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Public Roads - November/December 2014

November/December 2014
Issue No:
Vol. 78 No. 3
Publication Number:
Table of Contents

Surviving An Interstate Bridge Collapse

by Jeffrey L. Horton

What do you do, when, in the blink of an eye, a major interstate and freight corridor on the west coast is severed? Learn from Washington State’s experience.

This north main through-truss span of the Skagit River Bridge in Mount Vernon, WA, collapsed into the river after a southbound combination vehicle carrying an oversize load struck elements of the bridge.

On May 23, 2013, around 7 p.m. Pacific daylight time, a 160-foot (49-meter)-long section of the Skagit River Bridge on I–5 in northwestern Washington State came crashing down into the river below. A combination vehicle hauling a permitted oversize load and traveling southbound over the bridge had struck the northern portal and several overhead sway braces, collapsing the northern span of the through-truss in its wake.

The combination vehicle managed to continue across the bridge, but a southbound pickup truck towing a trailer and a northbound car were not so fortunate. Both vehicles, along with the collapsed span, fell into the river below. Miraculously, no fatalities and only minor injuries resulted. Equally amazing was that so few vehicles were affected because the collapse occurred the Thursday before Memorial Day weekend, one of the busiest travel weekends of the year.

This aerial view looking east shows the Skagit River Bridge with the collapsed northern span of the through-truss section of the bridge (at left). In the background is the Riverside Drive Bridge, which was the primary detour for I–5 traffic after the collapse.

For oversized loads, such as the one in this case, the carrier is required to obtain a permit through a self-issuing process. The carrier also is required to check restrictions on the route prior to moving.  In this case, when the permitted oversize load struck the bridge, it was over the lane marker of the southbound right lane where vertical clearances were lower due to the parabolic shape of the overhead portal and sway brace.

 The I–5 Skagit River Bridge is 48 miles (77 kilometers) south of the U.S.-Canadian border and serves as a key connection for traffic and commerce between Seattle, WA, and British Columbia, Canada. In 2012, nearly $14 billion in U.S.-Canadian trade and, based on the average daily traffic of approximately 71,000 over the bridge, approximately 26 million vehicle trips crossed the bridge. In addition, I–5 is the only north-south interstate that runs through Washington State.

Both northbound and southbound lanes of I–5 cross the Skagit River on a single bridge. So when the span collapsed, the effects were far reaching. For example, trucking companies that transport goods locally and regionally were unable to move their goods. Many indirect consequences resulted, too. Businesses that rely on customers traveling in the corridor lost business due to the closure and resulting congestion from detoured traffic. Shippers, tourists, shoppers, and recreational users also had to reroute, reschedule, or cancel their trips.

 Prompt emergency response followed by swift action and collaboration among multiple agencies was imperative in order to reconstruct the bridge, return traffic to I–5, and get the region back up and running.

Coordinating a Response

Immediately following the collapse, the Washington State Department of Transportation (WSDOT) partnered with the Skagit County Department of Emergency Management, the Washington State Patrol, the U.S. Coast Guard, Naval Air Station Whidbey Island, and the cities of Burlington and Mount Vernon to secure the scene, which included rescuing the vehicles and motorists in the river and establishing initial detour routes for I–5 traffic. WSDOT mobilized bridge engineers and inspectors to assess the damage. WSDOT also worked to provide frequent updates and travel alerts to the media to keep the public informed.

As word about the collapse spread, staff at the Federal Highway Administration (FHWA) Washington Division mobilized to respond to the emergency. The division’s bridge engineer, construction engineer, and the emergency relief program manager developed FHWA’s response plan, and the division administrator kept leadership at FHWA headquarters informed about response efforts.

“The team we have here in Washington epitomizes FHWA’s response to emergencies,” says Dan Mathis, administrator of the Washington Division. “They launched into action quickly upon learning of the Skagit River Bridge collapse and stepped up on behalf of the agency to respond.”

FHWA Washington Division staff immediately reached out to WSDOT, offering assistance and opening dialogue between the two agencies. FHWA also began gathering the necessary documentation and information on the history of the structure.

