Glossary of Seismic Terminology
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GLOSSARY OF SEISMIC TERMINOLOGY Acceleration – The time rate of velocity Center of Resistance – Resultant of change, commonly measured in “g” (an resistance provided by walls and frames. acceleration of 32 ft/sec/sec or 980 cm/sec/sec = gravity constant on earth). Configuration Terms: Accelerogram – The record from an Building Configuration – Size, shape and accelerograph showing acceleration as a proportions of the building; size, shape and function of time. location of structural elements; and the type, size and location of nonstructural Accelerograph – A strong motion earthquake elements. instrument recording ground (or base) acceleration. Regular Configuration – Building configurations resisting lateral forces with Aftershock – One of a series of smaller shear walls, moment resistant frames or quakes following the main shock of the braced frames - all in simple and near earthquake. symmetrical layout. Amplification - The period (or frequency) of Irregular Configuration – Deviation from the ground motion coinciding with the period simple symmetrical building configurations of the building causing significant increase of with repetitive plan and volume. (See acceleration and damage. WBDG Seismic Design Principles resource page for examples). Amplitude – Maximum deviation from mean of centerline of a wave. Structural Configuration – The size, shape and arrangement of the vertical load Attenuation – Reduction of amplitude or carrying the lateral force resistance change in wave due to energy dissipation components of a building. over distance within time. Core – The central part of the earth below a Axial Load – Force coincident with primary depth of 2,900 kilometers. It is thought to be axis of a member. composed of iron and nickel and to be molten on the outside with a central solid inner core. Base Isolation – A method using flexible bearings, whereby a building superstructure is Creep (along a fault) - Very slow periodic or detached from its foundation in order to episodic movement along a fault trace without reduce earthquake forces. earthquakes. Base Shear or Equivalent Lateral Force Crust - The lithosphere, the outer 80 (ELF)– Total shear force acting at the base of kilometers of the earth’s surface made up of a structure. crustal rocks, sediment and basalt. The general composition is silicon-aluminum-iron. Brittle Failure – Failure in material due to limited plastic range; material subject to sudden failure without warning signs. Center of Mass – Point in the building plan at which the building would be exactly balanced. Damping – The rate at which natural vibration Dynamic - The opposite of “static”, when a decays as a result of the absorption of energy. body (building) is in motion. In buildings it is an inherent nature to resonate inefficiently to vibration depending on structural Eccentric Braced Frame – A steel frame in connections, kinds of materials and which diagonal bracing is arranged eccentric to nonstructural elements used. “Damping” column/beam joints. design measures can reduce the magnitude of seismic forces. Effective Peak Acceleration – A coefficient shown on NEHRP maps used to determine Critical Damping – The minimum damping seismic forces. that will allow a displaced system to return to its initial position without oscillation. Elasticity – The ability of a material to return to its original form or condition after a Deflection – The horizontal or vertical displacing force is removed. Materials have an displacement of a member due to the elastic range. application of external force. Elastoplastic – The total range of stress Deformation – Permanent distortion due to (deformation), including expansion beyond seismic forces. elastic limit into the plastic range. In the plastic range deformation is permanent. Depth of Focus – the depth of the focus or hypocenter beneath the earth’s surface Energy Dissipation – Reduction in intensity of commonly classes Earthquakes: Shallow (0- earthquake shock waves with time and 70 kilometers), intermediate (70-300 distance, or by transmission through kilometers), and deep (300-700 kilometers). discontinuous materials with different absorption capabilities. Design Earthquake – Generally defined as 2/3 of the maximum considered earthquake. Epicenter – The point of the earth’s surface directly above the focus or hypocenter of an Diaphragm – Generally a horizontal member, earthquake. such as a floor or roof slab, which distributes lateral forces to vertical resisting elements. Equivalent Lateral Force (ELF) – The representation of earthquake forces on a Displacement - Lateral movement of the building by a single static force applied at the structure caused by lateral force. base of a building; also referred as Base