Abdeldjelil Belarbi (belarbi@mst.edu), Suriya Prakash Shanmugam (spsg33@mst.edu) and Young-min You (youngmin@mst.edu), Department of Civil, Environmental and Architectural Engineering, Missouri University of Science and Technology

Reinforced concrete (RC) columns of skewed and curved bridges and bridges with unequal spans and column heights can be subjected to combined loadings including axial, flexural, shear and torsional loadings during an earthquake.  Multi-directional earthquake motions with significant vertical excitations, structural constraints due to stiff deck, movement joints, soil condition and foundations may also lead to combined loadings.  The combination of axial, bending, shear and torsion in RC bridge columns can result in complex failure modes.  Under an NSF-NEES research project with objectives to understand the behavior of bridge columns under seismic loadings, experimental and analytical studies are conducted to investigate the performance of RC columns under combined loadings including torsion.  The main variables being considered in the experimental study are (i) the ratio of torsion to bending moment (T/M), (ii) the ratio of bending moment to shear (M/V), and (iii) level of detailing for high and moderate seismicity (low or high spiral ratio).  The experimental results are used to develop and calibrate the design interaction equations.  Based on the experimental results, damage and ductility models that account for the combined loading effects are also being developed from design point of view.  Normalized interaction diagrams for displacement and rotational ductility levels are also presented and discussed.  Analytical study focuses on developing mechanical models to predict the interaction effects for RC columns under combined loadings. However, knowledge of the interaction between axial, bending, shear and torsion in RC bridge columns is very limited.  This paper presents an overall summary of the major findings and relevant results from both the experimental and analytical studies and provides new directions in the design and detailing of RC bridge columns under seismic loading.