I-5 Skagit River Bridge Collapse Truck Impact Causes Fracture Critical Collapse, Mt Vernon, WA - May 23, 2013 Sean M. Gillham, The Pennsylvania State University, Fall 2014
In the early evening of May 23, 2013 an over-sized tractor trailer load attempted to cross the south bound side of the Skagit River Bridge. The over-sized load impacted the superstructure of the fracture critical bridge and resulted in the subsequent collapse of a single section of the bridge. (While referred to as one ‘bridge,’ the Skagit River Bridge is actually composed of several structurally independent spans of superstructure) (NTSB 2014). The impacting tractor trailer and several other vehicles were able to make it off the span before it collapsed. However, two other vehicles fell into the river during the collapse. Fortunately all survived with either no or relatively minor injuries (Carter et al). There are a number of issues and coincidences that lead to the events that triggered the collapse of the bridge. Lack of low clearance signs, low vertical clearance on edge of road, errors on estimating the real size of the transported load, and passing vehicles altering the intended path of the over-sized load all played a role in the events that occurred on May 23, 2013.
Fig 1: Skagit River Bridge before collapse. (Courtesy Walter Siegmund, GNU Free Documentation License version 1.2)
Keywords
I-5 Skagit River Bridge; Bridge Collapse; High Load Strike; Fracture Critical; Mt Vernon;
Bridge Overview
The Skagit River Bridge was constructed in 1955 as a two lane bridge with large shoulders. This was later changed to 4 narrower travel lanes and much reduced shoulder width. (National Transportation Safety Board (NTSB) 2014) This bridge spans the Skagit River, which separates the towns of Mt Vernon and Burlington. The main superstructure is composed of a series of four structurally independent steel through-trusses, with four approach spans on either side of the truss section.(National Bridge Inventory Database 2012). The central four through-truss spans are themselves fracture critical structures. The requirements for traffic lanes on bridges (clearances, lane widths etc) have changed over the years since the opening of the bridge in 1955. As a result the bridge has been deemed functionally obsolete (but not structurally deficient) due to these of clearances relative to new standards. (NTSB 2014).
Fig 2: Skagit River Bridge Vertical Clearance (Courtesy: Washington State Department of Transportation)
Located to the north of Mount Vernon WA, this bridge is part of the Interstate 5 highway network. Interstate 5 is classified as a principle arterial – interstate (NBI 2012), and is essentially the only major highway along western Washington running north south. As such, this road is the de facto thoroughfare to travel north/south, and consequently heavy freight traffic is likewise reliant on this route.
Interestingly enough, an impact to the bridge’s through-truss superstructure by a vehicle was not unique to the May 23, 2013 incident. In fact, the superstructure of this bridge was hit by vehicles on multiple prior occasions. One such impact caused significant structural damage to necessitate 'priority 1' repair and occurred not too long before the impact that ultimately collapsed the bridge (WSDOT 2012). Evidently, relatively speaking, bridge strikes from trucks are not that uncommon of an occurrence. According to an article from the Seattle Times there were numerous bridge strike incidents in years prior. “The state DOT said there were 21 bridge-strikes involving trucks last year, 24 in 2011 and 14 in 2010.” (Manuel Valdes and Mike Baker 2013). While most these strikes did not do any ‘structural’ damage, they do serve to continue to highlight the importance, and potential severity of vertical clearance issues. For example, since 2008 in Washington State there were 12 different incidents that did cause structural damage as a result of ‘high load strikes’ and one collapse (NTSB 2014).
The Collapse
Fig 3: Oversized Load Vehicle. (Courtesy: National Transportation Safety Board).
A commercial shipping company was tasked with transporting a large load from Nisko, Alberta CA to a port in Vancouver Washington (not to be confused with Vancouver, British Columbia) via I-5. The permit filed for oversized load stated the dimensions, which were measured by the driver as 15' 9" tall, and 11' 6" wide. As per regulations, a pilot vehicle was needed for the trip south along I-5. The pilot vehicle operator set the height pole to 16' 2", to meet state guidelines of 3" to 6" above the permit-vehicles vertical dimension.
