ABSTRACT
Several two-dimensional analytical beam column joint
models with varying complexities have been proposed in quantifying joint
flexibility during seismic vulnerability assessment of non-ductile reinforced
concrete (RC) frames. Notable models are the single component rotational spring
element and the super element joint model that can effectively capture the
governing inelastic mechanisms under severe ground motions. Even though both
models have been extensively calibrated and verified using quasi-static test of
joint sub-assemblages, a comparative study of the inelastic seismic responses
under nonlinear time history analysis (NTHA) of RC frames has not been thoroughly
evaluated. This study employs three hypothetical case study RC frames subjected
to increasing ground motion intensities to study their inherent variations.
Secondly, the issue of super-element joint models, causing numerical divergence
in non-linear time history analysis of reinforced concrete frames, is
investigated. The rigid joint assumption and a single rotational spring model
are implemented for comparison. Reinforced concrete joint sub-assemblages and a
one-third scaled frame have been employed for model validation. Results
indicate that the super element joint model overestimates the transient drift
ratio at the first storey and becomes highly un-conservative by
under-predicting the drift ratios at the roof level when compared to the single-component
model and the conventional rigid joint assumption. In addition, between these
storey levels, a decline in the drift ratios is observed as the storey level
increased. However, from this limited study, there is no consistent evidence to
suggest that care should be taken in selecting either a single or multi
component joint model for seismic risk assessment of buildings when a global
demand measure, such as maximum inter-storey drift, is employed in the seismic
assessment framework. Probabilistic seismic demand analysis also indicates that
super-element joint model may be less vulnerable relative to the
single-component joint model. Furthermore, the shift in fragility function may
lie in between the rigid joint and single-component joint modelling schemes,
implying non-divergence.
CHAPTER 1: INTRODUCTION
1.1 Background
In the present wake of
performance-based earthquake engineering (PBEE), the assessment of the
vulnerability of a structural system to withstand seismic forces has been
addressed by employing probabilistic models to quantify the level of
uncertainties associated with the estimation of the seismic demand imposed on a
structure given an intensity of ground shaking (Liel et al., 2009). Reliable
quantification of the seismic performance of existing reinforced concrete
buildings has been one of the major challenges within the earthquake
engineering research community. The performance-based earthquake engineering
(PBEE) methodology, since its inception, has provided engineers with a systemic
way to incorporate and propagate uncertainties relating to, for instance, the
estimation of seismic responses of structures subjected to severe ground
shaking. The process culminates in a probabilistic framework for seismic
assessment. This developed probabilistic framework decouples the risk
assessment problem into four key areas; hazard, structural, damage and loss
analysis. The final output may be the conditional mean annual frequency of
repair cost exceeding a specified percentage of the total replacement cost of a
specific structural system given the intensity of the ground motion (Liel et
al., 2009). Usually, a global scalar parameter, with a prescribed probability
distribution, is used to interface the various stages of the assessment
framework. In order to systematically quantify the degree of uncertainties,
such as modelling of structural elements and record to record variability in
selected ground motions, past researches have assumed the conditional
distributions of the parameters in the PBEE methodology to be markovian dependent
(Baker and Cornell, 2003). This assumption
allows, for instance, to estimate the probability of execeedance of the
structural response quantity (structural analysis), given a parameter
describing the intensity of ground shaking (hazard analysis), without
necessarily requiring knowledge of pertinent information such as the
distribution of magnitudes and source-to-site distances during the
probabilistic seismic hazard analysis (PSHA) as well as the ground motion
attenuation model used. Hence, one can analytically estimate the fragility of
the structural system without necessarily requiring certain site specific
information.
In order to reduce the dispersion
in the modelling uncertainties associated with structural components, past
researches have emphasized the importance of modelling the behaviour of
beam-column connections in a bid to predict the seismic demand efficiently (Park,
2010). This is due to the fact that recent earthquakes have shown that older
type non ductile reinforced concrete buildings are very vulnerable and do
sustain significant damage under seismic action (see Fig. 1.1). Existing
earthquake reconnaissance surveys (Moehle and Mahin, 1991; Sezen et al., 2002)
have stressed that non-ductile detailing of structural components should not be
tolerated in highly seismic zones.
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