Heidi Underwood 540-387-5493
Jason Bond 540-387-5493
July 2, 2009
VDOT REPORTS ON FINDINGS FROM BRIDGE FAILURE
SALEM— The Virginia Department of Transportation (VDOT) and the Virginia Transportation Research Council (VTRC), VDOT’s research division, have completed an investigation and report into the failure of the pedestrian bridge over Walker Creek in Giles County that occurred on Sunday, Oct. 12, 2008. Ten people were on the bridge at the time of the failure. Investigators have found that three factors contributed to the bridge collapse; a small defect in the material of one of the bridge’s four anchor bolts, an unusual pedestrian load, and the way in which the eye bolts were configured.
“Determining the factors that led to the bridge failure is extremely important to VDOT. This investigation was undertaken with great care, utilizing all available resources,” said Kendal Walus, P.E., VDOT State Structure and Bridge Engineer.
The investigation included four components: an examination of the bridge design, analysis of the broken bridge bolt, testing of one of the unbroken bridge bolts, and a comparison of the strength of the bolt to the amount of tension present at the time when it broke.
The pedestrian bridge over Walker Creek was constructed with two parallel cables that run across the top of the bridge and connect on each side of the creek. Anchor bolts connect each cable end to heavy concrete blocks buried on both sides of the creek. One of the four anchor bolts broke causing the bridge to collapse.
During analysis of the broken bridge bolt, investigators identified a small defect in the material of the broken bolt, which is believed to have been the primary cause of the bolt’s sudden failure. The defect was located on the inside of the hook and would not have been visible before the bolt broke. Further testing that compared the strength of the bolt to the amount of load present at the time when the bolt failed indicates that the anchor bolt would have supported the load present at the time of its breaking if there had not been a defect in the metal.
Also, during the comparison of the strength of the bolt to the amount of tension present at the time when it failed, investigators identified two other factors that contributed to the bolt breaking. The analysis showed that a higher than typical pedestrian load and swinging and shaking of the bridge also contributed to the bolt breaking and the failure of the bridge.
Finally, the pedestrian bridge over Walker Creek, and 10 other pedestrian bridges across Virginia are secured with open-hook anchor bolts. The report suggests that a closed-eye anchor bolt would have created an additional factor of safety on the bridge. However, the investigators found that the open hook anchor bolt was adequate to hold the load without breaking had there been no flaw in the material.
The investigation shows that the factors that contributed to the pedestrian bridge failure could not have been detected during inspections, and were not the result of flaws in the design of the bridge. However, VDOT is committed to preventing similar incidents. In the coming months, VDOT will work to implement the recommendations provided in the VTRC report. The recommendations include:
Determining which other suspension bridges in the state use the open hook anchor bolt detail. Develop a method to modify the open hook detail to prevent it from opening under an extreme loading event, should one take place. Future construction of pedestrian bridges of similar design should not use the open hook detail. Post a maximum number of people allowed on a bridge at one time.
“VDOT is committed to ensuring the safety of the public no matter what type of structure they are crossing. I appreciate the extensive effort and exhaustive research that went into the investigation of this incident. I want to acknowledge the positive cooperation between the Federal Highway Administration and the Research Council. VDOT’s bridge division has begun the effort of implementing the recommendations proposed in the report,” said Walus.
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