July 06, 2011
Quake study may lead to safer building designs
CARBONDALE, Ill. -- Pluck a guitar string, and it will vibrate at a certain rate, creating pitch based on how fast or slow these oscillations occur. This vibration rate, or frequency, depends on several factors, including the structure of the string and its length.
Like guitar strings, building have frequencies, too. So when the earth moves violently during an earthquake, buildings also vibrate at their frequency, which is also based on factors such as structure, size and the type of ground in which it is anchored. In an earthquake, those vibrations can prove catastrophic.
The phenomenon, known as fundamental frequency, is a fairly well understood variable in the world of engineering earthquake-resistant structures. But there are many other variables -- perhaps 20 or more -- that are not.
Two researchers at Southern Illinois University Carbondale are hoping to change that by crunching earthquake data gathered from the United States and around the world and combining it with imaging technology and software. What comes out the other side of this effort could potentially change building codes everywhere and allow engineers to build better, safer structures to withstand earthquakes.
Jale Tezcan, assistant professor of civil and environmental engineering, and Qiang Cheng, assistant professor of computer science, recently received a three-year, $260,000 federal grant form the National Science Foundation aimed at preventing structural damage like the kind Japan suffered this spring. The two researchers, along with graduate students, have begun pulling data together and formulating software to analyze and it.
“This is a really a creative way of looking at this problem,” Tezcan said. “We are converting earthquake signals into pictures and applying image processing techniques to them.”
The research could fill a critical gap in the knowledge set that engineers use when designing earthquake resistance into new structures.
When they design buildings, engineers typically rely on recorded earthquake signals to ensure their buildings can take the stress of future temblors. Often, however, engineers must rely on data from earthquakes that occur in such seismic hotspots as California, even if they are designing a building for Southern Illinois.
But when it comes to how earthquakes cause damage, one size far from fits all.
Instead, many factors -- soil types, bedrock depth, plate locations and boundaries and others -- can affect how earthquake energy travels, dissipates and ultimately impacts structures. And ultimately, how it affects structures.
“The problem is we don’t have enough earthquakes here that are large enough to provide such test signals,” Tezcan said. “So what happens is engineers just look at earthquake signals from California, make some adjustments based on building codes and go from there. There are a few dozen variables. But building codes only use three or four. That’s really unrealistic because everything is different.”
California, for instance, is located along a plate boundary. The motion that occurs during a quake, therefore, would be different in character than one that occurs in the Midwest. The geological conditions also are very different. California’s bedrock lies deep, which tends to isolate quake energy nearer to its source. In the Midwest, however, quake energy tends to travel much further. An earthquake that originated near Southern Illinois a few years back, Tezcan said, was felt as far away as Florida.
“A strong earthquake here would impact six to seven states,” she said. “Our goal is to provide engineers with more realistic earthquake signals so they have better scenarios to design their structures.”
To do this, Tezcan and Cheng will look at both the better-understood variables such as magnitude, distance and soil types, as well as the many that are not as well known, through the prism of their new approach.
“Magnitude is important, of course, as is distance and soil type as some tend to amplify seismic energy while others do not,” Tezcan said. “But there are other variables too, some of which probably haven’t even been discovered yet and some that researchers think may even be more important” than the ones that are known.
These other variables include factors such as the depth at which the focus of the quake occurs, as well as whether the motion is toward or away from structures.
The variables basically live in an array of databases kept both at home and abroad. In the United States, the U.S. Geological Survey keeps extensive tabs on quake variables, yet the information is not used in building codes because little is known about their effects on structures.
After gathering the USGS data, as well as data from Europe, Japan and other areas, the researchers will use computer analysis to take the quake signals, apply them to variables and turn them into pictures to figure out which variables are the most important to engineers.
Tezcan will collect the databases while Cheng will put the data through programs and processes he is creating that will identify patterns in earthquake signals using imaging technology, basically turning the signals into visual representation that researchers can interpret.
After that, Tezcan will use the data to create models that engineers will be able to apply to their unique geological situations. The work also will quantify uncertainty, which will help engineers make calculations, as well.
One key aspect will examine a quake’s amplitude as distributed over time and frequency. This might potentially tell an engineer what type of destructive fundamental frequencies a quake might put out in a certain area, bringing the vibrating guitar string example back into focus.
“Every structure likes to vibrate at a certain frequency, and if the earthquake provides that particular frequency the buildings become really happy,” and move back and forth more readily, Tezcan said. The work will alert engineers to such hazards, allowing them to design buildings to avoid or withstand such issues.
“We will create some scenarios of ground motions for regions,” Tezcan said. “Even though there are not enough actually earthquakes to test, we hope this will allow engineers to apply the model in other areas, too.”