New Tech to Help Protect Bridges, Other Infrastructure from Scour

"The 'in situ scour evaluation probe' (ISEP) is the first technology that allows technicians in the field to measure the scour potential of soils without the need for excavation," says Dr. Mo Gabr, a professor of civil, construction and environmental engineering at NC State and co-author of a paper describing the new device."Previous technologies required engineers to take samples and process them in a lab."

Understanding scour potential is important because it can help authorities prepare for, or minimize the impact of, events such as the failure of the levees in the wake of Katrina. Scour has also been linked to approximately 60 percent of the bridge failures in the United States, as documented by the Federal Highway Administration.

"The ISEP's ability to measure scour potential at different depths helps us predict how the soil will behave in the future as a support media, as various layers of soil are eroded or scoured," Gabr says.

The ISEP will also allow end-users such as federal and state agencies and private consultants to perform scour assessment more frequently, since they will not have to take physical samples back to a lab for analysis. More testing data means researchers will have a larger data set to work with, which should help them to more accurately predict scouring rates and behavior.

The new probe uses a water jet to burrow a hole into the soil. Researchers can track the rate at which the water displaces the soil to determine the scour rate. Researchers can also manipulate the velocity and flow rate of the water to simulate various natural events -- from normal stream flow to hurricane-induced surges.

The researchers plan to take the ISEP to North Carolina's Outer Banks later this month to help with research efforts related to dune erosion.

The paper,"In Situ measurement of the scour potential of non-cohesive sediments (ISEP)," was presented Nov. 8 at the 5th International Conference on Scour and Erosion in San Francisco, Calif. The lead author is NC State graduate student Cary Caruso. The ISEP was developed under a grant from the U.S. Department of Homeland Security (DHS), as part of the work being done by the DHS Center of Excellence on Natural Disasters, Coastal Infrastructure and Emergency Management.

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Engineers Test Effects of Fire on Steel Structures

Building fires may reach temperatures of 1,000 degrees Celsius, or more than 1,800 degrees Fahrenheit, said Amit Varma, a Purdue associate professor of civil engineering who is leading the work.g1

"At that temperature, exposed steel would take about 25 minutes to lose about 60 percent of its strength and stiffness," he said."As you keep increasing the temperature of the steel, it becomes softer and weaker."

One project focuses on how a building's steel-and-concrete floor and its connections to the building behave in a fire. Another project concentrates on how fire affects steel columns and a building's frame.

Such testing is customarily conducted inside large furnaces.

"However, in a furnace it is very difficult to heat a specimen while simultaneously applying loads onto the structure to simulate the forces exerted during a building's everyday use," Varma said.

To overcome this limitation, Purdue researchers designed a system made up of heating panels to simulate fire. The panels have electrical coils, like giant toaster ovens, and are placed close to the surface of the specimens. As the system is used to simulate fire, test structures are subjected to forces with hydraulic equipment.

In practice, beams and other steel components in buildings are covered with fireproofing materials to resist the effects of extreme heating.

"Because the steel in buildings is coated with a fireproofing material, the air might be at 1,000 degrees but the steel will be at 300 or 400 degrees," Varma said."We conduct tests with and without fire protection."

The work is funded by the National Science Foundation and the U.S. Department of Commerce's National Institute of Standards and Technology.

The heating system is being used to test full-scale steel columns at Purdue's Robert L. and Terry L. Bowen Laboratory for Large-Scale Civil Engineering Research. It is believed to be the only such heating system in the world, Varma said.

Each panel is about 4 feet square, and the system contains 25 panels that cover 100 square feet. Having separate panels enables researchers to heat certain portions of specimens, recreating"the heating and cooling path of a fire event," Varma said.

The Bowen Lab is one of a handful of facilities where testing can be performed on full-scale structures to yield more accurate data. The 66,000-square-foot laboratory is equipped with special hydraulic testing equipment and powerful overhead cranes.

The research group also has tested 10-foot-by-10-foot"composite floor systems" -- made of steel beams supporting a concrete slab -- inside a furnace operated by Michigan State University. The composite design is the most common type of floor system used in steel structures.

Findings from that research will be compared with floor-system testing to be conducted at the Bowen Lab. Results from both experiments will be used to test and verify computational models used to design buildings.

