Engineering For Safety
Metal fatigue and fracture research
Robert J. Connor, associate professor of civil engineering, has studied metal fatigue and metal fracture specifically as they relate to bridges, highway sign structures and high-mast lighting towers.
Robert J. Connor, associate professor of civil engineering, discusses high-mast tower retrofitting procedures with Ryan Sherman, a research engineer at Bowen Lab, and graduate student Lindsey Diggelmann.
Connor conducts much of his research at Purdue's Bowen Laboratory for Large-Scale Civil Engineering Research. At 66,000 square feet, it is one of only five university laboratories in the country to conduct large-scale investigations. Experts there are shaking, breaking, and even burning full-sized models to gain a greater understanding of how structures behave in crisis moments.
Much of Connor's current research is looking at the effects of wind on high-mast lighting towers — the structures lining interstate highways and high-speed roadways throughout the country. This summer Connor and two graduate students concluded a two-year study of selected towers in multiple states. Strain gauges and anemometers were installed earlier to measure wind speed and direction in order to find daily stresses on the towers. Also engineering for safety by della Pacheco | Photos by Andrew Hancock at each site was a cellular modem that worked on Verizon's 3G network sending back real-time data to a server at Purdue.
Ryan Sherman, a research engineer at Bowen Lab, and graduate student Allen De Schepper drove the mobile lab over 3,000 miles retrieving the equipment. This vehicle is an extension of the laboratory and is outfitted with computers connected to Purdue servers and equipment necessary to conduct assessments in the field.
In addition to fieldwork, tests were done in the lab on a mast tower to simulate wind conditions over a period of years. By operating the test 24/7, results on fatigue and fracture can be gained more quickly and various methods of retrofitting can be studied.
"We tested a high-mast tower in the lab that was retrofitted with bolts at the bottom," Connor says. "When you have hundreds of these by interstate highways and they begin to crack, what can you do? You can't just take them down."
The results of the studies will be sent to a panel at AAHSTO (American Association of State Highway Transportation Officials), a nonprofit, nonpartisan association representing highway and transportation departments in the 50 states, the District of Columbia and Puerto Rico.
"We write the results in specification language that is proposed to AASHTO and they will decide if they want to accept the changes and add it into the code for future design of these towers," Sherman says. "You really get to see where the rubber meets the road. You have a problem, find a solution and implement it into future design."
Testing effects of fire on steel structures
Amit Varma, associate professor of civil engineering, is a leading expert in the field of fire resistance research.
For many years Amit Varma, associate professor of civil engineering, has studied the field of fire-resistance research. Bowen Lab allows researchers like him to study the effects of fire on steel structures using a one-of-a-kind heating system and other innovative systems.
Typically such testing is conducted inside large furnaces, but Varma says that provides challenges. "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," he says.
Varma and his research team study the effects of fire on steel structures using a oneof-a-kind heating system and other innovative systems.
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.
Varma's research team is conducting a multiyear study looking at the effect of fire on steel girders used in highway bridges. The focus is on using heat to straighten, repair and rehabilitate girders damaged by collision with trucks. In the large-scale setting of Bowen Lab, the team can simulate damage and apply heat to straighten and make repairs. The results from the analytical investigations will provide design guidelines for heat straightening repairs, estimating the final residual stresses and evaluating the performance of damaged-repaired steel bridges.
Varma and Connor were called in to evaluate just such a situation after a tanker truck struck a bridge on I-465 in Indianapolis and burst into flames. Increasing truck traffic on highways could mean more such incidents.
The team uses coils to simulate fire and then uses heat to straighten, repair and rehabilitate bridge girders.
But not all failures in bridges are related to intense heat. Growing vehicular traffic and increased weight are causing stress on aging bridges. Through field instrumentation, Connor and his students install sensors on bridges to study stresses and recommend retrofits that can keep a small problem from becoming a catastrophic one.
To study this in the lab, a full-scale bridge deck was constructed and on it was a two-axle vehicle loaded down with the weight that a tractor-trailer would carry. Day after day the vehicle was moved back and forth, simulating the load on the bridge.
Inside and underneath, about 200 sensors were installed, strain gauges that show how the concrete stretches and compresses beneath the truck's weight. A laser tracked the truck's position. By shifting the truck's path from one side to the other, the test provided a full look at how the bridge twists and bends under pressure.
"It allowed us to see how the bridge reacts in real life," Connor says. "We took that data and compared it to computer models. That data allows us to improve the safety, reliability and durability of bridges so that we can have better infrastructure that will last us decades."
But what they do in the laboratory is only part of the picture. Sensors also are installed on different types of bridges and monitored to "let those structures tell us how are responding over time," Connor says.
History lessons for engineers
The old adage says, "Those who cannot remember the past are condemned to repeat it." In the case of infrastructure, many of the people responsible for repairing and maintaining roads and bridges weren't even born when the structures were built. That's why it's important, Connor says, to teach students about older materials while at the same time studying new high-performance ones.
"A lot of bridges we work on were built in the 1940s and 50s," Connor says. "If you don't know the issues from the past, then you will have a difficult time figuring out what the problems are and making sure that you don't repeat them in the future. There is a need for students to know what was done in the past, why we aren't using certain materials or designs anymore, or why we continue to use them."
Sherman agrees. He cites the Tacoma Narrows bridge in Washington as a good example. Built in 1940 and nicknamed "Galloping Gertie," it collapsed four months after it opened due to vertical movement of the bridge deck. Sherman says, "Knowing the past and doing the research for the future is where it all comes together to improve safety."