New Materials, Designs Create Earthquake-Resistant Bridges
1/10/2012 5:00:00 AM
Screen captures from video of Assistant Professor Bassem Andrawes' tests comparing the damage to a bridge pier with seismic loading for a traditionally built pier compared with alternative methods. Left to right: The as-built column was non-ductile, so it sustained severe damage at low drift levels; a traditional retrofit method using passive confinement resulted in severe damage to the column at moderate drift levels; a hybrid method using a shape memory alloy spiral as a supplementary retrofit measure proved to be very effective; no signs of significant damge were observed in the column retrofitted with shape memory alloys (active confinement), even at extremely large drift levels. (Visit the department's YouTube channel to view a video of Andrawes talking about his work with shape memory alloys.)
By Leanne Lucas
Bridges are a critical component of infrastructure, ensuring the transportation of goods as well as people. Two research projects underway in the Department of Civil and Environmental Engineering address the design for safety and functionality of bridges in the aftermath of an earthquake.
Associate Professor James LaFave and Assistant Professor Larry Fahnestock have worked with the Illinois Department of Transportation (IDOT) on a three-year project, funded through the Illinois Center for Transportation, to evaluate IDOT’s earthquake resisting system (ERS) strategy for the design and construction of new bridges in Illinois.
“The nature of the earthquake hazard is more highly variable than almost any other type of loading,” said LaFave, “and at its upper limits, it’s quite severe. It’s not practical to design structures that sustain no damage at all, but rather one should try and have a ‘hierarchy’ of damage. We’re evaluating IDOT’s ERS strategy to determine how favorable their idea of the hierarchy of damage is to what would actually happen in an earthquake. One aspect of that is to understand experimentally how the bearing assemblies behave [the components between the main bridge superstructure and the concrete piers/abutments], and to then evaluate that analytically in the context of the whole bridge as a system.”
“On the west coast of the U.S., they use specialized isolation bearings that have unique shapes and material properties to allow movement and to dissipate the earthquake-induced energy,” Fahnestock said. “They’re expensive and wouldn’t necessarily be viewed as an acceptable approach here. We would like to achieve a response in bridges here in Illinois that is similar to what is achieved in higher seismic regions, but we’d like to achieve it with more typical kinds of bearings and components.
They are investigating the use of elastomeric bearings, which in their simplest forms are just a steel plate attached to the girder above with a thick rubber pad that sits on the concrete. The basic function of elastomeric bearings is to allow thermal movement, or expansion and contraction due to ambient temperature, of the bridge. The rubber pads deform and allow the bridge to move and not build up stress during normal service life.
“We’re studying the idea that these ‘ordinary’ components can potentially function as isolation bearings,” Fahnestock said. “They’re not very expensive, and they have the potential to allow the bridge to move a good deal during an earthquake and dissipate energy.”
“We’re developing a greater base of fundamental understanding as to how the key components of these bridges behave, based on sound research testing and analysis,” said LaFave. “We’re then using that knowledge in conjunction with our IDOT partners to ensure that there can be a predictable, favorable hierarchy of damage that develops so that a bridge remains serviceable after a large seismic event.”
A second project that will advance research in the area of retrofitting and repairing current structures, is one being undertaken by Assistant Professor Bassem Andrawes, who received the 2011 National Science Foundation Faculty Early Career Development (CAREER) Award to develop and study a new technology that uses smart materials to reinforce lifeline concrete structures with the aim of mitigating damage from strong earthquakes.
“When an earthquake hits, a bridge needs to deform in a ductile manner. Bridges that were built 40 or more years ago lack this ductility,” said Andrawes.
Current technology used to improve the ductility in old bridges is to wrap the bridge columns in steel or fiber reinforced polymer (FRP) jackets, a technique called passive confinement.
“With passive confinement, an earthquake must hit the bridge in order for the column to start deforming or dilating laterally," Andrawes said. "That’s when these jackets start confining the concrete and improving the ductility. But this dilation process causes damage to the bridge columns, so for this technology to work, the concrete has to sustain some kind of damage first.”
Andrawes’ research applies active confinement, but not with traditional materials. Andrawes’ approach uses shape memory alloys (SMA), a class of metallic alloys that remembers its original shape after being deformed and returns to the pre-deformed shape through heating.
“The idea is to stretch the wires made of SMAs and wrap them around the bridge column in the plastic hinge zone, which is the most critical region in the column,” said Andrawes. “Once that’s in place, we apply heat until it starts to shrink to its original length. But because of the column, it cannot do that, so it squeezes the column and confines the concrete, causing the column to behave in a more ductile manner.
This method was so effective that Andrawes tested it as a quick emergency repair to damaged bridge columns.
“Right now, the technologies that are in place for repair take days or even weeks to restore functionality of the columns,” Andrawes said. “In our lab, we caused excessive damage to some of our columns, then repaired them with the shape memory alloy spirals. We were able to do it in less than 15 hours.”
To date, all of Andrawes’ work has been done in the lab, but, in cooperation with IDOT, the next phase of the project will be to implement this technology in one or more IDOT bridges.
“We will monitor these bridges over the next few years to see how the SMA spirals perform in real service conditions,” he said.