By Dr. Alexander Tarics
Article originally published in the September-October 1987 issue of The Military Engineer.
In 1980, the National Security Council investigated what the consequences of a major California earthquake would be and concluded that they “would surpass those of any natural disaster thus far experienced by the nation. Indeed, the United States has not suffered any disaster of this magnitude on its own territory since the Civil War.” Geologists and earthquake planners place the odds at 50 percent that a tremor measuring 8.3 on the Richter scale will hit California within the next 20 to 30 years.
Recent observations made in the walls of trenches excavated across the San Andreas Fault and measurements of the concentration of carbon isotopes provided new information on the approximate time of the occurrence of major earthquakes. Kerry E. Sieh of the California Institute of Technology determined that the average reoccurrence interval of major earthquakes along the San Andreas Fault in Southern California is about 140 to 150 years. The last serious one on the Mojave Segment of the San Andreas Fault happened 130 years ago in 1857. It was estimated that, since then, the accumulated strain in the earth’s crust is about 190 inches and, if released, could cause a major earthquake. But moderate quakes, which occur more frequently, can also cause a major disaster if they happen close enough to populated areas.
The 1971 San Fernando earthquake in California registered only a 6.4 on the Richter scale and accordingly was called “moderate”; yet many buildings, among them 17 hospitals, were damaged or destroyed. Four major hospitals were within 10 miles of the epicenter and most of them had up-to-date earthquake-resistant structural systems. All were significantly damaged and had to be evacuated. Also, the 1986 earthquake in San Salvador, registered only 5.4, yet over 1,000 people died and structural damage was in excess of $2 billion. Considering the heavy concentration of military assets in California, predictions about seismic occurrences must be taken seriously.
The consequences of a major earthquake could be devastating to the national defense. About 340,000 people work in 105 major military installations in California and more are employed in the associated industries. The Mare Island, Oakland, and Alameda Navy Bases; the Presidio of San Francisco; the Lawrence Livermore and Sandia Laboratories; and the Silicon Valley, where most of our microchips are produced, are close to the San Andreas and Hayward faults. In the Los Angeles area, military and industrial facilities are concentrated near the Newport-Inglewood Fault. Edwards, Norton, and Vandenberg Air Force Bases and the military installations around San Diego Bay are all in heavy earthquake-prone areas. This list is far from complete. The Department of Defense and the State of California both emphasize recovery planning to cope with the military and civilian disruption that would follow an earthquake.
The Secretary of Defense’s 1987 fiscal report to Congress recognized the “potential direct impact on our defense industrial base and national security posture.” Fortunately, recent developments and a major breakthrough in seismic engineering offer great promise that earthquake damage, at least to buildings and other structures such as bridges, can be significantly minimized and in many cases even prevented in the future, if proper actions are taken now.
Conventional engineering provides only limited seismic protections.—Construction is one of our oldest activities, yet we did not learn until recently how to erect buildings that can be expected to remain undamaged and their contents safe after an earthquake. The basic reason is the engineering principle on which current seismic engineering tries to protect our buildings. The traditional practice is to tie structures firmly to their foundations and make them strong enough to resist the forces produced by earthquakes. Experience has taught us that such a design generates very high earthquake forces in the buildings which often exceed the capacity of the structural system.
To prevent buildings from collapsing, we rely on the ductility of the structural materials. These materials stretch before they break; consequently, the buildings undergo large and permanent inelastic distortions during earthquakes. They may not collapse, but they are so badly damaged that they are unsafe for occupancy and must be demolished. Hospitals, communication and emergency operations centers, and police and fire stations cannot be used when they are needed most: immediately after the earthquake.
Taming earthquakes with new technology.—A recent breakthrough in seismic engineering called “base isolation” is revolutionizing how structures are engineered in earthquake-prone areas. Buildings are mounted on rubber-steel combination shock-absorbing pads that, during an earthquake, prevent most of the horizontal ground movement from being transmitted to the structures. Unlike a conventional design, the loose contents of the buildings are also protected; therefore, buildings can be expected to remain undamaged and functional after an earthquake.
Base isolation is predicated on a new principle: instead of tying a building to its foundation, the two are separated; this permits the building to “float” on flexible pedestals during earthquakes. This principle opens the door to further discoveries and improvements in our search for better seismic safety. The first building on base isolators in the U.S. was completed in 1985. Located at the crossroads of three of Southern California’s most active fault zones and only 11 miles from the San Jacinto Fault, where 17 significant earthquakes have occurred since 1890, the $38 million Foothill Communities Law and Justice Center (FCLJC) in San Bernardino County is mounted on 98 shock-absorbing pads called base isolators.
On October 2, 1985, an earthquake rocked the countryside at Redlands, California, with the epicenter 19 miles from the FCLJC. The quake registered only 4.9 on the Richter scale; consequently, it did not do any damage. However, it triggered the seismic sensors installed in the building by the Division of Mines and Geology of the State of California.
Base isolation is predicated on a new principle: instead of tying a building to its foundation, the two are separated; this permits the building to “float” on flexible pedestals during earthquakes.
Five conventionally designed and constructed buildings in the area were also similarly instrumented. While these structures amplified the ground accelerations from two to five times as expected, the base isolated building reduced them by one-third, according to design. This is one of the two known and documented evidences, other than theoretical considerations or laboratory tests, that base isolation can be effective in protecting buildings and their contents from earthquakes.
