Environmental Engineering Reference
In-Depth Information
damping. A design response spectrum may be applied that represents an average of sev-
eral appropriate spectra developed into a design envelope. The influence of the frequency
content is important, and some investigators recommend that instead of scaling the entire
response spectra to ground accelerations, a better overall picture of ground response is
obtained by specifying in addition the maximum ground velocity for intermediate fre-
quencies and the maximum ground displacement for the lower (long-period) frequencies.
Magnitudes for design spectra can be determined from equations that estimate ampli-
tudes directly, instead of scaling them from the estimates of peak ground velocity.
Alternatively, complete input motion can be obtained from actual accelerograph records
or produced synthetically from given base-rock motion for a particular site.
Seismic Hazard Analysis ( Section 11.4.5)
For foundations supported on rock or strong soils, response is evaluated by time-motion
methods or response-spectrum analysis. For structures founded on relatively deep and
weak soils, the feedback from structural oscillations is evaluated by SSI analysis or soil
amplification factors. The base-rock motion, given in terms of g varying with period, is
used to evaluate the motions at any point, such as the structure base, employing values for
the dynamic shear modulus and damping ratios of the soils. An alternative procedure is
to specify a control or input motion at some point in the “free field” from which compa-
rable rock motions are determined.
11.5.5
Limitations in the Present State of the Art
General
Limitations of knowledge regarding earthquakes as they pertain to engineering are
severe and require the application of considerable judgment based on experience as well
as generous safety factors on any project. The limitations are well-recognized, and sub-
stantial effort is being applied by various disciplines involved with earthquake engi-
neering to improve the various ignorance factors, some of the more significant of which
are given below. Unfortunately, capabilities in rigorous mathematical analysis appear to
be far in advance of the capabilities of generating accurate and representative data.
Earthquake Characteristics
Focal-Depth Effects
Procedures to define the design earthquake are considered approximately valid for the
continental western United States, where modern earthquakes are generally shallow and
fault-related. The applicability to intermediate and deep-focus events, which usually are
not associated with surface faulting, is not well-defined.
Occurrence Prediction
Seismicity data in many locations are meager, usually based on “felt” reports, and cover
only a relatively brief historical period (200 years or so in the United States) in compari-
son with “recent” geologic time of say, about 10,000 years from the last glacial age. Seismic
activity has been found to be cyclic in many areas, with cycle lengths longer than the time
interval of data in some areas. (How can it be known when essentially singular events
such as New Madrid or Charleston may return, or may occur in some other region with
“historically” low activity?)
Felt reports are based on the response of people and structures; therefore, they depend
on development and demography for registry. As the world population increases, so does
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