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Cypress Street Viaduct, Collapse Section
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
Collapsed Section of Cypress Structure
Dashed red line indicates collapsed section of Cypress structure in 1989

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.
Upper Deck of Cypress Street Viaduct
Collapsed section of upper deck of
Cypress Street Viaduct in 1989

The Cypress Street Viaduct was designed as a two-tier multi-lane highway constructed of reinforced concrete (Moehle, 1997). Upper and lower levels were connected by two-column bents in a combination of cast concrete and four pin (shear key) connections. The upper deck in some sections was not securely fastened to the lower deck, making this concrete susceptible to vibrations. As the bridge vibrated during the earthquake, the pins connecting the upper level to the lower level also began to vibrate, causing the concrete surrounding the pins to crumble and break away. Without the presence of concrete under the support columns, the columns slid sideways under the weight of the upper deck and allowed a large portion of the upper deck to collapse (Yashinsky, 1998).

Several aspects of the design and construction of the structure have been suggested as contributing to its failure: inadequate transverse reinforcement in the columns; ineffective bent cap and pin connection design (Moehle, 1997); and improper compensation for the weak soil conditions (Yashinsky, 1998). The prevailing conclusion is that each of the factors, coupled with the extreme conditions played a crucial role. Tests were performed on pieces of concrete extracted from the wreckage to assess structural integrity; many components of the Viaduct were found to be structurally sound. It was concluded that the concrete used had more than satisfactory strength. In addition, micro structural analysis of concrete samples taken from undamaged columns within the region of collapse showed that the concrete was produced and cast according to the proper procedures at the time of construction (Monteiro et al., 1991).


The Cypress Street Viaduct collapse disaster may have been avoided had the City of Oakland followed the repeated recommendation that the Cypress Street Viaduct be upgraded (Peterson, 1990). Earthquake engineers had suggested many times to the City of Oakland that the Viaduct should be retrofitted with the new technologies that had been developed to counteract the type of concrete breakaway that occurred during the quake. One such technology that was available, and that could have helped to inhibit that type of failure, was steel reinforcing plates that could have been retrofitted to the existing columns (Peterson, 1990). Another, lead/rubber isolators, would have minimized the vibrations the Viaduct experienced during the quake (Peterson, 1990).

It is unknown if additional reinforcement would have been effective. Due to the original design, the viaduct was susceptible to strong external driving forces matching its natural resonant frequencies. Reinforcement may have averted collapse had it been designed to counteract the effects of amplification of the seismic waves created by the soft fill of the valley floor, but the extent of amplification had not yet been realized. Also, the problem of matched resonant frequencies was unknown at the time and would not have been taken into account.


The process of rebuilding the viaduct proved to be a Herculean task; the new viaduct had to be completely redesigned, from the soil conditions to the pin joints. The new viaduct was again required to span several areas of soft mud; large, stiff foundations with 42 inch diameter steel pipe piles were designed to compensate for the weak soil (Yashinsky, 1998). To control large torsional moments that can occur during earthquakes, pinned bent cap connections, which join the upper and lower decks of the viaduct, were incorporated into the design (Yashinsky, 1998). Lack of knowledge on the interaction between reinforced concrete columns and bent caps required extensive testing by the University of California at San Diego. An overall cost of the rebuilding was estimated to be almost one billion dollars. The new viaduct was scheduled to be complete by the end of 1998.

Though the structural design of the Cypress Street Viaduct was incapable of withstanding the conditions met during the Loma Prieta earthquake, only a lack of knowledge and understanding of the geotechnical area and the structures angular frequencies are to be blamed for this unfortunate incident. It is a lesson for engineers and geologists who must prepare for destructive earthquakes in all conditions (Halliday et al., 1993).


Philip A. James, Luk M. St. Onge, Mark E. van Voorst, and Matthew P. Walker


Doyle, Kevin, (Ed.), Oct. 30, 1989, "The Day The Earthed Roared", Maclean's Magazine, pp. 54-62.

Halliday, David; Resnick, Robert; Walker, Jearl, 1993, Fundamentals of Physics, Extended, 4th ed., John Wiley & Sons Inc., 1306 pp., ISBN 0-471-57578-X.

Monteiro, Paulo; Asselanis, Jon; MacCracken, William, May-June 1991, "Investigation of the Microstructure and Mechanical Properties of the Structural Materials of the I-880 Double-Deck Viaduct", ACI Materials Journal, Vol. 88, No. 3, pp. 288-293, ISSN 0889-325X.

Moehle, J.P., updated Dec. 9, 1997, accessed Oct. 19, 1999, "Preliminary Observations on the Performance of Concrete Freeway Structures", National Information Service for Earthquake Engineering, University of California, Berkeley, Internet address:

Peterson, Alan, (Ed.), Mar. 1990, "Excuses on Shaky Ground", International Construction, Vol. 29, No. 3, pp. 40-41, ISSN 0020-6415.

Stewart, Jonathan, updated Dec. 8, 1997, accessed Oct. 24, 1999, "Key Geotechnical Aspects of the 1989 Loma Prieta Earthquake", National Information Service for Earthquake Engineering, University of California, Berkeley, Internet address

Yashinsky, Mark, 1998, "Cypress Street Viaduct", US Geological Survey Professional Paper, No. 1552-8, pp. 19-26, Library of Congress catalog-card No. 92-32287.

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

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

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