During an earthquake, which buildings will stand? Which will fall? Finding the answers to these life and death questions is at the heart of Abbie Liel's work in earthquake engineering.
An assistant professor of civil, environmental and architectural engineering, Liel specializes in structural engineering and structural mechanics, studying how concrete buildings withstand seismic events. Building design that mitigates the impact of natural hazards, such as earthquakes and flooding, can prevent casualties, reduce economic losses and lessen the time it takes for communities to recover.
One component of her research looks at older buildings that were constructed according to the required seismic codes of the time, but are not necessarily considered safe by today’s standards. Another area of her research is examining the impact of earthquakes at the community level when a large number of homes and businesses are affected.
“If we can understand seismic risks to communities,” she says, “we can use the information to be smarter about retrofitting and managing older buildings. Because older buildings are expensive to upgrade and building owners are not typically under legal obligation to do so, our research looks at how to modify buildings in a more cost-effective way.”
Each year there are about 16 earthquakes bigger than magnitude 7 worldwide. In the past decade, more than 740,000 people have died in earthquakes. This figure is much higher than in previous 10-year periods due to the devastating Indian Ocean tsunami and the Haiti earthquake. The last big U.S. earthquake was in Northridge, Calif., in 1994, killing 60 people and significantly damaging buildings and transportation infrastructure. Estimates put the damage at $25 billion.
Liel’s research group at CU-Boulder focuses on using performance-based earthquake engineering methods to assess the risk of structural collapse and the threat to public safety. The resulting measures of structural performance taken from detailed analysis models can be used to improve building code provisions and develop policies for taking care of risky structures.
By taking accelerometer recordings of the ground shaking from past earthquakes, Liel inputs the recordings into computer models of buildings. And then she makes the buildings in the models fall down to see what happens.
Her analysis is based on reinforced concrete frame models that represent past and present building codes and a database of seismic motions.
“If the building code says x, y and z,” says Liel, “we evaluate how well x, y and z are achieving the objectives of building codes and standards. If we change something about a design rule, does that improve the safety or reliability of the system?”
After the powerful 2009 L’Aquila earthquake in central Italy, which damaged upwards of 11,000 buildings, Liel traveled there to study the damage. By looking at patterns of structural failure and the types of buildings—residential, business or mixed use—that suffered the most damage, she can validate her computer models.
“It’s very difficult to validate our models,” she says. “There are few buildings that we have enough instrumentation on. Fortunately, there aren’t that many earthquakes. We’re challenged by a lack of data, so we deal with a lot of statistics and uncertainties.”
Liel believes an improved assessment of seismic collapse risk is needed to develop the next generation of building codes that more accurately account for near-fault activity.
Many major urban areas worldwide are interlaced by large seismic faults. However, the risk of earthquake- caused collapse for buildings near these faults is not well understood, says Liel. In California for example, it is estimated there are 40,000 older concrete buildings built before the mid-1970s that are vulnerable in an earthquake.
Liel has recently been studying the differences between good and bad concrete buildings and what characteristics make a concrete building dangerous in a seismic event. The strength or ductility of the reinforcement inside the concrete and how the reinforcement is arranged and tied down are critical factors. She is working with a team of researchers to establish criteria to identify which of California’s 40,000 older concrete structures are a definite threat to safety.
“These buildings are not in good shape and everybody knows that,” she says. “But we don’t know if they are all really bad or whether some are in better shape than others. It’s politically infeasible to tell all of the buildings’ owners that they need to fix the deficiencies.”
“Every single building is different,” says Liel. “It takes time and energy to make a computer model of the building. We look at a lot of photos and drawings to make sure we’re being representative of the building style to be as accurate as possible.”