A series of hairline cracks discovered on a $120-million bridge project will force a one year delay in opening the first of a pair of twin bridges over a key Mississippi River crossing in Minnesota.
The bridges, each to carry five lanes of east and westbound I-494 traffic over the river, will replace a narrow, four-lane bridge, which has long out-lived its useful capacity.
The first bridge, to carry westbound I-494 traffic and now nearly completed, will not open until sometime later this year while workers complete a design retrofit.
Construction on the twin, eastbound bridge also will be set back because demolition of the existing bridge to make way for the second bridge cannot start until traffic is switched over to the completed westbound bridge.
Completion of the entire project, originally scheduled for late 2007 has been set back to sometime in 2008 because of the design modifications now being made.
The good news is that half of the approximately 8 mi. (13 km) of complementary I-494 and Minnesota (MN) TH-61 road work is complete along with a mammoth highway interchange on the east side of the new bridge connecting east and westbound I-494 to north and southbound MN TH-61.
Located in the southeastern I-494/I-694 loop around the St. Paul/Minneapolis metro area, the bridge construction is part of a $250-million project to alleviate monstrous, 2 mi. (3.2 km) traffic back-ups at the I-494 Mississippi River bottleneck and greatly improve traffic flow through the I-494/MN TH-61 interchange.
The hairline cracks, attributed by state officials and designers to be stress related, did not jeopardize the structural stability of the bridge.
If not repaired, however, the cracks could reduce the 100-year life expectancy on a structure measured in thousands of feet and tons of concrete poured.
According to Dan Dorgan, Minnesota Department of Transportation (MnDOT) bridge engineer, an inspector discovered the series of cracks during a routine bridge inspection on the center wall inside one of the cavernous box girders that will carry the bridge deck.
The cracks, “all lining up in the same direction indicated to us that we probably had a stress issue rather than a random shrinkage crack.”
After engineers looked into materials going into the bridge, the method of construction and the original design, they determined that the assumed design load was incorrect, Dorgan said.
Designers assumed “that all three webs carried the same load so they reinforced them equally,” Dorgan explained. In reality, “the center web carries approximately 40 percent of the load.”
The fix to all this was to add vertical post tensioning to the center web portion of the box girder for all remaining segments to be poured on the first bridge, Dorgan said.
For the portion of the bridge already completed, Lunda workers are now anchoring cables to the top of each supporting pier just below the road deck and at the center of the span on the bottom of the box girder.
Tension applied to the cables transfers a portion of the bridge’s weight from the center to the piers, to relieve the over stress according to a drawing supplied by MnDOT engineers.
Bridge engineers from HNTB, with an office in Minneapolis and the primary bridge designer have modified plans of the second bridge to incorporate these changes.
Mn/DOT also has had an independent consulting company review the redesign to ensure all issues are addressed.
Dubbed the Wakota project, named from a combination of letters of two participating counties, road work will continue to roll forward despite the design delay.
Along with the erection of the twin bridges, other construction includes rebuilding approximtely 4 mi. (6.4 km) of I-494 east and west of the bridges and improving to freeway standards 4.3 mi. (6.9 km) of MN TH-61, the north/south intersecting highway of the interchange.
MnDOT is the owner/operator of the bridges and highways under reconstruction and awarded four contracts to build the project.
Local governments awarded five additional contracts for local street construction.
Three counties and eight communities are participating in the project along with the state and federal governments.
Lunda Construction Company of Black River Falls, WI, won the biggest of the contracts; the construction of the twin bridges and reconstruction of I-494.
It also won the contract to reconstruct a short piece of MN TH-61 north of the I-494/MN TH-61 interchange.
C.S. McCrossan of Maple Grove, MN, took the two contracts totaling $62 million to reconstruct MN TH-61 south of the interchange.
Originally bid at $120 million, the bridge construction is the largest of the contracts for this project and the cost will increase because of the design changes now being made.