Fig 4: Skagit River Bridge after collapse. (Courtesy: National Transportation Safety Board)
The pilot vehicle and oversized load combo traveled south down I-5 to the Skagit River Bridge. The lanes narrow for the bridge from the standard 12' to 11' 4". As the pilot/truck combo entered the approach spans of the bridge, another tractor trailer started to pass the oversized load in the adjacent lane. At this point the driver said he
‘felt “squeezed” by this passing vehicle so he moved his vehicle to the right’ (NTSB HR1401 p6).
When entering the though-truss, the oversized load impacted with, and significantly damaged, multiple sway braces of the through-truss on the North-most span, and several on the following span as well. This in turn lead to the almost immediate collapse of the north most truss span.
As the superstructure began to collapse, the two trucks and several other vehicles were able to make it off the span in time. Two other vehicles, however, were not so fortunate and plunged in to the river. A total of three people luckily only sustained minor injuries (the occupants of the two vehicles that fell into the river), and the rest involved were unharmed. The semi carrying the oversize load proceeded to wait in the shoulder to the south of the bridge. (NTSB 2014)
Investigation
The National Transportation Safety Board immediately began an investigation into the collapse of the Skagit River Bridge. The investigation looked through a multitude of possible contributing factors including but not limited to: vehicle size and bridge clearance, operator error, vehicle malfunctions, signage etc.
Geometry:
Fig 5: Dimensions of the pilot vehicle with height pole. (Courtesy of the National Transportation Safety Board)
After the crash the maximum height of the oversize load was measured to be 15’ 11”. NTSB notes though that the trailers load had local deformations from the impact damage, and as such the dimensions before the crash may have been different. For reference what the driver had reported, and what the trucking company listed on the permit application was a height of 15’ 9”. (NTSB 2014).
The pilot vehicles height pole was mounted 9” in from the rightmost edge of the vehicle. Investigators determined that due to the lean of both the poll and the antenna above, the ball at the top was 25” from the rightmost edge. Because the overhead sway bars of the Skagit River bridge were arched instead of straight meant that the right edge of the pilot vehicle would have had to have been on the fogline in order for the height pole of 16’2” to make contact. (NTSB 2014).
Additionally, based on the lane widths, measured clearances, and the dimensions of the over-sized load, NTSB found that in order "to clear the vertical bridge structure... the truck would have had to “intrude into the left travel lane a minimum of 21 inches.” (NTSB HAR 1401 p41)
Fig 6: Position of Vehicles to Impact Sway Bars. (Courtesy: National Transportation Safety Board)
Permit Process:
NTSB also examined the permitting process that Washington State Department of Transportation (WSDOT) uses. Generally, and in this case too, WSDOT automatically approves the permit based off of the given information, however the permit issued does explicitly state “WSDOT Does Not Guarantee Height Clearances." (NTSB HR1401 p24). Per Washington State Legislature, Washington Administrator Code 468-38-070 “It is the responsibility of the permit applicant to check, or prerun, the proposed route and provide for safe maneuvers around the obstruction or detours as necessary.” From the I-5 Corridor Bridge list, (a reference of the bridges clearances published by the Washington State Department of Transportation to be used by commercial shipping), the Skagit River Bridge has a maximum clearance of 17’ 3” and minimum clearance of 14’ 5”. The document however does not mention the lane width or position where these heights occur. (WSDOT 2014)
Signage:
Leading up to the bridge there were no signs were posted about the bridges vertical clearance. This though is in line with WSDOT policy of only posting height warning signs for vertical clearances less than 15’3” (as the fog line (shoulder) clearance height was 15’6”). Note though that the vertical height at the curb side is 14’8”. A conservative value of 14’5” is listed on WSDOT bridge database for the minimum clearance of the the edge of the bridge, but this is not posted at the bridge itself. Reduced lateral clearance signs were posted in advance, as lanes narrowed for the bridge from the standard 12’ to 11’4”.
