Washington Avenue Bridge likely to bear the load of light rail

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Following the Met Council’s February 27 approval of a $909 million Central Corridor light rail plan, it now appears likely that the route will have to rely on the Washington Avenue Bridge as its means for crossing the Mississippi River. To bear the additional load of light rail, however, the bridge will require structural modification—at a considerable cost of time and money.

Although the University of Minnesota is funding a study of an alternative northern alignment through Dinkytown that would possibly bypass the Washington Avenue Bridge, a final report on that study isn’’t expected until summer—and at this point, route changes could delay the project significantly, boosting the overall cost of the project by as much as $45 million, Met Council chairman Peter Bell has said. The task, therefore, of stringing the line across the Mississippi River now seemingly rests on the Washington Avenue Bridge.

Last year, the Met Council requested a study of the bridge’’s potential for accommodating light rail. The findings of that study indicated that the structure would require, among other things, “a major rehabilitation that would add new primary load-carrying members under the lower deck.” An initial estimate placed the cost of rehabilitation somewhere between $25-—30 million.

This isn’t the first time the Washington Avenue Bridge would have to be strengthened to accommodate a new kind of transit. Built in 1884, the first Washington Avenue Bridge—which has since been torn down—had to be retrofitted to accommodate the weight of streetcars just 16 years after it was originally built.

As the original Washington Avenue Bridge became outdated and incapable of handling heavy traffic, the necessity for a new bridge became increasingly clear. As early as 1945, talk of a new bridge had already reached the Minneapolis City Council; little more than a decade later, preliminary sketches of the new bridge began to emerge and by 1963, construction on the new double-deck bridge had started. Just over two years later, in October 1965, the current Washington Avenue Bridge opened—with a lower deck dedicated to vehicular traffic and an upper deck for pedestrians.

Funded by the Minnesota Highway Department and the Federal Bureau of Public Roads, the bridge was built for just over $3.3 million.

Added weight posed problems in the past

Even before the Met Council commissioned a 2007 feasibility study of the Washington Avenue Bridge for bearing the weight of a light rail transit line, there were indications that the current Washington Avenue bridge could not withstand any added structures without considerable reinforcement.

When the University of Minnesota decided to build an enclosed pedestrian walkway on the top level of the new bridge, the university spent close to $80,000 reinforcing the structure to make it capable of handling the added physical stress of the glass enclosure. Plans for the $385,000 enclosure were to have included a bookstore, shops, and even restaurants.

While the enclosure itself was built the year after the bridge was completed, the bookstore and restaurants never came to fruition—apparently because they would have proved too heavy to be added safely, a coordinator from the University’’s physical planning office said in a 1973 interview with the Minnesota Daily.

In the 1980s, there was renewed talk of adding a café atop the bridge, but the idea was again quashed when a structural study of the bridge revealed that the bridge in its current form could not withstand the extra weight.

Feasibility study results

Last year, the Met Council paid $100,000 to the engineering consulting firm URS to investigate what modifications would be necessary to incorporate a light rail line into the Washington Avenue Bridge—and how much such renovations might cost. The report, released by URS in September 2007, includes analysis of the bridge as it currently stands, as well as analysis of a hypothetical modified structure— that would include a composite lower deck. The URS report included a simulation with two lanes of vehicular traffic and two lanes of LRT on a rebuilt lower deck. The URS report recommends “replacing the existing lower deck with a composite deck and adding three lines of trusses between each two main girders.”

University of Minnesota civil engineering professor Cathy French—who is not involved with the project but reviewed portions of the report—summarized the report’s recommendation as adding some additional members to reduce the load on the current members.”

French said the difference between a non-composite deck—which the bridge currently has as its bottom roadway—and a composite deck is that a composite deck engages the rest of the structure, rather than merely adding to the load. Instead, it works with the supports.”With a composite deck, “you’’re getting the deck to work together with the girders or beams,”” she explained. A non-composite deck just rests on the supports.

According to the report, modifying the existing bridge might be more time-consuming than building a new bridge. “”When you have an existing structure,” said French, “you’’re taking away some of the parts that are supporting it” as you make modifications. Therefore, extra precautions must be taken to keep the bridge intact while the changes are made——precautions that can sometimes complicate construction.”

French also emphasized that the $25—-30 million estimate for strengthening the bridge was a rough estimate, noting that URS admitted in the report that attempting to calculate the weight and quantity of materials without a specific design is difficult.

Further, the report includes a disclaimer that the strengthening of pier columns and footings—which might be necessary——was not included at all in the overall cost estimate. The Met Council, however, does not believe this additional work will be necessary. ““The underwater foundations of the bridge,” wrote Met Council Laura Baenen in an e-mail, “were recently inspected under MnDOT direction and the foundations appear to be in good condition.””

Nearly six months have passed since the report was completed. Baenen said the $25-—30 million estimate for retrofitting the bridge “has not changed,” though further analysis of the bridge’’s superstructure and deck are currently in progress and are expected to be completed by summer—before the September deadline for project partners to finish the initial engineering design.

$30 million for strengthening the Washington Avenue Bridge has been budgeted in the $909 million Central Corridor plan that was approved by the Met Council Feb. 27. While the Met Council still intends to begin construction on the line in 2010 if all goes according to plan, there’’s no word yet on where or when the Washington Avenue Bridge rehabilitation would fit into that schedule.

Liz Riggs is a free-lance writer who also has written for Minnesota Daily and The Bridge.

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