Environmental Engineering Reference
In-Depth Information
are considered in comprehensive analytical methods. Input can be from actual strong-
motion records from simulated earthquake motion, or from response spectra.
Analysis considers the structure as being linear-elastic and having single or multiple
degrees of freedom. Ground shaking causes base excitation, which is established by equa-
tions expressing the time-dependent (dynamic) force in terms of the structure's character-
istics (mass, stiffness, and damping factors) as related to time-dependent acceleration,
velocity, and displacement. A dynamic mathematical model is developed representing the
structure and all of its elements, closely simulating the interaction effect of components on
each other and the response of each to the dynamic forces.
Deterministic Seismic Hazard Analysis (DSHA)
This site-specific approach uses the known seismic sources (usually faults) sufficiently
near the site and available historical seismic and geologic data to prepare models of
ground motion at the site. One or more earthquakes are specified by magnitude and loca-
tion, and are usually assumed to occur on the portion of the source closest to the site. The
ground motions are estimated deterministically, given the magnitude, source-to-site dis-
tance, and site conditions (USACE, 1995).
Probabilistic Seismic Hazard Analysis (PSHA)
This approach uses the elements of the DSHA and adds an assessment of the potential for
ground motions during the specified time period. The probability or frequency of occur-
rence of different magnitude earthquakes on each significant seismic source and inherent
uncertainties are directly accounted for in analysis. Ground motions are selected based on
the probability of exceedance of a given magnitude during the service life of the structure
or for a given return period (USACE, 1995).
For both the DSHA and PSHA methods, site-specific ground motion studies are
required to provide magnitude, duration, and site-specific values for the PGA, PGV, PGD,
and design response spectra and time histories in both the horizontal and vertical direc-
tions at the ground surface or on a rock outcrop. Studies should also consider Soil
Structure Interaction (SSI) effects which may reduce ground motions at the base of the
structure.
Time-History Analysis
The response spectrum mode superposition fully accounts for the multi-mode dynamic
behavior of the structure, but is limited to the linear elastic range of behavior and
provides only maximum values of the response quantities. The time-history method is
used to compute deformations, stresses, and section forces more accurately considering
the time-dependent nature of the dynamic response to earthquake ground motion
(USAEC, 2003).
The basic data input may be ground motion from an actual strong-motion record, or
simulated earthquake motion. The corresponding response in each configuration of the
vibrating system (mode) is calculated as a function of time. The total response, obtained
by summing all significant modes, can be evaluated for any desired instant.
Seismic Analysis Progression
The USACE (1995) provides a summary of the method of seismic analysis applicable for
the various phases of investigation for the seismic zones given in
Figure 11.13.
These rela-
tionships are given in
Table 11.13.
