10-10-2014, 02:05 PM
An important step in the earthquake resistant design of structures is the
specification of the induced ground excitation. For very important structures, the
input specification obviously must be quite realistic and rigorous. In this thesis,
some contributions have been made in the field of probabilistic specification of
earthquake ground motion.
Chapter I, presents a brief review of literature on the methods currently available
for specifying seismic inputs. Attention is focused mainly on earthquake
specification through spectrum compatible power spectral density (PSD)
functions. The concept of stochastic critical excitation has also been reviewed.
While these developments are described in detail, limitations in the existing
methods are brought out to state the motivation behind the present thesis. This is
followed by Chapter II wherein an iterative procedure is developed for arriving
at the compatible PSD function in a discretized form, from specified design
response spectra. The procedure is based on a stationary random process model
for the earthquake ground motion. Numerical results have been presented on the
generation of PSD function compatible with typical available response spectra.
The uniqueness of the generated compatible PSD function is examined by using
two different mathematical representations for the PSD function. The internal
consistency of the United States Nuclear Regulatory Commission (USNRC
(1973)) and Indian Standards (IS (1984)) spectra with respect to different
damping ratios has also been examined with the help of the compatible PSD
function.
In Chapter III, the solution of the previous chapter is improved by including
nonstationarity into the earthquake model in the form of a stationary random
process modulated by a known function. The compatible PSD function of the
stationary part is generated on a more rational basis by accounting for the
probability level of exceedance associated with the design response spectra. It is
shown that the incorporation of the precise probability level of exceedance
associated with the design spectra into the analysis, ensures proper estimation of
the compatible PSD function.
The determination of seismic threat for important structures through the critical
excitation method is presented in Chapter IV. The investigation is limited to
study the complicating factors involved in the application of stochastic critical
excitation method to a multi-degree degree of freedom structure. The possibility
of arriving at a common ground response spectra via an enveloping procedure is
suggested. The thesis concludes in Chapter V with a summary and a few
suggestions for further research.