The University of California San Diego is set in a breathtaking location, nestled on 1200 acres of coastal woodland near the ocean. However, at the university’s center is a canyon that separates the main campus from student housing and the East Campus Medical Center, and historically, this separation created long travel times for pedestrians and bikers. The new $10 million Mesa Housing Pedestrian and Bicycle Bridge solves this problem, improving access between the neighborhood and campus.
“The bridge design is inspired by the canyon it spans and the long-standing San Diego tradition of pedestrian and bike bridges that span from one mesa to the next,” says Eric Naslund, project architect for Studio E Architects. “The simple and elegant spans of the concrete beams make a perfect foil for an organically shaped deck that morphs as it arrives at either landing.”
Several bridge types were considered for this project, including steel truss, steel girder, precast concrete girder, and stress ribbon. The designers ultimately settled on a three-span, precast concrete spliced-girder bridge with a 190-ft-long middle span. The design was selected for its combined aesthetics, durability, and cost effectiveness.
“Use of precast concrete was the option with the highest cost savings, resilience, and durability, and the least maintenance,” says Sami Megally, project engineer for Kleinfelder. “Use of precast concrete girders allowed the addition of all aesthetics features that make this bridge an architectural icon of the UC San Diego campus.”
An extensive analysis of the precast concrete design demonstrated its constructability amid adjacent project construction and steep grades, and proved that it could accommodate environmental restrictions in the canyon, which is home to a protected wetland and coastal sage scrub and gnatcatcher habitats. “Permanent and temporary supports could not be used within the canyon,” says Keith Gazaway, project manager for Kleinfelder. “The 190 ft span length was the minimum feasible length to avoid encroaching into the environmental ‘no-touch’ zone.”
Spliced in the air
Each line of girders in the bridge consists of three segments: two segments over the bents and end spans, and a middle drop-in segment. To accommodate the limiting site requirements, the precast concrete girders for the end spans were designed to be erected before the main span, and to span over the bents and into the main span. Partial construction of the deck slab was then completed, so that the main span girders could be spliced in the air—an arrangement that avoided the need for temporary supports. To further minimize the project’s impact on the environmentally sensitive area, the team located bridge supports outside of the wetlands’ limits during and after construction.
During construction, a survey error resulted in the bridge being 6 ft longer than originally planned. “This was a challenging situation, especially when using precast concrete,” Gazaway says. His team solved the problem with a rapidly deployed abutment retrofit scheme, keeping the project on schedule.
The finished design features significant curvatures in the edge of the deck, with varying inclination of metal railings along the length of the bridge to create a more compelling visual experience. Shallow precast girders concrete were used to reduce crane-lifting requirements and to accommodate architectural features. To enhance the experience of bridge users, paths for pedestrians and bicyclists were distinguished from each other through the innovative use of a two-tone glass-seeded Lithocrete finish with two different colors.
By bringing innovative ideas and solutions to the bridge design and construction process, the project team achieved a balance of safety, functionality, environmental sensitivity, community value, aesthetics, and cost savings. The bridge enhances access and connectivity between the university and the surrounding community, and it encourages walking and biking by providing a significantly shorter, safer route to almost every part of the campus.
Gazaway believes this project demonstrates how a precast concrete girder bridge project can evoke a positive image of engineering excellence in terms of architectural design and structural feasibility. “The project also is a testimony to the fact that precast concrete girders with extended span limit can be used for architecturally innovative bridge crossings over areas where conventional formwork is not feasible or too costly.”