Abstract:
Past earthquake events have highlighted vulnerabilities of reinforced concrete buildings with
irregularities prone to damage localisation. Torsional irregularities in building structures are
generally created by asymmetric arrangement of structural elements, resulting in torsional
response under earthquake excitations. Localised collapse can be caused by high seismic
demand concentrating on the critical side, specifically in existing concrete buildings which
include structural vulnerabilities such as torsional asymmetricities or nonductile elements. In
the Canterbury Earthquakes in New Zealand, localised damage to relatively symmetric
reinforced concrete buildings were observed because of irregularities created by inelastic
behaviour of structural elements causing damage or nonductile response. These examples
raised the need for estimating inelastic torsional demand on existing concrete structures.
In current seismic assessment guidelines, torsional amplification mostly relies on amplification
due to accidental eccentricity, hence there are uncertainties in determining the effect of torsion
associated with nonlinear response. Although New Zealand Seismic Assessment Guidelines
provide inelastic torsional assessment methods, accuracy of these methods has not been
validated. This is particularly due to lack of experimental studies on inelastic torsion requiring
large-scale and high-performance experimental facilities for system-level testing.
This research, therefore, investigates the nonlinear torsional response of existing reinforced
concrete buildings through shake-table tests and numerical analysis. Two half-scale sevenstorey
reinforced concrete structures were tested under uni-directional excitations to capture
the response of stiffness, damage and ductility irregularities. The goal of this research is to
evaluate the ability of current seismic assessment procedures to estimate the observed
displacement demand, including the effects of nonlinear torsion, and suggest improvements for
torsional assessment procedures.
The shake-table tests presented localised damage and collapse due to torsional response,
which was quantified by the torsional irregularity ratio. It was found that the numerical models
need to include proper stiffness reduction due to damage, ductility capacity of nonductile
elements, and bidirectional interaction to simulate the observed torsional response. The issues
with estimating inelastic torsional demand by the seismic assessment guidelines included
limited applicability of inelastic torsional assessment and unconservative nonlinear demand estimates. Improvements to the seismic assessment guidelines are recommended based on the
findings for better estimates of inelastic torsional demand.