In north Dallas, the Northaven Trail Bridge has provided a long-awaited key connection for multiple regional bicycle/pedestrian trails. Before the bridge opened in November 2023, the popular and heavily used Northaven Trail on the west side of U.S. 75 and the Cottonwood Creek and White Rock Creek trails on the east side essentially terminated at the roadway. To cross and access the trails on the other side of the highway, one of the busiest in Texas, cyclists and pedestrians had to go out of their way to reach a dark and muddy underpass.

The stunning Northaven Trail Bridge spans the highway, providing a safe crossing for pedestrians and cyclists and a link to the DART Light Rail Station at Forest Lane. The Texas Department of Transportation (TxDOT) called the bridge “a regional example of the positive benefits of appropriate location and design aesthetics for future bicycle and pedestrian trails and amenities.”

Capturing a community’s vision

The idea for a pedestrian bridge over the highway was proposed in the early 2000s by a former City of Dallas councilmember, in whose district the span would reside. The Dallas Park and Recreation Department, the Friends of Northaven Trail group and others joined the councilmember in championing the project.

Working with TxDOT, HNTB served as engineer of record and construction phase services provider on the project. The firm participated in public outreach to stakeholders, including trail users and communities and organizations located near the trails, to gather input on their needs and wants.

“Our early design workshops and public engagement captured the needs and desires of TxDOT, city leaders and communities on both sides of U.S. 75,” said James Frye, HNTB project director during conceptual design. “That began to inform everything from trail geometry and bridge design to construction methods and how it all eventually came together.”

The councilmember liked the idea of representing a bicycle wheel in the bridge’s design. Based on all stakeholders’ feedback, urban design and bridge design team members from multiple HNTB offices developed the original bridge concept in 2017, designing a network tied arch main span that uses cables, reminiscent of the spokes on a bike’s wheel. The network cable arrangement, a term used when the cables cross one another more than once, also greatly reduces bending in the arch and deck, allowing both to be very thin while still being resilient to accidental overloads.

“It’s surprising how much the final design adhered to some of the early sketches and modeling concepts developed from conversations with the City of Dallas and other stakeholders,” said Tom Kramer, HNTB project manager during the plans, specifications and estimates portion of the project. “Often, an early concept and what is ultimately built are quite different. In this case, we developed a unique, initial concept and saw it all the way through.”

The only known bridge of its kind in the world

The site designated for the Northaven Trail Bridge had a unique geometry that required the bridge to be built at an angle across the eight mainlanes and four frontage road lanes of U.S. 75. Engineer of record and HNTB Chief Bridge Engineer Ted Zoli developed the concept to create a reverse tied arch structure that gives the bridge a soft, reverse “S” curve and reduces the skew at the piers. As characteristic of tied arches, the bridge deck ties the ends of the arch ribs together, just like the string in an archery bow.

“With tied arches, that “string,” the deck, normally must be straight,” said Kira Larson, HNTB project manager of construction phase services. “However, in the Northaven Trail Bridge, we see an S-curve geometry. Within that S-curve are straight post-tensioning strands running through the deck, acting as the tie. It’s a highly innovative way our design team created this signature project for TxDOT.”

The structure is the only known network-tied arch bridge in the world with a doubly curved tie. The bridge also has skewed ends, making it an incredibly complex structure not seen anywhere else.

Minimizing traffic impacts during construction

TxDOT’s two critical priorities during construction were the structure’s integrity and public safety. A key project goal was to deliver the Northaven Trail Bridge with minimal impact to the traveling public. The agency required that a frontage road lane or a lane both northbound and southbound on U.S. 75 remain open throughout construction and that any mainlane closure period be limited to Saturday overnights.

Those requirements drove early design choices to employ accelerated bridge construction techniques, which would allow the bridge to be built off-site and moved into place.

HNTB ensured that the structure selected was a lightweight, redundant system that could be safely transported to its final location on self-propelled modular transporters (SPMTs). The bridge was constructed on timber shoring in the back parking lot of a nearby furniture store.

As construction phase services provider, HNTB reviewed shop drawings, responded to challenges and requests for information and coordinated work with TxDOT and the contractor. HNTB also oversaw adaptations required in the field.

“HNTB provided accurate design for the project in order for us to safely construct it with minimal change orders or plan revisions,” said TxDOT Dallas County Area Engineer Nathan Petter. “Because this was such a unique bridge, challenges came up that we had to work through during construction. HNTB was a great partner in helping us find solutions. They were there with us every step of the way.”

"The Northaven Trail Bridge is a highly complex structure with inherent erection challenges, and HNTB, TxDOT and the contractor needed to proactively communicate throughout construction to keep the project on schedule," Larson said. "When changes needed to be made at the construction site, the team would quickly collaborate to find solutions."

When COVID-related supply chain issues delayed material deliveries, HNTB provided recommendations that reduced fabrication lead times. In the case of cable adjustments, the firm proposed purchasing turnbuckles that allow cable tension to be adjusted in just a few turns without the cable being disconnected every time a tension change is needed.