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Geotechnical Earthquake Contents:
Engineering
1. Introduction
2. Effects of local site conditions on ground motion
3. Design Parameters
4. Developments of design parameters
Chapter: 8
Local site effects and design ground motions
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Introduction Effects of local site conditions on ground motion
Local site effects play an important role in earthquake- Local site conditions can profoundly influence all of the
resistant design and must be accounted for on a case by important characteristics
case basis.
Amplitude
Local site effect is usually accomplished by the Frequency content and
development of one or more design ground motions. Duration of a strong ground motion
Design ground motions: Motions that reflect the levels of The extent of their influence depends on
strong motion amplitude, frequency content and duration
that a structure or facility at a particular site should be The geometry and material properties of
designed for. the subsurface materials
The site topography and
Characteristics of input motion
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Effects of local site conditions on ground motion Evidence from theoretical ground response analyses
Nature of local site effects can be illustrated by 1. At most sites the density and s-wave velocity of materials
near surface are smaller that at greater depths.
Simple, theoretical ground response analyses
Measurements of actual surface and subsurface If scattering and material damping are neglected, the
motions at the same site and conservation of elastic wave energy requires that energy
Measurements of ground surface motions from flux from depth to ground surface be constant. i.e.
sites with different subsurface conditions.
Since,
Then,
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Evidence from theoretical ground response analyses Evidence from theoretical ground response analyses
2. Characteristics of local soil deposits can also influence the Softer site (site A) will amplify low frequency (long period)
extent to which ground motion amplification will occur bedrock motion than the stiffer site.
when specific impedance is constant.
Stiffer site will amplify high frequency (short period)
bedrock motion than the softer site.
For more realistic condition of elastic bedrock, the nature
of local site amplification will be influenced by specific
impedance of bedrock.
Local site conditions should also include the density and
stiffness of the bedrock.
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Evidence from measured amplification functions:
Theoretical amplification functions are smooth because of
the assumptions – uniform material, horizontal layering
and vertical s-wave propagation.
But actual conditions are different - so actual
amplification functions are not smooth.
Interpretation of strong motion data from sites where both
surface and subsurface instruments had been installed The strong amplification at natural frequencies of the soil
allows actual amplification functions to be computed. deposit clearly illustrates the importance of local soil
condition on ground response.
The effect of soil non-linearity also cause amplification
functions from strong motion to differ those from weak
motions.
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Evidence from measured surface motion:
Evidence from measured surface motion:
UNAM site – rock site
Evidence of importance of local site conditions can be STC site – soil site
gained by comparing ground surface motions measured at Peak acceleration at SCT is
different sites. around 5 times than at UNAM
The ground surface
motions at the rock
outcrops are quite
similar, but the
Predominant period for SCT is
amplitude and
around 2 sec.
frequency content of the
Spectral acceleration at STC is
motions at sites
about 10 times than that at
underlain by thick soil UNAM.
deposits were markedly
different.
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Effect of surface topography and Basin geometry: Effect of surface topography and Basin geometry:
For a triangular infinite wedge subjected to vertically
The effects of topographic irregularities and alluvial basin propagating SH waves (with particle motion parallel to its
geometry on ground motions can be significant. axis), apex displacements are amplified by a factor 2π/φ,
where φ is the vertex angle of wedge.
Topography: This approach can
be used to
The topographic effects caused by simple irregularities can approximate
be estimated from exact solutions to idealized problems. topographic effects
for certain cases of
ridge valley
terrains.
Increased amplification is observed near the crest of the
ridge – damages in ridge are high.
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Effect of surface topography and Basin geometry: Design parameters:
Basin:
Earthquake resistant design of new structures and
The curvature of basin in which softer alluvial soils have evaluation of the safety of existing structures require
been deposited can trap body waves and cause some analysis of their response to earthquake shaking.
incident body waves to propagate through the alluvium as
surface wave.
These waves can produce stronger shaking and longer Evaluation of geotechnical hazards – liquefaction and
durations than would be predicted by one-dimensional slope failure – also requires analysis with respect to some
analysis. level of shaking.
The level of shaking for which satisfactory performance is
expected is often referred to as design level of shaking and
is described by a design ground motion.
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Design parameters: Design parameters:
Design earthquakes
The parameters most commonly used to specify design
ground motions are Historically, design parameters were most commonly
determined form a specified design earthquake.
Peak horizontal acceleration
Peak horizontal velocity Design earthquake implies a level of determinism in the
Predominant period seismic hazard analysis – after the design earthquake is
Response spectrum ordinates and characterized, its effects at the site of interest are
computed deterministically.
Duration
Historically, design earthquakes have been associated
with two level design, in which a structure or facility is
required both to remain operational at one level of
motion and to avoid catastrophic failure at another, more
severe level.
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Design parameters: Design parameters:
Design earthquakes Design earthquakes
A lower but more likely level of shaking would be
Many different terms have been in use:
produced by an operating basis earthquake (OBE)
The maximum credible earthquake (MCE): is usually OBE: an earthquake that should be expected during the
defined as the largest earthquake that can reasonably be life of a structure.
expected from a particular source.
