Bi-directional Testing of Circular and Interlocking
Spiral RC Columns
by Juan Arias, PhD Research Asst and David Sanders, PhD, Professor
RESULTS: Eight scale model (1:4 & 1:3) reinforced concrete columns were tested under
dynamic loading on a shake table. The experiments were based on seismic loading in the
transverse and axial directions of a single bent bridge column. Two configurations were
investigated: 1) Circular Columns (16” diameter, scale 1:3) and 2) Interlocking Spiral Columns
(17.5” x 12” oval, scale 1:4). The first four columns (2 circular and 2 interlocking) were tested
under bidirectional shaking without axial loading and without p-delta effects. The second four
columns were tested under bidirectional shaking with axial loading and p-delta effects.
Another main aspect of this testing was the use of a new mass rig termed the Bidirectional
Mass Rig (BMR) which was able to transmit transverse inertial loading and torsion while
supporting the axial load to a frame instead of the column. It was found that bidirectional
motions do not appreciably change the seismic performance compared to unidirectional
motions. It was also determined that analytical models which were composed of certain
elements (a plastic hinge zone and shear transfer mechanisms) performed well in comparison
to the experimental models.
Why We Pursued This Research
The research at UNR was conducted as part of a larger effort funded by the National Science Foundation that is
dedicated to studying the combined actions of forces on column performance. This study included research from six
institutions, further information of the overall project can be found here:
Hyperlink to the CABER synopsis
The work at UNR focused on the development of refined analysis and shaking table tests of cantilever-type
scale models of bridge columns subjected to different levels of biaxial, torsion and vertical loads through real
time earthquake motions. The performance of the specimens was assessed in terms of strength, deformation,
energy dissipation and failure mode. The results will be used to validate analytical tools, develop new inelastic
models for reinforced concrete columns under combined loadings and to propose new design methodologies.
Description of Research
Two sets of specimens of circular and double interlocking columns were constructed using current bridge
design details typical of bridges in California in accordance with the Seismic Design Criteria (CALTRANS,
2006). For circular columns the scaling factor selected was 1:3. The diameter of the specimens was 406 mm
(16 in) and the height 1830 mm (72 in), thus the aspect ratio was 4.5, which allows for flexural dominated
behavior. For double interlocking columns a scale factor of 1:4 was used. Consequently, the height was 1830
mm (72 in) and the width in the short side was 305 mm (12 in), while that in the long dimension was 445 mm
(17.5 in).
As part of the project a new inertial loading system was developed at UNR to test single cantilever-type
columns on shake table under biaxial excitations. The aim of the test setup is to have a supporting structure that
carries safely the vertical component of the inertial mass (superstructure weight) but allows transfer the inertial
forces from the structure to the specimen. A similar structure that allows dynamic excitation in one direction was
developed at UNR ten years ago (Laplace, 1999).
The new system is composed by a 3D four columns frame and a platform that sits on ball bearings located at
the top of the columns. The platform is connected to the RCC specimen through links in two perpendicular
directions, which transfer shear and torsion but not axial load. Additional mass is set on the platform to simulate
the weight of a portion of the bridge superstructure and this can be distributed in an asymmetric configuration to
induce torsion in the system. In addition, a safety system was designed to catch the platform in the event of
large displacements or specimen collapse.
Analytical models were developed to anticipate the seismic performance of the specimens and to determine
the appropriate input loadings to be used during the tests. Time history inelastic analysis
have been performed using OpenSees (Mazzoni et al.,2006). Analytical models of single cantilever-type
columns with lumped mass as well as models of the specimens including the inertial loading system were
studied under different levels of earthquake excitations and mass distribution to determine limit states in the
behavior of the columns during the tests.
Research Results
The seismic performance of the circular and interlocking specimens designed according to the 2006 Caltrans BDS and
SDC was satisfactory. It was controlled by the biaxial effect of bending with small influence of shear deformations. The
columns resisted several repetitions of earthquakes with spectral amplitude equal to or larger than the maximum
considered earthquake with sufficient drift and displacement ductility capacity.
The behavior between pairs of specimen was similar; it was characterized by good energy dissipation and high levels
of deformation without collapse. At low amplitude earthquakes, the specimens remained essentially elastic, the concrete
cover was spalled at a scale factor of 1.2 times Petrolia or Sylmar for the circular and interlocking columns, respectively;
which represents design level earthquakes (DE) in California. Application of accelerations of increasing amplitude and
larger than the DE, progressively deteriorated the section until the failure. This included longitudinal bar buckling, and
rupture of longitudinal and/or spiral reinforcement, accompanied by core concrete degradation.
For all the specimens, it was observed that after yielding, the maximum lateral capacity remained almost constant for
increasing earthquake intensities until the longitudinal bar buckling occurred. After this point, the response was
characterized by a pronounced stiffness and capacity degradation, leading to failure. Similar behavior was observed for
a column subjected to unidirectional excitations and constant axial load tested at the University of Nevada, Reno.
The available elements and material models for concrete and reinforcement in OpenSees were successful in predicting
the nonlinear behavior of the columns studied. It was found that the modeling of the specimens with a nonlinear beamcolumn element with fibers, hysteretic material model with strength degradation, bond slip and viscous damping leads to
the best estimation of the measured performance.
In order to model the plastic hinging and take into consideration the reduction in lateral stiffness due to initial cracking, a
beam-with-hinges nonlinear element in OpenSees was found to be the best representation of the RC columns. The
plastic hinge length and effective lateral and torsional stiffness calculated from the Caltrans SDC provisions were
sufficient to capture the nonlinear behavior of the specimens.
A more detailed synopsis of the project can be found here:
Hyperlink to detailed report
A full .pdf file of the dissertation of this project can be found here:
Hyperlink to dissertation
a) Interlocking Circular Column
c) Column C1 (after testing)
b) Interlocking Spiral Column
d) CAD Drawings of Testing Apparatus