NECESSITY TO DETERMINE GROUND RESPONSE <GRA> DURING STRONG EARTHQUAKE
Engineer.Anjum.Zafar
SOCIETY OF AMERICAN ENGINEERS IN ASIA [SAEA]
SOCIEDAD DE INGENIEROS AMERICANOS EN ASIA [SAEA]
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NECESSITY TO DETERMINE GROUND RESPONSE <GRA> DURING STRONG EARTHQUAKE
By Engineer Anjum Zafar, CEng
Seismic Structures Specialist
One of the most important and most commonly encountered problems in
Seismic Structures is the Evaluation of Ground Response which is
expected during severe seismic conditions. Ground Response Analyses
<GRA> are used aiming to predict Ground Surface Motions for the
development of Design Response Spectra, so as to evaluate Dynamic
Stresses <developed during the course of a strong earthquake> and
Strains <i.e. deformations> for the evaluation of liquefaction
hazards. Not only this, thru this analysis <i.e. GRA>, an engineer is
able to determine the earthquake induced forces…the forces that can
lead to instability of earth and earth-retaining structures.
Now, let us consider the ideal conditions. Under ideal conditions, a
complete Ground Response Analysis <GRA> would model the rupture
mechanism at the SOURCE of an earthquake. The propagation of stress
<i.e. different type of FORCE Waves, induced by an earthquake> waves
thru the earth to the TOP of the bedrock, just below a particular
site; and we will then be able to determine how the ground surface
motion is influenced by the soils that lie just above the bedrock. In
fact, the mechanism of fault rupture is so complicated; and the nature
of energy transmission between the source and the site is so uncertain
that this approach is NOT practical for common engineering
applications. In practice, empirical methods <based on the
characteristics of previously recorded earthquakes> are used, so as to
develop the predictive relationships. These predictive relationships
are often used in conjunction with a seismic hazard analysis, in order
to predict bedrock motion characteristics at any site. The problem of
Ground Response Analysis <GRA> then becomes one of determining the
response of the soil deposit to the motion of the bedrock immediately
just below it at a site. Despite the fact that Seismic Waves <p-waves,
s-waves, surface waves, etc.> may travel thru more than thousands of
miles of rock and often even less than 100 miles of soil, play a very
important role in determining the characteristics of the ground
SURFACE motion.
The influence of local soil conditions, on the NATURE of earthquake
damage, has been recognized for many many years. Since the 1920s,
Seismic Structural Engineers and engineers specialized in Earthquake
Engineering have worked toward the development of QUANTITATIVE
methods, so as to predict the influence of local soil conditions at
the time of STRONG ground motion. Over the years, a number of
techniques have been developed for Ground Response Analysis <GRA> .
These techniques are usually grouped according to the dimensionality
of the problems whatsoever they can address, although many of the 2-
dimensional and 3-dimensional techniques are relatively
straightforward extensions of corresponding 1-dimensional techniques.
However, at the end, I’d emphasize that the engineer must be familiar
with the most commonly used methods for calculating 1-dimentional, 2-
dimentional and 3-dimensional ground response:::
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Uploaded By::: Eng. Anjum Zafar
President:::<SAEA>:::
For:::NYC Center [United States]
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Subido Por::: Inglés. Anjum Zafar
Presidente:::<SAEA>:::
For:::NYC Center [United States]
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