On May 23, 2013, at approximately 19:00 local time, the bridge span that took Interstate 5 onto the Skagit River in the US state of Washington collapsed. There were no casualties, but three people in two different vehicles fell into the river below and rescued by boats, escaping from serious injuries. The cause of catastrophic failure is determined to be a huge burden that befalls some of the overhead bridge support beams, leading to the collapse of the northernmost span.
The through-truss bridge was built in 1955 and connects the Skagit County cities of Mount Vernon and Burlington, providing important links between Vancouver, British Columbia and Seattle. It consists of four spans in a structurally independent sequence. Only the northernmost span that collapses into the river; adjacent spans also suffered collision damage from the same vehicle, but not severe enough to produce collapse. The overhead support structure is known to have been hit by a truck recently in October 2012.
Not long before the accident, the bridge was considered safe. Although not structurally lacking, it is considered "functionally outdated", which means it does not meet current design standards. The design of the bridge is a "critical fracture," meaning that it has no excessive structural members to protect its structural integrity in the event of failure of one of the bridge support members.
Within a month after the fall, two temporary bridges were erected and placed in a span support column that failed while a permanent bridge was built. In September 2013, a permanent bridge was installed and work began to prevent similar failures from the remaining three ranges.
Video I-5 Skagit River Bridge collapse
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The bridge is intersecting between Mount Vernon and Burlington, in Washington State, about 60 miles (97 km) north of Seattle. It is part of a major land transport route between the Seattle and Vancouver metropolitan areas of British Columbia. Before the collapse, about 71,000 vehicles crossed the bridge every day.
The bridge was built in 1955, a year before the Interstate Highway System began. The bridge carries four lanes of traffic, two lanes in each direction. The top of the river is four consecutive spans, each along 160 feet (49 m). The landscape is constructed from triangular steel beams, using a skeletal design in which the road passes through an open tunnel between the left and right scrolls and between the bottom and top frames. The highway has relatively limited vertical permits for high-rise vehicles due to the upper frame members. The adjacent range shares the dock foundation and appears to be one continuous bridge, but four spans are completely independent.
The bridge was recently considered safe and in good condition despite being 58 years old; it is not listed as structural deficiency. This bridge is classified as functional obsolete, in this case because the bridge does not meet the current design standards for lane width and vertical clearances on new highway bridges. The bridge is not a candidate for significant or well-maintained improvement or replacement.
Maps I-5 Skagit River Bridge collapse
Design through-truss
The steel bridge through the truss has a "critical fracture" design with non-redundant load beams and connections that are each important for the whole structure intact. Initial failures (possibly with cracks) from one important part can sometimes overload other parts and make them fail, which quickly trigger a chain reaction of more failures and cause the entire bridge span to collapse. In 2007, the Mississippi River I-35W bridge in Minneapolis collapsed suddenly from the slow cracks of too large and too pressed gusset plates. In steel, these early fractures started small and took years to grow large enough to become dangerous. Following the Minneapolis incident, such age-related disasters in critical crash bridges are now avoided by finding and repairing cracks in the required thorough examination every two years. Eighteen thousand bridges in the United States are labeled critical fractures (from their designs) and require crack inspections. The last Skagit River Bridge has been checked for cracks in August and November 2012 with just a little bit of work required.
In addition to fracturing, some bridges with critical non-redundant parts can also suddenly fail from members pressing bend (opposite of crack members). At the through-truss bridge, the important compression parts are the top-chord rods that run horizontally along the top of the bridge, parallel to the curb. They carry most of the bridge weight and traffic. Chords usually stay aligned and held in place by vertical posts, diagonals, and struts swaying to the side. The chord button will quickly fold if the connection somehow becomes uneven. Buckling damage is cumulative, but mostly due to collision damage or excess pressure rather than age and corrosion.
The vertical distance for the vehicle is limited by the portal and the sideways tilt sideways. It's relatively low on the older bridge. On the Washington State bridge, the wobble strut often curves downward at the outer end, with little distance above the outer and outer lanes. The high load then needs to use the inner line for maximum exemption. These bridges are susceptible to impact by vehicles that are too high, and such impacts are common. There was a known strike on this bridge that occurred on October 22, 2012, and investigators found evidence of some other impacts in previous years. The bridge inspection reports from 1979 often noted the damage caused by overloaded vehicles, and inspection reports from the end of 2012 noted three-inch injuries to steel.
According to Charles Roeder, a professor of civil engineering at the University of Washington in Seattle, the bridge through the truss is a general bridge design in the 1950s (there are 10,200 bridges through the roof-dots in the US), but "[i] f you take some top framing, You set the bridge for failure of stability. "Before the computer, bridge engineers analyzed the skeletal power with slide rules, with each time-consuming calculation. Although the finite element method and plastic design theory, both capable of analyzing redundant structures, have recently been formulated and have been used occasionally, they require much larger calculations than simple calculation methods for decisive static structures, which hinder the use of excessive structural members.. A large number of bridges were being designed at the time, and there were inadequate design engineers available to design many bridges as indeterminate structures.
