In Figure 1, the demolition crew has already started removing the bridge before the reconnaissance team could inspect the damage (the team started in Sacramento and were escorted by the California Highway Patrol down I-5 through various detours to a hotel in Pasadena). During the earthquake, the outer frames rotated, failing the restrainers, followed by unseating and collapse of the cantilevered spans. A retrofit in 1974 added restrainers at the hinges. These two parallel 741-foot-long bridges (53 1797L/R) built in 1955, were designed with table-like center frames that supported the cantilevered spans of the four end frames on highly skewed, 8-inch-long hinge seats. Seven bridges, five of which were designed before 1971, were severely damaged during the earthquake but all 60 bridges in the Los Angeles area that had been retrofitted after the Loma Prieta Earthquake performed very well. The need to quickly complete the retrofit program was demonstrated again when the 1994 Northridge Earthquake occurred before the program was completed. Thus, the Caltrans Seismic Advisory Board was formed and continues to advise Caltrans on seismic issues. The Board of Inquiry recommended that a standing board of experts should be created to advise Caltrans on its earthquake engineering practices. During the earthquake, the soil shook violently, dragging the piles from their connection with the superstructure which resulted in the pile extensions punching through the bridge deck.Ĭoncerns about the bridge damage prompted the California governor to create a Board of Inquiry that found that Caltrans was doing a good job addressing seismic issues but needed to accelerate the seismic retrofit program. The Struve Slough Bridges (36 0088L/R) in Watsonville were T-girder bridges on piles founded on very soft soil. Also, a 50-foot span over Pier 9 on the East Crossing of the San Francisco Oakland Bay Bridge (33 0025),īuilt in the 1930s, collapsed due to inadequate 4-inch-wide seats, reiterating the lesson that seats have to be long enough to support the resulting displaced bridge members during earthquakes. Several other double-deck viaducts around San Francisco sustained severe damage to the superstructure-to-column connections that resulted in their closure and removal after the earthquake. This resulted in the collapse of a mile-long segment of the viaduct during the earthquake. The main reinforcement in these connections was not sufficiently developed, and the shear reinforcement was inadequate. The earthquake damaged the double-deck Cypress Viaduct (33 0178), built in the 1950s, that had been designed with vulnerable pinned connections to make it structurally determinant and easier to analyze. Unfortunately, the 1989 Loma Prieta Earthquake occurred before many bridges were retrofitted. The previous retrofit program relied on cable restrainers to limit displacement and prevent column damage, but shear damage to the short columns on the Route 605/5 Separation (53 1660) generated enough concern to begin a new retrofit program to wrap bridge columns in steel (or fiber-reinforced polymer) casings on older bridges. The 1987 Whittier Narrows Earthquake was another turning point in Caltrans seismic design of bridges. The San Fernando Earthquake was also the start of the practice of Caltrans sending out a reconnaissance team of licensed engineers to study the damage and write a report with lessons learned, a practice that has continued for every subsequent large earthquake. Caltrans also started a seismic retrofit program to address the many existing bridges that had been under-designed for earthquakes. Other changes included the development of a site-specific ground shaking hazard for designing bridges and a capacity-based design method that relied on structural column fuses to limit seismic forces. Also, the minimum seat length at expansion joints, abutment seats, and hinges went from 12 inches to 18 inches (and later to 24 inches). Immediately afterward, Caltrans wrote construction change orders requiring more transverse reinforcement and continuous main reinforcement in bridge columns and eliminating a vulnerable lap splice connecting the footing to the column. Bridges at that time were designed for a small seismic force, which resulted in extensive damage to bridges and interchanges during the earthquake. The most significant changes occurred after the 1971 San Fernando Earthquake. Every damaging California earthquake has resulted in changes to the California Department of Transportation’s (Caltrans’) seismic practice.
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