NSPE Code of Ethics Case Study (Cypress Street Viaducts)

Collapsed section of Cypress Street Viaduct, 1989
In October 17, 1989 the Loma Prieta earthquake hit the San Francisco Bay area, causing over $12 billion in damages and claiming 64 lives. Over half the victims were on the Cypress Street Viaduct during the quake. What follows is an overview and analysis of the disaster, with reference to the materials and design features used in the construction of the viaduct. The knowledge gained from this tragedy has benefited subsequent structures not only in the technical data that it provided but in the research that such a public disaster is bound to initiate.
History of Bridge In 1949, the design of a new highway that was to service the City of Oakland, California began; by 1957, the construction of the Nimitz Freeway, or I-880, was complete (Yashinsky, 1998). A portion of the Nimitz Freeway that linked the I-880 to the I-80, known as the Cypress Street Viaduct, was a 2 km two-tier highway with five lanes per deck, and traffic flowing at ground level. The path that the Cypress Street Viaduct was required to follow resulted in certain portions of the bridge being constructed on soft mud; in much of the area, the bedrock was over 150 m below the surface (Yashinsky, 1998). The bridge was constructed using cast-in-place concrete with multi-celled reinforced box girders that typically spanned 80 feet (Moehle, 1997). At the time of construction, the California State seismic criteria required designing for a lateral force of 0.06 times the dead load (Yashinsky, 1998). Over subsequent years there was great advancement in both construction and earthquake engineering technology, and although new technology was becoming available, the Cypress Street Viaduct was not being properly restructured to withstand a large-scale earthquake. After a 1971 earthquake in the San Fernando Valley, the State of California initiated a two-phase program to increase the resistance of highways and bridges to earthquakes: Phase 1 involved strengthening the connections between elevated road ways and their support columns; Phase 2 was to strengthen the support columns themselves (Doyle, 1989). While the structure was retrofit with cable restrainer units in 1977, Phase 2 was never carried out on the Viaduct (Yashinsky, 1998).
The Loma Prieta Earthquake