WSDOT’s Emergency Operations Center also contacted the FHWA emergency coordinator liaison, who prepared a situation report and worked side by side with WSDOT staff at the emergency center. In addition, within 2 hours, FHWA had deployed staff to the scene of the incident to meet with WSDOT staff.

FHWA staff members worked well into the night collecting data on the bridge, such as its Structure Inventory and Appraisal Sheet from the National Bridge Inventory, structural plans, and inspection history. The team pulled together technical specialists from around the country and participated in conference calls with WSDOT and FHWA leadership. The compiled information would help to brief the National Transportation Safety Board (NTSB) team who arrived onsite the next day.

As with any significant highway incident, NTSB launched an investigation into the probable cause of the collapse and later issued safety recommendations. The report issuing the recommendations is available at WSDOT opened a dialogue with the FHWA division emergency relief program manager and both began drafting requests for Federal Emergency Relief funding.

By the following day, FHWA division staff members were onsite meeting with the NTSB team to determine additional needs for the investigation. A representative from FHWA’s Office of Bridges & Structures accompanied NTSB to provide additional technical expertise. FHWA also continued to provide assistance to WSDOT staff to quickly move through the process.

Detours and Next Steps

WSDOT established two detour routes. The primary detour routed northbound I–5 traffic east on SR–538 at exit 227 then north on Riverside Drive in Mount Vernon. Traffic crossed the Skagit River on Riverside Drive and entered the city of Burlington on Burlington Boulevard where the detour continued north to George Hopper Road. Once at George Hooper Road, the detour directed traffic west to rejoin with I–5 at exit 229. For southbound traffic, the detour was in the reverse direction along the same route. The detour added only a half mile (0.8 kilometer) of additional travel.

A secondary detour routed southbound traffic off of I–5 at exit 230 onto SR–20, then west to Avon Allen Road and south toward SR–536. At SR–536, the detour directed traffic east where it crossed the Skagit River over a narrow truss bridge and wound through downtown Mount Vernon to rejoin I–5 at exit 226. For northbound traffic, the detour was in the reverse direction along the same route. The secondary detour added 3 miles (4.8 kilometers) of travel.

This combination vehicle with a permitted oversize load is the one that struck the through-truss bridge and caused its collapse. (Inset) The closeup shows the damage to the upper front right corner of the load where it struck the overhead bridge elements.

The detours posed a number of challenges for travelers and the affected communities. Riverside Drive and Burlington Boulevard are commercial corridors, so posted speeds were slow and the roads were heavily congested with the unanticipated I–5 traffic. The congestion would pose challenges for both communities, prompting Washington State Governor Jay Inslee to secure economic support for several businesses that experienced related hardships.

With the challenges of the detours immediately apparent, WSDOT began the arduous task of figuring out how to fix the collapsed span and return traffic to I–5 as quickly as possible. Since the I–5 Skagit River Bridge’s construction in 1955, the structure had not been rebuilt or rehabilitated.

The 1,112-foot (339-meter)-long, 12-span bridge had 4 steel through-truss spans in the main unit and 8 concrete approach spans (4 at each end of the main spans). These 12 spans carry 4 lanes of I–5 traffic on 2 northbound lanes and 2 southbound lanes separated by a concrete median barrier. The overall width of the bridge deck is 60 feet (18.2 meters) with a roadway width of 56 feet (17 meters).

Here, a crane hoists a pickup truck from the river. The truck plunged into the water when span 8 of the bridge collapsed.

The span that collapsed was span 8, the northernmost through-truss span. Each through-truss span (spans 5, 6, 7, and 8) is a simple, independent span measuring 160 feet (49 meters) in length. When span 8 collapsed, the adjacent spans of the through-truss bridge remained in place because of their independent design. With that in mind, the specific challenges WSDOT faced were to remove the collapsed span and replace it. WSDOT also had to address some damage to an overhead sway brace on span 7, which it did in addition to the work on span 8.

Using emergency contracting methods, WSDOT hired a contractor within 24 hours to remove the collapsed span from the river and also to provide input as WSDOT developed a plan of action to get the bridge back into service.

