ABSTRACT

Earthquake engineering studies the effects of ground movement on buildings, bridges, underground pipes, and dams to determine ways to build future structures or reinforce existing ones so that they can withstand tremors. Earthquake damage and injury are aggravated by the fact that neither the time nor the location of major tremors can be precisely predicted by earth scientists. Damage to human-made structures may be lessened, however, using proper construction techniques.

KEY CONCEPTS

EARTHQUAKES AND SOIL CONDITIONS

Earthquake engineering attempts to minimize the effects of earthquakes on large structures. Engineers study earthquake motion and its effects on structures, concentrating on the materials and construction techniques used, and recommend design concepts and methods that best permit the structures to withstand the forces.

One might logically expect that the structures nearest an earthquake fault would suffer the most damage from the earthquake. Actually, structural damage seems to bear little direct relation to the faults or to their distance from the structure. It is true that buildings near the fault are subject to rapid horizontal or vertical motion and that if the fault runs immediately beneath a structure—which is more likely in the case of a road or pipe than a building—and displaces more than a few inches, the structure could easily fail. The degree of damage, however, has more to do with the nature of the local soil between the bedrock and the surface. If the soil is noncohesive and sand-like, vibrations may cause it to compact and settle over a wide area. Compaction of the soil raises the pressure of underground water, which then flows upward and saturates the ground. This “liquefaction” of the soil causes it to flow like a fluid so that sand may become quicksand. Surface structures, and even upper layers of soil, may settle unevenly or drop suddenly. Sinkholes and landslides are possible effects.

NATURAL FREQUENCY

Ground vibration and most ground motion are caused by seismic waves. These waves are created at the earthquake’s focus, where tectonic plates suddenly move along an underground fault. The waves radiate upward to the surface, causing the ground to vibrate. Wave vibrations are measured in terms of frequency—the number of waves that pass a given point per second.

Much earthquake damage depends on what is known as natural frequency. When any object is struck or vibrated by waves, it vibrates at its own frequency, regardless of the frequency of the incoming waves. All solid objects, including buildings, dams, and even the soil and bedrock of an area, have different natural frequencies. If the waves affecting the object happen to be vibrating at the object’s frequency, the object’s vibrations intensify dramatically—sometimes enough to shake the object apart. For this reason, an earthquake does the most damage when the predominant frequency of the ground corresponds to the natural frequency of the structures.

At one time it was thought that earthquake motion would be greater in soft ground and less in hard ground, but the truth is not that simple. Nineteenth-century seismographers discovered that the natural frequency of local ground depends on the ground’s particular characteristics and may vary widely from one location to another. The predominant frequency of softer ground is comparatively low, and the maximum velocity and displacement of the ground are greater. In harder ground, the predominant frequency is higher, but the acceleration of the ground is greater. When the ground is of multiple layers of different compositions, the predominant frequency is quite complex.

FREE- AND FORCED-VIBRATION TESTS

In order to determine the effect of vibrations on a building, an engineer must do the obvious: shake it. Whereas the effects on a very simple structure such as a pipe or a four-walled shack may be computed theoretically, real-life structures are composed of widely diverse materials. By inducing vibrations in a structure and measuring them with a seismograph, one can easily determine properties such as the structure’s natural frequency and its damping (the rate at which vibrations cease when the external force is removed).

The simplest type of test is the free vibration, and the oldest of these is the pull-back test. A cable is attached to the top of the test structure at one end and to the ground or the bottom of an adjacent structure at the other. The cable is pulled taut and suddenly released, causing the structure to vibrate freely. Other tests cause vibrations by striking the structure with falling weights or large pendulums or even by launching small rockets from the structure’s top.

Forced-vibration tests subject test structures to an ongoing vibration, thereby giving more complete and accurate measurements of natural frequencies. In the steady-state sinusoidal excitation test, a motor-driven rotating weight is attached to the structure, subjecting it to a constant, unidirectional force of a fixed frequency. The building’s movements are recorded, and the motor’s speed is then changed to a new frequency. Measurements are taken for a wide range of different frequencies and forces. Surprisingly, the natural frequencies for large multistory buildings are so low that a 150-pound person rocking back and forth will generate measurable inertia in the structure, thereby providing an adequate substitute for relatively complex equipment.