OBE has been taken as an earthquake with half the peak
The two level design approach generally requires that acceleration of the SSE (safe shutdown earthquake), as an
structures or facilities be designed to avoid catastrophic earthquake that produces motion with a 50% probability
failure at the levels of shaking produced by these upper of exceedance in 50 years and as an earthquake with a
level design earthquakes. return period of about 110 years. (Christian, 1978)
Two level design requires that structures and facilities be
designed to remain operational after being subjected to
the levels of shaking associated with these lower level
design earthquakes.
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Design parameters: Design parameters:
Design spectra: Design spectra:
A Newmark-Hall
Response spectra are used to represent seismic loading
design spectrum is
for dynamic analyses of structure.
obtained by
So as a result, design ground motions are often expressed multiplying the PGA,
in terms of design spectra. PGV and PGD values by
the factors shown in
Design spectra and the response spectra of actual table
earthquakes are not the same.
At periods below about 0.17 sec (fre above about 6 Hz), the
o Response spectra from earthquakes are highly spectral accelerations are tapered down to the peak
irregular; their shapes reflect the details of their ground acceleration,
specific frequency content and phasing.
o Design spectra, on the other hand, are generally quite A PGV of 48 in/sec (122 cm/sec) and PGD of 36 in (91 cm)
smooth. (after smoothing, averaging and or are assumed to be consistent with a PGA of 1.0g.
enveloping the response spectra of multiple motion)
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Design parameters:
Design spectra example: Design spectra:
Develop a Newmark-Hall design spectrum for 5%
damping and a peak ground acceleration of 0.25g.
A tripartite plot of the resulting design spectrum is shown
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Development of design parameters: Development of design parameters:
Characteristics of design ground motion at a particular Design ground motion can be developed by two ways
site are influenced by
o Site-specific development
o The location of site relative to potential seismic o From the provisions of building codes and standards
sources
o The seismicity of those sources Site specific development:
o The nature of rupture at the source
Site specific design ground motion reflect the detailed
o Travel path effects between source and site
effects of the particular subsurface conditions at the site
o Local site effects and of interest.
o Importance of structure or facility for which the
ground motion is to be used. The usual process for developing site specific ground
motion involves
o Seismic hazard analysis
o Ground response analysis
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Development of design parameters: Development of design parameters:
If the site of interest is located on a similar profile, the
parameters from seismic hazard analysis may be taken
directly as the design ground motion parameters.
If not so, then the parameters from the seismic hazard
analysis must be modified to account for the effects of
local site conditions.
This parameter modification process may be performed
empirically or analytically.
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Development of design parameters: Development of design parameters:
Code based development: Code based development:
Alternatively, design ground motions can be developed
Uniform Building Code (UBC) allows two basic
on the basis of building code provisions.
approaches to the earthquake resistant design of
buildings –
Design ground motions developed from code provisions
o Static approach – the effects of ground motions are
are usually more conservative ( corresponding to stronger
represented by static lateral forces and
levels of shaking) than those developed from site specific
analyses. o Dynamic approach – the ground motion is
characterized by a design response spectrum.
The simpler static approach is allowed only for certain
conditions of geometric regularity, occupancy and height
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Development of ground motion time histories Development of ground motion time histories
On many occasions, ground motion parameters alone do The most commonly used methods for generation of
not adequately describe the effects of ground shaking. artificial ground motions are
For analysis of nonlinear problems such as the response 1. Modification of actual ground motion records
of inelastic structures or the permanent deformation of 2. Generation of artificial motions in the time
an unstable slope, time histories of motion are required. domain,
3. Generation of artificial motions in the frequency
The strong ground motions can directly be used if the
domain
local and regional geologic and tectonic conditions of the
site of interest is similar to the site where actual strong 4. Generation of artificial motions using Green’s
motion have been measured. function techniques.
Artificial ground motion can be developed when above
case is not valid
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Development of ground motion time histories Development of ground motion time histories
1. Modification of actual ground motion records 1. Modification of actual ground motion records
• Rescaling of peak acceleration and peak velocity of
Simplest approach to ground motion generation. actual ground motion time histories.
Peak acceleration and peak velocity have been used to
The artificial ground motion is generated by rescale actual strong motion records to higher or
lower levels of shaking.
• Rescaling of peak acceleration and peak velocity of
actual ground motion time histories. A scaling factor of 0.25 to 4.0 is suitable for linear
elastic structures and 0.5 to 2.0 is recommended for
liquefiable soils.
• Rescaling of time scale of the original time history
Actual time history must be selected carefully – a
desirable ground motion time history will have close
peak acceleration and velocity as well as similar
frequency content and duration to the target motion.
SMS 2012 SMS 2012
Development of ground motion time histories Development of ground motion time histories
1. Modification of actual ground motion records
• Rescaling of time scale of the original time history
Rescaling of time scale has been used to modify the
frequency content of an actual ground motion
records.
Usually accomplished by multiplying the time step of
a digitized actual record by the ratio of predominant
period of the target motion to the predominant
period of the actual motion.
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Development of ground motion time histories Development of ground motion time histories
2. Generation of artificial motions in the time domain
It typically involves multiplying a stationary,
filtered white noise signal by an envelope function
that describes the buildup and subsequent decay of
the ground motion amplitude.
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Development of ground motion time histories
2. Generation of artificial motions in the frequency domain
It can be generated by combining Fourier amplitude
spectrum with a Fourier phase spectrum.
Useful for generating motions that are consistent
with target response spectra.
End of chapter 8
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