The existing bridge was long overdue for replacement because “this portion of the road has been a bottleneck for a long time now and the new bridges will eliminate it. The road and bridge improvements will increase safety and traffic capacity,” said Adam Josephson, MnDOT project engineer.
Each of the twin bridges will be 1,880 ft. (573 m) long with each bridge carrying an average deck width of 85 to 99 ft. (26 to 30 m).
In contrast, the current bridge deck is only 66 ft. (20 m) wide. Each bridge will carry three through lanes of traffic and two auxiliary lanes.
The westbound bridge also will feature a pedestrian/bicycle trail, an amenity not part of the existing bridge.
The twin bridges will be cast in place segmental box girder construction, only the third time in Minnesota this type of bridge has been built.
The first was the Plymouth Avenue Bridge in Minneapolis followed by the Wabasha Bridge constructed across the Mississippi River in downtown St. Paul.
The Wakota Bridge will feature two main spans at 466 ft. (142 m) and side spans of 266 ft. (81 m), 328 ft. (100 m) and 353 ft. (108 m). The superstructure for the bridges is a combination cast-in-place balanced cantilever using form travelers for the interior spans and cast-in-place on falsework for the end spans.
A pair of Norwegian Strukturas form travelers owned by Lunda are pouring the box girder segments outward from each pier until they meet in the middle of the span. Each form traveler weighs 160 tons (145 t).
The extended deck widths on the new bridges will alleviate one of the biggest bottlenecks in the Twin Cities metro region and quickly lower the bar of frustration for the thousands of commuters crossing the existing bridge.
Currently, 82,000 travelers and commuters cross this bridge daily and fight traffic back ups during rush hour periods.
The new bridge crossing will easily handle the current ADT and the projected ADT of 116,000 by the year 2020, according to Mn/DOT engineers.
Though bridge construction has been delayed, it’s much smoother sailing through the I-494/MN TH-61 interchange.
Named the North Ring project because frontage roads form a perimeter around the interchange, it consumes 90 acres (36 ha) of space, equivalent to more than 90 football fields.
Drivers have seen a complete geographical overhaul of this huge interchange. To improve traffic flow and capacity at this busy interchange, workers constructed 22 additional bridges, loops and ramps.
New ramp alignments through the hilly topography forced the construction of several retaining walls, the largest approximately 2,000 ft. (610 m) in length and 50 ft. (15 m) high.
Over the past three years, hundreds of backhoes, graders, dozers and trucks, dug, sliced, dragged and hauled thousands of tons of earth at the North Ring project.
The result is an interchange easier to negotiate and looking substantially different from the old one.
And most of the construction has been completed, much to the relief of the thousands of daily commuters who drive it.
“Workers have completed all 11 retaining walls of the North Ring Road interchange and are substantially complete with the construction of the seven associated bridges,” said Dustin Thomas, MnDOT project engineer.
Likewise just north of the interchange along westbound I-494, where construction crews rebuilt the freeway on both sides of the river and added an additional lane in each direction to make it three through lanes of traffic.
“Most of the construction is complete on I-494 westbound except for some approach work coming up to the river bridge,” said Eric Embacher, Mn/DOT project engineer. “The design retrofit will delay some of the work on I-494 eastbound.”
When construction is completed on the two bridges alone, approximately 80,000 cu. yd. (60,000 cu m) of concrete and 15 million lb. (6.8 million kg) of reinforcement will have been consumed.
Another 81,000 cu. yd. (61,000 cu m) of concrete and 82,500 tons (74.845 t) of bituminous paving will be placed on the I-494 approach roads to the Mississippi River crossing.
Retaining wall construction required for the bridge work alone requires approximately 26,000 cu. yd. (19,500 cu m) of concrete and 2.7 million lb. (12.2 million kg) of reinforcement out of a project total of approximately 87,000 cu. yd. (76,000 cu m) of concrete and 8 million lb. (3.6 million kg) of reinforcement.
