Cypress Street Viaduct - Loma Prieta Earthquake

Loma Prieta Earthquake

On October 17, 1989, the portion of the structure from 16th Street north all the way to the MacArthur Maze collapsed during the 1989 Loma Prieta earthquake, due to ground movement and structural flaws.

When it was in use, the upper tier was used by southbound traffic, and the lower tier was used by northbound traffic. Some sections of the Cypress Street Viaduct were largely supported by two columns on either side, but some sections were only supported beneath by a single supporting column. The design was unable to survive the earthquake because the upper portions of the exterior columns were not tied by reinforcing to the lower columns, and the columns were not sufficiently ringed to prevent bursting. At the time of its design, such structures were not analyzed as a whole, and it appears that large structure motion contributed to the collapse. It was built on filled land on top of bay clay, which the filled land is highly susceptible to soil settlement during an earthquake and the bay clay exhibits larger ground motion.

After the earth stopped moving, local residents and workers began crawling into and climbing upon the shattered structure with the goal of rescuing those left alive. Many were saved; some only by amputation of trapped limbs. The collapse of the upper tier onto the lower tier resulted in 42 fatalities—two-thirds of the total quake death toll of 63.

After the viaduct was torn down, Cypress Street was renamed Mandela Parkway, in honor of Nelson Mandela, and a landscaped median strip was planted where the viaduct once stood. Before reconstruction occurred, the viaduct ended at the Eighth Street exit on the southern end, with the two roadways going over Seventh Street, while the southbound exit off the MacArthur Maze onto Cypress Street at 32nd Street remained open to local traffic on the northern end.