Selecting a Course of Action

Discussion about the best and quickest fix began as early as the night of the collapse. WSDOT recognized early on that the detoured traffic was resulting in negative economic impacts to both Burlington and Mount Vernon, prompting urgent discussions to develop an action plan. Multiple meetings were held throughout the holiday weekend to discuss various options for rebuilding the span and restoring function to the structure. Attendees included staff from FHWA; WSDOT’s Bridges and Structures Office, Maintenance Operations Office, Construction Office, and the Northwest Region and Mount Baker Area offices; and WSDOT Secretary of Transportation Lynn Peterson.

Although the participants evaluated several possibilities, the decision came down to two options: (1) Keep traffic detoured and install a permanent replacement as quickly as possible, or (2) install a temporary span and return traffic to I–5, and then follow with a permanent span at a later date. WSDOT considered both options seriously.

The department attempted to find existing bridge beams that would work for immediate installation of a permanent replacement span rather than having beams fabricated, which would take too much time. WSDOT located some beams at a fabrication plant in Oregon. However, although the beams were the correct size to carry the required loads, they were not the correct length and did not have the appropriate camber, so time-consuming modifications would have been required.

The temporary spans are shown here during installation.

The second option was to install a temporary span while a permanent replacement span was fabricated. One advantage of installing the temporary span would be to quickly relieve the negative impacts on the local communities as a result of the heavily traveled detours.

After gathering input from Governor Inslee, FHWA, and the contractor, WSDOT elected to proceed with installing a temporary replacement span first and following with a permanent replacement span later. On May 26, Governor Inslee announced the plan to replace the collapsed portion of the bridge. The Governor also challenged WSDOT to install the temporary span by mid-June, followed by the permanent span in mid-September.

Returning Traffic to I–5

Before WSDOT could install the temporary span, the contractor had to remove the collapsed span carefully from the river under the direction of NTSB. Because the incident was under investigation, the debris was treated as evidence and was removed slowly to preserve its integrity.

On June 5, NTSB completed its onsite investigation and released the site to WSDOT. While the NTSB investigation continued offsite, the contractor completed the removal of all of the debris from the river bottom by June 7.

For the temporary replacement span, the department elected to use two Acrow spans placed side by side. The temporary spans are modular steel prefabricated sections, which WSDOT assembled on the adjoining approach spans of the Skagit River Bridge and launched over the gap. Each of the temporary replacement spans had a width of 24 feet (7.3 meters) and carried two lanes of traffic.

The emergency contractor procured the temporary spans and began assembly on May 28. Working around the clock, WSDOT and its contractor installed the temporary spans and reopened the Skagit River Bridge to traffic on June 19--just 27 days after the collapse.

Although the temporary replacement spans enabled traffic to return to the interstate, they were not ideal. Because of the narrow width of the lanes and the need to reduce truck impact loadings, all oversize loads were required to use the detours, and the speed limit was reduced to 40 miles (64 kilometers) per hour in the vicinity of the bridge.

Even with the reduced speed limit and load restrictions, reopening the bridge with the temporary spans was an improvement over the detours. According to Jay Drye, then the acting assistant regional administrator for the WSDOT Northwest Region, “Restoring I–5 and getting traffic back onto the interstate was a tremendous relief to the communities, the businesses, and the travelers,” he says. “It was a challenging time for all the people affected. The temporary bridges provided much-needed relief as we went into the summer travel season.”

A Permanent Fix

Shortly after work began on the temporary span, WSDOT switched its focus to developing a plan for the permanent replacement span. To take advantage of contractor ingenuity and efficiency, WSDOT elected to proceed with a design-build method for project delivery. Instead of designing the replacement span, assembling plans, and advertising the project for bid, WSDOT would provide a framework, design parameters, and time expectations, and select a design-build team based on best value to design and then construct the span.

WSDOT was able to reopen I–5 to traffic once these temporary Acrow replacement spans were in place.

For the permanent span, WSDOT considered three options: a truss span similar to the original span; prestressed concrete girders with a concrete deck; or steel girders with a concrete deck. Although a truss span would have matched the existing spans, WSDOT determined that fabricating and installing it would take too long to meet the mid-September goal. So WSDOT officials decided to use either prestressed concrete girders or steel girders supporting a concrete deck.

