The 48-year-old dual-bridge replacement near Bozeman, Mont., was originally scheduled to be completed in two phases.
Being able to complete a nearly $8 million, 18-month project in only eight months was no easy feat, but was made possible because of an accelerated labor plan.
The 48-year-old dual-bridge replacement near Bozeman, Mont., was originally scheduled to be completed in two phases. Phase 1 was set to take place from March through October or November of 2013 and Phase 2 was scheduled to take place from March of 2014 until late July 2014.
“We were able to accelerate the project so much that we could finish both phases in one season,” said Allen Frankl, project manager of Dick Anderson Construction of Bozeman, Mont. “In 2014 we did some sidewalk work, seeding and grading; things we couldn’t finish in the last season of 2013. We also had to put in some joint sealant and do some slope protection.
“The whole project was accelerated. We started on March 11, 2013, and by July 23, 2013, we opened the first half of the bridge,” Frankl said. “Crews worked six days a week and 16-hour days to accelerate the construction so we were able to finish the first phase and get the bridge open, which was the most critical part.”
The 7th Avenue dual-bridge along I-90 in West Bozeman was replaced with one single bridge that includes four driving lanes, two in each direction, two left-hand-turn lanes onto the I-90 westbound on-ramp, a raised median, 5-ft. (1.5 m) sidewalks and 10-ft. (3 m) shoulders on each side of the bridge to accommodate bicycles and pedestrians. It also is ADA accessible, in conjunction with the Bozeman Area Transportation Plan.
“This really was a great project and we’re proud of what the crews could accomplish. The State of Montana was a great partner on the project. They made some great decisions and went above and beyond while working with us,” Frankl said.
Lori Ryan, communications specialist of the Montana Department of Transportation, explained that each phase involved detouring traffic, removing the old structure, and placing half of the new structure. The original interchange consisted of two adjacent two-lane bridges. One bridge was demolished and two-way traffic was maintained on the other existing structure. Half of the new structure was constructed that was wide enough to handle three-lanes of traffic during Phase 2. Phase 1 also including adding a lane to the westbound ramp.
Phase 1 included the removal of the northbound structure, the east bridge, which crosses I-90, and handling two lanes of North 7th Avenue traffic on the existing southbound structure while half of the new bridge was being built.
Phase 2 included shifting traffic to the new half of the structure that handled three lanes of North 7th Avenue traffic while the existing southbound structure, the west bridge, was removed and replaced with the other half of the new structure, information from the Montana Department of Transportation said.
Bridge removal was accomplished by using a specialized sub-contractor who was able to demolish and drop the bridge onto the Interstate below with a cushion of sand for protection, Ryan added. The rubble and sand cushion was then removed in one operation. Using this method, the bridge was demolished overnight.
But finishing both phases in one season was no easy task.
“Most everything on the project was as was anticipated and the state did a good job of letting us know what to anticipate,” Frankl said. But one of the biggest things that no one anticipated was that when we placed the second half of the bridge deck in late November that we would have 30-below zero temperatures to deal with. We thought it would get down to zero but it reached 28-below for about 10 days.
“The cold temperatures made it difficult to keep the deck warm enough to allow proper concrete curing,” Frankl added. “We used three glycol loop-fed ground heaters and we covered the deck with the heater hoses and quadruple covered them. We also had monitors spread out over the deck and we had to move the monitors around to make sure there were no cold spots. We also had to clean off the bottom of the deck to keep air movement down so if a truck went under the deck it didn’t move warm air out and replace it with cold air.”
A wide array of excavators with 330 and 355 demo hammers and numerous forklifts and graders were used to excavate 25,821 cu. yd. (19,741 cu m) of excavation backfill. A 125-ton (113 t) Lorain crane and a 100-ton (90.7 t) Manitowoc crane could be seen on site along with an 830 diesel impact hammer for driving the 1,978 ft. (602.9 m) of 24-in. (61 cm) diameter steel pipe pile for the project, Frankl said. A big row 4800 paver was used on the deck.
The project also required cast-in-place reinforced concrete for the substructure, while the superstructure was constructed of 2,024 ft. (616.9 m) of prestressed concrete beams and cast-in-place deck that is reinforced with epoxy coated rebar. Materials needed in the project included 678,353 lbs. (307,695.7 kg) of reinforcing steel, which included black, epoxy and seismic, 2,8803 cu. yd. (22,021 cm) of concrete and 6,891 tons (6,251 t) of asphalt, Ryan said.
The new bridge was built with an integral pier table, which is a slim, horizontal cast-in-place concrete section supported on the intermediate pier that the girders frame into at the middle of the bridge, Ryan said. It allows for longer spans and eliminates the usual blocky-looking section supporting the spans that is used in many bridges. It maintains vertical clearance for future interstate widening.
Choosing a single-lane bridge vs. the previous two-lane bridge “allows for turn lane storage on the bridge against the median and provided the necessary width for the traffic, bicycle and pedestrian lanes within a ’footprint’ that did not require additional right-of-way,” Ryan said.
The project was intended to address seismic vulnerabilities, structural deficiencies, functional obsolescence and traffic safety.
The previous bridge had been outgrown and Bozeman needed more traffic capacity, ADA accessibility and bicycle lane capacity, Frankl added.
“The old bridges were seismically substandard and the deck was in need of rehab work.”
Retrofitting was considered at this bridge location, including widening, but replacement was chosen over retrofit based on a benefit-cost analysis, Ryan said. The geotechnical and structural analyses included a peak ground acceleration for a design event with a 7 percent probability of excellence in 75 years at this location with its Class C soil.
Some of the subcontractors on the project include: Knife River Corporation; Poteet Construction Inc.; H & H Earthworks Inc.; Grizzly Steel Inc.; High Mark Construction; Penhall Company; Montana Lines Inc.; Becker Landscaping; and Olson Communications.