Improved ground motion intensity measures for reliability-based demand analysis of structures
Creators: name: Bianchini, Marcello <1976>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: ICAR/09 Tecnica delle costruzioni

description: In Performance-Based Earthquake Engineering (PBEE), evaluating the seismic performance (or seismic risk) of a structure at a designed site has gained major attention,
especially in the past decade. One of the objectives in PBEE is to quantify the seismic
reliability of a structure (due to the future random earthquakes) at a site. For that
purpose, Probabilistic Seismic Demand Analysis (PSDA) is utilized as a tool to estimate the Mean Annual Frequency (MAF) of exceeding a specified value of a structural
Engineering Demand Parameter (EDP).
This dissertation focuses mainly on applying an average of a certain number of spectral acceleration ordinates in a certain interval of periods, Sa,avg (T1,…,Tn), as scalar
ground motion Intensity Measure (IM) when assessing the seismic performance of inelastic structures. Since the interval of periods where computing Sa,avg is related to the more
or less influence of higher vibration modes on the inelastic response, it is appropriate to
speak about improved IMs. The results using these improved IMs are compared with
a conventional elastic-based scalar IMs (e.g., pseudo spectral acceleration, Sa (
T(¹)), or peak ground acceleration, PGA) and the advanced inelastic-based scalar IM (i.e.,
inelastic spectral displacement, Sdi). The advantages of applying improved IMs are:
(i ) "computability" of the seismic hazard according to traditional Probabilistic Seismic
Hazard Analysis (PSHA), because ground motion prediction models are already available for Sa (Ti), and hence it is possibile to employ existing models to assess hazard in
terms of Sa,avg, and (ii ) "efficiency" or smaller variability of structural response, which
was minimized to assess the optimal range to compute Sa,avg. More work is needed to
assess also "sufficiency" and "scaling robustness" desirable properties, which are disregarded in this dissertation. However, for ordinary records (i.e., with no pulse like
effects), using the improved IMs is found to be more accurate than using the elastic-
and inelastic-based IMs. For structural demands that are dominated by the first mode
of vibration, using Sa,avg can be negligible relative to the conventionally-used Sa (T(¹)) and the advanced Sdi. For structural demands with sign.cant higher-mode contribution,
an improved scalar IM that incorporates higher modes needs to be utilized.
In order to fully understand the influence of the IM on the seismis risk, a simplified
closed-form expression for the probability of exceeding a limit state capacity was chosen
as a reliability measure under seismic excitations and implemented for Reinforced Concrete (RC) frame structures. This closed-form expression is partuclarly useful for seismic
assessment and design of structures, taking into account the uncertainty in the generic
variables, structural "demand" and "capacity" as well as the uncertainty in seismic excitations. The assumed framework employs nonlinear Incremental Dynamic Analysis
(IDA) procedures in order to estimate variability in the response of the structure (demand) to seismic excitations, conditioned to IM. The estimation of the seismic risk using
the simplified closed-form expression is affected by IM, because the final seismic risk is
not constant, but with the same order of magnitude. Possible reasons concern the non-linear model assumed, or the insufficiency of the selected IM. Since it is impossibile to
state what is the "real" probability of exceeding a limit state looking the total risk, the
only way is represented by the optimization of the desirable properties of an IM.