Stiffness degradation of shear walls under cyclic loading: experimental study and modelling

Stiffness is an important parameter for the seismic design of shear walls, which is related to the shear force distribution of each wall member. The stiffness degradation occurs in shear walls under the effect of earthquakes, and the effect of stiffness degradation is considered by a constant reduction factor in many design codes. However, the constant reduction factor cannot consider the whole stiffness degradation process of shear walls. The aim of this paper is to experimentally study and model the stiffness degradation of shear walls under cyclic loading. Five wall specimens were tested under cyclic loading to study the influence of reinforcing bar strength, axial load ratio, failure mode, and cross-section type on the stiffness degradation. Based on the experimental stiffness degradation curves of rectangular shear walls failing in flexure, a four-line stiffness degradation model controlled by crack, yield, peak, and ultimate points was proposed, and the calculation methods for the values of each point were established. The four-line stiffness degradation model was used to obtain the analytical stiffness degradation curves of the wall specimens, which were compared with the experimental stiffness degradation curves. Study shows that increasing reinforcing bar strength decreases the initial stiffness, and increasing axial load ratio significantly improve the initial stiffness while accelerating the stiffness degradation rate with drift ratio. The stiffness of shear walls failing in flexure is more fully degenerated than that of shear walls failing in shear. The stiffness of T-shaped shear walls is larger loaded in the positive direction than that loaded in negative direction. The flange improves the stiffness while accelerates the stiffness degradation rate with drift ratio. The analytical and experimental stiffness degradation curves were in the reasonable agreement.