Shear (V). Drift - Horizontal displacement of basic building elements due to lateral earthquake Failure Mode – The manner in which a forces. structure fails (column buckling, overturning of structure, etc). Ductility – Ability to withstand inelastic strain without fracturing. Ductility is a material Fault Terms: property to fail only after considerable inelastic (permanent) deformation which process Fault – A fracture plane in the earth’s crust dissipates the energy from the earthquake by across which relative displacement has design. occurred. (Location of slippage between the earth’s plates). Duration – The period of time within which ground acceleration occurs. Normal Fault – A fault under tension Eccentric Bracing – The centerlines of where the overlying block moves down the brace, beam and of column and do not dip or slope of the fault plane. coincide allowing deformation, thereby utilizing ductility. Strike-Slip Fault (or lateral slip) – A fault whose relative displacement is purely Moment Frame – Frames in which horizontal. structural members and joints resist lateral forces by bending. There are “ordinary”, Thrust (Reverse) Fault – A fault under “intermediate” and “special” moment compression where the overlying block frames. The latter provide the most moves up the dip or slope of the fault resistance. plane. Frequency - The number of wave peaks or Oblique-Slip Fault – A combination of cycles per second. The inverse of Period. normal and slip or thrust and slip faults whose movement is diagonal along the dip Fundamental or Natural Period – The of the fault plane. elapsed time, in seconds, of a single cycle of oscillation. The inverse of Frequency. Faulting – The movement which produces relative displacement of adjacent rock masses "g" - see Acceleration. along a fracture. Graben (rift valley) - Long, narrow trough Fault Zones – The zone surrounding a major bounded by one or more parallel normal faults. fault, consisting of numerous interlacing small These down-dropped fault blocks are caused faults. by tensional crustal forces. FEMA – Federal Emergency Management Ground Acceleration - Acceleration of the Agency. Free publications available at: ground due to earthquake forces. http://www.fema.gov/ Ground Displacement - The distance that Flexible System – A structural system that will ground moves from its original position during sustain relatively large displacements without an earthquake. failure. Ground Failure - A situation in which the Focal Depth – Depth of the earthquake (or ground does not hold together such as land hypocenter) below the ground surface. sliding, mud flows and liquefaction. Focus (of an earthquake) or Hypocenter – Ground Movement - A general term; includes The point at which the rupture occurs; (It all aspects of motion: acceleration, particle marks the origin of the kinetic waves of an velocity and displacement. (The plates of the earthquake). earth's crust move slowly relative to one- another accumulating pressure or strain Frame Terms: resulting in slippage and complex vibration inducing forces in a building.) Braced Frame – One having diagonal braces for stability and capacity to resist Ground Velocity - Velocity of the ground lateral forces. during an earthquake. Concentric Braced Frame – The Hypocenter or Focus - The point below the centerlines of brace, supporting beam and epicenter at which an earthquake actually column coincide. begins; the focus. Input Motion - A term representing seismic Machine Isolators – Calibrated mountings forces applied to a structure. with springs used to attenuate vibration generated by machines. For seismic locations Inelastic - Behavior of an element beyond its they are modified in order to absorb lateral elastic limit, having permanent deformation. movement and to keep the machine or equipment upright. These devices are Inertial forces - Earthquake generated available commercially. vibration of the building's mass causing internally generated inertial forces and building Magnification Factor - An increase in lateral damage. Inertial forces are the product of forces at a specific site for a specific factor. mass times acceleration (F = m a). Magnitude - A measure of earthquake size Intensity - A subjective measure of the force which describes the amount of energy of an earthquake at a particular place as released. See Richter Scale. determined by its effects on persons, structures and earth materials. Intensity is a Mantle - The main bulk of the earth between measure of energy. The principal scale used the crust and the core. in the United States today is the Modified Mercalli, 1956 version. MM (or Modified Mass – A constant quantity or aggregate of Mercalli) scale is based on observation of the matter. effects of the earthquake MM-I thru MM-XII (MM-I = not felt, MM-XII = damage nearly