Pedestrian and bicycle access on the Washington Avenue bridge has been restricted in recent weeks because of safety issues discovered by engineering firm URS in the course of studying how to make the bridge work for light-rail transit (LRT). According to URS, which is conducting an updated study for the Met Council about the Washington Avenue bridge LRT retrofit, buckling capacities of the exterior columns supporting the upper deck are “critically” inadequate.
According to Hennepin County, the bridge’s owner, and the Met Council, which is behind the Central Corridor LRT project, both the county and the Federal Transit Administration (FTA) were aware of the need to structurally reinforce the pedestrian bridge, and the county was in the process of determining what to do when URS alerted the county to the potential severity of the problem.
Restricted access to the bridge — the main pedestrian artery between the University of Minnesota’s East and West banks — has presented problems for people who rely on the bridge’s upper deck. (The lower deck is dedicated solely to vehicular traffic and is not affected by the restrictions.) In mid-September, Hennepin County Transportation Director Jim Grube said access would likely remain limited until at least April 2009, the time needed for repairs to be designed and implemented.
But those working to design a repair for the problem at the source of that closure face multiple challenges. First, they must devise a plan to strengthen the columns supporting the pedestrian deck. They also are attempting to keep modifications consistent with LRT designs. The Central Corridor plan — preliminary engineering design plans were submitted Sept. 5 — still needs FTA approval; a response is expected early next year.
The LRT situation “does complicate” the bridge repair issue for the county, Grube said. However, having a plan that addresses the issue in the context of the bridge’s future uses is worth the added hassle at this stage, he said.
Repairs to the pedestrian bridge fall under Hennepin County’s jurisdiction, meaning the county will foot the bill to reinforce the pedestrian deck. Meanwhile, Central Corridor LRT plans are in the province of the Met Council. Rich Rovang, deputy project director of the Central Corridor LRT project, said in an e-mail that the Met Council and Hennepin County are “approaching the problem as a team.” He said both entities are using the same consulting firm for their designs — the Met Council for its major bridge retrofit and the county for its repairs.
Grube said in the event that an LRT-compatible solution isn’t possible, the county will likely select the most cost-effective temporary solution to safely address the pedestrian bridge reinforcement issue.
Hennepin County is still analyzing the best way to strengthen the upper deck and a final design is expected by December 2008. But even with the county and the Met Council working closely together, any strengthening that does occur will almost certainly add weight to or alter the bridge’s current design. That means any LRT design scenarios based on studies of the bridge before county-funded safety modifications are implemented, will likely require some revision.
Other Uncertainties Loom
A September 2007 engineering study conducted by URS on behalf of the Met Council’s Central Corridor Project Office recommended that the bridge’s current non-composite lower deck — the deck where the current roadway for vehicles rests — be rebuilt as a composite deck. According to the study, a composite deck would better handle the added weight of light rail. The Met Council has subsequently pursued that as a central option.
A composite deck engages the girders and beams below it in a bridge, unlike a non-composite deck, which merely rests on its supports, according to University of Minnesota civil engineering professor Cathy French. French is not involved with the project, but reviewed sections of the latest URS engineering report, as well as a 2007 URS report, for the Daily Planet.
The latest URS report, dated August 18, 2008, suggests further investigation of safety and feasibility issues is needed. One feasibility issue involves the welding of shear studs into the existing bridge structure to create the needed engagement between a new composite deck and the existing girders. According to the report, “Should this welding be found not feasible, the proposed new composite deck would not be possible.”
Additionally, the $30 million estimate the Met Council included in its recent project application to the FTA, did not budget for strengthening pier columns and footings. The September 2007 report noted that this strengthening might be necessary with the addition of light rail.
“[The Minnesota Department of Transportation] is the definitive investigator on that type of inspection, and we are proceeding on their findings,” Rovang said, explaining the Met Council’s logic in not budgeting for pier column reinforcement. But according to online records, MnDOT’s most recently contracted inspection was six years, and that six-year-old report is now outdated, as the report itself recommends re-inspection at least every five years. Still, Rovang said material testing on the concrete and footings is currently underway. It is unclear what those results will yield.
CORRECTION: After meeting with several Central Corridor officials — Gary Erickson, assistant project director for design and engineering, Rich Rovang, deputy project director, and Charles Hymes, Jr., vice president of DMJM Harris, the prime consultant for the Central Corridor Light Rail Transit project — late last week, it came to my attention that there was, in fact, a more recent underwater bridge inspection commissioned by MnDOT than the 2002 report I cited in this article.
Both the 2007 and the 2002 reports were prepared for the Minnesota Department of Transportation by Collins Engineers, Inc.
The 2007 report says: “It is recommended that existing plans be reviewed to determine the foundation type and depth at both piers in regard to the footing exposure. Normally, the foundations of bridges in the Mississippi River are deep and/ or founded on piling, which would suggest no reason for concern at this time, if that is confirmed to be the case. If not, countermeasures could be warranted, such as placing riprap around the exposed footing at both piers to prevent further exposure.” The ’02 and ’07 reports have similar findings, and this paragraph is verbatim from both reports.
My original statement that no study had been done since 2002 was also based on information from Rovang. In an e-mail interview, I asked specifically whether there had been any studies of the pier footings since the 2002 inspection. Rovang responded: “There have been no additional studies on the footings. MnDOT is the definitive investigator on that type of inspection, and we are proceeding on their findings. Material testing on the concrete and the footings is presently underway.”
Thus, at least three issues still await definitive solutions:
1) Strengthening or reinforcing the columns supporting the upper deck;
2) Whether strengthening pier columns and footings for light rail traffic is necessary;
3) Whether rebuilding the current lower deck as a composite deck is feasible.
What is known is that the project budget potentially leaves little leeway, should retrofitting the Washington Avenue bridge become increasingly costly, due one of the scenarios above, or other uncertainties.
Though the overall cost of the Central Corridor project increased by approximately $6 million from the $909 million plan approved by the Met Council in February, the budget had to be kept especially tight in order to meet the necessary federal cost-effectiveness index. “We have scrubbed it and scrubbed it and scrubbed it again,” Met Council Chair Peter Bell said, referring to the budget, in a statement on the Central Corridor website. “We have no additional dollars to work with.”
Liz Riggs contributes regularly to the Daily Planet.