By Kayla Hanson, P.E. and Mason Nichols
For thousands of years, bridges have been used to span obstacles and link people with places. Thanks to advancements in construction technology, modern bridges are as aesthetically pleasing as they are functional, evidenced by iconic structures across the world. Still, whether it’s the Brooklyn Bridge in New York or the Great Belt Bridge in Denmark, all bridges are designed to create connections, bringing people together that would otherwise be separated by mountains, rivers or valleys.
Spanning the calm waters of the St. Croix River in Minnesota, the Stillwater Lift Bridge is no different. The bridge links Minnesota and Wisconsin and has enabled millions of travelers to navigate between the two states since its construction in 1931. But it provides only one lane in each direction, creating havoc for drivers of the 18,000 vehicles that traverse it each day. The Minnesota Department of Transportation partnered with Lunda/Ames JV to help reduce traffic congestion with the new St. Croix River Bridge. Anchored by the use of nearly 1,000 massive precast concrete segments, the bridge will provide a more efficient experience for all travelers when completed in fall 2016.
An extra dose of precast
Construction of the St. Croix River Bridge is part of a large-scale, $600 million project that includes extensive roadway work in Minnesota and Wisconsin, the creation of a trail system and conversion of the old Stillwater Lift Bridge to a pedestrian and bike path.
According to Michael Beer, project director with MnDOT, the new bridge will have two lanes in each direction to handle projected growth of up to 48,000 vehicles per day by 2036. Additionally, thanks to the use of the precast concrete segments, the bridge will boast a 100-year lifespan.
“One of the benefits of precasting is constructing the pieces in a more controlled environment,” said Paul Kivisto, bridge construction engineer for MnDOT. “Using the precast concrete will help enable the bridge’s extended lifespan and also enable us to put segments up at a faster rate than cast-in-place.”
The bridge will be more than 5,000 feet in length when it is completed, making it one of the longest bridges in the state. Kivisto said it features an extradosed design, a style that limits tower heights and results in a smaller angle of inclination for the stay cables. The extradosed design will be the first for Minnesota and one of the first in the U.S.
Recipe for success
Each of the approach pieces for the St. Croix River Bridge is manufactured with a 0.32 water-cementitious materials ratio using one of two mix designs – either 6,000 psi or 8,000 psi. Cementitious materials for both mixes consist of 70% Type I/II cement, 20% ground granulated blast-furnace slag and 10% fly ash.
The main span segments, which are about twice the size of the approach pieces, are also produced using two mix designs – either 8,000 psi or 9,000 psi – both with a 0.30 w/cm ratio. The cementitious materials used in the 8,000-psi mix consist of 70% Type I/II cement and 30% fly ash. In the 9,000-psi mix, the cementitious materials are comprised of 65% Type I/II cement, 20% fly ash and 15% ground granulated blast-furnace slag.
The mixes contain varying quantities of air-entraining admixtures and water reducers. Three types of reinforcing steel are also present in the bridge components: traditional reinforcing steel is used in the footings, epoxy-coated rebar is used in the above-ground components up to the top layer of the deck and the top surface of the deck uses stainless steel.
Putting it all together
Tackling a project of this magnitude comes with a number of challenges, ranging from casting techniques for manufacturing such large segments to the logistical issues associated with connecting them together. With dimensions measuring up to 18 feet high, 10 feet long and 48 feet wide, each of the 656 main span segments requires a single continuous pour of 90 cubic yards that can take up to 4 hours. Though casting the segments in place was possible, doing so would have significantly delayed the project timeline.
“The contractor had the ability to design cast-in-place on all of the bridge if they had wanted, but our plans were prepared with precast for a time component,” Kivisto said. “Precast has benefits – you can produce the segments while you’re constructing the substructures and then more quickly finish the superstructure during erection.”
Massive pieces such as these require significant care and attention not only in casting and curing, but also in transportation and placement. The smaller segments, which each consist of 45 cubic yards of concrete, are used in the bridge approach on land and manufactured at an on-site casting yard next to the construction site. The main span segments are produced 30 miles away from the construction site and travel by barge via the Mississippi and St. Croix Rivers to their final location.
“A single barge will be able to take 4 to 6 segments, but they’ll be shipping together with a couple of different barges,” Kivisto said. “So it could be 10 to 12 in a shipment.”
When the segments arrive on site, a crane positioned on the top deck of the bridge lifts them into position. Construction crews apply epoxy to the male face of one segment and the female face of the match-cast segment. After the epoxy is applied, the two shear key pieces are positioned together and post-tensioned.
Much like the match-cast precast segments, residents in both Minnesota and Wisconsin will benefit from the connection that will be made once the St. Croix River Bridge is completed in 2016. While much work remains, Beer said he believes both states will experience opportunities for growth made possible by the new structure.
“It’s really exciting to see everything that’s taken place and how quickly everything has gone up,” Beer said. “There’s a lot of interest and excitement about the project among the folks that live and work in the area.”
Construction will intensify as the summer progresses, with crews placing precast segments on three of the bridge’s piers at the same time. The magnitude of the work should serve to further engage residents on both sides of the river.
“There will just be a buzz of activity going on out there for anyone that comes to look at the new bridge,” Beer said. “It’s nice to see that high level of excitement from not only the construction workers and owners’ personnel, but the public as well.”
Kayla Hanson, P.E. is a technical services engineer with NPCA.
Mason Nichols is the managing editor of Precast Solutions magazine and is NPCA’s external communication and marketing manager.