This paper presents a theoretical model for reinforced concrete members under cyclic torsion. It is based on the softened truss model (STM) but has been extended to include the tension stiffened portion of the torsional response. The STM was derived to predict a member's response under a monotonically increasing torsion. It uses a softened stress-strain relationship for concrete in compression derived from shear panel tests and neglects the tensile strength of concrete. The model proposed in this paper uses a bilinear relationship for concrete in tension and additional compatibility equations for the tension stiffened region. It provides an improved prediction of both the pre-cracking and post-cracking torsional behavior. The model is compared to the envelope curve of a reinforced concrete (RC) girder tested under pure torsion using full-reversal cyclic loading. It is one girder tested as part of an experimental investigation aimed at studying the behavior of RC girders loaded in combined shear and torsion under seismic-like cyclic loading. The hollow box girder tested was 14.7 meter long and loaded under several full-reversal torsional cycles. The model is also calibrated to three other torsional members found in literature.
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