Existing domestic experimental studies on the torsional behavior of reinforced concrete members are limited to members subjected to pure torsion, bending, shear, and torsion. However, in actual members, axial force, bending, shearing, and torsion are compounded by various load combinations such as fixed load and live load. In order to confirm the torsional behavior of reinforced concrete members subjected to such combined loads, in this study, an experimental study of members subjected to combined loads was conducted.
Structural members act in various combinations of axial force, bending, shearing, and torsion depending on the location and use of the member. did. Since the cross-sectional characteristics also affect the torsional strength of a member, it is suggested in the lateral torsional reinforcement ratio and yield strength, which are the main variables of the torsional strength evaluation formula presented in KDS 14 20, the current domestic concrete design standard, and ACI 318-19. The torsional reinforcement ratio of the specimen to the maximum strength ratio of the specimen and the longitudinal torsional reinforcement muscle ratio of the bending-shear-torsion load correlation equation described in Section II.2.2 of this study were selected as additional variables.
In order to realize the combined load of axial force, bending, shearing, and torsion, an experimental equipment was produced that induces a subject subjected to torsion to rotate while maintaining a constant central axis. The experimental equipment is described in detail in Chapter III of this paper.
To evaluate the torsional behavior of reinforced concrete members subjected to combined loads, the experimental results were summarized as torsional moment-angle of twist relationship, crack pattern, and strain distribution of torsion reinforcing bars. Based on this, the effect of the selected variable on the torsional behavior of the member was confirmed by comparing the torsional secant stiffness and ductility. To evaluate the accuracy of the correlation equation by comparing the experimental results with the predicted results by the existing combined load correlation equation. The experimental results and analysis are described in detail in Chapter IV of this paper.
The conclusions obtained through this study are summarized as follows.

1) Although the effect of axial force on the strain rate of torsional reinforcing bars was insignificant, it was confirmed that the effective compressive strength of concrete was increased and the occurrence of torsional cracks was delayed, thereby increasing the torsional strength of the member, suppressing cracks, and increasing the secant stiffness and ductility.

2) The effect of the bending moment ratio on the torsional cracking pattern was insignificant, but as the bending moment ratio increased, the applied shear force increased and the torsional resistance capacity of the member decreased. Accordingly, it was confirmed that the torsional strength was lowered, the secant stiffness and torsional ductility were reduced, and as the shear stress overlapped with the torsional shear stress on the right side of the member increased, the torsional moment sharing ratio of the transverse torsional reinforcing rod increased and the strain increased.

3) It was confirmed that the effect of the upper and lower longitudinal torsional reinforcement muscle ratios on the torsional strength, crack pattern, strain distribution, secant stiffness, and torsional ductility of the member was negligible. This is considered to be because the longitudinal torsion reinforcing muscle ratio is the same, so the contribution of the longitudinal torsional reinforcing muscle to torsion is the same.

4) It was confirmed that the torsional strength, secant stiffness, and ductility increased as the lateral torsional reinforcement rate increased, and the distance between torsional cracks became denser. This is considered to be because the torsion resistance capacity increased as the lateral torsional reinforcement rate increased, the spacing between the reinforcing bars became denser, and the torsional share of the reinforcing bars decreased. However, the effect of the transverse torsional stiffening rate on the strain distribution was insignificant, which is thought to be because the torsional strength of the member increased as the torsional resistance capacity of the transverse torsional stiffener increased, resulting in an increase in the torsional strength to be resisted.

5) It was confirmed that the torsional strength of the specimen with increased lateral torsional reinforcing muscle ratio and reduced yield strength was also set to the same, and the spacing between torsional cracks became denser. This is thought to be due to the fact that as the reinforcing bar ratio increases, the spacing of the reinforcement becomes dense, the area directly resisting torsion is widened, and the cracks are dispersed. However, the effect on strain distribution, secant stiffness and ductility was insignificant. This is considered to be because the transverse torsional reinforcement muscle ratio was increased by planning the same, but the yield strength and yield strain decreased and thus the strain, secant stiffness, and ductility were not affected.

6) The existing bending-torsion correlation equation underestimated the experimental results. This is because the specimen in this study is in the shape of a cantilever beam, and there is a strong axis and a weak axis, and even if the non-yielding strong axis resists torsion, even if it can not resist torsion due to the yielding of the weak axis lateral torsional reinforcing rod, where the shear stress and the torsional shear stress are overlapped, It is judged that the torsional strength is increased and the ductile behavior is exhibited. On the other hand, the existing correlation equation is based on the simple beam experiment in which the maximum torsional moment is reached with the yielding of the transverse torsion reinforcing bar due to the ambiguous boundary between the weak axis and the strong axis.

7) The torsional strength evaluation formula of ACI 318-19 underestimated the experimental results regardless of the moment ratio. For a rational design, it is necessary to reflect the effect of the axial force and the change in the internal strength of the actual member according to the change in the reinforcement ratio in the equation to prevent over-reinforcement for the required performance, and to prevent the crushing of concrete before the yield of the reinforcement. The limit value of 100% reinforcement ratio for this purpose is judged to be appropriate.