Richmond-San Rafael Bridge

Richmond-San Rafael Bridge. . History , The Richmond-San Rafael Bridge opened to traffic in 1956, following 2 years of planning and design and 4 years of construction. At the time, its 4.5 mile-length and double-decker design made it one of the most complex bridges in the state and California’s longest steel structure. The bridge serves as a vital link between Marin and Contra Costa Counties. Today, over 75,000 commuters use the bridge daily to cross the northern end of the San Francisco Bay., Loma Prieta Earthquake , On October 17, 1989, the Loma Prieta Earthquake jolted the Bay Area. The earthquake, which measured 7.1 on the Richter scale and lasted 15 seconds prompted a review of the seismic safety of all California bridges, including the Richmond-San Rafael Bridge., The Richmond-San Rafael Bridge is sandwiched between the San Andreas Fault, located 10 miles to the west, and the Hayward Fault, located 4 miles to the east. A “no collapse criterion” was used for the seismic retrofit of the nearly 50 year old structure. Under this strategy, the bridge will withstand an earthquake of 7.25 magnitude on the Richter scale on the Hayward Fault and 8.0 magnitude on the San Andreas Fault., Seismic Retrofit , The seismic retrofit of the Richmond-San Rafael Bridge involved strengthening all parts of the structure from the foundations and bridge deck to the steel towers and trestle. Retrofitting the foundations presented the most daunting challenge for the project. The strait is subject to strong currents, high winds, and low to no underwater visibility. Under these difficult conditions, workers installed large diameter piles in between the bell-shaped foundations of eleven of the bridge’s piers. Forty-seven bridge piers were retrofitted with micropiles, in which smaller diameter piles were drilled through the existing foundation. A total of 480 micropiles were installed throughout the structure., Another challenging aspect of the retrofit involved modifications that allowed controlled movement of the structure during an earthquake. Viscous dampers were installed at the existing towers’ anchorages. In an earthquake, the dampers allow the bridge to expand or contract by as much as six feet. The action absorbs the earthquake’s energy preventing additional structural damage. Some of the bridge’s dampers are as large as a semi-truck, weighing two tons each. Isolation joints and bearing further strengthen the main bridge structure, and allow appropriate movement during an earthquake.