Another useful device is the vibration table: a spring-mounted platform several meters long on each side. Although designed to hold and test model structures, some tables are large enough to hold full-scale structural components—or even small structures themselves. Useful forced vibrations are also provided by underground explosions, high winds, the microtremors that are always present in the ground, and even large earthquakes themselves.

EARTHQUAKE-RESISTANT DESIGN

Structures can be designed to withstand some of the stresses put upon them by large ground vibrations. They must be able to resist bending, twisting, compression, tension, and shock. Two approaches are used in earthquake-resistant design. The first is to run dynamic tests to analyze the effects of given ground motions on test structures, determine the stresses on structural elements, and proportion the members and their connections to restrain these loads. This approach may be difficult if no record exists of a strong earthquake on the desired type of ground or if the research is done on simplified, idealized structures.

The other approach is to base the designs on the performance of past structures. Unfortunately, new buildings are often built with modern materials and techniques for which no corollary exists in older ones. It follows that earthquake-resistant design is easier to do for simple structures such as roads, shell structures, and one-story buildings than for complex skyscrapers and suspension bridges.

BASIC CONFIGURATION OF A STRUCTURE

The first concern in examining a structure is its basic configuration. Buildings with an irregular floor plan, such as an “L” or “I” shape, are more likely to twist and warp than are simple rectangles and squares. Warping also tends to occur when doors and windows are nonuniform in size and arrangement. Walls can fail as a result of shear stress, out-of-plane bending, or both. They may also collapse because of the failure of the connections between the walls and the ceiling or floor. In the case of bearing walls, which support the structure, failure may in turn allow the collapse of the roof and upper floors. Nonstructural walls and partitions can be damaged by drift, which occurs when a building’s roof or the floor of a given story slides farther in one direction than the floor below it does. This relative displacement between consecutive stories can also damage plastering, veneer, and windowpanes.

Lateral (sideways) cross-bracing reduces drift, as do the walls that run parallel to the drift. Another way to avoid drift damage is to let the nonstructural walls “float.” In this method, walls are attached only to the floor so that when the building moves laterally, the wall moves with the floor and slides freely against the ceiling. (Alternatively, floating walls may be affixed only to the ceiling.) Windows may be held in frames by nonrigid materials that allow the frames to move and twist without breaking the panes. The stiffness and durability of a wall can be improved by reinforcement. For reinforcement, steel or wooden beams are usually embedded in the wall, but other materials are used as well. If the exterior walls form a rectangular enclosure, they may be prevented from separating at the top corners by a continuous collar, or ring beam.

STRUCTURAL ELEMENTS

Frame buildings are those in which the structure is supported by internal beams and columns. These elements provide resistance against lateral forces. Frames can still fail if the columns are forced to bend too far or if the rigid joints fail. Unlike bearing walls, frame-building walls are generally nonstructural; the strength of the frame, however, can be greatly enhanced if the walls are attached to, or built integrally into, the frame. This method is called “in-filled frame” construction. Roofs and upper floors can fail when their supports fail, as mentioned earlier, or when they are subjected to lateral stress. An effective way to avoid such failure is to reduce the weight of the roof, building it with light materials.

Another danger to walls is an earthquake-induced motion known as pounding, or hammering, which can occur when two adjacent walls vibrate against each other, damaging their common corners. The collision of two walls because of lateral movement or the toppling of either is also called pounding. Columns and other structural elements may pound each other if they are close enough; in fact, the elements pounding each other may even be adjacent buildings. If the natural vibrations of the two structures are similar enough, the structures may be tied together and thus forced to vibrate identically so that pounding is prevented. Because such closeness in vibration is rare, the best way to avoid pounding is simply to build the structures too far apart for it to occur.

Shell structures are those with only one or two exterior surfaces, such as hemispheres, flat-roofed cylinders, and dome-topped cylinders. Such shapes are very efficient, for curved walls and roofs possess inherent strength. For this reason, they are sometimes used in low-cost buildings, without reinforced walls. When failure does occur, it is at doors and other openings or near the wall’s attachment to the ground or roof, where stress is the greatest.