On May 30, WSDOT informed FHWA of its intent to use the design-build contracting method for the permanent span. In the following days, WSDOT developed a request for proposals (RFP) and made it available to six short-listed design-build teams. FHWA worked with WSDOT during the development of the RFP and approved it for release on June 3 with a proposal deadline of June 17.

The design-build team constructed the permanent replacement span on a work platform (right) next to the temporary side-by-side spans (left).

The design-build teams were required to submit a proposal with a cost that included construction of the span to meet the criteria established in the RFP, a plan to minimize impacts on the traveling public during construction, and a timetable for opening the span to traffic no later than October 1, 2013.

Four teams submitted proposals. Two proposed a prestressed girder span, and two proposed a steel girder span. The winning proposal included constructing a prestressed concrete girder span at a cost of $6.8 million and closing I–5 for only 12 hours to install the permanent span. The contract was executed on June 18, and the notice to proceed was issued on June 19, the same day that traffic was restored to I–5 on the temporary spans. Work on the permanent replacement span began immediately.

Constructing the New Span

The design of the permanent span was completed on July 9, and the next step was to construct it. The design-build team installed a temporary work platform just west of the temporary span, where the team would build the permanent span. When the permanent span was completed, the team would detour traffic, pick up and slide out the temporary spans, pick up and slide the new permanent span into place, and then reopen the bridge to I–5 traffic.

To begin, the design-build team drove a number of temporary piles into the river on both sides of the span. On the west side it assembled the temporary work platform to construct the permanent span. East of the bridge, it assembled a second temporary platform to hold the temporary spans when it was time for installation of the permanent span.

The permanent replacement span consists of eight 65-inch (1.6-meter)-deep prestressed lightweight concrete bulb-tee girders with a silica fume concrete overlay. The lightweight concrete is a significant design element because the new span is set on the existing bridge substructure. To avoid additional seismic inertia loads, the weight of the permanent replacement span could not exceed that of the original truss span by more than 5 percent. The lightweight concrete used in the girders, the diaphragms, and the bridge rail kept the dead load within that limit.

These elements were constructed on the west work platform using cranes located on the river’s north bank and a barge. In spite of the complexity of building the permanent span over the river, the design-build team was ready to put the finishing touches on it by mid-September.

On the night of Saturday, September 14, the team detoured traffic and closed I–5 at the bridge at 7 p.m. The design-build team then moved the two temporary spans to the platform east of the bridge using hydraulic jacks to slide them on Teflon® pads on long steel rails. Next, the team used a similar system to slide the permanent 900-ton (816-metric ton) span into its permanent location.

The transition took 19 hours and encountered some minor complications, such as an unexpected and time-consuming process to cut through thick metal plates in order to lift the temporary span. However, once the permanent span was in place, the design-build team completed the final roadway striping and cleanup to open the roadway to traffic at 2 p.m. on September 15--only 115 days after the collapse.

Mitigating Structural Risk

Even though the permanent replacement span was in place, work on the Skagit River Bridge was not complete. An analysis of the remaining truss structure, performed while the permanent span was being fabricated, revealed that the bridge was still vulnerable. When span 8 collapsed, the north portal also came crashing down. In addition, the overhead sway braces on the remaining truss spans continued to be a potential obstacle for vertical clearance outside of the lanes. Before oversize vehicles could once again traverse the bridge safely, WSDOT chose to replace the north portal, modify the parabolic sway braces, and modify the south portal.

WSDOT awarded a $2.7 million project to complete the needed modifications to the remaining truss spans. On Tuesday, September 17, the contractor began additional overnight closures of I–5 at the Skagit River to complete the retrofitting work. The retrofitting crew raised and reinforced the bridge’s overhead structural support system to provide for 18 feet (5.5 meters) of vertical clearance across the remaining spans. In addition to the overhead work, they completed some maintenance work on the rivets below the bridge deck, along with some painting. With the exception of painting the repaired or modified areas, the work for this phase was completed by mid-November. The painting was completed on July 19, 2014.