Previous Incidents:
Prior bridge inspection reports for the Skagit Bridge were examined as well. Earlier truck impacts were not unique to this incident, and in this case it was found that each of the six prior inspection reports since 2003 indicated that there were high load impacts to the cross members. (NTSB 2014) Additionally there were two recent damage inspection reports (as a result of truck impacts), one of which did significant enough structural damage to necessitate 'priority 1' repairs. (WSDOT 2013).
Operator Error:
The NTSB also investigated the relevant drivers for operational error along with serviceability of the oversize load carrying vehicle. The driver of the oversize vehicle was experienced and had also driven other large loads over this bridge on several prior occasions. The report found through video evidence that there was inadequate distance between the pilot vehicle and the truck transporting the oversize load; such that there would not have been time for the oversize load to stop had the pilot vehicle indicated any warning. (NTSB HAR1401 2014)
Through interviews and phone records it was determined that the operator of the pilot vehicle was using a handsfree cell phone for a significant portion of the trip (including the immediate time prior to the impact). “When interviewed by NTSB investigators the pilot/escort vehicle driver stated she was not sure where within the right lane she was traveling, but she believed that she was in the center of the lane as she crossed the bridge” (NTSB HAR1401 p 42). There are conflicting reports on whether the height pole hit the sway bars of the bridge or not, and NTSB concedes it is impossible to know one way or the other. The driver of the pilot vehicle stated the height pole did not make contact, however the driver of a vehicle passing the pilot vehicle asserted that height pole hit several sway bars. (NTSB 2014).
Conclusions
The cause of the collapse was, unsurprisingly, a result of the impact of the oversize load into the arched sway bars bracing the compression members of the truss. The NTSB investigation report though also focuses on many contributing factors, the more significant of which are outlined below as well.
Fig 7: Skagit River Bridge Truss Diagram. (Courtesy National Transportation Safety Board)
The National Transportation Safety Board Highway Accident Report 1401 covers the progression of events during the collapse. In this report, NTSB determined from available evidence that the oversize vehicles rightmost edge was 2 inches past the fog line. The vehicle then proceed to hit successive sway bars as it traversed the span, and 5 on the following span (that span did not collapse). NTSB concluded that the “collapse sequence began when the load struck sway brace 4 in span 8 on the west truss, causing deformation of the adjacent vertical member (L4-U4), which pulled the attached upper chord member (U3-U5) downward, causing a buckling failure in that upper chord member at node U4. The NTSB further concludes that, because the Skagit River Bridge was a non-load-path-redundant structure, buckling of the upper chord on the west truss resulted in failure of the east truss and the collapse of span 8” (NTSB HAR 1401 p40).
Since the bridge has reduced lane widths and varying vertical clearance,in order for the oversized load to cross the bridge safely both lanes would have needed been secured from public traffic. NTSB concluded there was a lack of consideration for this on the part of the trucking company. (NTSB 2014).
NTSB also focused on the responsibility of the pilot vehicle operator. WSL WAC 468-38-100, notably section 6 governs these responsibilities. This sections states, along with many other requirements, that the pilot vehicle driver must inform the driver of the permit vehicle of any condition that could affect the safe movement (explicitly including overhead clearances and lane widths reductions as an example). (WSL WAC 468-38-100). NTSB found that the operator of the pilot vehicle did not communicate any warning of the narrowing lanes or reduced overhead clearance to the oversize load operator, thus not meeting the responsibilities outlined in WAC 468-38-100. Additionally, the NTSB states the driver of the pilot vehicle was distracted by use of the cellphone from fulfilling the required duties. To compound this issue, the oversize vehicle was following too close to the pilot vehicle to have had adequate braking distance if any communication of the hazards were even to be given. (NTSB 2014)
The procedure WSDOT uses to issue permits (lack of oversight), along with the detail of information readily available to the commercial drivers is also an issue NTSB found. “The protection of bridge infrastructure is too vital a state concern to leave the responsibility for assessing the risk associated with the transportation of oversize loads entirely with the motor carrier” (NTSB HAR1401 p58)
National Bridge Inventory information pertaining to the Skagit River Bridge.
