During the past decade, earthquakes in California and Kobe, Japan have caused the dramatic collapse of freeways, bridges and other structures built according to stringent earthquake-resistant design codes. Theoretical simulations and scale models have only limited usefulness in predicting structural damage, so MIT researchers and their collaborators have prepared a conceptual design for a 30-by-30-meter "shake table" that can subject a full-scale building or other structure to the forces of a real earthquake.
To determine the force that the table must deliver, the researchers performed extensive analyses of past earthquake data and found that ground motions can be more intense and variable than previously thought possible. For the past two years, collaborating teams of experts in civil, electrical, and mechanical engineering, seismology and electromagnetic energy systems have been doing this research and examining the feasibility of building an electromagnetic seismic simulator (EMSS). The device is intended to be located at the Idaho National Engineering and Environmental Laboratory (INEEL) as part of a major facility for testing the responses of full-scale structures to earthquakes, wind and aging due to humidity, salt spray and solar radiation.
Leading the teams are M. Nafi Toksoz, professor of geophysics and director of the Earth Resources Laboratory; Professor Eduardo Kausel of civil and environmental engineering; and Emmanouil A. Chaniotakis, research scientist in the Plasma Science and Fusion Center. The researchers are also working with collaborators at INEEL.
The first step in designing a full-scale shake table is to understand the ground motions that it must replicate. Professor Toksoz and his coworkers analyzed data on peak ground accelerations (PGAs) measured during 15 significant earthquakes worldwide. (Strong-ground-motion instruments generally measure accelerations; velocities and displacements must be calculated from the accelerations.)
Analyzing ground motion
Their analysis led to some unexpected observations. First, the maximum PGAs in recent earthquakes were much larger than traditional earthquake models predict. Moreover, earthquakes with similar magnitudes did not necessarily yield the same ground motions. In fact, for earthquakes with similar magnitudes at similar locations, the maximum PGA varied by factors of two to five. And at some sites, vertical accelerations - usually thought to be less important - were about the same as and occasionally greater than the horizontal.
Perhaps most surprising was the dramatic variability of ground motions over small distances. Seismologists have long recognized that waves from an earthquake can get absorbed, reflected and redirected by subsurface geologic structures, leading to variations in surface ground motions from place to place. But recent data from closely spaced sensors showed that variations up to fivefold can occur at sites as little as 100 meters apart, even for locations as much as 100 kilometers from the epicenter.
Such observations explain the shortcomings of current building codes during recent earthquakes. First, the ground motions were greater than the codes are designed for. Second, vertical motion was significant, yet codes focus on withstanding horizontal motions such as sliding. Most important, ground motions varied over distances as small as the footprints of large structures such as freeway spans, parking garages and shopping malls. That finding could explain the widespread damage to such structures. If, for example, all the supports of a bridge move in the same direction, damage may be minimal. However, if one corner of the bridge twists or slides in a different direction from another, the likelihood of damage or collapse greatly increases.
Getting an accurate picture of how such motions affect structures requires a shake table that can carry larger loads than current tables can, impose stronger forces and larger accelerations with precision, and create complicated motions, with different forces imposed at different points on the structure being tested. The EMSS designed by Dr. Chaniotakis and his coworkers can hold a 10-story building weighing 1,000 tons. It can operate for 30 seconds and achieve an acceleration of 1 g, a velocity of 3 meters per second and a displacement of half a meter.
Each of its nine panels can shift up and down and sideways, rock front to back and side to side, and twist. The system is modular so the panels can be arranged in any pattern. For example, to test a bridge, the panels can be placed side by side to create a long platform.
Professor Kausel and his coworkers performed simulations that describe the propagation of seismic waves through the ground and the differential motions they create at points on the surface&emdash;points that may be only a few meters apart, thus within the footprint of a sample structure being tested on the EMSS. He also calculated the components of "feedback effects" that would not occur in natural systems and has defined the forces needed to offset them. For example, the structure could begin to move the table rather than vice versa.
A major challenge was finding a means of moving the panels. Existing shake tables are generally driven by hydraulics - a method that does not yield the precision and flexibility needed for a large, articulated shake table. The MIT researchers came up with a novel solution: they used "actuators" driven by electromagnets.
Drawing on their experience with using very large magnets in fusion energy experiments, Dr. Chaniotakis and his colleagues in the Plasma Science and Fusion Center designed an actuator for the EMSS that consists of two magnets. One is a stationary hollow cylinder; the other is a rod inside the cylinder that moves up and down like a piston. Current passing through the outer cylinder generates a magnetic field. Current flowing through the central rod interacts with the magnetic field, generating a force that causes the rod to move. By altering the polarity, the researchers can make the rod go up and down.
Several actuators are placed on each panel, oriented in different directions. By controlling the magnetic fields in the individual actuators, the researchers can make a given panel move in all directions. Altering the pattern of motion requires changing the way electric power flows to the system - a far simpler task than changing hydraulic fluid flows. The result is large, well-controlled forces capable of moving a heavy weight.
Computer simulations - including one using the real signal of the 1995 Kobe earthquake - have shown that the EMSS is capable of delivering the necessary forces to a test structure. The power system can satisfy the requirements of other facilities at the INEEL site, particularly the equipment that will test the response of structures to high winds like those during hurricanes. Dr. Chaniotakis and his team have also designed and built a table-top seismic simulator to demonstrate certain principles inherent in the full-scale concept.
An important practical question is how the EMSS will affect the region around it. The researchers estimate that operating the table will produce forces equivalent to those of a magnitude 4.5 earthquake. The geology at INEEL would seem capable of handling such a disturbance: layers of hard basalt are interspersed with layers of soft volcanic ash that are ideal for damping seismic motion. The MIT researchers determined that most of the ground motion caused by the shaking of the table would be confined to a small region close to the EMSS.
Future tasks include improving the table-top model, developing a half-scale prototype of the actuator included in the conceptual design, and further refining computer simulations of the operation of the EMSS and the interactions between a sample structure, the table and the ground beneath. The research was supported through the Energy Laboratory by the University Research Consortium of the Idaho National Engineering and Environmental Laboratory.
(This article originally appeared in the April/June issue of e-lab, the Energy Laboratory's quarterly research bulletin.)
A version of this article appeared in MIT Tech Talk on September 10, 1997.