TOP 10 BRIDGES: Going for gold (No. 4)

By: Mary Beth Nevulis

Any major bridge project has expectations to live up to, but when a project is next to the Golden Gate Bridge, the bar is set a little higher.
The Doyle Drive High Viaduct is a concrete bridge deck that is part of a larger project to connect the Golden Gate Bridge and the city of San Francisco. The site is in an urban national park, and there were aesthetic and historical requirements that had to mesh with engineering concerns.
Being in California, seismic concerns were a priority. Seismic joints at the beginning, middle and end of the bridge will allow movement during tremors or earthquakes.
“The existing bridge was not seismically safe, so that will be totally demolished and the new bridge will be built to current Caltrans [California Department of Transportation] seismic standards,” Jyotsna Sharma, resident engineer, told Roads & Bridges.
The bridge is situated in Presidio Park, where some of the oldest buildings in California are. Because the bridge foundations were being built next to historical buildings, columns had to be oscillated down, not impact-hammered down. The problem was, Caltrans needed the foundations to be 12 ft in diameter.
“An oscillator with a 12-ft diameter doesn’t exist in the U.S., and we didn’t know it existed at all,” Dave Pang, Caltrans construction manager, told Roads & Bridges. “The world’s largest oscillator had to be specially made for this project in Germany.”
The foundations needed to be so large because they were supporting 275-ft-wide spans, Molly Graham, public information officer, told R&B.
“You look through the high viaduct and you’re looking at the Golden Gate Bridge,” Graham said. “The large spans opened the view.”
The superstructure of the deck was designed to be slender, with a lot of curves on the bottom, which requires a lot of prestressing, said Pang.
“We have both longitudinal and latitudinal transverse prestressing in both directions, which is pretty difficult to do,” Pang said.
The project is expected to be finished by the end of 2011, with most major construction completed in mid-November.