lecture#15
DESCRIPTION
Lecture#15. CE-312 Engineering Geology and Seismology Instructor: Dr Amjad Naseer. Department of Civil Engineering N-W.F.P University of Engineering and Technology, Peshawar. Outlines of the Presentation. Ground Motion Parameters Amplitude, Frequency and Duration. - PowerPoint PPT PresentationTRANSCRIPT
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CE-312
Engineering Geology and Seismology
Instructor:
Dr Amjad Naseer
Lecture#15
Department of Civil Engineering
N-W.F.P University of Engineering and Technology, Peshawar
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• Ground Motion Parameters
• Amplitude, Frequency and Duration
Outlines of the PresentationOutlines of the Presentation
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• Ground motion parameters are essential for describing the
important characteristics of strong ground motion in
compact, quantitative form.
• Amplitude, Frequency and Duration
Ground Motion ParametersGround Motion Parameters
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• Amplitude
Parameters:
The most common way of describing a ground motion is with a time history. The motion parameter may be acceleration, velocity, or displacement, or all three may be displayed. Typically, only one of these quantities is measured directly with the others computed form it by integration and/or differentiation.
(A) Amplitude Parameters(A) Amplitude Parameters
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• The most commonly used measure of the amplitude of a
particular ground motion is the peak horizontal acceleration
(PHA). The PHA for a given component of motion is simply
the largest (absolute) value of horizontal acceleration
obtained from the accelerogram of that component.
(a) Peak Acceleration(a) Peak Acceleration
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Horizontal accelerations
have commonly been used to
describe ground motions
because of their natural
relationship to inertial forces;
indeed, the largest dynamic
forces induced in certain
type of structures (that is
very stiff structures) are
closely related to the PHA.
(a) Peak Acceleration(a) Peak Acceleration
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• The PHA can also be correlated to earthquake intensity.
Although this correlation is far from precise, it can be very
useful for estimation of PHA when only intensity information
is available, as in the cases of earthquakes that occurred
before strong motion instruments were available (pre-
instrumental earthquakes). A number of intensity-
acceleration relationships have been proposed.
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(a) Peak Acceleration(a) Peak Acceleration
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• Vertical acceleration have received less attention in
earthquake engineering than the horizontal acceleration,
primarily because the margins of safety against gravity-
induced static vertical forces in constructed works usually
provide adequate resistance to dynamic forces induced by
vertical accelation during earthquakes. For engineering
purposes, the peak vertical acceleration (PVA) is often
assumed to be two third of PHA.
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(a) Peak Acceleration(a) Peak Acceleration
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• Ground motions with high peak accelerations are usually,
but not always, more destructive than motions with lower
peak accelerations. Very high peak accelerations that last
for only a very short period of time may cause little damage
to many types of structures. A number of earthquakes have
produced peak acceleration in excess of 0.5 g but caused
no significant damage to structures because the peak
accelerations occurred at very high frequency and the
duration of the earthquake was not long. Although peak
acceleration is a very useful parameter, it provides no
information on the frequency content or duration of the
motion; consequently, it must be supplemented by
additional information to characterize a ground motion
accurately.
(a) Peak Acceleration(a) Peak Acceleration
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Peak Velocity:
The peak horizontal velocity (PHV) is another useful parameters
for characterizations of ground motion amplitude. Since the
velocity is less sensitive to the higher-frequency
components of the ground motion, the PHV is more likely
than the PHA to characterize ground motion amplitude
accurately at intermediate-frequencies. For structures or
facilities that are sensitive to loading in this intermediate
frequency range (for example, tall or flexible buildings,
bridges, etc), the PHV may provide a much more accurate
indication of the potential for damage than the PHA.
(b) Peak Velocity(b) Peak Velocity
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Peak Displacement:
Peak displacements are generally associated with the lower-
fequency components of an earthquake motion. They are,
often difficult to determine accurately due to signal
processing errors in the filtering and integration of
accelerograms and due to long-period noise. As a result,
peak displacement is less commonly used as a measure of
ground motion than is peak acceleration or peak velocity.
