At about 7 p.m. on Thursday, May 23, 2013, while crossing the Interstate 5 Skagit River Bridge at Mount Vernon, a tractor-trailer's oversized load dings part of the truss and the northernmost truss span collapses into the river. The truck makes it to the other side but two vehicles carrying three people fall into the river along with the span. They are bruised and shaken, but not badly injured. The bridge is located in Skagit County between Mount Vernon and Burlington. Interstate 5 is a major north-south highway that carries some 71,000 vehicles per day; traffic backs up in both directions, and will ultimately be detoured. Governor Jay Inslee (b. 1951) will issue an emergency proclamation, enabling WSDOT (Washington State Department of Transportation) to begin to clean up and rebuild immediately using pre-designated contractors without putting out bids, and enabling the use of emergency funds. Within a month, two temporary steel prefabricated modular spans will be installed side by side, one northbound and one southbound, and on June 19, 2013, the bridge will reopen to traffic. In the next weeks a new span will be built at the site and in a marathon 19-hour work effort will be slid into place on September 15, 2013.The new permanent bridge will reopen to traffic on Sunday, September 15, at 2 p.m.
All Fall Down
At about 7 o'clock on May 23, trucker William Scott proceeded to drive his oversized load -- housing for drilling equipment -- south across the Interstate 5 Skagit River Bridge. He arrived at the other side and, looking into his mirrors, watched "in horror" as the bridge collapsed behind him, taking with it two vehicles. He immediately called his wife, Cynthia Scott, who told reporters, "He looked in his mirrors and it just dropped out of sight .... I spoke to him seconds after it happened. He was just horrified" ("'Horrified' Trucker Watches..."). William Scott was an experienced trucker who worked for Mullen Trucking in Alberta, Canada. He had been properly following his local escort car and had obtained a permit for transporting the oversized load. As it turned out, his load had hit part of the bridge's truss.
Dan Sligh and his wife, Sally Sligh, of Oak Harbor, were heading southbound for a camping trip near Granite Falls in their 2010 Dodge Ram pickup truck towing a 2009 Jayco camper trailer, when the bridge span in front of them disappeared "in a big puff of dust" and they went into the river ("'Horrified' Trucker Watches..."). Sally Sligh could not swim, and her husband pulled her out of the water and kept her calm until help arrived in rescue boats. Later, outside the hospital, Dan Sligh reflected, "You talk miracles. ... I don't know what you want to call it. When you're sitting down in the water and all that mangled metal of the bridge. You look around and you pinch yourself" ("'Miracles ..."). His wife told news media that her husband was her hero.
Bryce Kenning of Mount Vernon, a 20-year-old college student, was driving northbound to a pick-up hockey game in Bellingham in his 2013 Subaru XV Crosstrek. He reported that the bridge "seemed to explode" in front of him. He hit the brakes but could not stop his vehicle from going off the edge into the river. "It was like time was frozen," he said, "like a roller coaster where you're not attached to the tracks. ... I'm sure it was just one of the loudest sounds ever to hear this thing explode and fall into the water like that, but I didn't hear a thing. I just witnessed it happening in front of me" ("'Horrified' Trucker Watches..."). The airbags erupted and his SUV began filling with water. Kenning tried opening his door, which did not budge. He tried kicking out the windshield. No luck. Then he tried the passenger door, pulling the handle and kicking it hard. He got out, and sat on top of his car until help arrived.
The three were shaken and bruised but suffered only minor injuries.
"People and Cars in Water"
The inaugural bridge-collapse tweet was made by Washington State Trooper Mark Francis: "N/B and S/B lanes of I-5 Skagit River Bridge collapsed. People and Cars in the water" (Puget Sound Business Journal, May 23, 2013).
The mishap made instant national and international news and dominated news channels in the state. Washington Governor Jay Inslee said in a statement that he was thankful there were no fatalities, and added, "We will be involved in a vigorous and diligent effort to get traffic flowing again through the Skagit bridge corridor ... . This is an opportunity for us to pull together to show strength of character and patience and good citizenship as we deal with this disruption" ("Governor Jay Inslee's statement ...," May 23, 2013).
The following day Governor Inslee proclaimed a state of emergency in Skagit, Snohomish, and Whatcom counties and directed that "the plans and procedures in the Washington State Comprehensive Emergency Management Plan be implemented" ("Proclamation ...," May 24, 2012). The governor instructed the Washington State Military Department, Emergency Management Division, to coordinate all incident-related assistance to the affected areas.
The emergency proclamation meant that WSDOT could spring into action using predesignated contractors without putting out bids for the bridgework, and it prompted the immediate release of $1 million in United States Department of Transportation (USDOT) emergency funds. Later, on June 23, USDOT, Federal Highway Administration, announced that it would release $15.6 million in Emergency Relief Funds to help rebuild the bridge.
Over the few days following the collapse a lengthy discussion ensued in media outlets as to what inspections the bridge had received and on the question of its structural sufficiency. The bridge had been inspected in August and November of 2012, and repairs had been made. It was a "fracture-critical" bridge, which means that components of the bridge were "non-load-path-redundant" ("Preliminary Report"). Bridges bear loads, and the load passes from one component to the next on down to and through the piers. The bridge may be structurally sound, but if one component is removed, the bridge collapses. In this and other fracture-critical bridges there is no structural redundancy: If one of the essential components goes, the bridge goes.
