To replace the deteriorating Sixth Street viaduct, Milwaukee wanted a bridge that would be a signature for the city as well as a stimulus for the development of the Menomonee River Valley and a gateway to the thriving community on the city’s near south side. They also wanted to get the project designed and built in 24 months with only a 15-month traffic disruption.
After considering several options, the planners decided on a pair of cable-stayed bridges to give the look they were aiming for and to deliver traffic to a pair of at-grade double-leaf bascule bridges in the valley. As the method for delivering the bridge complex on schedule, they chose design-build. Sixth Street is the first publicly funded road construction project in Wisconsin to use design-build.
The city of Milwaukee was the major proponent of design-build for the Sixth Street viaduct: "The design itself is unique. The cost is large. And the timetable upon which we had to construct it is very tight," Mariano Schifalacqua, Milwaukee’s commissioner of public works, told ROADS & BRIDGES. "So we felt that this was one of those projects that really lent itself to design-build."
"If this was standard DOT, this would be a two-year construction project, which is about what we have to do it," said Scott Piefer, P.E., vice president of Zenith Tech Inc., Waukesha, Wis. "So from a construction standpoint, I’m not sure we’re actually delivering the project any faster, but when you add in real estate, utility relocation and the design, and you’re doing all of that in two years, conservatively we’re cutting out a year and I wouldn’t doubt at all that we’re cutting out closer to two years from project conception to ribbon cutting."
HNTB Corp. developed the design for the bridges out of its Milwaukee office, but some of the work also was done at its offices in New York and New Jersey. Zenith Tech is constructing the bascule bridges. Lunda Construction Co., Black River Falls, Wis., is constructing the cable-stayed bridges.
The owners group includes the Wisconsin DOT, the city of Milwaukee and Milwaukee County.
The first hurdle to constructing a bridge by design-build is that the state of Wisconsin does not permit design-build to be used on publicly funded road and bridge construction projects.
The officials of Milwaukee and Milwaukee County discussed the plan and then approached the state to request special permission from the legislature to employ the design-build strategy in constructing the Sixth Street viaduct.
The state had some experience with design-build. Milwaukee had previously built a water treatment plant using the method, so the officials could present a concrete success to the legislature when asking for permission to use the strategy on another major project.
The process has worked well so far. The owners group gave HNTB a design that was 30% complete, with certain elements that were fixed, and the rest was up to HNTB. The flexibility has meant that the contractors and designers have been able to adjust mid-stream to changing conditions and unexpected developments.
For instance, several piles failed during driving because the load was too great for the pile to withstand.
"My guess is this will be the highest-capacity piling ever used in the state," said Ted Zoli III, P.E., associate vice president and director of long span bridges at HNTB. "It’s appropriate, but it’s also an attempt to be as efficient as we can be. That extends to a number of elements of the structure in terms of what we’ve changed and how we’ve designed the structure. We really have taken it as part of the challenge of design-build to develop something that’s not only very efficient but also be able to make adjustments in the field in terms of making things more constructible."
When a pile failed, the contractor’s person in the field was able to call HNTB, talk directly to the designer and quickly get a solution to the problem.
"In a conventional job, had you run across that, you’re probably down for a week at least," said Zenith Tech’s Piefer. "Here we were down for less than a day. We kept driving adjacent piles."
The piles are friction piles, driven closed-ended and then filled with concrete. Most of them were driven down 100-120 ft. They have outside diameters of 123Ú4 or 103Ú4 in. On the cable-stayed bridge pylons, the piles have capacities of more than 200,000 tons working load.
When a pile failed, it was assigned a lower capacity. There was enough flexibility in the design to allow the contractor to compensate by driving more piling near a failed pile or to employ some other remedy, unlike on a conventional job.
"We’ve had great interaction with field people on a continual basis," added Zoli. "Those lines of communication are specific from designer to either foreman or even the guys beneath the foreman in some cases."
The builders also have been flexible on originally fixed design elements. The towers for the cable-stayed bridges started out with a hollow center to allow inspection of the cable anchor heads. There are two towers for each of the two cable-stayed bridges. After HNTB received the design contract, the company approached the owners and suggested making the towers solid.
"Making the tower sections solid removes some form work and also allows us to proceed a little faster and build a little more strength into the tower," explained Zoli. "Because this bridge is quite low, it offers the possibility of inspecting the anchor heads from an inspection vehicle, from a man lift."
There are doors to the anchor area that provide access from the outside for inspection.
The constructors had more ideas for the design: "As part of our proposal, we recommended an additional strand in every cable that in the future could be taken out and inspected," said Tom Braun, vice president of the Wisconsin Bridge Division of Lunda, "so we can see what’s going on in the cable."
The "reference" strands can be removed at 5, 10, 25 and 50 years, for instance, to see if the strands are corroding.
To accommodate an inspection vehicle, the sidewalks on the cable-stayed and bascule bridges are wide, strong structural members of the bridge. The decks of the bascule bridges are steel grate, but the sidewalks are lightweight, high-performance concrete (HPC) cantilevered onto the sides. In fact, the bascule sidewalks are the first use of glass fiber-reinforced polymer concrete on the deck of a moveable bridge.
Construction began on Oct. 27, 2000. At the beginning of October 2001, the design-build organization had three of the four cable-stay towers completed, and the fourth was 75% done. The north cable-stayed structure was scheduled to be poured by the end of the year. The north bascule bridge foundations were in and ready for setting structural steel in October. The south bascule piers were being installed. The contractors planned to pour the approach sections over winter from the valley to the north and south bascule bridges. Spring of next year should see them pouring the deck of the south cable-stayed bridge and finishing off the project at a total cost of $52 million and a design life of 75 years. The new Sixth Street viaduct is schedule to be opened to traffic next fall after a traffic closure of only 15 months.
You can get there from here
Community involvement has been an important part of the process from the beginning. In fact, a roundabout at the south end of the Sixth Street viaduct was prompted by a request from the mayor and the community at the south end of the viaduct to change the confusing intersection that was there before.
The Sixth Street viaduct connects downtown Milwaukee with the ethnically Hispanic near south side across the Menomonee River Valley. The viaduct is in effect the gateway to this thriving residential and business community.
The owners group came up with the idea of a roundabout. It will be the first two-lane roundabout in Wisconsin.
The great majority of road and bridge projects in Wisconsin will continue to be constructed using the conventional process. But the state’s experience in the world of design-build has been positive, and the Sixth Street viaduct design-build organization said that future major projects would probably be able to get permission to use the strategy again.