Emergency Response Funding

To date, the total cost to restore the Skagit River Bridge to full operation is approximately $20 million. Even though this emergency was not caused by a natural disaster (such as a flood, earthquake, hurricane, or landslide), the majority of the costs to repair the bridge were eligible for reimbursement with FHWA Emergency Relief funds.

Congress authorized the Emergency Relief Program in Title 23, United States Code, Section 125, to provide funds from the Highway Trust Fund for the repair or reconstruction of Federal-aid highways and roads on Federal lands that suffered serious damage from natural disasters or catastrophic failures from external causes. Emergency Relief funds can supplement resources from States, localities, and other Federal agencies to help in the repair of facilities damaged by eligible events.

During an overnight closure, the design-build team removed the temporary spans and slid the permanent span into position, as shown here.

The collapse of span 8 of the Skagit River Bridge was viewed as a catastrophic failure prompted by an external cause. Title 23 Code of Federal Regulations (CFR) 668.103 defines a catastrophic failure as the sudden failure of a major element or segment of the highway system because of an external cause such as a barge striking a bridge pier causing the sudden collapse of the structure, a truck crash resulting in a fire that damages the roadway, and acts of terrorism that result in structural damage to the highway facility.

From the beginning, the FHWA Washington Division treated the emergency as an eligible event. On May 24, Governor Inslee declared a state of emergency. The declaration along with a letter of intent was sent to FHWA. FHWA responded to WSDOT with a letter of acknowledgement and eligibility. Through a formal request for Emergency Relief funds, FHWA announced a quick release of $1 million for the Skagit River Bridge.

Although not all of the expenses were eligible, Emergency Relief funds reimbursed WSDOT and other agencies for their eligible expenses and efforts to correct the situation. FHWA Emergency Relief funds covered nearly $18 million of the total expenses.

“The quick release of Emergency Relief funds enabled WSDOT to completely focus on initially mobilizing resources without the burden of trying to determine how to cover the significant costs required to maintain traffic safety and establish alternate routes to keep traffic moving,” says John Jeffreys, Federal-aid planning manager with WSDOT. “Throughout the entire event, the responsiveness and cooperation WSDOT received from the FHWA [Washington] Division Office was outstanding and contributed greatly to the reopening of the I–5 Skagit River Bridge just 27 days after the bridge collapsed.”

Innovations in Action

Several innovations led to the successful handling of the emergency and the restoration of the Skagit River Bridge. The use of design-build contracting to deliver an Emergency Relief project was a first for WSDOT. Design-build contracting enabled WSDOT to tap into the creativity and efficiency of the contracting community. As time was of the utmost importance, giving the design-build teams the opportunity to design a new permanent replacement span that could be constructed and installed quickly and cost-effectively, allowed for the restoration of all traffic to I–5 only 115 days after the collapse.

This illustration depicts the modification necessary to raise the overhead sway braces of the main truss span to an 18-foot (5.5-meter) vertical clearance across all travel lanes.

WSDOT’s use of lightweight aggregate in the concrete beams, deck, and barriers made it possible to keep the span’s weight within the allowable loads on the existing bridge substructure. This innovation also made the replacement span light enough to be lifted off the work platform and slid into its permanent location.

The slide-in methodology used to construct and move the permanent span into place is an accelerated bridge construction technology being promoted by FHWA’s Every Day Counts initiative. This technology enabled interstate traffic to use the temporary spans while the permanent span was constructed on a temporary work platform. It also minimized traffic disruptions, as the permanent span was slid into place in less than 24 hours. Traditional methods of construction would have taken several weeks while traffic remained detoured through congested city streets.

Another innovative solution was WSDOT’s use of the temporary spans on the interstate, which was a first for the department. Although there were load restrictions in place for the temporary spans, this was an emergency and restoring traffic to I–5 was a high priority.

On September 6, WSDOT and FHWA hosted a showcase in Bellevue, WA. The showcase highlighted the project’s use of the accelerated bridge construction technology, underscoring the important effect this technology has had at a national level through Every Day Counts. The showcase also included a project overview, discussions on emergency response, the development of design-build contracts, and the design and construction of the permanent replacement span.  An open panel forum discussion offered participants a chance to ask questions. The participants also had an opportunity to attend an onsite tour of the project.

