Would you fight CalTrans to save historic arches? This #seismicsaturday we feature the Georgia St Bridge in the Hillcrest neighborhood of San Diego.
In the early 1900s, city planners in a rapidly-expanding San Diego wanted a streetcar line down University Ave to connect the new suburb of North Park with the city. The biggest roadblock was a steep hill between the Hillcrest and North Park neighborhoods. As a solution, a 40 ft cut was made through the hill (fig. 2). Retaining walls were built on either side of the cut and in 1914, the Georgia St. Bridge was built above the cut.
The bridge was designed by engineer James Comley. 3 parabolic arches span the cut, each one anchored into either side of the hill (fig. 4). The main arches support secondary arches, rising at regular intervals to support the deck (fig. 5). A beautiful reinforced-concrete cantilever extends outward to support the bridge crosswalks. It is curious that the top of the cantilever seems to be the only major structural component of the whole bridge in tension.
There have been two major threats to the bridge: earthquakes and CalTrans. By the 1990s, after over 85 years of heavy use, the bridge was deemed unsafe in an earthquake. CalTrans decided to tear the bridge down and replace it with a modern earthquake-resistant overpass. However, community members fought hard to stop this demolition. Included in the effort to save the bridge was Toni G. Atkins, then a small-time assistant to a City council member. She recalled a meeting with a city engineer, where the engineer said that he would be around long after she was out of politics and the bridge was torn down. Atkins said that the city engineer is now gone, and Atkins is now our San Diego representative in the California Senate, and Senate Pro-Temp.
As a result of the community campaign to save the bridge, the bridge underwent a 14 million dollar seismic upgrade from 2015 to 2017 (one can see the point of view of Caltrans, who would have saved taxpayer money by just tearing the bridge down and building a new one). Everything but the 3 initial parabolic arches was demolished and rebuilt (fig. 6). According to a report by Kleinfelder, the contractor on the project, fiber-reinforced concrete was used (fig. 7). This type of concrete is strengthened with fiberglass and carbon fiber in the concrete mix to give the concrete more strength in tension and resistance to cracking/fracture.
Structures shouldn’t just be practical, they should also be beautiful. The Georgia Street Bridge is proof that we can save our beautiful historic structures while at the same time making them earthquake safe.