National Transportation Safety Board. (July 15, 2014). "Collapse of the Interstate 5 Skagit River Bridge Following a Strike by an Oversize Combination Vehicle; Mount Vernon, Washington; May 23, 2013; Accident Report HAR1401" <http://www.ntsb.gov/doclib/reports/2014/HAR1401.pdf> (September 28, 2014).
NTSB Investigation Report on the collapse of the Skagit River Bridge.
Bridge inspection report done after a vehicle hit superstructure. (This report is of an impact several months prior to the one that caused the collapse).
Truck Impact Causes Fracture Critical Collapse,
Mt Vernon, WA - May 23, 2013
Sean M. Gillham, The Pennsylvania State University, Fall 2014
Table of Contents
Introduction
In the early evening of May 23, 2013 an over-sized tractor trailer load attempted to cross the south bound side of the Skagit River Bridge. The over-sized load impacted the superstructure of the fracture critical bridge and resulted in the subsequent collapse of a single section of the bridge. (While referred to as one ‘bridge,’ the Skagit River Bridge is actually composed of several structurally independent spans of superstructure) (NTSB 2014). The impacting tractor trailer and several other vehicles were able to make it off the span before it collapsed. However, two other vehicles fell into the river during the collapse. Fortunately all survived with either no or relatively minor injuries (Carter et al). There are a number of issues and coincidences that lead to the events that triggered the collapse of the bridge. Lack of low clearance signs, low vertical clearance on edge of road, errors on estimating the real size of the transported load, and passing vehicles altering the intended path of the over-sized load all played a role in the events that occurred on May 23, 2013.Keywords
I-5 Skagit River Bridge; Bridge Collapse; High Load Strike; Fracture Critical; Mt Vernon;Bridge Overview
The Skagit River Bridge was constructed in 1955 as a two lane bridge with large shoulders. This was later changed to 4 narrower travel lanes and much reduced shoulder width. (National Transportation Safety Board (NTSB) 2014) This bridge spans the Skagit River, which separates the towns of Mt Vernon and Burlington. The main superstructure is composed of a series of four structurally independent steel through-trusses, with four approach spans on either side of the truss section.(National Bridge Inventory Database 2012). The central four through-truss spans are themselves fracture critical structures. The requirements for traffic lanes on bridges (clearances, lane widths etc) have changed over the years since the opening of the bridge in 1955. As a result the bridge has been deemed functionally obsolete (but not structurally deficient) due to these of clearances relative to new standards. (NTSB 2014).Located to the north of Mount Vernon WA, this bridge is part of the Interstate 5 highway network. Interstate 5 is classified as a principle arterial – interstate (NBI 2012), and is essentially the only major highway along western Washington running north south. As such, this road is the de facto thoroughfare to travel north/south, and consequently heavy freight traffic is likewise reliant on this route.
Interestingly enough, an impact to the bridge’s through-truss superstructure by a vehicle was not unique to the May 23, 2013 incident. In fact, the superstructure of this bridge was hit by vehicles on multiple prior occasions. One such impact caused significant structural damage to necessitate 'priority 1' repair and occurred not too long before the impact that ultimately collapsed the bridge (WSDOT 2012). Evidently, relatively speaking, bridge strikes from trucks are not that uncommon of an occurrence. According to an article from the Seattle Times there were numerous bridge strike incidents in years prior. “The state DOT said there were 21 bridge-strikes involving trucks last year, 24 in 2011 and 14 in 2010.” (Manuel Valdes and Mike Baker 2013). While most these strikes did not do any ‘structural’ damage, they do serve to continue to highlight the importance, and potential severity of vertical clearance issues. For example, since 2008 in Washington State there were 12 different incidents that did cause structural damage as a result of ‘high load strikes’ and one collapse (NTSB 2014).
