Chris Higgins, Oregon State engineer (Photo: Frank Miller)

Growing greenery on roofs brings many benefits. Buildings stay cooler, saving energy. Roofs last longer, saving money and materials. Birds and insects find new habitat, helping ecosystems. And green roofs make urban spaces more aesthetically and spiritually pleasing, as well as reducing heat-island effects for city dwellers.

But there are some costs that need to be considered, too. “Eco-roofs carry higher gravity loads and must support more moisture for longer periods than traditional roofs,” says Oregon State structural engineer Chris Higgins. “That changes the probabilities that need to be considered during design. In order to extract all the benefits of eco-roofs, we need to ensure their structural safety. That requires research.”

Photo courtesy of City of Portland

One big question: Are green roofs safe during earthquakes? Led by Higgins, engineers in the School of Civil and Construction Engineering at Oregon State are undertaking the first comprehensive study of the seismic performance of eco-roofs with funding from the National Science Foundation. Using a full-scale simulated eco-roof, they will investigate drainage characteristics, load distribution of water-saturated soils, long-term service performance and the behavior of different planting materials during lateral shaking. Their findings will guide the development of standards for eco-roofs in seismic zones.

The Astoria-Megler Bridge carries U.S. Hwy. 101 across the mouth of the Columbia River from Astoria, Oregon, to Point Ellice, Washington.

Whether you venture onto a few wooden planks over a trout stream, a steel colossus over a swift river or a concrete viaduct carrying bumper–to–bumper commuters, you trust the beams and girders to hold you up. This act of faith, made daily by millions of motorists on U.S. highways, was shaken last summer when a steel truss bridge in Minneapolis plunged into the Mississippi River during rush hour. As media coverage raged and pundits called for reform, Oregon Governor Ted Kulongoski ordered an immediate inspection of 34 similar bridges across the state.

Meanwhile, just 30 miles south of the statehouse, some of the world’s most advanced studies in bridge science were in full tilt. At OSU, researchers in multiple disciplines (civil and structural engineering, ocean and coastal engineering, computer modeling) are investigating destructive forces and possible countermeasures. Human impacts — the loads exerted by cars and trucks, as well as the occasional collision by a boat or barge — comprise just one set of challenges. Equally critical to bridge safety are the myriad processes of nature. Most are routine: currents, tides, wind, erosion, salt air, sub–zero winters, simmering summers. Others are rare but often devastating: floods, hurricanes, earthquakes, tsunamis.

For guidance on bridge evaluation, repair and replacement, as well as design for worst–case scenarios, state and federal transportation officials have turned to OSU.

High off the ground, a guy in a hardhat sits at the controls of a 35–ton yellow crane. As though born to the task, he pushes and pulls the levers, maneuvering the 100–foot hydraulic boom into position over a 40,000–pound concrete beam. Workers grab the bulky hook dangling from the boom and attach it to the massive slab. They give the thumbs–up. With a deafening roar that makes earplugs standard equipment here, the crane hefts the load and swings it into position. Construction site? No, engineering lab.

At OSU’s Structural Engineering Research Laboratory, experimental precision depends on tools that pound, lift, shake and cut: diesel–powered machines, hydraulic rams, welding torches, rebar benders and shears (trade name, Rodchompers). The guy at the crane’s controls, an engineering professor studying the physics of bridges, reveals that his supply lists (which recently included Arctic parkas for his crew of graduate students) have raised a few eyebrows with Research Office accountants.

“I may be the only structural engineering professor in the U.S. who’s a certified hydraulic crane operator,” says a grinning Christopher Higgins as he climbs down from the cab.

Pointing toward the concrete girder, now encased in a cage of steel columns and rods resembling a giant Erector Set, Higgins projects an almost paternal air. “This guy,” he says proudly, “is our Goliath.” An intermediate bridge support called a “bent cap,” the kind that sits mid–river to support bridges with long spans, carries the integrity of the whole structure. As Higgins explains, “If it fails, you can lose the whole bridge.”

“We’re doing some things that no lab in the world has ever done before. We built a moving load simulator that can actually roll, acting like a truck traveling across full-size girders.”
Chris Higgins

As part of Higgins’ comprehensive research program on concrete bridge components funded by the Oregon Department of Transportation (ODOT) and the Federal Highway Administration (FHA), Goliath will undergo a series of strength and rehabilitation experiments inside the steel cage, the structural–engineering equivalent of a test tube. The futures of the 155,000 U.S. bridges rated “structurally deficient” or “functionally obsolete” by the FHA could depend on the findings.

Size Matters

To reduce risks of catastrophic collapses on our highways, OSU researchers have taken bridge experiments to a whole new level: life–size. Historically, most studies have been done on miniature replicas. Many models are only a fraction of the size of the real structure, says Higgins, a professor in the School of Civil and Construction Engineering. Trouble is, tests on these scaled–down versions have inherent limitations. A pencil–thin wooden beam, for instance, doesn’t act like a two–ton timber, no matter how carefully you design the experiment. That’s because the physical properties of wood, concrete and steel differ geometrically with size. So do the forces that impinge on them.

