The North Torrey Pines Road Bridge in Del Mar, Calif., is a sweeping 550 feet (168 meters) long structure that curves along the Pacific Coast, crossing an active railroad line. The original structure was built in 1933; and after standing for more than seven decades in the harsh salt air, it was suffering from severe corrosion.
The bridge had been classified as structurally and seismically deficient, and analyses showed that the majority of the bents and abutments needed to be rehabilitated. Even if the corrosion had been mitigated, the existing columns would not be able to handle large seismic demands without catastrophic failure, says Keith Gazaway, senior engineer for Kleinfelder, the engineer for the project.
With the goal of preserving this historic structure, a specialized rehabilitation strategy was developed that would allow for minimal change to the visual character of the bridge while providing a far more seismically stable design. That would not be easy, Gazaway says. “Developing a constructible retrofit strategy that satisfied the complex seismic demands without affecting the bridge’s appearance and historic eligibility was our biggest challenge.”
They chose a precast concrete system to meet the needs of the project because precast concrete would deliver the high-performance seismic and durability they needed, with the design flexibility to maintain the historic look and feel of the original bridge. “The use of precast allowed the contractor to demolish and replace the historic structure in a sensitive marine environment in close proximity to a very active rail line,” Gazaway says.
Construction of the bridge included replacing the superstructure with a precast, posttensioned concrete replica, supplemental “hidden” abutments, ground improvements to mitigate seismic liquefaction, and shear retrofit of all columns. “The precast concrete bridge girders were pretensioned to introduce a compressive force in the units but then were posttensioned into continuous spans, thus providing a structure that should perform well in the aggressive marine environment,” says Jon Grafton of Oldcastle Precast. The historic barrier rail was preserved in a customized concrete barrier, which incorporated the original bridge pilasters. Existing substructure elements were repaired and protected with an impressed current cathodic system to extend the service life at least 50 years.
All of the precast concrete pieces featured complex geometry, including warped soffits, varying end bearing conditions, and multiple girder shapes, Grafton says. “The complex detailing of the skewed and haunched girders was nicely accomplished in the controlled environment of the precasting yard.” To replicate the historic board form finish on the original structure, a textured surface was added at the bottom of all of the precast concrete components.
This project was complex; but after three years of construction and more than a decade of technical, seismic, and environmental studies, the rehabilitation and retrofit of this historic bridge is finally complete. The resulting rehabilitation delivered an elegant and redundant earthquake-resisting system that preserved the historic qualities of the bridge, Gazaway says. “We overcame a multitude of political, logistical, and constructability issues to achieve a beautifully restored landmark structure.”