Decay of Torsional Stiffness in RC U-Shaped Walls

Reinforced concrete (RC) U-shaped walls are a popular construction choice, commonly used to resist the lateral loads from wind and earthquakes. In many buildings, the center of stiffness of a floor is eccentric from the center of mass and the building will therefore undergo some twist. Often, U-shaped walls contribute significantly to the torsional stiffness of the building and the analysis of the structure therefore requires an estimate of the torsional stiffness. This research investigates the dependency of the torsional stiffness of U-shaped walls on the translational displacement demands using experimental evidence and advanced numerical models. The experimental results show that the torsional stiffness decreases with increasing translational displacement. Furthermore, the numerical and experimental results indicate that the torsional stiffness degrades at a similar rate as the translational stiffness of the walls. The rotational stiffness of the wall is smaller when it is centered than when it is subjected to a translation. This effect is attributed to the longitudinal stresses throughout the cross section that are already present due to the imposed flexural displacement. In particular, the compression zone resulting from the translational displacement imposed on the wall stiffens the wall with regard to the twisting. As a result, the effective torsional stiffness increases with increasing axial load ratio. Current codes do not provide reduction factors for the torsional stiffness, but the results show that the reduction factors for the shear stiffness can serve as estimate, albeit they do not capture the dependence on the axial load ratio. (C) 2020 American Society of Civil Engineers.