Making older buildings safer during earthquakes

Buildings being built now in earthquake-prone regions are designed better to withstand tremors. Trouble is, for a long while yet, most of the buildings in which people live and work were built before new earthquake-related design concepts and new materials were available. How do we make these buildings safer? Physorg reports that Recent simulated earthquake tests conducted by University of California San Diego engineers are expected to lead to retrofit schemes that make historic buildings safer. The structural engineers tested a structure similar to those that were built in California in the 1920s — structures that have masonry-infilled walls and reinforced concrete frames.

Based on data collected from tests performed on the world’s only outdoor shake table, the engineers develop new seismic assessment tools and critical retrofit designs for these kinds of structures, which were not designed according to current standards. As part of the project, the engineers subjected a 3-story structure with non-ductile reinforced concrete frames with unreinforced masonry infill walls to shaking representative of a series of different seismic events. Infill walls can generally improve the seismic safety of a building up to a certain level of earthquake intensity depending on the number of walls and their locations. Once the strength of the walls is exceeded by the earthquake force, the failure of such structures could be sudden and catastrophic as demonstrated in the recent UC San Diego tests. Owing to the frame-panel interaction, the earthquake load resisting mechanism of these structures is complicated and it is difficult for engineers to assess their seismic resistance. The objectives of this project are to investigate the resistance of this type of structure under realistic seismic load conditions with large-scale tests and develop and calibrate reliable analytical models to assess their seismic performance.

“We will also look into retrofit methods to push the performance envelop of these structures. In reality, some of these structures may not have sufficient walls to resist earthquake loads or some walls may be missing in critical locations of a building. Hence, we need reliable means to assess and improve their performance,” said Benson Shing, a structural engineering professor at the UC San Diego Jacobs School of Engineering, and the lead researcher on the project.

Currently, there is no reliable analysis methods to evaluate the seismic performance of these older structures and validated retrofit methods to improve their seismic behavior. In California, construction of unreinforced masonry buildings including those with brick infill walls came to a halt after the 1933 Long Beach earthquake — which was a 6.4 magnitude — but many of them still exist today. The tremor was only moderate in terms of magnitude, but it caused serious damage to unreinforced masonry structures on land fill from Los Angeles south to Laguna Beach. Property damage was estimated at $40 million, and 115 people were killed.