Research conducted at the George E. Brown Jr. Network for Earthquake Engineering Simulation suggests that roughly 1,500 nonductile concrete buildings in Los Angeles may be vulnerable to collapse during a large earthquake and should be more closely examined to determine the precise risks. Wikimedia Commons/Andy
A multifaceted project in Los Angeles seeks to assess the performance of older concrete buildings in earthquakes and develop an accurate inventory of structures that remain vulnerable.
April 1, 2014—There are approximately 1,500 nonductile concrete buildings in Los Angeles that should be examined more closely to determine their vulnerabilities to collapse during earthquakes. That was one of the key conclusions of a long-term, multifaceted research project that made headlines recently when the team presented Los Angeles city leaders with an inventory of the structures.
The Grand Challenge Project was conducted via the George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES) and funded by the National Science Foundation. The researchers examined the prevalence of nonductile concrete buildings in the city, the vulnerabilities of such structures to collapse during earthquakes, and the mechanisms that can lead to such collapses.
The project was led by principal investigator Jack Moehle, Ph.D., P.E., M.ASCE, a professor of civil engineering at the University of California at Berkley. The team included Mary Comerio, a professor of architecture at UC Berkeley, Jonathan P. Stewart, Ph.D., P.E., F.ASCE, the chair of the Civil and Environmental Engineering Department at UCLA, and Thalia Anagnos, Ph.D., A.M.ASCE, a professor of engineering at San Jose State University.
“Nonductile concrete buildings are pervasive around the world. It’s a common method of construction. And they’ve been observed in many previous earthquakes to be a significant collapse hazard,” says Stewart. But procedures to assess that risk have been too conservative, predicting a collapse rate that far exceeds the approximately five percent that has been observed in actual earthquakes.
“This is critically important, because if you overstate the magnitude of a problem, it becomes practically intractable,” Stewart notes. “Somewhat paradoxically, if a problem seems to be too big to solve, then nothing happens to solve it.”
The team approached the analysis from a variety of perspectives. Experimental testing of concrete elements and systems and a review of past test results on nonductile concrete joints yielded new information about how the structures perform in an earthquake and the creation of numerous new analytical models.
Soil-structure interaction effects were considered as part of this project “because often these concrete buildings are fairly stiff and their response is strongly influenced by flexibility at the base,” Stewart notes. “The traditional method of structural modeling—where the structure is fixed at the base—can be significantly in error. What we were trying to do was improve the procedures for capturing the flexibility and damping and the foundation-soil interface.”
The project also included an extensive inventory effort in Los Angeles that began with searches of public records from a variety of agencies and employed the mapping abilities of a geographic information system. That initial list was validated by a team consisting of Christine Goulet, Ph.D., a postdoctoral researcher at UCLA, and a group of undergraduate students who visited each property either in person or virtually via satellite imagery that was available through the Internet. The results were then validated by professional engineers.
“We wanted to make a case study of a large city in California as a way of not just treating concrete buildings like they were all concrete beams and columns but to really understand them in context, because of the significance of that for policy,” says Comerio.
“The idea was to try to understand the inventory—to understand what a city might have to deal with,” Comerio explains. “We initially thought the numbers were going to be much higher. But by the time we eliminated either buildings that had been torn down for other developments or retrofitted, we ended up with about 1,500 buildings in Los Angeles.”
Many of the buildings date to either the 1920s or the 1960s, two periods of expansion in the city, and encompass an array of types, including residential, commercial, industrial, office, and educational.
One of the key challenges of the project was finding commonalities between groups of structures. “Because we were working with large numbers of buildings, we could not do an individual analysis of each single building for a region-wide risk assessment,” Anagnos says. “We had to lump the buildings into categories. We were trying to find some common characteristics of the way these buildings are built, and their occupancies, and their value. It’s a major modeling challenge in estimating losses. Conversely, to decide if a specific building is at risk of collapse, individual analyses are required that consider the specific building characteristics and the ground-shaking hazards at the site. A detailed analysis of this type for specific buildings was beyond the scope of the study.”
Once the team had established commonalities and grouped the structures accordingly, they assigned fragility curves to them based on Hazus, the Federal Emergency Management Agency’s (FEMA’s) methodology for estimating potential losses from disasters. The team computed ground motions from three earthquake scenarios, including a 7.8 magnitude earthquake on the San Andreas Fault and a 7.15 earthquake on the Puente Hills Fault.
Loss estimates in these scenarios reached as high as $20 billion with between 300 and 2,000 casualties. The Concrete Coalition, a volunteer organization within the Earthquake Engineering Research Institute, in Oakland, California, estimates as many as 22,000 such structures exist throughout the state, approximately 17,000 in areas of high seismic risk. Santa Monica recently approved funds to conduct an inventory of buildings there.
“We hope that the conversation about these types of buildings continues to spread through California,” Anagnos says. “It was very exciting to see that Santa Monica started talking about it almost immediately. Hopefully some other cities in the state of California will start to discuss how to address this very severe seismic problem.”