyon sides, creating a more stable structure. The fractured bedrock and steep slopes lent themselves to a foundation system consisting of piers hand-mined into the canyon's sides and continuous with the columns. Reinforcing bars in the columns were based on Caltran's seismic standards.

Although constructing the new bridge cost more than retrofitting, Caltrans will realize savings through many years of low maintenance. The result is a beautiful, modern bridge that meets all current seismic design standards.


  • Owner: California Department of Transportation
  • Engineer: DMJM+Harris, Sacramento, Calif.
  • Contractor: MCM Construction, Inc., Sacramento