MAP-21 at Work

The passage of the Moving Ahead for Progress in the 21st Century Act (MAP-21) facilitated a few more “firsts” for WSDOT on Emergency Relief projects.

For example, WSDOT added additional train service around the collapsed bridge to help relieve some of the detour congestion. Section 1107 of MAP-21 allows such additional transit service to be eligible for Emergency Relief funding. Prior to MAP-21, that was not possible. WSDOT took advantage of the change.

MAP-21 also expanded the actions that could be categorically excluded from requiring a National Environmental Policy Act environmental impact statement under Title 23 CFR 771.117(c)(9). The expanded actions include the repair or reconstruction of any road, highway, or bridge damaged by an emergency that is declared by the Governor of the State, and the repair or reconstruction activity occurring within the existing right-of-way and in a manner that substantially conforms to the preexisting design, function, and location as the original. This project met those requirements and was categorically excluded. This was the first permanent repair project in Washington State and the Nation that took advantage of this flexibility allowed for Emergency Relief projects.

Collaborating to Meet Goals

Several actions contributed to the successful delivery of this project and communication was key. From the beginning, staff from the FHWA Washington Division and WSDOT held frequent (daily or even multiple times a day) conference calls. The continuous communication kept all parties informed and up to date, which made it possible to find a solution that was acceptable to both WSDOT and FHWA.

Here, vehicles travel underneath the modified portals and sway braces of the main truss spans after completion of the clearance upgrade project.

“It’s all about relationships,” says FHWA’s Mathis. “The relationship we have with WSDOT allows us to work as a team to do what’s best for the traveling public.”

One of the best examples of the success of these collaborative efforts is the speed at which WSDOT was able to develop the RFP for the permanent repairs. WSDOT assembled the RFP over a weekend because the RFP writers and reviewers, including FHWA, were all working together at the local WSDOT area office. After the technical specialists developed a section, they forwarded it to the RFP writers and reviewers who edited the language as necessary. Everyone was committed to working long hours to prepare the document quickly. To their credit, it was completed on time.

“The success of the rapid assembly of the Skagit River Bridge emergency design-build contract package hinged on several indispensable factors,” says Derek Case, then the engineering manager for the Skagit River Bridge emergency design-build project. “One, subject matter experts for the key areas--structures, foundations, geotechnical, traffic--were either present or on call and committed to the project throughout the weekend. Two, the decisionmakers for those key areas and for the agency as a whole were also present or on call and equally committed. And three, the veteran core project development team was all in the same room and committed to the task. Without any of these key elements, the speedy decisionmaking necessary for such an endeavor would have [been] bogged down, [delaying] publication of the RFP.”

Communication, collaboration, innovation, and FHWA Emergency Relief funding were critical to replacing the collapsed span and upgrading the overhead elements to prevent further incidents. Here is the I–5 Skagit River Bridge with the new span and the main truss spans with modified portals and sway braces.

Another example resulting from successful collaboration is WSDOT’s work with environmental resource agencies. The Skagit is an environmentally sensitive river containing numerous threatened and endangered species. When the collapse occurred, WSDOT immediately notified local, State, and Federal environmental resource agencies, a standard practice in the State. By involving these stakeholders from the start, the department could efficiently manage the environmental process in a timely manner.Completing such a complex project in such a short time was an intense but rewarding experience. Decisions with long-lasting consequences had to be made quickly. To be able to make those decisions, detailed information had to be gathered rapidly, and that information had to be processed in a brief period of time, all the while ensuring that the project met the need and remained eligible for Emergency Relief funds. Collaboration and cooperation between the various WSDOT offices, the FHWA division and headquarters offices, and many other stakeholders and affected parties helped to write this success story for the replacement span of the Skagit River Bridge.

Jeffrey L. Horton, P.E., is an area engineer in the FHWA Washington Division Office. He received a bachelor’s degree in civil engineering from Kansas State University.

For more information, contact Jeff Horton at 360–753–9411 or

The author would like to thank Susan Wimberly, field operations team leader and Emergency Relief manager, and Debbie Lehmann, bridge engineer, both with the FHWA Washington Division Office, for their contributions to the article.