Other then the added work caused by the design retrofit on the westbound bridge, the biggest challenge for Lunda workers has been working around the traffic.
“Keeping the traffic moving and staging our work around in-place traffic on I-494 and MN TH-61 has been one of the more challenging aspects of this project,” said Dale Even, P.E., Lunda project superintendent. “I-494 is part of the major interstate loop around the metro area and is a critical part of the traffic system.”
Lunda alone has had as many as 200 workers on site during peak construction months, Even said.
Dozens of pieces of heavy equipment of every variety, size and manufacture have been on the bridge site alone, Even added.
“For bridges and retaining walls on the project we have had up to 20 cranes on site ranging from 50 to 300 tons in size,” Even said. “For the Mississippi River crossing we are also using barges and tug boats to move equipment and materials. We utilize both lattice boom crawlers and RTC rubber tire cranes. Most of the concrete is placed with concrete pumps.”
For excavation, embankment, utility, grading and paving work a wide variety of equipment including dozers, blades, backhoes, pavers, scrapers, batch plants, belly dumps, tandem-axle dumps and rollers have been on site, Even said.
Lunda Construction brought on Shafer Contracting out of Shafer, MN, for the excavation and grading work along with another 16 sub-contractors working the job site.
Most of the reconstruction of the 3-mi. stretch of MN TH-61 south of the bridge also is complete, said Eric Sellman, chief estimator of McCrossan.
McCrossan used mostly Cat heavy equipment on this project including D5, D6, D8 dozers; 615 and 627 scrapers; 14H and 140H graders, a half dozen types of Cat excavators; a couple of loaders and Manitowac and Grove cranes.
With a peak work force of 125 workers, McCrossan and its sub-contractors “reconstructed the highway to freeway standards and added a lane in each direction to make the road three through lanes of traffic in each direction,” Sellman said.
Additionally, two key interchanges connecting the communities of Newport and St. Paul Park were reconstructed.
The single-point diamond design interchange at Newport includes three steel bridges and one pedestrian bridge.
A couple miles south, McCrossan workers constructed a conventional diamond interchange featuring two prestressed girder bridges.
McCrossan and Mn/DOT won an Association of General Contractors (AGC) Bridge Construction Award for the bridge crossing MN TH-61 at Newport.
According to a Mn/DOT memo, the bridge won because of “the workmanship and appearance of the Mankota-Kasota simulated stone masonry aesthetic treatments.”
Construction challenges noted in this memo cited that the bridge is “a single-span steel girder bridge bounded by four railroad tracks with over 60 trains per day and by local frontage roads resulting in a very confined construction space.
“None of the tracks or roads could be closed making complex coordination of staged construction involving night work, traffic bypasses and detours necessary.”
The memo stated further “the need to stabilize the subsurface clay layer to obtain the required foundation bearing capacity resulted in development of an extensive QC/QA (quality control) program that utilized deep soil mixing, excavation, backfilling and compaction operations along with numerous innovative tests to verify performance of the foundations.”
Other work by McCrossan crews included the construction of 3 mi. (4.8 km) of CIP (cast in place) retaining walls and 5 mi. of new asphalt frontage roads and trails.
Some of the estimated final quantities on the MN TH-61 road and bridge construction include 2 million cu. yd. (1.6 million cu m) of borrow, 1.5 million cu. yd. (1.2 million cu m) of borrow, 60,000 cu. yd. (46,000 cu m) for bridge and wall concrete, 50,000 cu. yd. (38,000 cu m ) of concrete pavement and 60,000 linear ft. (18,300 m) of underground storm sewer.
Though the entire construction schedule has been delayed because of the design retrofit, Lunda crews and subcontractors will continue with some structural work on a retaining wall and bridge through the winter months.
This spring, while the design retrofit continues on the westbound bridge, workers will be back to begin the reconstruction of a good portion of eastbound I-494. CEG