To get around this problem, Higgins tests bridge components that are as big as the ones holding up the phalanx of ramps and overpasses that crisscross every major city in the country. Access to real–size data lets engineers correct the assumptions and interpolations that plague analytical models built on sub–size experiments. When we think of bridges, behemoths come to mind, like Portland’s I–5 Marquam Bridge, which curves dramatically to a knee–weakening summit high above the Willamette River. But the structures Higgins typically deals with are not “the striking or soaring long–span kind that grab people’s attention,” he says. Rather, Higgins focuses on the mundane and unsung, the “bread and butter” of the highway system, “the ones you cross under and over without even realizing it.”

Whether they are aesthetic masterpieces or unlovely chunks of pure functionality, bridges are being asked to withstand the ceaseless crush of ever–bigger, ever–heavier and ever–more–numerous vehicles. So Higgins and his students punish their experimental girders (bent caps like Goliath, along with smaller T–shaped girders called

“The forces that threaten bridges are not always visible to the naked eye, like a rusty beam or a semi–truck, or to a weather satellite, like a windstorm or a hurricane. Instead, they are distant, invisible and unpredictable.”

T–beams) with mega–forces and maxi–stressors. They pound them with hydraulic cylinders, pummel them with tons of rolling force, and subject them to extremes of heat and cold, down to minus 13 degrees Fahrenheit (hence the need for Arctic parkas). They even use sound to detect invisible defects. By listening to acoustic emissions, the researchers can analyze internal noise sources and pinpoint structural weaknesses.

“We’re doing some things that no lab in the world has ever done before,” says Higgins. “For instance, we built a moving load simulator that can actually roll, acting like a truck traveling across full–size girders. We found that a moving load affects the bridge structure differently than a single load pushing at one spot. The internal stresses change as the load moves across.”

More than 70 of these full–size T–beams, the workhorses of concrete bridges, have been subjected to loads as heavy as 500,000 pounds (the equivalent of about 100 SUVs) in the OSU lab. The idea is to make them fail, determine how to predict that failure and then figure out how best to fix them. “I’m all about existing structures, how to squeeze more life out of them, figure out how much strength is left in them and quantify the risks associated with them,” says Higgins.

As greater and greater force is applied, hairline cracks form at the concrete surface like networks of varicose veins. But causing a 26,000– to 40,000–pound hunk of reinforced concrete to crack is no mean feat. The researchers do it in several ways. To simulate the movement of continuous, everyday traffic (what engineers call “high–cycle fatigue”), the researchers apply millions of repeated bounces to the T–beams with a hydraulic cylinder. To imitate the impact of heavily loaded triple–tractor–trailer rigs (“low–cycle fatigue”), they apply a half–million pounds of downward pressure (a million pounds for the massive Goliath). To test the effects of temperature and shrinkage on strength, they pull the girders lengthwise, using as much as a quarter–million pounds of force.

Once cracked, some of the beams are mended. The purpose is to test the performance of both novel and existing repair techniques. Some are applied internally, others externally. The researchers inject epoxy and insert steel rods. They wrap cracked beams in sheets of a polymer, a composite material reinforced with carbon fibers originally developed for aerospace applications, that bonds to the surface and restricts the cracking like, he says, “a Band–Aid across a cut.” The lab–induced fissures mimic the fatigue cracks that inspectors have found on some 500 of Oregon’s 1,800 concrete bridges, most of which date from the 1950s when President Dwight D. Eisenhower launched the Interstate Highway System. In 2000, ODOT hired Higgins and OSU’s multidisciplinary Kiewit Center for Infrastructure and Transportation to help it assess the state–owned spans. The result of the $1.5 million study was another milestone for Higgins and his team, one that produced a more accurate bridge assessment process and has already saved up to a half–billion dollars for the state. The breakthrough was twofold: better prediction of load capacity for existing bridge components and the development of a load–rating tool known as a “load factor” (a number that statistically represents the expected loading on the bridge). The research has produced the first state–specific load factors in the nation. By using actual traffic data in place of generic figures, the new load factors have brought unprecedented precision and specificity to Oregon’s bridge rating process. “Site–specific load factors are more refined because they are characteristic of a particular bridge site, route or jurisdiction,” wrote former graduate research assistant Jordan Pelphrey (who now designs and fabricates bridges for Knife River in Harrisburg, Oregon) in a paper coauthored with Higgins. “They reflect the actual truck traffic and likely maximum loadings over the exposure period.”

“Based on Professor Higgins’ research, we were ableto reduce the number of bridges that were required to be replaced or repaired,” explains Bruce Johnson, state bridge engineer at ODOT.

In September, Higgins submitted written testimony to the U.S. House Committee on Science and Technology, calling on Congress to create “a national research center focused on safety evaluation of existing bridges that draws on expertise from across the country.” Such a center, modeled after the National Science Foundation’s Earthquake Engineering Research Center, would be “a logical and fruitful” nexus of university research and federal support, Higgins told the committee.