On October 17, 1989 at 5:04 p.m., a devastating earthquake, measuring 7.1 on the Richter Scale, rocked the city of Oakland, causing billions of dollars worth of damage and a death toll of 64. Geological Scientists determined the epicenter of the quake to be in Nisene Marks State Park, along the San Andreas Fault (Doyle, 1989). During the earthquake, a 1.4 km section of the Cypress Viaduct collapsed. As the upper level fell, slabs of concrete trapped many unsuspecting motorists. A survivor gave a firsthand account of the experience as "like being inside an exploding building" and saw the vehicles ahead of her "go like dominoes" (Doyle, 1989). The deaths on the Cypress Viaduct, 35 in total, accounted for more than fifty percent of the 64 lives taken during the earthquake. Fortunately, only about eighty vehicles were on the affected stretch of highway during the earthquake — a fact attributed to a sporting event: the San Francisco Giants were playing in the World Series and people were indoors anticipating the commencement of the game (Doyle, 1989).
The collapse of the bridge was one the most devastating effects of the earthquake, requiring immediate action by all levels of assistance. Emergency crews worked nonstop to free people from the rubble; local residents provided ladders and helped extinguish fires and locate survivors. The final survivor was located and rescued approximately 90 hours after the initial earthquake (Doyle, 1989). The State of California and the Federal government declared the region a disaster area, making it eligible for $300 million in immediate relief (Doyle, 1989). Damage caused by the quake was extensive, and rebuilding the damaged transportation infrastructure continued for many years following the disaster.
Causes of Failure
Two major factors led to the collapse of 1.4 kilometers of the Cypress structure, part of the Nimitz freeway (Interstate I-880). The first was the geotechnical aspect of the central San Francisco Bay area. The second was the design of the concrete Viaduct and its response to strong ground shaking.
|During the Loma Prieta earthquake, the entire Viaduct structure began to vibrate tremendously (Peterson, 1990). Whereas well-graded soils helped to dampen vibrations (Yashinsky, 1998), the soft "bay mud" upon which most of the structure was constructed actually served to increase the amplitude of vibrations by up to five times in comparison to that of the rest of the freeway which was built on rock. In addition, it was later determined that the angular frequency of the seismic waves almost exactly matched a natural angular frequency of the individual horizontal sections of the structure (Halliday et al., 1993). It is suggested that these sediment resonances could have played a significant role in contributing to the freeway collapse, since the resulting forces were not anticipated in the original design.
Collapsed section of upper deck of
Cypress Street Viaduct in 1989
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Figure 1: Wilshire, H.G., updated Dec. 5, 1995, accessed Oct. 22, 1999, "Aerial view of collapsed sections of the Cypress viaduct of Interstate Highway 880", US Geological Survey, Internet address http://wrgis.wr.usgs.gov/docs/geologic/ca/dds-29/oakland.html.
Figure 2: Prescot, Will, updated Apr. 9, 1996, accessed Oct. 22, 1999, "The part of the Cypress freeway structure in Oakland, California, that stood on soft mud (dashed red line) collapsed in the 1989", US Geological Survey Fact Sheet-176-95 1995, Internet address http://quake.wr.usgs.gov/QUAKES/FactSheets/BetterDesign/.
Figure 3: Wilshire, H.G., updated Dec. 5, 1995, accessed Oct. 22, 1999, "Side view of support-column failure and collapsed upper deck, Cypress viaduct", US Geological Survey, Internet address http://wrgis.wr.usgs.gov/docs/geologic/ca/dds-29/oakland.html.