Glossary

Attenuation — The rate at which earthquake ground motion decreases with distance. Base shear — The total lateral force for which a structure is designed using equivalent lateral force techniques.

Characteristic earthquake — A relatively narrow range of magnitudes at or near the maximum that can be produced by the geometry, mechanical properties, and state of stress of a fault (Schwartz and Coppersmith 1987). Completeness — Homogeneity of the seismicity record.

Cripple wall — A carpenter's term indicating a wood frame wall of less than full height T, usually built without bracing.

Critical damping — The value of damping such that free vibration of a structure will cease after one cycle (ccrit = 2 mo).

Damage — Permanent, cracking, yielding, or buckling of a structural element or structural assemblage. Damping — Energy dissipation that occurs in a dynamically deforming structure, either as a result of frictional forces or structural yielding. Increased damping tends to reduce the amount that a structure responds to ground shaking. Degradation — A behavioral mode in which structural stiffness or strength is reduced as a result of inelastic behavior.

Design (basis) earthquake — The earthquake (as defined by various parameters, such as PGA, response spectra, etc.) for which the structure will be, or was, designed. Ductile detailing — Special requirements, such as for reinforced concrete and masonry, close spacing of lateral reinforcement to attain confinement of a concrete core, appropriate relative dimensioning of beams and columns, 135° hooks on lateral reinforcement, hooks on main beam reinforcement within the column, etc. Ductile frames — Frames required to furnish satisfactory load-carrying performance under large deflections (i.e., ductility). In reinforced concrete and masonry this is achieved by ductile detailing.

Ductility factor — The ratio of the total displacement (elastic plus inelastic) to the elastic (i.e., yield) displacement.

Elastic — A mode of structural behavior in which a structure displaced by a force will return to its original state upon release of the force. Fault — A zone of the earth's crust within which the two sides have moved — faults may be hundreds of miles long — from one to over one hundred miles deep, and not readily apparent on the ground surface.

Ground shaking — A random, rapid cyclic motion of the ground produced by an earthquake.

Hysteresis — A form of energy dissipation that is related to inelastic deformation of a structure.

Inelastic — A mode of structural behavior in which a structure, displaced by a force, exhibits permanent unrecoverable deformation.

Lateral force resisting system (LFRS) — A structural system for resisting horizontal forces due, for example, to earthquake or wind (as opposed to the vertical force resisting system, which provides support against gravity).

Liquefaction — A process resulting in a soil's loss of shear strength due to a transient excess of pore water pressure.

Magnitude — A unique measure of an individual earthquake's release of strain energy, measured on a variety of scales, of which the moment magnitude Mw (derived from seismic moment) is preferred.

Mass participation — That portion of total mass of a multidegree of freedom structure that is effective in a given mode of response.

MCE — Maximum considered earthquake — the earthquake intensity forming the basis for design in the NEHRP Provisions.

Mode shape — A deformed shape in which a structure can oscillate freely when displaced.

Natural mode — A characteristic dynamic property of a structure in which it will oscillate freely.

Nonductile frames — Frames lacking ducility or energy absorption capacity due to lack of ductile detailing — ultimate load is sustained over a smaller deflection (relative to ductile frames) and for fewer cycles.

Participation factor — A mathematical relationship between the maximum displacement of a multi-degree of freedom structure and a single degree of freedom structure.

Peak ground acceleration (PGA) — The maximum amplitude of recorded acceleration (also termed the ZPA, or zero period acceleration).

Period — The amount of time it takes a structure that has been displaced in a particular natural mode and then released to undergo one complete cycle of motion.

Pounding — The collision of adjacent buildings during an earthquake due to insufficient lateral clearance.

Response spectrum — A plot of maximum amplitudes (acceleration, velocity, or displacement) of a single degree of freedom oscillator (sdof), as the natural period of the sdof is varied across a spectrum of engineering interest (typically, for natural periods from 0.03 to 3 or more seconds, or frequencies of 0.3 to 30+ Hz).

Reverse fault — A fault that exhibits dip-slip motion, where the two sides are in compression and move away toward each other.

Seismic risk — The product of the hazard and the vulnerability (i.e., the expected damage or loss, or the full probability distribution).

Soft story — A story of a building signifiantly less stiff than adjacent stories (i.e., the lateral stiffness is 70% or less than that in the story above, or less than 80% of the average stiffness of the three stories above; BSSC 1994).

Spectral acceleration — The maximum response acceleration that a structure of given period will experience when subjected to a specific ground motion.

Spectral displacement — The maximum response displacement that a structure of given period will experience when subjected to a specific ground motion.

Spectral velocity — The maximum response velocity that a structure of given period will experience when subjected to a specific ground motion.

Spectrum amplification factor — The ratio of a response spectral parameter to the ground motion parameter (where parameter indicates acceleration, velocity, or displacement).

Viscous — A form of energy dissipation that is proportional to velocity.

Yielding — A behavioral mode in which a structural displacement increases under application of constant load.

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