ground shaking and thus potential damage at a site. These are the size of the earthquake, the type of earthquake, the distance from the source of the earthquake to the site, and the types of soil at the site. Larger earthquakes will shake longer and harder, and thus cause more damage. Experience has shown that the ground motion can be felt for several seconds to a minute or longer. In preparing for earthquakes, both horizontal (side to side) and vertical shaking must be considered.

Generally, the farther from the source of an earthquake, the less severe the motion. The rate at which motion decreases with distance is a function of the regional geology and inherent characteristics of the earthquake and its source. The underlying geology of the site can also have a significant effect on the amplitude of the ground motion. Soft, loose soils tend to amplify the ground motion and in many cases also make it last longer. In such circumstances, building damage can be accentuated. In the San Francisco earthquake of 1906, damage was greater in the areas where buildings were constructed on loose man-made fill and less at the tops of the rocky hills. Even more dramatic was the 1985 Mexico City earthquake. This earthquake occurred 250 miles from the city, but very soft soils beneath the city amplified the ground shaking enough to cause weaker midrise buildings to collapse. Sites with rock close to or at the surface will be less likely to amplify motion. The type of motion felt also changes with distance from the earthquake. Close to the source the motion tends to be violent rapid shaking, whereas farther away the motion is normally more of a swaying nature. As expected, buildings will respond differently to the rapid shaking than to the swaying motion.

There are many ways to describe the size and severity of an earthquake and associated ground shaking. Perhaps the most familiar are Richter magnitude and Modified Mercalli Intensity (MMI, often simply termed "intensity"). Richter magnitude is a numerical description of the maximum amplitude of

ground movement measured by a seismograph (adjusted to a standard setting). On the Richter scale, the largest recorded earthquakes have had magnitudes of about 8.5. It is a logarithmic scale, and a unit increase in magnitude corresponds approximately to a thirtyfold increase in total energy of the earthquake.

Modified Mercalli Intensity (MMI) is a subjective scale defining the level of shaking at specific sites on a scale of I to XII. (MMI is expressed in Roman numerals, to connote its approximate nature.) For example, slight shaking that causes few instances of fallen plaster or cracks in chimneys constitutes MMI VI. It is difficult to find a reliable relationship between magnitude, which is a description of the earthquake's total energy level, and intensity, which is a subjective description of the level of shaking of the earthquake at specific sites, because shaking severity can vary with building type, design and construction practices, soil type and distance from the event.

The following analogy may be worth remembering: earthquake magnitude and intensity are similar to a lightbulb and the light it emits. A particular lightbulb has only one energy level, or wattage (e.g, 100 watts, analogous to an earthquake's magnitude). Near the lightbulb, the light intensity is very bright (perhaps 100 footcandles, analogous to MMI IX), while farther away the intensity decreases (e.g., 10 footcandles, MMI V). A particular earthquake has only one magnitude value, whereas it has many intensity values.

MMI is a subjective measure of seismic intensity at a site that cannot be measured using a scientific instrument. Rather, MMI is estimated by experts based on observations, such as the degree of disturbance to the ground, the degree of damage to typical buildings and the behavior of people. A more objective measure of seismic intensity at a site, which can be measured by instruments, is the force or acceleration caused by the ground motion. In this handbook, Effective Peak Acceleration (EPA) is employed

as the measure of seismic intensity. The maximum EPA likely to occur during the "life" of a building for any particular region of the United States has been estimated, and divided into seven levels termed "NEHRP Map Areas" 1 to 7. In conducting a survey of seismically hazardous buildings for a specific city, only one Map Area will be involved, and the actual EPA values involved need not be specifically addressed. EPA values are discussed in some detail in the handbook technical supporting document, ATC-21-1 (Appendix B).

15 Seismicity of the United States

It is evident from Figure 1-1 that some parts of the country have experienced more and larger earthquakes than others. The boundary between the North American and Pacific tectonic plates occurs along the west coast of the United States. The San Andreas fault in Califomia and the Aleutian Trench off the coast of Alaska are part of this boundary. These active seismic zones have generated earthquakes with Richter

magnitudes greater than 8. There are many other smaller fault zones throughout the western United States that are also helping to release the stress that is built up as the tectonic plates move past one another. Because earthquakes always occur along faults, the seismic hazard will be greater for those population centers close to active fault zones.

On the east coast of the United States, the cause of earthquakes is less understood. There is no plate boundary and very few locations of faults are known. Therefore, it is difficult to make statements about where earthquakes are most likely to occur. Several significant historical earthquakes have occurred, such as in Charleston, South Carolina, in 1886 and New Madrid, Missouri, in 1811 and 1812, indicating that there is potential for very large earthquakes. However, most earthquakes in the eastern United States are smaller magnitude events. Because of regional geologic differences, eastern and central U.S. earthquakes are felt at much greater distances than those in the western United States, sometimes up to a thousand miles away.

2

Many different types of damage can occur in buildings. Damage can be divided into two categories: structural damage and non-structural damage, both of which can be hazardous to building occupants. Structural damage means degradation of the building's structural support systems (i.e., vertical and lateral force resisting systems), such as the building frames and walls. Non-structural damage refers to any damage that does not affect the integrity of the structural support system. Examples of nonstructural damage are a chimney collapsing, windows breaking or ceilings falling. The type of damage to be expected is a complex issue that depends on the structural type and age of the building, its configuration, construction materials, the site conditions, the proximity of the building to neighboring buildings, and the type of non-structural elements. These possible contributions to the hazard of the building will be discussed in more detail below.

