The bridge that carries Pullen Road over westbound Western Boulevard near N.C. State University’s main campus was built in 1952 mostly of concrete and steel.

But hidden under the concrete foundations at either end of the bridge are piles made of a material people don’t normally associate with highway bridges: wood. And this bridge is far from unique in North Carolina.

The Pullen Road span is one of 2,777 bridges maintained by the N.C. Department of Transportation that are considered “timber bridges,” meaning they have wood somewhere in their structure, often out of sight of motorists.

The use of wood is generally considered outdated in road bridge construction, and a large majority of the highway bridges that contain wood in North Carolina are decades old. NCDOT’s efforts to improve the overall condition of bridges in the state include replacing many of these older timber bridges with ones made exclusively of concrete and steel.

About a quarter of all timber bridges in the state are rated in “poor condition,” compared to 8.4% of bridges overall. While timber bridges can be found all across the state, more than two-thirds of them are in 25 mountain counties, where they mostly carry secondary roads over creeks and streams.

It’s no coincidence that those mountain counties also have the highest number of bridges in poor condition, accounting for more than half of the state’s total.

“If we’re trying to address the condition of our bridges and improve it, it looks like a big chunk in the western part of the state are these timber bridges,” said Brian Hanks, NCDOT’s state structures engineer.

Bridges are rated on a scale developed by the federal government as either being in poor, fair or good condition. Inspectors look at the three components that make up a bridge — the foundation, the support beams and the deck or riding surface — and assess their condition to help determine when a bridge needs repairs or replacing.

While NCDOT wants to minimize their numbers, bridges rated in poor condition are not in danger of collapse, Hanks says.

“They’re still safe,” he said. “If it’s not safe, we close it.”

Condition of state bridges gradually improving

A decade ago, 16.4% of bridges owned and maintained by NCDOT were considered in poor condition. That number has improved to 8.4%. About 80% of those poor condition bridges are on less-traveled secondary roads.

And more than half of those poor condition bridges contain some wood, a sign of their age but also that wood is not as durable as concrete or steel.

About 20% of NCDOT’s 13,701 road bridges are classified as timber. That number represents bridges 20 feet or longer and doesn’t include another 5,000 or so culverts that the state maintains.

Wood was once commonly used in bridge construction for good reasons, says Gregory Lucier, a research professor who manages the Constructed Facilities Lab at N.C. State University’s Department of Civil, Construction and Environmental Engineering.

“Wood is fairly cheap, is highly available in this part of the world, is easy to work with, and is light to handle,” Lucier wrote in an email.

It’s especially good for building short bridges that handle small amounts of traffic, Lucier said. A concrete box culvert might be stronger and more durable, but building one on a remote mountain road can be difficult and probably unnecessary if the bridge handles only a few cars a day.

Which is why, Hanks said, when NCDOT looks to replace aging timber bridges on lightly traveled, dead-end roads in the mountains it will likely build at least parts of them out of wood.

”It’s a useful material in the right location,” he said.

Timber bridges found across North Carolina

Outside of the mountains, the most common use of wood in a highway bridge is in the piles that were driven into the ground as part of the foundation. Timber piles can be found in all sorts of bridges; the twin spans that carry Interstate 95 over U.S. 421 in Dunn were built on wooden piles in the early 1950s, Hanks said.

In Durham, Orange and Wake counties, 44 NCDOT bridges are classified as timber bridges. Most of them are at least 50 years old; the oldest, which carries N.C. 42 over Middle Creek in southern Wake County, was built in 1947, though it’s listed in fair condition by bridge inspectors.

All but a handful carry secondary roads over creeks and streams.

When those bridges are replaced, contractors won’t use wood, Hanks said. That goes for the I-95 bridge in Dunn, which is being replaced now, and for the Pullen Road bridge over Western Boulevard.

That bridge was rated in fair condition during its last inspection, and is due for another inspection in June. Because of its age and condition, NCDOT has begun preliminary engineering on a replacement, Hanks said, with construction tentatively scheduled to begin in February 2029.

NC Reality Check is an N&O series holding those in power accountable and shining a light on public issues that affect the Triangle or North Carolina. Have a suggestion for a future story? Email realitycheck@newsobserver.com