Spectral acceleration (SA) is a unit measured in ''g'' (the acceleration due to

Earth's gravity The gravity of Earth, denoted by , is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a vector qua ...

, equivalent to

g-force The gravitational force equivalent, or, more commonly, g-force, is a measurement of the type of force per unit mass – typically acceleration – that causes a perception of weight, with a g-force of 1 g (not gram in mass measur ...

) that describes the maximum acceleration in an

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

on an object – specifically a damped,

harmonic oscillator In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force ''F'' proportional to the displacement ''x'': \vec F = -k \vec x, where ''k'' is a positive const ...

moving in one physical

dimension In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coord ...

. This can be measured at (or specified for) different oscillation

frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is e ...

and with different degrees of

damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples i ...

, although 5% damping is commonly applied.Intensity Measure Type (IMT)
OpenSHA, accessed 2011-04-14 The SA at different frequencies may be plotted to form a response spectrum. Spectral acceleration, with a value related to the natural frequency of vibration of the building, is used in

earthquake engineering Earthquake engineering is an interdisciplinary branch of engineering that designs and analyzes structures, such as buildings and bridges, with earthquakes in mind. Its overall goal is to make such structures more resistant to earthquakes. An earth ...

and gives a closer approximation to the motion of a building or other structure in an earthquake than the

peak ground acceleration Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a par ...

value,FAQs – What is "spectral acceleration" or SA?

United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

, accessed 2011-04-14 although there is normally a

correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...

between hort periodSA and PGA. Some

seismic hazard A seismic hazard is the probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold. With a hazard thus estimated, risk can be assessed and incl ...

maps are also produced using spectral acceleration.

External links

Spectral Acceleration Hazard Map of California – for 1 sec period

2005 National Building Code of Canada – Spectral Acceleration Hazard Maps for various periodsRevision of Time-Independent Probabilistic Seismic Hazard Maps for AlaskaWhat is a ground shaking hazard map? – Includes explanations of SA and PGA

References

Seismology Earthquake engineering {{seismology-stub

See also

External links

References