Summer 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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Page 21 of 55

On February 7, 2019, representatives of University of California San Diego (UCSD), San Diego Association of Governments (SANDAG), and California Department of Transportation (Caltrans) cut the ribbon on the Gilman Drive Overcrossing, a bridge that had been in the campus plan for over 40 years. The structure completes the campus loop and provides a second crossing over busy Interstate 5 (I-5). Background Founded in 1960, UCSD is a university known for its cutting-edge research. In the 1970s, Caltrans built the I-5 freeway and an overcrossing at Voigt Drive to service the small campus. By 2011, the campus had grown to almost 40,000 students, and a new crossing at Gilman Drive was needed. UCSD and its design team, led by Moffatt & Nichol, partnered with Caltrans and SANDAG to develop a new bridge for this location. Design Goals and Process The stakeholders agreed that the structure would need to be functional and cost-effective, and its design should have a strong character that would be compatible with the world-class architecture on campus. The project goals led the design team to propose a concrete arch. The simple lines of an arch are beautiful, elegant, and timeless. The arch has been used for thousands of years and is one of the most robust structural forms. The funicular shape produces axial compression under uniform gravity loads—the type of inter nal loads for which concrete is ideally suited. A funicular structure is one that achieves an ideal equilibrium state by adopting the right form." For example, an arch under uniform gravity loads should be parabolic in shape, and, in that case, all sections of the arch will be under direct compression with no internal shear or bending forces. The arch naturally adapts to the funicular shape as loads are applied." Under self- weight or other uniform gravity loads, it takes the shape of a catenary and all sections of the cable are in direct tension, with no shear or bending."If a point load were applied, its shape would change to the new funicular shape for that applied loading (V-shape). For an arch to be viable, it needs to have enough rise for an efficient shape, and the large horizontal thrusts must be resisted by the foundations; therefore, strong foundation material is needed near the ground surface. For the Gilman Drive project, the bridge deck would be almost 40 ft above the eight-lane I-5, providing an acceptable rise-to-span ratio of 1:9. Because the existing I-5 is profile GILMAN DRIVE OVERCROSSING / SAN DIEGO, CALIFORNIA BRIDGE DESIGN ENGINEER: Moffatt & Nichol, San Diego, Calif. CONSTRUCTION MANAGER/GENERAL CONTRACTOR: Mid-Coast Transit Constructors, a joint venture of Stacy and Witbeck, Herzog, and Skanska, San Diego, Calif. POST-TENSIONING CONTRACTOR: DYWIDAG-Systems International, Long Beach, Calif. OTHER CONSULTANTS: Safdie Rabines Architects, San Diego, Calif. (bridge architect); Earth Mechanics Inc., San Marcos, Calif. (geotechnical engineer) OTHER MATERIAL SUPPLIERS: Condon-Johnson & Associates, San Diego, Calif. (micropiles); Gerdau Reinforcing Steel, San Diego, Calif. (reinforcing bars) Gilman Drive Overcrossing at the University of California San Diego by Dr. Tony Sánchez, Systra-International Bridge Technologies Completed Gilman Drive Overcrossing at the University of California San Diego. Photo: Paul Turang. 20 | ASPIRE Summer 2019 P R O J E C T

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