At present, framework design and other critical fracture designs are avoided in most new bridges for medium-sized spans. Using three or more parallel main beams or frames allows the structure to withstand the failure of a single component.
Incident
The collapse was caused by a southern semi-trailer truck from Canada carrying large payloads to Vancouver, Washington, which directly destroys the strut sway and, indirectly, the compression chord in the top steel frame (trusswork) in the northernmost span of the bridge. The vertical distance from the road to the upper curved beam on the outer lane is 14 feet 7 inches (4.45 m), and all trucks with large loads are expected to travel on the inner tracks at a distance of about 17 feet (5.2 m). m). The overgrown truck actually enters the bridge on the outside lane, while the second semi truck and BMW pass through it on the inner track. The oversized truck has received the State's oversized clearance for a wide and high load, for a height of 15 feet 9 inches (4.80 m), and after the collapse of the "upper corner of the dents and scratches along the upper side [seen] on the 'oversize' pulled on a truck. "The National Transportation Safety Council (NTSB) measures the height of the truck, after the crash, to 15 feet 11 inches (4.85 m). Pilot cars are employed to ensure loads can pass safely. The pilot's car never alerted the truck driver that there would be a problem crossing the Skagit bridge and not alerting the truck driver to use the lane inside.
The overgrown truck was over the bridge while the first span collapsed behind him. Both large load drivers and pilot vehicles remain on the scene and cooperate with investigators.
There were no fatalities, but three people were taken to a local hospital after being rescued from their fallen car. The cars remained on the deck of the flooded bridge after falling into the river.
The trucker, employed by Mullen Trucking, is hauling a large load containing housing for drilling equipment. The company's vice president, Ed Sherbinski, said the permit had been issued from Washington State which included permits for all bridge crossings on the route. The truck had been guided over the bridge by a pilot escort vehicle. A spokeswoman for the Washington State Department of Transport said there were no warning signs leading to the bridge regarding his permit height. In Washington, only overcrossings of less than 14 feet (4.3 m) (normal high law limit) are required to have high post height restrictions.
The overly large truck also destroys the second wobble shake, but not enough to start the collapse. The range was then fixed.
Investigation
The Washington State Patrol and the National Transportation Safety Council investigated the accident. The NTSB report linked collisions with high-loaded trucks that were on the wrong track, outside, and packed farther to the shoulders by passing trucks. This collapsed bridge attribute to the collision takes some swaying braces, which destabilizes the critical load section (above the chord).
Aftermath
Governor Jay Inslee declared an economic "emergency" for the three surrounding districts (Skagit County, Snohomish County and Whatcom County) to tackle local traffic and economic disruptions. Traffic on I-5 crashes around the site of collapse on the adjacent bridge upstream. State police Sean O'Connell was killed while driving traffic through a detour when his motorcycle hit a truck. Shortly after the accident, three state lawmakers proposed a bill that would rename the repaired bridge after him; this amendment was approved by the Washington State Transportation Commission.
The collapsed temporary range is replaced by a pair of two-lane bridges manufactured by ACROW, which are rolled onto the existing bridge dock. It started operating on June 19th. Inspection in the temporary range near the end of July 2013 found that part of the "L" connection that holds the asphalt between the temporary bridge and the permanent road has been detached. The right lane of the bridge was closed for about two hours while the crew welded back into place and spread the new asphalt.
The $ 6.87 million contract was awarded to Max J. Kuney Construction Spokane contractor to design and build a permanent replacement range. It is built along a temporary range without disturbing traffic, and moved into place during overnight closures on 14-15 September 2013. The planned changes to the three remaining overhead ranges should allow vehicles that are too high to operate on the outer lane.
The collapse raises the question of how the Washington State Department of Transport regulates too many vehicles. The department handed over to the driver to determine a safe route to their destination, unlike in many other countries where the route was assigned. Permission for the Mullen Trucking trip does not specifically record the Skagit River Bridge as a danger to the route, although the outer rolls are lower than trucks; this contrasts with the previous permission to the same company that includes many warning permits. State MPs are exploring changes in oversized state vehicle laws.
The bridge built to replace the collapsed bridge has been named the Memorial Force of Sean M. O'Connell Jr. Jr.
Washington infrastructure
Before the bridge collapsed, the Seattle Section of the American Society of Civil Engineers (ASCE) issued the 2013 Report Card for the Washington State infrastructure. The state bridge is rated "C-" (inter-state average score). There are 400 bridges that are structurally lacking in Washington. Thirty-six percent of all bridges are over 50 years old. The oldest bridge designed for life expected only 50 years; keeping them safe is getting harder and more expensive. The Transportation for America advocacy group reports that 5.1% of Washington's bridges are structurally lacking, which is the sixth best in the country.
See also
- List of bridge failures
References
External links
I-5 - Skagit River Bridge Override (and detour) , Washington State Department of TransportationSource of the article : Wikipedia
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