(c) Peak Displacement(c) Peak Displacement
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Frequency Content Parameters:
Dynamic response of buildings, bridges, slopes or soil deposit is
very sensitive to the frequency at which they are loaded.
Earthquake produces complicated loading with components
of motion that span a broad range of frequencies. The
frequency content describes how the amplitude of a ground
motion is distributed among different frequencies. Since the
frequency content of an earthquake motion will strongly
influence the effects of that motion, characterization of the
motion cannot be complete without consideration of its
frequency content.
(B) Frequency Content(B) Frequency Content
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Duration:
The duration of strong ground motion can have a strong
influence on earthquake damage. Many physical processes,
such as the degradation of stiffness and strength of certain
types of structure and the buildup of porewater pressures in
loose, saturated sands, are sensitive to the number of load
or stress reversals that occure during an earthquake. A
motion of short duration may not produce enough load
reversals for damaging response to build up in a structure,
even if the amplitude of the motion is high. On the other
hand, a motion with moderate amplitude but long duration
can produce enough load reversals to cause substantial
damage.
(C) Duration(C) Duration
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The duration of a strong ground motion is related to the time required for release of accumlagted strain energy by rupture along the fault. As the length or area of fault rupture increase, the time required for rupture increase. As a result, the duration of strong motion increases with increasing earthquake magnitude. While this relationship has been supported empirical evidence for many years, advances in source mjuechanism modeling, have provided theoretical support indicated that
(C) Duration(C) Duration
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RESPONSE SPECTRA The response spectrum is the most important characterisation
of seismic ground-motion in earthquake engineering and forms the basis for most design. This chapter introduces the concept of the response spectrum and the particular influence that certain features of the earthquake can have on have on its shape and amplitude.
Response SpectraResponse Spectra
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Definition of the elastic response spectrum
A single-degree-of-freedom (SDOF) system is a mechanical system with mass, m, that provides inertia, and stiff, k, that provides a restoring force, whose deformation can be fully described by a single coordinate. The natural period of vibration of such an SDOF system, T, is given by the following equation:
Real systems do not vibrate indefinitely when they are
perturbed because of the dissipation of energy by damping. The damping is usually expressed as a proportion of critical damping, which is the level of damping that will restore a system to its at rest position without vibrations. For reinforced concrete structures it is usually assumed that the damping can be taken as 5% of critical.
Response SpectraResponse Spectra
k
m2T
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Real systems do not vibrate indefinitely when they are perturbed because of the dissipation of energy by damping. The damping is usually expressed as a proportion of critical damping, which is the level of damping that will restore a system to its at rest position without vibrations. For reinforced concrete structures it is usually assumed that the damping can be taken as 5% of critical.
Response SpectraResponse Spectra
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If a series of SDOF systems with a given level of structural damping are all subjected to an acceleration time-history acting at their base, each mass will respond differently according to its natural period and the relationship between this period and the frequency content of the ground motion. The maximum absolute value of the response of each SDOF oscillator can be calculated and plotted against the corresponding value of period, T. The resulting plot, called a response spectrum, shows the maximum response that an SDOF system will experience when subjected to the ground motion represented by that particular accelerogram. This is illustrated in figure on the last slide. The response spectrum reflects the characteristics of the earthquake that generated the motion and the nature of the recording site.
Response SpectraResponse Spectra
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Response SpectraResponse Spectra
Figure 2. Elastic response spectra of absolute acceleration of the four accelerograms shown in Figure 9.8. 1 – Peru 1974, 2 – Yugoslavia
1979, 3 – Romania 1977, 4 – Mexico 1985
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Three different spectra can be defined according to how the response of each SDOF is measured: relative displacement, relative velocity or absolute acceleration. At zero period the spectra of relative displacement and relative velocity are equal to zero since for an infinitely rigid SDOF there is no vibration. At zero period the relative acceleration is also zero and the absolute acceleration is equal to the maximum acceleration of the ground. This is a very important point to grasp: the response spectrum of absolute acceleration anchors at PGA, as can be appreciated from fig.2: despite their very significant differences, all of the spectra converge to 0.18g at the period T=0.
Response SpectraResponse Spectra