Author: Assistant Professor Marios Panagiotou, Structural Engineering, Mechanics and Materials
By 2030, 5 billion people, or 60 percent of the world's total population, will live in urban areas, compared to 47 percent in 2000. The number of urban centers near major earthquake faults, as well as the bridges connecting these cities, will increase accordingly.
It is generally accepted that multistory buildings (buildings taller than 35 meters) are essential for sustainable urban growth. For shallow earthquakes (the most common type of earthquake), regions in the direction of the fault rupture and within 10 km (depending on the earthquake magnitude) from the fault plane are subjected to the most severe near fault ground motions (NFGMs).
These NFGMS contain strong pulses (lasting between 1 and 8 seconds, with velocity up to 3 meters per second) that determine the destructive potential of the NFGMs on flexible structures.
Current seismic design philosophy for multistory buildings and bridges focuses primarily on collapse prevention and occupant safety. It does not attempt to limit earthquake-induced damage and ensure prompt post-earthquake functionality.
Such design strategies may result in unprecedented and unwanted direct and indirect economic and social losses following an earthquake, due to costly, time-consuming, and disruptive repairs, as well as downtime, temporary evacuation, and even demolition of structures.
For example, after the 2011 earthquake in Christchurch, New Zealand (magnitude 6.3), 36 out of the 50 tallest buildings in the center of Christchurch (about 5 km from the fault rupture) had to be demolished. This caused major disruption for over two years. Damage from this earthquake exceeded 15 percent of the GDP of New Zealand.
What will happen to multi-story buildings and bridges located near the fault rupture of a larger earthquake (such as 7.2 or 7.8 magnitude quake)? And are there cost-efficient seismic designs of multistory buildings and bridges that would result in minimal damage when subjected to NFGMs?
CEE Assistant Professor Marios Panagiotou, together with PhD students Vladimir Calugaru, Yuan Lu, Grigorios Antonellis, Tea Visnjic (co-advised with Professor Jack Moehle), and William Trono (co-advised by Professor Claudia Ostertag), are working on answers to these questions.
Three of the main seismic design strategies used in their research are:
- designs using seismic isolation devices able to develop horizontal displacements up to 1 meter under concurrent large vertical forces;
- designs that use rocking structural components (walls, foundations, and columns);
- designs that use a combination of seismic isolation and rocking components.
In these strategies, the majority of large deformations that the structure is expected to develop during NFGMs are accommodated, without damage, in robust isolation planes designed to re-center the structure after the earthquake.
The design in the image at the top of the article combines piers supported on rocking foundations and isolation devices at the abutments. In May 2013, together with CEE Professor Stephen Mahin and researchers from UC San Diego and UC Davis, they tested the seismic response of such bridge piers supported on rocking shallow foundations. These tests included physical modeling of the soil using the large soil box and the earthquake simulator of the NEES facility at UC San Diego. The test specimens were able to develop the level of deformations expected during NFGMs (7-10% rotation) with negligible damage in the structure and the soil.
See video below summarizing the experimental test responses.
The image (above [a]) shows the two-dimensional elevation of a 20-story building hypothetically located in the San Francisco Bay Area, 1 km from the San Andreas Fault. As shown in the image (above [b]), the structural system uses a seismic isolation plane, located at the base of the building below the ground level, and a core wall designed to rock at the ground level.
The building was designed to result in prompt post-earthquake functionality after an earthquake event equivalent to the 1906 San Francisco earthquake (magnitude 7.8). Developed analytically by Calugaru and Panagiotou, this system will be tested by Lu and Panagiotou in a spring 2014 experiment.