The purpose of this study was to investigate the mechanism of shear strength degradation of a reinforced concrete (RC) member under cyclic loading. First, the shear failure after flexural yield of tension reinforcing bars of a RC column designed as bridge pier under cyclic loading was simulated by the method of three-dimensional rigid-body-spring-model (3D RBSM) and the shear strength degradation was quantitatively evaluated by a numerical approach. Afterward, based on the shear resistant mechanism, the degraded shear strengths after each load cycle were decoupled into the contributions provided by beam action and arch action, with the use of the numerical local stress results. As the important finding, the mechanism of shear strength degradation due to cyclic loading of the RC column was concluded as with the increase of deformation level, the arch action first degraded gradually whereas the beam action maintained its original capacity; then after losing most capacity of the arch action, the beam action began to decrease rapidly, which caused the shear failure. K E Y W O R D Sarch action, beam action, cyclic loading, RC column, shear strength degradation | INTRODUCTIONIn the seismic design of a reinforced concrete (RC) member, one of the basic requirements is that brittle shear failure is prevented and the shear strength exceeds the shear demand corresponding to the flexural strength. 1-3Although a well-designed RC member under monotonic loading ordinarily suffers flexural failure and shows good deformation performance, if subjected to cyclic loading, the load-bearing capacity often decreases within a relatively small displacement ductility after the flexural yield of the tension reinforcing bars and ultimately shear failure occurs (called shear failure after flexural yield in this study). Because the deformation ability of a RC member under cyclic loading is significant in seismic deformationbased design, many researchers have investigated the influences of several factors such as web reinforcement ratio, tension reinforcement ratio, axial load, and shear span-to-depth ratio for the ultimate deformation ductility of RC members by numerical and experimental methods. [4][5][6][7][8] As shown in Figure 1, the reason for shear failure after the flexural yield of a RC member has been conceptually explained as that the shear strength degrades gradually with the increase of displacement level and ultimately becomes less than the shear demand corresponding to the flexural strength, which leads to a decrease of loadbearing capacity. Corresponding to this failure mode, the relationship between degraded shear strength and displacement ductility (also called the shear capacity degradation curve) was proposed for the first time by the ATC seismic design guidelines. 9

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