By Laurie A. Shuster
Researchers in diverse fields at Columbia University are shooting neutrons into bridge cables to reliably characterize their behavior when small wires inevitably break.
Researchers at Columbia University are using neutron diffraction to determine how and when small wires that break deep inside bridge cables regain their load-bearing capacity. Carleton Laboratory, Columbia University
April 21, 2015—While civil engineers often concern themselves with such large-scale structures as skyscrapers, dams, and bridges, researchers in applied physics examine the other end of the physical scale, studying the behavior of some of the smallest particles detectible by humans. It may seem unlikely that these two fields would collide, but that is exactly what has happened at Columbia University, where a diverse team of researchers is using microscopic neutrons to investigate the behavior of broken wires inside suspension bridge cables. The results of the researchers' collaboration may be good news for the owners and operators of some of the nation's largest and oldest suspension bridges.
Raimondo Betti, Ph.D., M.ASCE, a professor of civil engineering and engineering mechanics at Columbia, and I. Cevdet Noyan, Ph.D., the chair of the university's Applied Physics and Applied Mathematics Department and a professor of materials science and engineering and of earth and environmental engineering, have long been friends. They began discussing a question over coffee one day: could they use the neutron diffraction machines at the Los Alamos and Oak Ridge national laboratories—which use subatomic particles, much like an X-ray machine, to illuminate materials under extreme conditions—to investigate the behavior of the small wires buried deep inside the main cables of suspension bridges?
The answer turns out to be yes, and with surprising results. The researchers are learning that when individual wires break, they may resume their load-carrying capacity at much shorter distances from the breaking point than once thought—which means older suspension bridges probably have a lot more life left in them than was once assumed.
The main cables from which the suspenders of suspension bridges hang are made of thousands of parallel, high-strength, steel wires, most with a diameter of about 5 mm, grouped into bundles and compacted together, Betti explains. Engineers have long known that over time, individual wires sometimes break, losing their ability to carry loads at that breaking point. But they have also observed that eventually, at some point along their length, those small wires regain their load-carrying capacity. Researchers aren't entirely sure why, Noyan says, but it probably relates to the friction, curvature, and twisting of the wires, and the compaction of the cable. "These broken wires, in the immediate vicinity of the break, don't count toward the strength of the cable, but at a certain distance from the break, they do," Betti explains. "So then the question is, when do we think the wire will regain strength? How do the broken wire and the other wires interact so that the broken wire can regain its load-carrying capacity?"
Betti says that the assumption has long been that that resumption of load-carrying capacity did not happen for perhaps the width of two suspender bays, or as much as tens of meters from the break. "The question we are asking is, is this true?" Betti says. If that capacity remains unreliable over long distances, then the overall remaining strength of the cable may be in question, especially on older bridges that may have been subject to many small and undetectable wire breaks over the years. "In the bridge engineering community, they are working on assumptions that are not validated by research," Betti explains. "With this work we are doing, at least we can provide a more scientific approach to understanding the problem of where the broken wires regain their load-carrying capacity."
Put another way, Noyan says, "The end result will be predictive models with realistic boundary conditions. There will be no assumptions; everything will have been tested."
Using neutron diffraction, the researchers have been able to investigate seven-wire strands made of galvanized, high-strength steel—strands that match those used in real-world suspension bridges. "It allowed us to investigate how the strains are transferred to the broken wire from the surrounding wires," Noyan says. "We could put the cable under a load and measure each wire, then increase the load and measure each wire again, and then do it again," Noyan says. "And when one breaks, you can see the load redistribution."
The broken wires investigated in this way regained their load-bearing capacities at much shorter distances—some as short as just a few meters—than anyone had previously thought possible. "We are quite excited about this," Noyan says.
The samples that the researchers tested were loaded in tension and in a combination of tension and torsion on custom-built machinery that enabled them to calibrate the loads and then use the neutron diffraction technology to observe what was happening to the wires. Neutron diffraction—which enables researchers to test material under such extreme conditions as stress, heat, or exposure to magnetic fields—has been used in the past to test jet engines, turbine rings, fertilizer tanks, railroad wheels, and even spent nuclear fuel rods, Noyan says. But this is the first time that the equipment has been used "with real, engineering-level samples" from civil structures, he says.
As a method of nondestructive testing, neutron diffraction has a significant advantage over other methods, Noyan says. "What neutron diffraction can do that other methods can't is measure buried wires, wires deep inside the core, without [us] putting in strain gauges or adding friction or changing the boundaries in any way," he says. "We are testing things without interfering with them."
Other investigative methods, including putting strain gauges on cables before they are placed in service or adding sensors to existing bridges, alter the cables in some way. With this method, Betti says, "I can measure the strains inside the specimens without touching them."
Using the results of their research, which is being funded by the National Science Foundation, Betti and Noyan hope to able to tell engineers exactly what to expect from suspension bridge cables as the years tick by. "The end point is to be able to tell the engineering community, once we are finished with our study, that if they find a wire broken inside a cable, they can account for the [the load-carrying capacity] just a few inches or meters away," Betti says.
"We are happy about these findings," Noyan says. "It is a small step toward making our bridges safer."