Last month’s old bridge collapse still under review
State transportation officials continue to review an Aug. 15 incident in which a 64-year-old pier unexpectedly gave out during demolition of the southernmost section of the old Grand Avenue bridge.
No one was injured and no property damage resulted when the 210-foot span of girders, from which the concrete bridge deck had already been removed, came down at once instead of in three separate segments as originally planned.
“It was an unplanned event, but it failed safely and that’s something we really need to emphasize,” said Steve Olson, program engineer for Colorado Department of Transportation Region 3.
Olson said the general contractor on the bridge project, Granite-Wadsworth, and demolition subcontractor, Cherry Creek Recycling, followed a demolition plan, and safety measures were in place just in case.
“We implemented those plans and by doing so were able to protect property and individuals and do a prompt response and recovery,” Olson added. “Any damage was limited and contained in the project area.”
Some of the girders did fall across the main line of the Union Pacific Railroad. However, railroad inspectors responded within hours of the late-evening event, and determined that no rails needed to be replaced.
“The Union Pacific did not require track protection for the girder removal operation over the railroad tracks,” Olson said. Freight trains were running later that night, and the next day’s Amtrak passenger train schedule was not affected.
By the next morning, most of the old girders had been cut into sections and were being hauled off as part of the demo project.
Also during the collapse, some of the exposed rebar on two of the pier supports for the new Grand Avenue bridge was damaged. The damage was repaired within the next couple of days, and no project delays resulted.
“On video, it does look like a big deal,” Olson said of dramatic video taken from a camera mounted atop the Colorado Mountain College building at Eighth and Grand that night.
“But it really did fail safe, and it failed in a confined area,” he reiterated.
The method of removing the girders over Seventh Street and the railroad tracks, using backhoes to pull the sections away from the pier, was also according to plan, he said.
By contrast, the remainder of the bridge span over the Colorado River and Interstate 70 was done using cranes to lift the girders vertically from the bridge supports.
“It is our understanding that common practice is to remove girders vertically or horizontally, but not both,” Olson said.
“If engineered, though, it may be possible to do both,” he said, adding that safety considerations such as the potential for cranes to tip over into nearby buildings in that particular section also had to be taken into account.
The failure of the third pier, as crews were attempting to detach the girder section, also was not an indication that the pier was compromised to begin with, Olson said.
“During the demolition, that one particular pier got loaded incorrectly,” he said.
Because it was an unplanned incident, Olson said what’s called an “after-action review” is being done, and that no outside agency review is necessary. The contractor also has a procedure to do a follow-up safety review and “root cause analysis,” he said.
Olson said there is no particular time frame to complete the internal reviews.