CA Seismic key terms

The largest acceleration the ground experiences during an earthquake, usually expressed as a fraction of gravity. It is a key input for site-specific design spectra and for evaluating how strongly a structure's base will be shaken.

The peak acceleration response of a single-degree-of-freedom oscillator at a given natural period and damping when subjected to a ground motion. Design codes plot Sa against period to form the response spectrum used for base shear.

A graph showing maximum response (acceleration, velocity, or displacement) of simple oscillators of varying natural periods subjected to one ground motion. Smoothed design spectra generalize this for code-based seismic design across many possible earthquakes.

The longest natural period of vibration of a structure, governed mainly by its mass and stiffness. Code base shear depends heavily on this value, and long-period structures may be sensitive to soft-soil amplification.

A soil or rock category (A through F) assigned from measured shear-wave velocity, standard penetration resistance, or undrained shear strength in the upper 100 feet. It modifies the design spectrum to reflect local ground conditions.

The increase in ground shaking amplitude caused by soft soils relative to firm rock, especially at periods matching the soil column's natural period. It is captured in codes through site coefficients applied to spectral values.

The earthquake ground motion level a structure must resist without collapse and with limited damage, typically associated with a specified probability of exceedance over a reference period. It anchors the design response spectrum.

The most severe earthquake ground motion considered in codes, often defined at a very low probability of exceedance. Collapse-prevention provisions are calibrated to it, and it underlies the mapped spectral values modified by site factors.

The total lateral force at the base of a structure that the seismic-force-resisting system must resist, computed from the design spectrum and structural weight. It is distributed over the height for member and story design.

A simplified code method that represents earthquake effects as static lateral forces applied at each floor, scaled by base shear. It is permitted for regular structures where higher-mode effects are modest.

A coefficient that accounts for a system's ductility, overstrength, and redundancy by reducing elastic seismic demand to a design-level force. Systems expected to deform inelastically safely receive higher R values.

A multiplier applied to elastic lateral displacements to estimate realistic inelastic drift during design-level shaking. It is used to check story drift limits and separation between adjacent structures.

The horizontal displacement of one floor relative to the floor below, often expressed as a ratio to story height. Excessive drift damages nonstructural elements and can cause instability, so codes set limits.

The additional overturning moment and lateral demand created when gravity loads act on a laterally displaced structure. It grows with drift and height and can amplify forces enough to require strength adjustment or stability checking.

The structural assembly of elements designated to resist earthquake forces, such as moment frames, braced frames, or shear walls. Codes assign each system specific detailing requirements and response modification factors.

The ability of a member or system to undergo large inelastic deformation without losing significant strength. It lets structures dissipate seismic energy and is the basis for force reductions in design codes.

A localized region of a structural member that yields and rotates inelastically while maintaining moment capacity. Controlled hinge formation in beams before columns supports the strong-column/weak-beam concept.

A design philosophy ensuring columns remain stronger than the beams framing into them so that yielding occurs in beams rather than columns. This promotes a ductile, stable mechanism instead of a brittle story collapse.

A design approach in which selected elements are made stronger than the elements intended to yield, using overstrength factors. It forces inelastic behavior into ductile locations and protects critical components from unexpected failure.

The extra resistance a structure possesses beyond its nominal design strength due to material factors, strain hardening, and conservative detailing. It is used in capacity design to protect non-yielding elements.

A map showing ground-motion values with a stated probability of exceedance, used to derive code design spectra. Values are adjusted for site class and may include near-fault factors.

Ground-motion characteristics close to a rupturing fault, including large velocity pulses and directivity that can impose severe demands on long-period structures. Codes may require amplified design forces in such zones.

Loss of soil shear strength when saturated loose sands or silts are shaken and pore pressures rise, causing the ground to behave like a fluid. It can cause settlement, lateral spreading, and foundation failure.

Horizontal ground displacement that occurs when liquefied or softened soil moves down a slope or toward a free face. It can pull apart foundations, utilities, and bridge approaches even when shaking itself is moderate.

A segment of an active fault that has not ruptured for a long time relative to its typical recurrence interval, suggesting accumulated strain. Such gaps are treated as elevated-hazard sources in seismic evaluation.

Surface displacement along a fault during an earthquake, which can shear foundations, pipelines, and structures straddling the trace. Setbacks and avoidance are typical mitigation measures.

Elastic energy traveling through the earth as body waves (P and S) and surface waves, each with distinct velocities and shaking characteristics. S-waves and surface waves generally cause the strongest structural shaking.

Energy dissipation in a vibrating structure from internal friction, connections, and nonstructural elements, expressed as a percentage of critical damping. Higher damping lowers response peaks and widens design spectra.

The characteristic deformation pattern of a structure vibrating at one of its natural frequencies. Higher modes matter for tall or irregular buildings and are combined using modal analysis.

A dynamic analysis method that computes a structure's natural periods and mode shapes, then combines modal responses to estimate total seismic demand. It suits tall or irregular structures where the equivalent lateral force method is inadequate.

A strategy using flexible bearings at the base to lengthen a structure's period and shift it away from dominant ground-motion frequencies. It reduces accelerations transmitted to the superstructure, though displacements at the isolation level increase.

A supplemental damper, such as a viscous, friction, or metallic-yielding device, installed in a frame to absorb seismic energy. These devices reduce drift and damage to the primary structural system.

The presence of multiple load paths or elements so that failure of one component does not cause disproportionate collapse. Codes may reward redundant systems with reduced design forces.

A building with plan or vertical discontinuities, such as soft stories, offsets, or torsional imbalance, that complicate seismic response. Codes impose stricter analysis and design requirements for such configurations.

A story significantly more flexible than those above it, often from open ground-floor framing, which concentrates inelastic drift. It is a common collapse mechanism unless strengthened or designed for the demand.

A condition where a building's mass and stiffness centers are offset, causing uneven lateral displacement across the plan during shaking. It increases demands on perimeter elements and triggers stricter analysis requirements.

A ratio comparing P-delta overturning effects to story shear, used to judge whether second-order effects can be ignored. Exceeding code thresholds requires amplified design forces or a more rigorous analysis.

Building elements not part of the structural system, such as ceilings, partitions, cladding, and mechanical equipment, that must be anchored and braced. Their seismic qualification protects occupants and maintains function after an event.