The isolators in the Center are cylindrical, 30 inches in diameter, 16 to 17 inches high, and are made of many layers of approximately 1/8-inch-thick steel plates and ½-inch-thick, special composition high-damping rubber. The layers are vulcanized to the adjacent steel plates. The base isolator bearings have very high vertical stiffness to support the weight of the building and very low horizontal stiffness to prevent high ground accelerations from being transmitted into the building during an earthquake. The bearings are similar to those commonly used in bridge construction to control the effects of temperature change on the bridges.
How does base isolation work?—When an earthquake occurs, much of its energy is released in rapid, violent ground shaking. This back-and-forth movement occurs at different rates at different building sites. A typical “California” quake would have a dominant frequency of about 2 to 4 cycles per second. The 1985 Mexico City earthquake, however, had a dominant frequency of only about 0.5 cycle per second. A cycle per second is referred to as a hertz, so much of the energy of an earthquake is released at a frequency of from 2 to 4 hertz.
Every building also has a natural vibration frequency. If the top of a conventional building were pulled and then released, the building would vibrate at a rapid constant rate or frequency. For more structures of less than about 10 stories, this frequency would be in the range of from 1 to 3 hertz, which is close to that of an earthquake.
The relationship between the frequency of an earthquake and that of a building is important and significantly affects the behavior of the structure during the earthquake. When the natural frequency of the building is close to the dominant earthquake frequency, the building resonates and the deceptively harmless movement of the ground violently shakes it, particularly on the higher floors. By putting base isolators between the building and the ground, we move the natural frequency of the building far from the dominant frequency of the earthquake.
How much does it cost?—Base isolation is economical. Several studies indicated that the added costs of base isolation design, the isolators, and the additional construction at the foundation level caused by installing the isolators are offset by savings in erecting the building above the isolators, since it can be designed for drastically reduced earthquake forces. This is, of course, only for the initial cost; the economical benefits are overwhelming after the first, even moderate, earthquake.
Base isolation and its associated costs usually represent a small percent of the total expense of constructing a building. The added costs around the foundation could vary between 2 and 5 percent of the total price of the building, and the savings in the superstructure are about the same. The cost of the base isolators on the FCLJC project was less than 1 percent of the total construction cost.
Range of application.—Base isolation is most effective on buildings that have high natural frequencies. Reinforced concrete or masonry shear wall buildings, or structural steel buildings with braced frames in the range of five to 10 stories in height, are examples. Soil conditions under the building determine the vibration characteristics of the earthquakes. Careful geotechnical studies must be made on any site for which a base isolated structure is considered.
Base isolation can be used to rehabilitate old buildings. If a building has a structural system that can resist minor earthquakes, the installation of base isolators can provide additional protection for major earthquakes. Most of the construction work is at the foundation level; therefore, interference with everyday use of the building is minimal.
The acceptance of base isolation.—Base isolation was first officially recognized in the U.S. in California. The State Legislature passed an Assembly Concurrent Resolution requesting that the State Architect give full consideration to base isolation “…that can mitigate the effects of major earthquakes on new or existing buildings….” Also, the Building Safety Board of the Office of Statewide Health Planning and Development at its January 13, 1987, meeting approved a document specifying the design and basis for design review of hospitals with base isolation for earthquake protection.
A recent survey indicated that at least 25 countries have ongoing research programs on base isolation and the number of structures, mainly buildings and bridges, already constructed on base isolators is somewhere between 75 and 100.
Other developments.—The high-damping rubber base isolators used on the FCLJC project are the latest development in base isolation technology. Several other base isolation mechanisms have been successfully tested and installed in buildings and bridges. Others are in the conceptual development and early testing stages. The various devices are not identical in their performance and do not accomplish the same thing. An in-depth comparative study should be made before selecting the most appropriate base isolator mechanism for a particular project.
Base isolation strategy for earthquake protection is not the only recent development in seismic engineering. Passive energy dissipators provide protection by absorbing earthquake energy, while a building not on base isolators deforms laterally during an earthquake.
When the natural frequency of the building is close to the dominant earthquake frequency, the building resonates and the deceptively harmless movement of the ground violently shakes it, particularly on the higher floors.
The most developed and tested such device is the one recommended by Dr. Avtar Pall of Montreal, Canada. Friction brake lining pads are placed in the structure at the intersection of frame cross braces. The device slips during a major earthquake at a predetermined optimum load and thereby extracts kinetic energy from the moving building. This energy would otherwise force the structural system to yield and cause permanent damage. The device was successfully tested at the University of British Columbia and at the Earthquake Engineering Research Center of the University of California, Berkeley.
Conclusion.—The acceptance and incorporation of such a radical change as base isolation, or passive energy dissipation, in our building practices are very complex processes. They require the transfer of technology from specialists to practicing structural engineers. The specialists developed this new construction technology and worked out appropriate computer programs for predicting the performance of these buildings during earthquakes. It is also necessary that this new information be transferred to architects who advise building owners on matters related to building designs.
Representatives of federal, state, and local government agencies also must be informed because they make the final decisions on what kind of earthquake protection will be provided for our public buildings and military installations. Much of the time and effort needed to transfer this information is already behind us. The technology has been adequately developed and is available to contribute significantly to solving what was until now an unsolvable problem—how to keep out military and supporting defense industry installations undamaged and functional after an earthquake.
[Article originally published in the September-October 1987 issue of The Military Engineer.]