Nonlinear Finite-Element Analysis of RC Bridge Columns under Torsion with and without Axial Compression Tarutal Ghosh Mondal 1 and S. Suriya Prakash, Ph.D. 2 Abstract: Finite-element (FE) modeling of RC structures under combined loading has received considerable attention in recent years. However, the combination of torsion and axial compression has been rarely studied in spite of its frequent occurrence in bridge columns under earthquake loading. This paper aims at creating a nonlinear FE model to predict the behavior of RC bridge columns under combined torsion and axial compression. A number of circular and square columns were analyzed. The developed FE model was calibrated on local and global behavior through comparison with test data. The overall torque–twist behavior of the members was captured well by the developed FE models. The predicted values of strain in the longitudinal and transverse reinforcement matched closely with the experimental results. An increase in transverse steel ratio was found to increase the torsional capacity and limit the damage of columns under torsion. It was further observed that at a low level of axial compression, the torsional capacity of columns is enhanced. In addition, the FE analysis showed a good agreement on the identification of the damage mechanism and the progression of failure. The shape of the cross section is found to play a major role in the distribution of torsional damage in the columns. Square columns exhibited a more localized damage due to presence of warping, whereas circular columns exhibited damage distributed along their length. DOI: 10.1061/(ASCE)BE.1943- 5592.0000798. © 2015 American Society of Civil Engineers. Author keywords: Finite-element analysis; Reinforced concrete columns; Torsion; Axial compression; Shear flow thickness; Spalling of cover. Introduction The damage observed after earthquakes indicates that torsional oscillations are often the cause of distress in buildings and bridges. RC bridge columns with irregular three-dimensional (3D) bridge configurations can undergo significant torsional moments in ad- dition to axial, bending, and shear forces during earthquake events. The addition of torsion is more likely in skewed or horizontally curved bridges, bridges with unequal spans or column heights, and bridges with outrigger bents. Torsion in bridges with outrigger bents occurs because of the eccentricity of the reaction force de- veloped in the footing, which is due to lateral movement of a superstructure under seismic vibration. [If the super structure is subjected to a lateral force (P), reaction force (R) is developed at the footing (Fig. 1). Torsional moment (T ) is produced in the beam owing to this eccentric reaction force (R) in the footing]. In skewed bridges, the collision between bridge deck and abutment may cause inplane rotation of superstructures and consequently induces torsion in the bridge columns [Fig. 1(b)]. Torsion effects due to rotation of the superstructure can be sig- nificant when shear keys restrain the bridge superstructure at the abutment, and/or if there is a significant decrease in the torsion stiffness in relation to the bending stiffness of the column. Con- struction of bridges with these configurations is often unavoidable because of site constraints. The force produced in bridge columns because of dead and live loads is primarily axial. Bridges near the earthquake epicenter can be subjected to a significant vertical load (Saadeghvaziri and Fouch 1990), which is typically neglected in design. Lateral seismic loads will cause the single-column bents to translate laterally and rotate slightly when the bridge abutment has significant stiffness. Spread footings and pile footings have adequate torsional restraint to be considered when they are fixed against rotation. As such, the superstructure rotation will cause compatibility torsion in the columns. The load on the columns will, therefore, include axial compression, shear, flexure, and torsion. Axial loads can be considered constant in the absence of a vertical component owing to near-field effects, whereas other loads act cyclically. Torsional loadings can significantly affect the flow of internal forces and the deformation capacity of RC columns. This, if not considered in design, can influence the performance of vital com- ponents of bridges and consequently affect the daily operation of the transportation system. Moreover, the presence of torsional loading increases the possibility of brittle shear–dominated failure, which may result in a fatal catastrophe. However, a review of previously published studies indicates that the torsional behavior of RC members has not been studied in as much depth as the behavior under flexure and shear. The possibility of significant torsional loadings was illustrated in an analytical study carried out to investigate the seismic torsion response of skewed bridge piers by Tirasit and Kawashima (2005). The results from their analysis show that pounding between skewed bridge deck and abutments takes place, resulting in inplane deck rotation that increases seismic torsion in skewed bridge piers. Moreover, they found that the consideration of the locking of bearing movement after failure could extremely amplify the seismic torsion in skewed bridge piers. This necessitates a clear understanding of the effect of torsion combined with bending, shear, and axial compression on the behavior of bridge columns. 1 Graduate Student, Dept. of Civil Engineering, Indian Institute of Technology, Hyderabad 502205, Andhra Pradesh, India. E-mail: ce13m1023@iith.ac.in 2 Assistant Professor, Dept. of Civil Engineering, Indian Institute of Technology, Hyderabad 502205, Andhra Pradesh, India (corresponding author). E-mail: suriyap@iith.ac.in Note. This manuscript was submitted on September 30, 2014; approved on March 31, 2015; published online on June 19, 2015. Discussion period open until November 19, 2015; separate discussions must be submitted for individual papers. This paper is part of the Journal of Bridge Engineering, © ASCE, ISSN 1084-0702/04015037(13)/$25.00. © ASCE 04015037-1 J. Bridge Eng. J. Bridge Eng., 2016, 21(2): 04015037 Downloaded from ascelibrary.org by INDIAN INSTITUTE OF TECHNOLOGY, HYDERABAD on 03/03/16. Copyright ASCE. 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