Spectral acceleration
Encyclopedia
Spectral acceleration is a unit measured in g (the acceleration due to Earth's gravity
, equivalent to g-force
) that describes the maximum acceleration in an earthquake
on an object – specifically a damped, harmonic oscillator
moving in one physical dimension
. This can be measured at (or specified for) different oscillation frequencies
and with different degrees of damping
, although 5% damping is commonly applied. 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
and gives a closer approximation to the motion of a building or other structure in an earthquake than the peak ground acceleration
value, although there is normally a correlation between SA and PGA.
Some seismic hazard
maps are also produced using spectral acceleration.
Earth's gravity
The gravity of Earth, denoted g, refers to the acceleration that the Earth imparts to objects on or near its surface. In SI units this acceleration is measured in metres per second per second or equivalently in newtons per kilogram...
, equivalent to g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
) that describes the maximum acceleration in an earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
on an object – specifically a damped, harmonic oscillator
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 constant....
moving in one physical dimension
Dimension
In physics and mathematics, the dimension of a 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 because only one coordinate is needed to specify a point on it...
. This can be measured at (or specified for) different oscillation frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and with different degrees of damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
, although 5% damping is commonly applied. The SA at different frequencies may be plotted to form a response spectrum
Response spectrum
A response spectrum is simply a plot of the peak or steady-state response of a series of oscillators of varying natural frequency, that are forced into motion by the same base vibration or shock. The resulting plot can then be used to pick off the response of any linear system, given its natural...
.
Spectral acceleration, with a value related to the natural frequency of vibration
Mechanical resonance
Mechanical resonance is the tendency of a mechanical system to absorb more energy when the frequency of its oscillations matches the system's natural frequency of vibration than it does at other frequencies...
of the building, is used in earthquake engineering
Earthquake engineering
Earthquake engineering is the scientific field concerned with protecting society, the natural and the man-made environment from earthquakes by limiting the seismic risk to socio-economically acceptable levels...
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
Peak ground acceleration is a measure of earthquake acceleration on the ground and an important input parameter for earthquake engineering, also known as the design basis earthquake ground motion...
value, although there is normally a correlation between SA and PGA.
Some seismic hazard
Seismic hazard
Seismic hazard refers to the study of expected earthquake ground motions at the earth's surface, and its likely effects on existing natural conditions and man-made structures for public safety considerations; the results of such studies are published as seismic hazard maps, which identify the...
maps are also produced using spectral acceleration.