Earthquake-Resistant Construction

construction in regions subject to earthquakes that takes into account the effects of seismic (inertial) forces on buildings and structures. A more precise term, “antiseismic construction” (anti-seismicheskoe stroitel’stvo), has come into use in Russian alongside the term “earthquake-resistant construction” (seismo-stoikoe stroitel’stvo). The supplementary requirements for buildings and structures being built in seismic regions are set by appropriate norms and rules.

Different countries use different seismic scales to measure earthquake intensity. According to the scale adopted in the USSR (All-Union State Standard [GOST] 6249–52), earthquakes registering seven and more are considered dangerous to buildings and structures. In regions where the maximum predicted intensity of earthquakes does not exceed six, no provision is generally made for antiseismic measures in design and construction. The seismicity of regions subject to earthquakes is determined by seismic-zoning maps. Appropriate surveys are made to determine the precise seismicity of construction areas. Because construction in regions whose seismicity registers greater than nine is very uneconomic, the norms limit themselves to regions whose seismicity ranges from seven to nine. Ensuring that buildings will survive earthquakes completely intact usually necessitates great expenditures for antiseismic measures, and in some cases this is not practically feasible. Because earthquakes, especially strong ones, occur comparatively rarely, the norms allow for the possibility of damage to structural members if this does not threaten human safety or the survival of valuable equipment.

The degree to which seismic forces act on buildings and structures depends in large measure on soils. Solid rocky soils are considered best in a seismic sense. Rocks that are greatly weathered or broken by geological processes provide unfavorable, and sometimes even unsuitable, bedding for structures, as do subsided ground and areas with fill material, quicksand, and mining excavations. When construction is carried on under such geological conditions, builders reinforce the bedding and take additional steps to protect the structures against seismic forces. This makes the construction significantly more expensive.

Builders make structures earthquake resistant by selecting seismically favorable construction sites and developing the most efficient designs and plans for the structures. They provide for special design features to increase the strength and uniformity of the supporting elements, which make it possible for structural members and joints to accommodate deformations and thus significantly increase the resistance of structures to the action of seismic forces. High-quality building materials and construction work are very important in improving the earthquake resistance of structures.

The correct choice of structural systems and cross-sectional dimensions is determined by appropriate calculations. According to existing norms, design of earthquake-resistant structures generally is based on load-carrying capacity and is aimed at calculating design seismic loadings. It is not possible to determine precisely the magnitudes of seismic forces and the directions in which the forces act on a structure because movement of the earth’s crust during an earthquake depends on many factors that can be estimated quantitatively only with certain assumptions. Various approximate methods of estimating seismic forces are used. The static method of determining seismic forces, which was common in the first half of the 20th century, proceeds from the assumption that the structure is an absolutely rigid body, all points of which have seismic accelerations equal to the acceleration of the bedding; therefore, the inertial forces that develop in the structure are equal to the product of the corresponding masses and the acceleration of the bedding.

The dynamic method of determining seismic forces, which is more sophisticated, is used in designs and calculations for earthquake-resistant construction in the USSR, the United States, and other countries today. Even this method, however, makes a number of assumptions, primarily because there is no reliable prior information on the maximum values of the basic characteristics of the movement of the beddings of buildings and other structures during earthquakes or on the way the basic characteristics change in the course of an earthquake (displacements, velocities, accelerations).

In view of the approximate nature of the techniques used to estimate the ability of structures to withstand seismic forces, the norms introduce a number of mandatory design restrictions and requirements. For example, there are restrictions on the dimensions of buildings in plan and elevation, and three categories have been established for the earthquake resistance of brickwork, the first indicating the greatest strength and solidity and the third indicating the least. Buildings that have brick walls of second-category brickwork and that are being built in regions of seven-point seismicity cannot be more than four stories tall; in a region of nine-point seismicity, the maximum is two stories. For brick and stone walls, the norms determine minimum cross-sectional dimensions of dividing walls, set distances between walls, and require reinforced-concrete belts on every floor. They do not restrict the height of buildings constructed from the most dependable structural members and materials, such as frame buildings constructed of steel and reinforced concrete or with cast reinforced-concrete walls.

The magnitudes of seismic loads and all design requirements are set by the norms in accordance with the seismicity of the construction site and the purpose of the building or structure. The design seismicity of most buildings is taken to be equal to the seismicity of the construction site. For especially important structures, the calculated seismicity is made higher than the seismicity of the contruction site; the increase is generally by one point, which corresponds to a doubling of the seismic load. The figure is lowered for temporary structures, such as storage buildings, whose destruction would not endanger human life.