Minnesota Bridge Conditions Were Extensively Documented
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 Nancy Soulliard/ENR Engineering reports identified suspect truss members of structure that was built in the 1960s using a design that has since fallen from favor. |
 Kent/ Kobersteen/Minneapolis Star Tribune/Zuma |
...conservative as today, still were inherently robust. Consulting engineers say testing them today is important because "sometimes structures that look pretty bad are not always that bad," says Donald Pearson-Kirk, technical director at London-based Mouchel Parkman and former PB engineer. "Testing doesn't necessarily show things are worse. What it does is identify things for action, what's the priority," especially with limited transportation budgets, he says.
URS recommended in a June 2006 draft report that MNDOT should retrofit 20 fracture-critical members in the main truss to add internal redundancy with high-performance steel plates and high-strength bolts. It advised to inspect fatigue-sensitive details at boxed chord diaphragms, where the toes of the fillet welds between the tabs and the truss chords were "primary" locations for potential fatigue cracks. Deck replacement, including an analysis of construction sequencing, also needed study.
Last year, engineers not quite satisfied with the 2006 recommendations for fear that wholesale retrofitting would require massive drilling and possibly compromise the structure, went back to the drawing board with URS. "That was an engineering decision, not a money decision," says Bob McFarlin, MNDOT assistant commissioner.
In January, URS issued an updated draft—not quite final—that offered three "equally viable" options: Retrofit 52 fracture-critical truss members on the main truss; use non-destructive testing to locate and remove defects in the "suspected" weld details; or perform combination of the two by retrofitting 24 fatigue-sensitive members and testing 28 fracture-sensitive members to identify cracks potentially lying under the surface.
MNDOT began a critical inspection in May, going over about 50% of the bridge and finding no unsafe details before calling it quits for the summer construction season.
"We intended to go back and resume inspection this fall," says Daniel L. Dorgan, the state's chief bridge engineer. The entire retrofit option was priced at about $1.5 million, "not a significant budget item," McFarlin adds.
| What URS Recommended in January: |
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| What MNDOT Did: |
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By halting inspections before they were complete, the situation on the bridge and conditions before its collapse became more complicated. When the collapse occurred, half of the roadway was shut down. Two lanes on the outside of the northbound side and two inside lanes on the southbound side were blocked off. Most of the crews and materials were staged on the southbound side.
PCI was working on the bridge under a $9-million contract to replace the deck's 2-in. concrete overlay and repair its 7-in.-thick structural slab. Work began in early June. It included shallow Type 1 and medium-depth Type 3 slab repairs, as well as about 728 sq ft of full-depth replacement, which would have required scaffolds and formwork wrapping around the underside of the slab. Investigators have not said whether or not that equipment was present during the collapse.
Engineering experts say that construction factors are not likely an issue. However, some agree that full-depth replacement might have inherently made the bridge less redundant in the case of a truss failure. The 2001 University of Minnesota report found that if "the fractured main truss deflected significantly the slab could prevent the complete collapse through catenary action." It noted that fatigue cracks were more likely to begin in floor trusses, which were easier to inspect.
The bridge "could likely tolerate the loss of a floor truss without collapse," concluded the report. Fatigue cracking was "not expected during the remaining useful life of the bridge." The state had no weight restrictions on the bridge. URS issued a statement that it "has not been involved in any of the recent work being performed on the bridge, nor was the recent work related to the company's recommendations."
Debris removal is now mobilizing. MNDOT on Aug. 5 inked a cleanup contract with Carl Bolander & Sons Co., St. Paul, that could cost $15 million.
Replacing the bridge is on a fast track. MNDOT will open proposals on Aug. 8 for its design-build replacement. It is slated for completion by late 2008 and will cost at least $150 million.
One confirmed team to propose is a joint venture of Lunda Construction Co., Black Water Falls, Wis., and Ames Construction Inc., Burnsville, Minn., says Chris Fox, Lunda corporate counsel. The firms are already teamed on the approximately $300-million Crosstown interchange project in south Minneapolis.
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