Bike commuters and weekend riders understand the perils of navigating busy roads.
Although bike trails and lanes have been popping up in cities around the country, traveling from one area to another isn’t always easy. Sharing the road with cars, passing through large intersections, or riding under sketchy underpasses can make your trip unpleasant –even unsafe at times.
View a 47-second time-lapse video of this project here
In Dallas, multiple bike trails converged around U.S. Route 75 – a major north-south route in the metro area. To get across the highway, bikers and pedestrians had to take an inconvenient detour, including passing through a dark and muddy underpass. A bridge arching over U.S. 75 was the perfect solution to connect the eastern and western trails.
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But there were challenges to building and installing a bridge, including designing one that could attach to angled piers and maintaining traffic flow during construction. Working together, the Texas Department of Transportation and engineering firms HNTB, Ragle Inc., and Mammoet USA, along with members of the community, designed a standout bridge that was installed in a single day.
Improving recreation and access
U.S. 75 connects the Dallas central business district to North Dallas, one of the fastest-growing areas in the region. On an average day, 293,000 cars drive on the eight-lane highway.
While car commutes are popular in the region, there are also light rail stations running along the corridor as well as bike and pedestrian trails that meander through neighborhoods to these stations. The highway separates four well-traveled trails – the Northaven Trail on the west of U.S. 75, and the Southern Pacific, Cottonwood Creek, and White Rock Creek trails to the east. Before the bridge was built, bikers and pedestrians had to use a muddy underpass below U.S. 75 to travel between trail systems.
“I used the trail connection before when it went under the highway – it was quite a challenging path,” said Tony Hartzel, TxDOT Dallas District spokesman, adding that the underpass connection was not an ideal long-term solution.
Dallas City Council members, parks and recreation staff, and local residents agreed. In the early 2000s, they proposed a connecting bridge that would link the trail system and provide a signature bridge for the area. Considering the purpose of connecting bike-friendly trails, a council member suggested the bridge could resemble the spokes of a bike wheel. The idea was a hit and after many community meetings and feedback from stakeholders, the design process began in 2017.
Building a bridge
Creating a bridge with a bike-wheel aesthetic was the first of multiple challenges in the Northaven project. The geometry of the bridge site required creative solutions. For instance, the median between the eight traffic lanes was not wide enough to host a support column. In addition, the piers on both sides of the highway and frontage roads were offset and at an angle.
To accommodate these conditions, HNTB designed an S-curved, network tied arch bridge. For a tied arch bridge like this, Kira Larson, P.E., M.ASCE, a project manager at HNTB, said to imagine a bow and arrow. “Typically, if you think about a bow and arrow, that string – the deck – has to be straight.” But in this case, the deck has a gentle S-shaped curve that accommodates the skewed piers at either end of the bridge.
“As far as we know, this is the first S-curved, network tied arch bridge in the world, certainly in the United States,” she said.
The installation itself had some stringent requirements – namely, keep traffic moving. “We didn't want to shut down U.S. 75 for any large amount of time,” Hartzel said. “At all points during this moving process, we kept at least one lane open in each direction.” The team also was required to complete the installation in a day, from late Saturday to midday Sunday.
Because of the need for quick installation, the network tied arch bridge was a great choice, explained Larson. “It’s an extremely resilient bridge type,” she said. “Even though it’s beautiful, it’s utilizing all the members to their maximum efficiency and allows the members to be very thin but also very strong.”
But the design also meant the bridge needed to be assembled near its installation site. The team put the bridge together behind a furniture store adjacent to the highway. After the 201-foot bridge was assembled, Mammoet was tasked with picking up the bridge, swinging it across U.S. 75, and carefully placing it on the piers.
The cabled Northaven bridge design required a bit more finesse than other installation projects.
“What we would ideally do is drive underneath (the bridge) with our specially designed ‘Mega Jacks’ on trailers, pick it up, and then drive to the piers and set it down,” said Matthew Berlanga, a mechanical engineer who was the lead engineer representing Mammoet during the installation process. “That's a very, very quick way to do things.”
Over the weekend of Sept. 9-10, 2023, the Mammoet team instead jacked up the bridge and loaded it onto double eight-line self-propelled modular transporter trailers, driving it adjacent to the highway. The team then used a gantry jack and track system on the trailers to carefully swing the bridge across the highway and fine-tune the final placement – all while keeping at least one lane of traffic free. Gantries and a skid track were built around the bridge and linked to the lift beam that was added to the bridge for installation. From start to finish, moving the bridge into place took about 20 hours.
Today, the bridge is a prominent landmark along U.S. 75. “This is a real centerpiece for North Dallas,” Hartzel said, adding that the bridge paints a striking view for drivers coming in and out of the downtown area. “For us at TxDot, there’s also a source of pride in being able to get this project built and installed.”