2.1 Earthquake Effects

When earthquake shaking occurs, a building gets thrown from side to side and/or up and down. That is, while the ground is violently moving from side to side, the building tends to stay at rest, similar to a passenger standing on a bus that accelerates quickly. Once the building starts moving, it tends to continue in the same direction, but by this time the ground is moving back in the opposite direction (as if the bus driver first accelerated quickly, then suddenly braked). Thus the building gets thrown back and forth by the motion of the ground, with some parts of the building lagging behind and then moving in the opposite direction. The force F that the building sustains is related to its mass m and the acceleration a, according to Newton's law, F = ma.The heavier the building the more

the force is exerted. Therefore a tall, heavy reinforced concrete building will be subject to much more force than a lightweight, one-story wood frame house, given the same acceleration. Damage can be due to structural members (beams and columns) being overloaded and/or differential movements between different parts of the structure. If the structure is sufficiently strong to resist these forces or differential movements, little damage will result. If the structure cannot resist these forces or differential movements, structural members will be damaged, and collapse may occur.

Building damage is related to the duration and the severity of the ground motion. Larger earthquakes tend to shake longer and harder and therefore cause more damage to structures. Earthquakes with Richter magnitudes less than 5 rarely cause significant damage to buildings, since acceleration levels and duration of shaking for these earthquakes are relatively small. In addition to damage caused by ground shaking, damage can be caused by buildings pounding against one another, ground failure that causes the degradation of the building foundation, landslides, fires and tidal waves (tsunamis). Most of these "indirect" forms of damage are not addressed in this handbook.

The level of damage that results from a major earthquake depends on how well a building has been designed and constructed. The exact type of damage cannot be predicted because no two buildings undergo identical motion. However, there are some general trends that have been observed in many earthquakes. Post-earthquake investigation teams have found that steel buildings perform significantly better than those built of unreinforced masonry, for example. Newer buildings generally sustain less damage than older buildings designed to earlier

codes. Common problems in wood frame construction are the collapse of unreinforced chimneys (Figure 2-1) and houses sliding off their foundations (Figure 2-2). Although such damage may be costly to repair, it is not usually life threatening. On the other hand, the collapse of load bearing walls that support the entire structure is a common form of damage in unreinforced masonry structures. Roofs have collapsed in many older tilt-up buildings. From a life-safety perspective, vulnerable buildings need to be clearly identified and strengthened or removed.

Each building has its own vibrational characteristics that depend on building height and structural type. Similarly, each earthquake has its own vibrational characteristics that depend on the geology of the site, distance from the source, and the type and site of the earthquake source mechanism. Sometimes the characteristics of the earthquake and the building are very similar and cause a sympathetic response, termed resonance. Resonance, which occurs when the frequency of earthquake excitation is equal to the natural frequency of the building, will cause an increase in the amplitude of the building's vibration and consequently increase the potential for damage. Resonance was a major problem in the 1985 Mexico City earthquake, in which the total collapse of many mid-rise buildings (Figures 23 and 2-4) posed a serious life-safety hazard.

22 How Earthquake Forces are Resisted

Buildings experience horizontal distortion when subjected to earthquake motion. When these distortions get large, the damage can be catastrophic. Therefore, most buildings are designed with lateral force resisting systems (LFRS), to resist the effects of earthquake forces. In many cases LFRS make a building stiffer and thus minimize the amount of lateral movement and consequently the damage. LFRS are usually capable of resisting only forces that result from ground motions parallel to them.

However, the combined action of LFRS along the width and length of a building can typically resist earthquake motion from any direction. LFRS differ from building to building because the type of system is controlled to some extent by the basic layout and structural elements of the building. Basically, LFRS consist of axial(tension and/or compression), shear- and/or bending-resistant elements.

In wood frame stud-wall buildings, plywood siding is typically used to prevent excessive lateral deflection. Without the extra strength provided by the plywood, walls would distort excessively or "rack," resulting in broken windows and stuck doors. In older wood frame houses, this resistance to lateral loads is provided by either wood or steel bracing.

The earthquake resisting systems in modern steel buildings take many forms. Many types of diagonal bracing configurations have been used. Examples of the use of single diagonal braces, cross-bracing and "k-bracing" are shown in Figure 2-5. Moment-resisting steel frames are also capable of resisting lateral loads. In this type of construction the connections between the beams and the columns are designed to resist the rotation of the column relative to the beam. Thus, the beam and the column work together and resist lateral movement by bending. This is contrary to the braced frame, where loads are resisted through tension and compression forces in the braces. Steel buildings are sometimes constructed with moment resistant frames in one direction and braced frames in the other.

In concrete structures, "shear walls" are sometimes used to provide lateral resistance, in addition to moment-resisting frames. Ideally, these shear walls are continuous reinforcedconcrete walls extending from the foundation to the roof of the building, and can be exterior walls or interior walls. They are interconnected with the rest of the concrete frame, and thus resist the motion of one floor relative to another. Shear walls can also be constructed of