Colorado Vehicle Collides With Fallen Overhead Bridge Girder

Colorado: A 2002 Dodge Durango sport utility vehicle (SUV) driven by a 34-year-old man eastbound on Interstate 70 (I–70) approached the Colorado State Route 470 (C–470) overpass. The driver’s 37-year-old wife and their 2-year-old child were also in the SUV. The interchange of I–70 and C–470 was in a temporary traffic control zone for a highway construction project, during which an additional entry ramp and two additional lanes were being constructed for the overpass.

As the SUV approached the overpass, a fabricated steel girder line composed of two joined sections, which had been erected during the evening parallel to the existing overpass, as a part of the bridge-widening project, rotated toward the overpass and sagged into the I–70 eastbound lanes. The girder struck the SUV about half the distance between the vehicle’s front end and its windshield and sheared off the vehicle’s top. The lower portion of the SUV continued east for 818 feet, coming to rest in the grassy median of I–70. All three vehicle occupants were killed.

The Federal Highway Administration (FHWA) and the State of Colorado funded the I–70/C–470 interchange construction project, which was intended to improve traffic capacity and safety at the interchange of these two routes and to make additional improvements along I–70 in this area. As a part of the project, the overpass of I–70 by C–470 was to be widened by adding two lanes. An additional loop ramp was also to be constructed. Under the terms of the stewardship agreement between the FHWA and the Colorado Department of Transportation (CDOT), CDOT was managing the project  and had contracted with Asphalt Specialties, Inc., to perform the actual construction work. Asphalt Specialties had been the general contractor for numerous projects in Colorado for CDOT and other agencies.

The FHWA Turner-Fairbank finite element analysis report contained the following conclusions:

The girder has low resistance to lateral bending and twisting. The girder is dependent on top flange lateral bracing for stability under most conditions. Brace number 2 appears to be the most critical to providing stability to the girder. Removal of brace 2 caused immediate instability for the out-of-plumb girder model. The plumb girder model remained stable when brace 2 was removed but became unstable when either brace 3 or 5 was subsequently removed. Braces 3 and 5 are redundant members and their removal would not be expected to result in collapse. The presence of braces 3 and 5 is insufficient to brace the girder in the event brace 2 is lost. The out-of-plumb girder model was more susceptible to instability compared to the plumb girder model. This indicates that erection of the girder in an out-of-plumb condition could be a significant factor in the girder failure. Wind loading created modest increases in brace forces, but in no case did it push the model into instability. All model runs that were stable under gravity only were also stable under added wind load. Therefore, wind load can be eliminated a primary reason for failure. Cyclic forces on the braces due to wind load may be a factor in reducing anchor bolt capacity over time. The girder was susceptible to lateral vibration at a natural frequency between 1 and 2 Hz [hertz]. It is possible that wind loading and truck gust loading could have excited these lateral vibration modes. This could create a cyclic force on the brace members that could have caused degradation in capacity over time. However, results from static analysis under wind load indicates the cyclic force variation was probably low due to vibration. The maximum force calculated on any of the brace members for any load case was less than 1/2 the allowable compressive force calculated by the AISC [American Institute of Steel Construction, Inc.] steel construction specifications. Therefore, brace capacity is not a significant factor in initiating this failure. The maximum pull-out and shear forces on the anchor bolts were well below the manufacturer’s allowable design loads for a correctly installed anchor bolt. Since examination of the evidence shows that the anchor bolts pulled out during the failure, this strongly suggests that the anchor bolts did not reach their design capacity. Based on the cases investigated in this study, the most likely cause of final collapse was the failure of brace 2. Failure of other braces changes the lateral deflected shape of the girder but would not be expected to cause collapse.