The Old Bridge
The I-5 Skagit River Bridge was a steel through-truss bridge, the type in which traffic drives through and under the truss. It had four 160-foot-long over-water steel-truss spans and four concrete approach spans on the north and south ends. The truss was of the common Warren design.
The bridge was originally built by Peter Kiewit Sons Co. of Longview, Cowlitz County. The contract was awarded on September 9, 1954. Work began on September 21 of that year and it was completed on August 26, 1956. During construction, bridge workers contended with the flooding of the Skagit River and with cold, windy, rainy, and snowy weather. It took a year longer to build than the contract specified.
Cause of Collapse
The National Transportation Safety Board issued the preliminary report on its investigation of the bridge collapse on June 11, 2013. In part the one-page document stated:
"Immediately prior to the collapse, a 2010 Kenworth truck-tractor in combination with a 1997 Aspen flatbed trailer loaded with a casing shed (oversize load) was following a pilot vehicle traveling southbound on Interstate 5. According to witnesses, as both vehicles approached the bridge, another southbound truck-tractor in combination with a semitrailer overtook and passed the oversize load in the left lane. The driver of the oversize load reported to investigators that he felt "crowded" by the passing combination vehicle so he moved his vehicle to the right. As the oversize load was being transported across the bridge, the top of the load collided with the overhead portal and multiple sway braces on the far right side of the truss structure. The impacts caused significant damage to load-bearing members of the bridge's superstructure, resulting in the failure and subsequent collapse of the northernmost bridge span. During the post-collision investigation, the driver reported to investigators that he thought the height of the oversize load was 15 feet 9 inches. The lowest portion of the sway braces, as measured over the active portion of the roadway, was determined to be 14 feet 8 inches. According to the operator of the pilot vehicle, the clearance pole mounted on the front of her vehicle was set at 16 feet 2 inches" ("Preliminary Report").
Sway bracing is diagonal bracing located at the top of a through truss, which resists horizontal forces such as wind. Sway bracing does not bear load but rather helps to keep the components of the truss aligned. But as the report specifies, load-bearing members were damaged and so the span went down.
The Temporary Span
In the month after the collapse, WSDOT cleaned up the wreckage from the water, repaired an adjacent span also damaged in the incident, and made new pier caps for the existing piers. It contracted with Acrow Bridge, a Parsippany, New Jersey, firm with an office in Camas, Clark County, Washington, to install two side-by-side 160-foot-long temporary bridge spans, one for northbound and one for southbound traffic. These were prefabricated modular steel spans that were assembled on site. Acrow Bridge specializes in "steel bridge solutions for temporary and emergency use ..." (Civil Engineering).
The bridges each had roadway widths of 24 feet. According to the National Steel Bridge Alliance newsletter, Acrow used its inventory located in both Camas and Lafayette, New Jersey, to supply parts to the temporary span. Acrow directed the assembly and placement of the bridge from its offices in Camas and in Richmond, B.C.
On June 19, 2013, the bridge reopened to traffic less than a month after the collapse. Vehicles were obliged to slow to 40 miles an hour and oversized loads had to continue taking the detours.
The New Span
On June 18, WSDOT awarded the contract to build the new span to Max J. Kuney Construction of Spokane, for $8.5 million supplied by federal Emergency Relief Funds. The Seattle firm Parsons Brinckerhoff served as lead designer.
The work was done from barges and from a floating work platform. The new span was built on pilings parallel to and a few inches from the temporary span.
First, pilings made of steel tubing were vibrated and hammered into the riverbed to a depth of 60 to 80 feet. Steel bracing was welded to the pilings and they were cut off to an equal length. On these temporary pilings, the new bridge deck would be built.
After the pilings were in place, eight concrete girders were trucked in (the first one arrived on July 13) and lifted into place on the pilings. The girders were made in Tacoma by Concrete Technology Corporation. Each was 162 feet long and weighed 168,000 pounds. After the girders were in place, the concrete roadbed was poured, and the replacement span was ready to be slid into place.
On Saturday night, September 14, 2013, beginning at 7 p.m., using a system of hydraulic jacks, Teflon pads, and long steel rails (skid beams), the two temporary bridges were slid off the piers and set on a work platform to be disassembled. Then, using the same system, the new span, which weighed 900 tons, was slid into place on the permanent bridge piers and locked down. The process was completed by 2 p.m. on Sunday afternoon. The new span weighed no more than the old one and provided the same navigational clearance.
The work crew for the great musical-bridges operation included contractor Max J. Kuney Co., lead designer Parsons Brinckerhoff, and the bridge-moving firm Omega Morgan. Governor Inslee acknowledged "the tremendous work of the entire team" and thanked the residents and businesses of the Skagit Valley for their patience and optimism during the disruptions to the local economy that the incident involved. ("In with the New ...").
Further work on the bridge was planned to raise parts of the truss, the better to avoid any repeat scenario.
