May 28, 2024

By Tony Hartzel

DALLAS – The new Northaven Trail Bridge has hit all the right notes since opening to pedestrians and cyclists last fall.

The elegant network tied arch bridge has struck the right chords with community leaders and bridge users, all of whom sing the praises of the unique pedestrian structure that spans U.S. 75/North Central Expressway.

But even a new bridge needs a little fine-tuning after installation.

Before it was moved into place in September, each of the bridge’s 64 cables were tightened to precise specifications.

After the move, and after installation of a thin pavement surface on the bridge deck, the cables were inspected again, and several cables needed slight adjustments.

So this month, crews “tuned” several of the bridge cables to bring the structure into full engineering harmony.

What does tuning a bridge actually involve? Turns out, it’s a little more complicated than tuning a piano, guitar or other musical instrument.

Specialized engineers with bridge design firm HNTB developed a matrix calculation table that crews used to adjust each cable to precise specifications. Quarter turn by quarter turn, each cable turnbuckle was twisted, tightened or tweaked, so that each cable functions as fully intended to support the 800,000-pound bridge.

Such precision is required to account for the interplay among the bridge’s chorus of cables -- tightening one cable can affect the tension and forces on one or more other cables.

“It’s a really complex optimization problem,” said Gregor Wollmann, an HNTB practice consultant and senior technical advisor for the Northaven Trail bridge. “We have to find the optimum combination on all 64 cables.”

When complete, the tuned bridge cables function together to support the bridge. And the benefit of the network tied arch bridge is that, like spokes on a bicycle wheel, the structure is stronger the more times each cable crosses one or more other cables.

“If you change one spoke or cable, you change the load on all the others,” said HNTB’s Chief Bridge Engineer Ted Zoli. “We’re adjusting the spokes in such a way to not only get the bridge deck in the right position, but also to make sure the cables are best able to share the loads.”

After extensive advance calculations and hours of adjustments, the Northaven Trail Bridge is now officially “tuned” and further adjustments are not anticipated. Construction of the bridge was a collaboration among TxDOT, the city of Dallas, Dallas County, the North Central Texas Council of Governments, designer HNTB and construction contractor Ragle, Inc.

While the final tuning effort can’t be heard, the end result will be seen and appreciated for decades as a gateway and architectural centerpiece for North Dallas.