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Structural design codes limit the maximum amount of torsional reinforcement of reinforced concrete (RC) members to prevent possible sudden torsional failure due to over-reinforcement. While the provisions in the ACI 318-14, EC2-04, CSA-14, and JSCE-07 codes are almost the same for torsional strength calculation of RC members based on the space truss model and thin-walled tube theory, different limits are used for the maximum amount of torsional reinforcement. The requirements of the maximum torsional reinforcement in the ACI 318-14 and JSCE-07 codes are based on the empirical equations, while those in the EC2-04 and CSA-14 codes are based on the truss model. To compare the accuracy of these four design codes, this paper presents the test results of 15 RC beams that have various torsional reinforcement ratios and concrete compressive strengths. The experimental and analytical results indicate that the ACI 318-14 and JSCE-07 design codes evaluate the torsional failure modes of 15 RC members with reasonable agreement, while the EC2-04 and CSA-14 design codes overestimate the boundary between two torsional failure modes.
Keywords: beams; reinforced concrete; torsional reinforcement; torsional strength; truss model.
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INTRODUCTION
In design codes such as ACI 318-14,1 EC2-04,2 CSA-14,3 and JSCE-07,4 different equations are used for the shear strength calculation of reinforced concrete (RC) members, while those used to calculate the torsional strength of RC members are almost identical. For shear strength calculations, the 45-degree truss model5,6 in ACI 318-14 and JSCE-07, the plasticity truss model7 in EC2-04, and the Modified Compression Field Theory8 in CSA-07 are used; nevertheless, the provisions for torsional strength in all of these codes are based on the space truss model9 and thin wall tube theory.10 The angle of the diagonal crack due to shear is taken as 45 degrees in the ACI 318 and JSCE-07 codes, while in the EC2-04 and CSA codes, it is calculated as 1 < cotØ < 2.5 (EC2-04, Eq. 6.7N) and 29 + 7000ex (ex is the strain at the center of section; CSA-14 Eq. (11) and (12)), respectively. However, in these four design codes, the torsional crack angle is calculated in terms of the transverse torsional reinforcement and longitudinal torsional reinforcement. In 1971, the ACI 318 code for the first time included the torsion...