Black locust (Robinia pseudoacacia) is among the strongest and most rot-resistant wood species in North America, yet it is rarely used in major construction projects because it doesn’t always grow straight and can warp.
One national engineering firm decided to take a chance on black locust, however, and through subcontractor E2PM Project Management LLC, recently hired the University of Maine’s Advanced Structures and Composites Center to conduct critical testing for strength and stiffness of black locust beams to be used in a project being constructed in the shadow of New York City’s legendary Brooklyn Bridge.
The center tested three types of beams that will be used in a new 400-foot overhead footbridge that will take pedestrians from the new Brooklyn Bridge Park under the Brooklyn Bridge to Squibb Park in the Brooklyn Heights neighborhood of the borough.
“It’s an interesting project and a little unusual for us, but it’s exactly the type of thing that we like to do,” says Russell Edgar, who directed the testing as the center’s wood composites manager with assistance from Jon Hill, the center’s laboratory engineering specialist, and UMaine wood science major Howard Friant. “Black locust has all of these unusual properties, so it was an exciting project for us to be a part of. As a wood guy, it was exciting to work with something a little different.”
The testing took place from March to June.
Black locust is considered an underutilized species, yet it can last much longer than others. Edgar says depending on the application, a structure made of black locust might have a life span of up to 70 years because of its rot-resistant properties, compared to structures from other wood species more commonly used in construction, which might have a life span of 40 years.
Ted Zoli, the MacArthur Fellow-winning engineer who designed the footbridge for engineering firm HNTB, said in a recent Popular Mechanics magazine article that the species’ propensity to warp slightly will be a “desired effect” for the bridge.
Bridge members are being installed in green condition, without seasoning or treatment.
“Black locust was used historically a lot for fence posts because of its rot resistance and because it doesn’t grow particularly straight as a large tree (e.g. possesses undesirable cross grain when cut into lumber), and that’s historically been one of the problems,” says Edgar, who earned his master’s degree in wood science from UMaine. “(HNTB is) turning a perceived negative into a positive, saying they know the bridge members are going to warp slightly as they are exposed to the natural elements.”
The black locust tested at UMaine came from the New York-Pennsylvania border area. Black locust is primarily found in the Southeast, mid-Atlantic and Northeast, including Maine.
Because black locust hasn’t often been used in bridge applications, the engineers wanted to do testing to confirm the design values that had been published in the U.S. Department of Agriculture’s Forest Products Laboratory Wood Handbook. The handbook lists values of strength, stiffness and other characteristics of hundreds of wood species.
“The testing in the handbook was done decades ago on small, clear, straight-grained, often first-generation pieces of wood, so they have relatively high values compared to what you would get if you went out into the forest and cut down a tree today,” Edgar says. “For a bridge of this size and importance in this location, the engineers wanted to make sure they did their due diligence and have a good handle on the proper flexural and compressive strength and stiffness design values of the actual resource that they will be using.”
The Advanced Structures and Composites Center tested four members being used in the bridge, including 6-inch and 10-inch diameter round pieces used for the truss, 4-inch by 10-inch crossbeams, and 2-inch by 8-inch stringers.
The testing occurred in two phases. In the first phase, Edgar, Hill and Friant conducted destructive testing, taking the wood pieces to failure to determine strength and stiffness. The 6-inch diameter pieces were tested under a 300,000-pound hydraulic actuator, with the average maximum load in compression parallel to grain testing coming in at 148,000 pounds of force, and an average ultimate compressive stress of more than 4,000 pounds per square inch (psi).
In the second phase, they took approximately 5 percent of the actual pieces that are going into the bridge and tested them nondestructively, meaning the beams were brought up to proof loads that the engineers designed as minimum strength values to ensure the pieces could sustain those loads.
The center also tested several small clear pieces of black locust from this resource to determine bending and compressive strength and stiffness, comparing the current values with those published decades ago in the wood handbook decades ago. Edgar hopes to publish the results of this testing program.
“The testing has derived design values that the engineers feel comfortable are sufficient for this bridge,” he says. “This is going to be a high-profile demonstration of the structural use of this interesting species.”
Contact: Russell Edgar, (207) 581-2101; Jessica Bloch, (207) 581-3777 or firstname.lastname@example.org