"Most of these experiments are showing that we have good models, and we are using data to benchmark the models and make sure the theory and experiment agree with each other," Varma said.

Models are needed to design composite floor systems, which can be heavily damaged by fire.

"When you have a floor supporting weight, the floor starts sagging from the heat," Varma said."It expands, but it's got nowhere to go so it starts bowing down, which produces pulling forces on the building's frame. It starts pulling on the columns and then it becomes longer and permanently deformed. After the fire, it starts cooling, and then it starts pulling on the columns even harder."

Recent research findings were detailed in a paper presented in June during the Structures in Fire conference at Michigan State University. The paper was written by graduate student Lisa Choe and Varma.

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Disaster Spawning New Concepts in Bridge Research, Testing and Safety

When testing is complete and the technology implemented, the system might allow a technician working for a day to produce a better analysis of a bridge's structural condition than a more expensive and highly-trained engineer could do in weeks.

Developed at OSU, the technology is being tested this fall by a simulated laboratory failure of the exact type of truss connecting plate that caused a bridge to collapse on Interstate 35W in Minneapolis in 2007, killing 13 people and injuring 145.

The work also brings focus to a little-understood aspect of bridge safety -- that most failures are caused by connections, not the girders and beams they connect, as many people had assumed. The issues involved are a concern with thousands of bridges worth trillions of dollars in many nations.

"The tragic collapse of the interstate bridge across the Mississippi River in 2007 brought a lot of attention to this issue," said Chris Higgins, a professor in the School of Civil and Construction Engineering at OSU."For decades in bridge rating and inspections, we've been concentrating mostly on the members, but in fact it's the connectors where most failures occur. And the failure of a single critical connection can bring down an entire bridge, just like it did in Minneapolis."

This is a growing concern, Higgins said, because thousands of bridges were built around the world in the 1950s and later that may be nearing the end of their anticipated lifespan, including many of those on the interstate highway system in the United States. Maintenance, repair and replacement of this infrastructure could cost trillions of dollars, he said, at local, state and federal levels.

But prioritizing which bridges are still safe and which most urgently need repair or replacement is not easy and has never been obvious, Higgins said.

"The failure of the bridge in Minneapolis was caused by a single connecting plate that inspectors saw repeatedly," Higgins said."They took pictures of it, actually had to touch it, because an access ladder was right next to it when they were doing inspections.

"But it still wasn't readily apparent that it had a deficient design and was distorted before the accident happened," he said."Then one day, as part of a repaving project, they had stockpiled material right above this weak spot, and the bridge collapsed."

To address this issue, Higgins has created a computerized plate analysis system that incorporates digital imaging and machine vision, and can be used by any trained technician. It can provide sophisticated data that are much more precise than a human eye could detect, analyzing connections to make sure they meet specifications and are still sound. It should allow for more widespread, low-cost and accurate inspections that will better identify trouble spots before another disaster occurs, he said.

The system works, researchers say, but now they are putting it to the ultimate test, using a state-of-the-art structural testing laboratory and other technology at OSU that will provide real data unlike any other available in the world. Using a copy of the failed connector from the Minneapolis bridge, they are going to test it this fall by applying enormous forces until it collapses. The data provided should prove the efficacy and accuracy of the system.

Similar technology, Higgins said, might also be used to inspect construction processes for new buildings or bridges to make sure they meet design standards, or even help create customized replacement parts more easily and at lower cost for existing structures.

"The bridges built 40 and 50 years ago used the design standards available at the time, which were based on the forces it was believed the bridges would be exposed to," Higgins said."Now we have better quality materials, different construction procedures, more precise analysis methods, and we ask tougher questions, like what forces will it take to actually collapse a bridge."

In other words, modern bridges are built better. But most of the world is still driving on older bridges that have to be maintained, used, and kept safe for some time, Higgins said. The new OSU technology may allow that to be done more cost effectively while increasing the accuracy of inspections.

The findings will be published soon, Higgins said, and the system may be used more broadly in the near future. Consultants and transportation agencies have already begun to deploy the system on bridges around the country.

The work has been supported in part by the Oregon Transportation Research and Education Consortium, a National University Transportation Center created by Congress in 2005, as a partnership between OSU, Portland State University, the University of Oregon and the Oregon Institute of Technology. Additional funding was provided by the Bridge Section of the Oregon Department of Transportation.

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