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The I-5 Skagit River Bridge collapse occurred on May 23, 2013 in the state of Washington, in the northwestern United States. At approximately 7:00 pm PDT, one bridge span carrying Interstate 5 over the Skagit River collapsed into the river below. Three people in two different vehicles fell with the span; they were rescued by boat and did not sustain serious injury. The cause of the collapse was determined to be an oversize load striking several overhead support beams on the bridge, which led to an immediate collapse of the northernmost span. The through-truss bridge was built in 1955 and connects the Skagit County cities of Mount Vernon and Burlington. It had recently been evaluated as safe and though not structurally deficient, it was considered "functionally obsolete", meaning it did not meet current design standards. The bridge's design was "fracture-critical"; i.e., it did not have redundant structural members to protect its structural integrity in the event of a failure of one of the bridge's support members. The overhead support structure was known to have been struck by a truck as recently as October 2012. From an engineering standpoint, the bridge actually consists of four consecutive spans which are structurally independent. Only the deck and overhead superstructure of the northernmost span collapsed into the river; the span south of the collapsed span also sustained damage from being struck by the same truck, though not severe enough to result in a collapse. Also, the piers below the deck were not damaged. Because the bridge collapse severed a vital transportation link between Vancouver, British Columbia and Seattle, replacing the collapsed span became a high priority for the Washington state government. In June 2013 two temporary bridges were erected and placed on the collapsed span's support columns while the permanent bridge was built. In September 2013 the permanent bridges were installed and work began to prevent similar failure of the remaining three spans. ==Background== The bridge crosses between Mount Vernon and Burlington, in Washington State, about north of Seattle. It is part of the primary road transportation route between the metropolitan areas of Seattle and Vancouver, British Columbia. Before the collapse, approximately 71,000 vehicles crossed the bridge every day. The bridge was built in 1955, a year before the Interstate Highway System was begun. The bridge carries four lanes of traffic, two lanes in each direction. The portions over the river are four consecutive spans, each long. The spans are built from triangulated steel girders, using a through-truss design where the roadway passes in an open tunnel between the left and right trusses and between the lower and upper truss work. The roadway has relatively limited vertical clearance for tall vehicles due to the upper truss members. The abutted spans share pier footings and appear to be one continuous bridge, but the four spans are actually independent. The bridge was recently evaluated as safe and in good condition despite being 58 years old; it was not listed as structurally deficient. The bridge was classified as functionally obsolete, in this case because the bridge does not meet current design standards for lane widths and vertical clearance in new highway bridges. The bridge was not a candidate for any significant upgrades or replacement and was well-maintained. 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「I-5 Skagit River Bridge collapse」の詳細全文を読む スポンサード リンク
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