The Collapse
A commercial shipping company was tasked with transporting a large load from Nisko, Alberta CA to a port in Vancouver Washington (not to be confused with Vancouver, British Columbia) via I-5. The permit filed for oversized load stated the dimensions, which were measured by the driver as 15' 9" tall, and 11' 6" wide. As per regulations, a pilot vehicle was needed for the trip south along I-5. The pilot vehicle operator set the height pole to 16' 2", to meet state guidelines of 3" to 6" above the permit-vehicles vertical dimension.
The pilot vehicle and oversized load combo traveled south down I-5 to the Skagit River Bridge. The lanes narrow for the bridge from the standard 12' to 11' 4". As the pilot/truck combo entered the approach spans of the bridge, another tractor trailer started to pass the oversized load in the adjacent lane. At this point the driver said he
‘felt “squeezed” by this passing vehicle so he moved his vehicle to the right’ (NTSB HR1401 p6).
When entering the though-truss, the oversized load impacted with, and significantly damaged, multiple sway braces of the through-truss on the North-most span, and several on the following span as well. This in turn lead to the almost immediate collapse of the north most truss span.
As the superstructure began to collapse, the two trucks and several other vehicles were able to make it off the span in time. Two other vehicles, however, were not so fortunate and plunged in to the river. A total of three people luckily only sustained minor injuries (the occupants of the two vehicles that fell into the river), and the rest involved were unharmed. The semi carrying the oversize load proceeded to wait in the shoulder to the south of the bridge. (NTSB 2014)
Investigation
The National Transportation Safety Board immediately began an investigation into the collapse of the Skagit River Bridge. The investigation looked through a multitude of possible contributing factors including but not limited to: vehicle size and bridge clearance, operator error, vehicle malfunctions, signage etc.Geometry:
After the crash the maximum height of the oversize load was measured to be 15’ 11”. NTSB notes though that the trailers load had local deformations from the impact damage, and as such the dimensions before the crash may have been different. For reference what the driver had reported, and what the trucking company listed on the permit application was a height of 15’ 9”. (NTSB 2014).
The pilot vehicles height pole was mounted 9” in from the rightmost edge of the vehicle. Investigators determined that due to the lean of both the poll and the antenna above, the ball at the top was 25” from the rightmost edge. Because the overhead sway bars of the Skagit River bridge were arched instead of straight meant that the right edge of the pilot vehicle would have had to have been on the fogline in order for the height pole of 16’2” to make contact. (NTSB 2014).
Additionally, based on the lane widths, measured clearances, and the dimensions of the over-sized load, NTSB found that in order "to clear the vertical bridge structure... the truck would have had to “intrude into the left travel lane a minimum of 21 inches.” (NTSB HAR 1401 p41)
NTSB also examined the permitting process that Washington State Department of Transportation (WSDOT) uses. Generally, and in this case too, WSDOT automatically approves the permit based off of the given information, however the permit issued does explicitly state “WSDOT Does Not Guarantee Height Clearances." (NTSB HR1401 p24). Per Washington State Legislature, Washington Administrator Code 468-38-070 “It is the responsibility of the permit applicant to check, or prerun, the proposed route and provide for safe maneuvers around the obstruction or detours as necessary.” From the I-5 Corridor Bridge list, (a reference of the bridges clearances published by the Washington State Department of Transportation to be used by commercial shipping), the Skagit River Bridge has a maximum clearance of 17’ 3” and minimum clearance of 14’ 5”. The document however does not mention the lane width or position where these heights occur. (WSDOT 2014)
Signage:
Leading up to the bridge there were no signs were posted about the bridges vertical clearance. This though is in line with WSDOT policy of only posting height warning signs for vertical clearances less than 15’3” (as the fog line (shoulder) clearance height was 15’6”). Note though that the vertical height at the curb side is 14’8”. A conservative value of 14’5” is listed on WSDOT bridge database for the minimum clearance of the the edge of the bridge, but this is not posted at the bridge itself. Reduced lateral clearance signs were posted in advance, as lanes narrowed for the bridge from the standard 12’ to 11’4”.