Much earthquake damage could be prevented if the stresses on a structure as a whole could be reduced. One of the more practical methods of stress reduction uses very rigid, hollow columns in the basement to support the ground floor. Inside these columns are flexible columns that hold the rest of the building. This engineering technique succeeds in reducing stress, but the flexible columns increase the motion of the upper stories. More exotic methods to reduce stress involve separating the foundation columns from the ground by placing them on rollers or rubber pads. Castles in feudal Japan, one of the most earthquake-prone areas in the world, were constructed in a similar manner. Each story rested on crossmembers that were firred to the floor below in such a way that each floor could slide back and forth during the oscillations produced by an earthquake, leaving the entire structure essentially unaffected afterwards. Structures with several of these lines of defense are much less likely to collapse; should a vital section of cross-bracing, bearing wall, or partition fail, the building can still withstand an aftershock. Overall, the earthquake resistance of a structure depends on the type of construction, geometry, mass distribution, and stiffness properties. Furthermore, any building can be weakened by improper maintenance or modification.

ALTERNATIVES TO UNREINFORCED MASONRY

Buildings using unreinforced masonry (URM) or having those types of veneers have a poor history in past earthquakes. Because unreinforced masonry walls are neither reinforced nor structurally tied to the roof and floors, they move excessively during an earthquake and often collapse. Similarly, ground floors with open fronts and little crosswise bracing move and twist excessively, damaging the building. Unreinforced masonry chimneys may fall to the ground or through the roof. Buildings with those specific kinds of bearing walls are now forbidden by California building codes, but unreinforced masonry is still common in many less developed areas of the world. There are several low-cost earthquake-resistant alternatives to such construction.

Adobe walls may be reinforced with locally available bamboo, asphalt, wire mesh, or split cane. Low-cost buildings should be only one or two stories tall and should have a uniform arrangement of walls, partitions, and openings to obtain a uniform stress distribution. The floor plan should be square or rectangular or, alternatively, have a shell shape such as a dome or cylinder. Roofs and upper floors should be made of lightweight materials—wood, cane, or even plastic, rather than mud or tile—whenever possible, and heavy structural elements should never be attached to nonstructural walls.

The Center for Planning and Development Research at the University of California at Berkeley noted certain features of modern wood-frame houses that make them especially susceptible to damage from strong ground motion. In addition to unreinforced masonry walls or foundations, such houses may have insufficient bracing of crawl spaces, unanchored water or gas heaters, and a lack of positive connections between the wooden frame and the underlying foundations. Porches, decks, and other protruding features may be braced poorly. Most of these deficiencies can be corrected.

PROTECTION AGAINST INJURY AND PROPERTY DAMAGE

Earthquakes are arguably the most destructive natural disaster on the planet. No other force has the potential to devastate so large an area in a very short time. Not only can it not be predicted, but there is also even less advance warning for the earthquake than for other types of disaster. A hurricane can be seen coming and tracked by radar and a volcano typically gives many warning signs before it erupts. An earthquake simply happens.

Yet the magnitude of the earthquake is not solely responsible for the destruction. Property damage and injury to humans also depend on the type and quality of construction, soil conditions, the nature of the ground motion, and distance from the epicenter. The tremor which struck Anchorage, Alaska, in 1964 measured 8.3 on the Richter scale and killed eleven people; by comparison, the earthquake that hit San Fernando, California, in 1971 measured only 6.6—less than a tenth of the force of the Anchorage quake—and fifty-nine people died. Most of the San Fernando deaths occurred in one building: a hospital that collapsed. It seems likely that the hospital had not been adequately constructed to withstand the stresses to which it was suddenly subjected. The higher damage toll resulted from the soil characteristics in San Fernando and an underground fault that had previously been unmapped.

The only protection earthquake-zone residents have against property damage is that given by the engineers who design and build their homes, workplaces, railway structures, dams, harbor facilities, and nuclear power plants and by the public officials who regulate them. Now that engineers can learn how ground movement affects engineered structures and can design new structures accordingly, many of the earthquake-prone regions have building codes mandating earthquake-resistant construction. In some communities, programs exist to determine which buildings are unsafe and how they may be made resistant. Unfortunately, not all quake regions have such rules and programs in place, because of apathy, high cost, or other reasons. The high costs of recovery after major quakes, however, provide a compelling rationale for better preparation.

Further Reading