FRP Composites Provide a Lighter Path for the Northampton Street Bridge

By Scott Reeve

The Northampton Street Bridge has connected Easton, Pennsylvania, and Phillipsburg, New Jersey, across the Delaware River since 1806. 

The steel, cantilever truss bridge replaced a wood bridge in 1895 and has been repaired or rehabilitated many times during its almost 150-year history to safely accommodate evolving forms of travel. It underwent significant repairs in 1924, 1951, 1955, and 2001  with minor repairs in intervening years. During its 2020 inspections, the bridge was listed in “fair condition.” It was time for an update.

Safe conduct for pedestrians was a core goal of the 2022-2023 bridge refresh project. But the sidewalk decking needed to fit the physical restrictions of this historical structure while meeting modern safety and performance standards. Composite decking, made from Fiber Reinforced Polymer (FRP), met the challenge.

Preserving History

Project owner Delaware River Joint Toll Bridge Commission (DRJTBC) shared that “the project’s core objective is to extend the 125-year-old bridge’s useful life.” The Northampton Street Bridge is the only three-lane vehicular bridge in their bridge system and is the most heavily used non-toll bridge. Each day in 2019, the bridge bore an average volume of 16,900 vehicles across its three-ton weight limit and 15-mile-per-hour speed limit.

History made this project a challenge; the bridge received its National Historic Civil Engineering Landmark designation by ASCE in 1995. Maintaining its structure was a core requirement of the project. 

But historic steel-truss bridges have weight restrictions that cannot meet the demands of modern vehicles or traffic. While the trusses provide great strength, the steel structures are also very heavy. This means that, compared to other bridge structures, steel truss bridges can accommodate fewer vehicles before reaching total weight capacity. And structures like the Northampton Street Bridge that have cantilevered sidewalks face further weight restrictions from the additional sidewalk weight.

The DRJTBC needed to reduce bridge weight wherever possible without compromising safety, longevity or the historical integrity of the bridge. Composites provided a lightweight solution.

On Firm Footing

The Northampton Street Bridge project called for wider sidewalk decking that worked with the planned steel structure without adding too much weight and that had built-in water control. Creative Composites Group (CCG) provided the project contractor, J.D. Eckman, Inc., with FRP composite panels to deck the sidewalks of the bridge.

The prefabricated sidewalk panels don’t just meet design requirements, including deflection and wind uplift loading. The panels also account for water control, and the CCG team included special details in the design like cover plates at the truss cut-outs, hatches and lamp post pedestals.

FRP was a strong solution for this historic bridge due to its very high strength-to-weight ratio and its flexible sizes. “Many of us were skeptical how the installation and fitment of the panels would go, but we were pleased with how it turned out,” said a representative of J.D. Eckman. “For the panel size, it was light and surprisingly rigid, which made for easy handling and placement. And the panels fit very well.” The strength-to-weight and weight-to-safety factors for FRP are superior to concrete or wood, which would have added too much weight to the total bridge weight before pedestrians or vehicles.

The FRP panels are so strong, in fact, that they can support more than the 90 PSF uniform pedestrian live load. “Another key thing that led us to CCG is definitely the strength-to-weight ratio,” reported a member of the project management team. “The sidewalks are cantilevered off the truss structure. This means we can’t access the facia of the bridge from the roadway; we have to do it from the sidewalks. We use some relatively small equipment that is slightly heavier than pedestrian loads, and if we went with a heavy concrete deck surface, the steel superstructure wouldn’t have supported the weight.”

Creative Composites Group brought two additional benefits to the project. Largely funded through the Build America, Buy America Act, the DRJTBC needed to source American-made materials for the project. CCG’s composite decking is made in the US, which complies with the BABAA requirement. A representative of CCG was also constantly available to the contractor team and, according to a representative of the company, “were great to work with and willing to discuss anything.” These composite panels, and the support behind them, are home-grown, just like the Northampton Street Bridge itself.

In the words of a representative of the contractor, “This project was not a typical rehabilitation. The bridge has lots of character, so installing the FRP panels was not on a level plane. The finished product turned out very good, and the project team is pleased.” Feedback shared with the DRJTBC shows the decking is doing its job. “The old product that we used held up for a while, but the traction on it had worn out and became very slippery. Parts were beginning to fail, and it felt flimsy,” said a member of the project team. “The new CCG composite decking feels very solid. The traction on the surface is outstanding, and so far, all the feedback has been really positive.”