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The eastern span replacement of the San Francisco–Oakland Bay Bridge was a construction project to replace an unsafe portion of the Bay Bridge with a new self-anchored suspension bridge (SAS) and a pair of viaducts. The bridge is located in the U.S. state of California, and crosses the San Francisco Bay between Yerba Buena Island and Oakland. It was built between 2002 and 2013, and does not have a name other than the unofficial name of the bridge as a whole ("San Francisco-Oakland Bay Bridge"). The eastern span replacement is the most expensive public works project in California history, with an estimated cost of $6.4 billion. Originally scheduled to open in 2007, several problems delayed the opening until September 2, 2013.〔(http://abcnews.go.com/US/wireStory/64b-sf-oakland-bay-bridge-opens-traffic-20139554 ) AB News website〕 With a width of ,〔(【引用サイトリンク】url=http://www.dot.ca.gov/hq/paffairs/news/pressrel/14pr095.htm )〕 comprising 10 general purpose lanes,〔(【引用サイトリンク】title= The San Francisco–Oakland Bay Bridge: Facts at a Glance )〕 it is currently the world's widest bridge, according to Guinness World Records.〔http://www.guinnessworldrecords.com/world-records/6000/widest-bridge〕〔http://www.dot.ca.gov/hq/paffairs/news/pressrel/14pr095.htm〕 The Bay Bridge has two major sections: the western suspension spans and their approach structures between San Francisco and Yerba Buena Island (YBI), and the structures between YBI and the eastern terminus in Oakland. The original eastern section was composed of a double balanced cantilever span, five through-truss spans, and a truss causeway. The original spans of the bridge east of Yerba Buena Island became the subject of concern after a section collapsed during the Loma Prieta earthquake on October 17, 1989. The replacement span is engineered to withstand the largest earthquake expected over a 1500-year period, and it was expected to last at least 150 years with proper maintenance.〔(SAS Maintenance Travelers ). Bay Bridge Info. Retrieved on 2013-07-15.〕 ==Background== It had been known for over 30 years that a major earthquake on either of two nearby faults (the San Andreas and the Hayward) could destroy the major cantilever span. Little was done to address this problem until the 1989 Loma Prieta earthquake. The earthquake measured 6.9 on the moment magnitude scale and while the epicenter was distant from the bridge, a 50-foot (15 m) section of the upper deck of the eastern truss viaduct portion of the bridge collapsed onto the deck below, indirectly causing one death at the point of collapse.〔(Oakland Bay Bridge Collapse ) This video shows the crash at 0:0:26. This was caused by the misdirection of traffic by the California Highway Patrol, over concerns about the stability of the western off ramps and freeway in San Francisco.〕 The bridge was closed for a month as construction crews removed and reconstructed the fallen section. It reopened on November 18, 1989, with a new stronger retrofit in place. The failure was at the transition between the easternmost through-truss and the westernmost double-deck causeway segment, a location where the inertial response character of the structure makes an abrupt change. Analysis of the event completed by internal staff has shown that the bridge was close to a far more catastrophic failure in which either the through-truss or the causeway segment would have dropped from their common support structure. It was clear that the eastern span needed to be made more earthquake resistant. Estimates made in 1999 placed the probability of a major earthquake in the area within the following 30 years at 70%, although recent studies announced in September 2004 by the United States Geological Survey have cast doubt on the predictability of large earthquakes based upon the duration of preceding quiet periods. A more recent (2008) analysis asserts an increased probability of a major event on the Hayward Fault. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Eastern span replacement of the San Francisco–Oakland Bay Bridge」の詳細全文を読む スポンサード リンク
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