Previous Incidents:
Prior bridge inspection reports for the Skagit Bridge were examined as well. Earlier truck impacts were not unique to this incident, and in this case it was found that each of the six prior inspection reports since 2003 indicated that there were high load impacts to the cross members. (NTSB 2014) Additionally there were two recent damage inspection reports (as a result of truck impacts), one of which did significant enough structural damage to necessitate 'priority 1' repairs. (WSDOT 2013).
Operator Error:
The NTSB also investigated the relevant drivers for operational error along with serviceability of the oversize load carrying vehicle. The driver of the oversize vehicle was experienced and had also driven other large loads over this bridge on several prior occasions. The report found through video evidence that there was inadequate distance between the pilot vehicle and the truck transporting the oversize load; such that there would not have been time for the oversize load to stop had the pilot vehicle indicated any warning. (NTSB HAR1401 2014)
Through interviews and phone records it was determined that the operator of the pilot vehicle was using a handsfree cell phone for a significant portion of the trip (including the immediate time prior to the impact). “When interviewed by NTSB investigators the pilot/escort vehicle driver stated she was not sure where within the right lane she was traveling, but she believed that she was in the center of the lane as she crossed the bridge” (NTSB HAR1401 p 42). There are conflicting reports on whether the height pole hit the sway bars of the bridge or not, and NTSB concedes it is impossible to know one way or the other. The driver of the pilot vehicle stated the height pole did not make contact, however the driver of a vehicle passing the pilot vehicle asserted that height pole hit several sway bars. (NTSB 2014).
Conclusions
The cause of the collapse was, unsurprisingly, a result of the impact of the oversize load into the arched sway bars bracing the compression members of the truss. The NTSB investigation report though also focuses on many contributing factors, the more significant of which are outlined below as well.The National Transportation Safety Board Highway Accident Report 1401 covers the progression of events during the collapse. In this report, NTSB determined from available evidence that the oversize vehicles rightmost edge was 2 inches past the fog line. The vehicle then proceed to hit successive sway bars as it traversed the span, and 5 on the following span (that span did not collapse). NTSB concluded that the “collapse sequence began when the load struck sway brace 4 in span 8 on the west truss, causing deformation of the adjacent vertical member (L4-U4), which pulled the attached upper chord member (U3-U5) downward, causing a buckling failure in that upper chord member at node U4. The NTSB further concludes that, because the Skagit River Bridge was a non-load-path-redundant structure, buckling of the upper chord on the west truss resulted in failure of the east truss and the collapse of span 8” (NTSB HAR 1401 p40).
Since the bridge has reduced lane widths and varying vertical clearance,in order for the oversized load to cross the bridge safely both lanes would have needed been secured from public traffic. NTSB concluded there was a lack of consideration for this on the part of the trucking company. (NTSB 2014).
NTSB also focused on the responsibility of the pilot vehicle operator. WSL WAC 468-38-100, notably section 6 governs these responsibilities. This sections states, along with many other requirements, that the pilot vehicle driver must inform the driver of the permit vehicle of any condition that could affect the safe movement (explicitly including overhead clearances and lane widths reductions as an example). (WSL WAC 468-38-100). NTSB found that the operator of the pilot vehicle did not communicate any warning of the narrowing lanes or reduced overhead clearance to the oversize load operator, thus not meeting the responsibilities outlined in WAC 468-38-100. Additionally, the NTSB states the driver of the pilot vehicle was distracted by use of the cellphone from fulfilling the required duties. To compound this issue, the oversize vehicle was following too close to the pilot vehicle to have had adequate braking distance if any communication of the hazards were even to be given. (NTSB 2014)
The procedure WSDOT uses to issue permits (lack of oversight), along with the detail of information readily available to the commercial drivers is also an issue NTSB found. “The protection of bridge infrastructure is too vital a state concern to leave the responsibility for assessing the risk associated with the transportation of oversize loads entirely with the motor carrier” (NTSB HAR1401 p58)
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