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 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 will come up with 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 Three-story structure with non-ductile reinforced concrete frames with unreinforced masonry infilled 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 present and their locations. Once the earthquake force exceeds the strength of the walls, the failure of such structures could be sudden and catastrophic as demonstrated in the recent UC San Diego tests. Due 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 envelope 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,” says 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 a lack of 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. Although only moderate in terms of magnitude, this earthquake caused serious damage to unreinforced masonry structures on land fill from Los Angeles to Laguna Beach. Property damage was estimated at $40 million, and 115 people were killed.
The impact of this $1.24 million project, funded by the National Science Foundation, is vast since a large number of such structures can be found in the Pacific Northwest, and in the Midwestern and Eastern United States, where big earthquakes could occur even though the recurrence frequency is lower. This type of structural system is also very common in areas of high seismicity around the world, including China and the Mediterranean region.
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