NUMERICAL STUDY ON CYCLIC SEISMIC RESPONSES OF UNREINFORCED MASONRY WALLS RETROFITTED WITH LOW-STRENGTH ENGINEERED CEMENTITIOUS COMPOSITES

Abstract

Most of the faults of Tien Shan Mountain are situated in Central Asia. Even the small faults of the mountain may cause risk to the nearby cities, such as Almaty, Kazakhstan, with a population of about 1.8 million people. Over half of the buildings in Almaty are unreinforced masonry buildings, followed by reinforced masonry, confined masonry, and reinforced concrete, and a minority of structures are from wood and steel. Unreinforced masonry buildings may demonstrate brittle behavior under seismic activities. Earthquakes such as those happened in the past of Almaty can damage most of the city and its inhabitants. The catastrophic destruction action of the earthquake can be mitigated by the construction of high strength, and ductile buildings, which can withstand large deformations with minor damages. Thus, to improve the response of existing unreinforced masonry structure, different studies propose the use a retrofitting, which can increase the deformation capacity and ductility of the unreinforced masonry walls. This thesis focuses on parametric study on cyclic responses of unreinforced masonry walls retrofitted with low-strength Engineered Cementitious Composite (ECC). The new retrofitting material, Engineered Cementitious Composite (ECC) with a high tensile strain hardening rate, was retrofitted to the unreinforced masonry wall in this study. Previous experimental studies have shown that the normal strength ECC can increase the strength of URM but has little effect on the deformation capacity due to the incompatibility of the masonry and composite. Moreover, a numerical study by Sailauova (2022) has shown that the low-strength ECC can increase the deformation capacity under monotonic loading because of stiffness compatibility between the masonry wall and composite. Considering the prior studies, this study addresses to the cyclic response of the masonry wall since the earthquake has a cyclic nature. The application of low-strength ECC approved the hypothesis that it can reach more deformation rather than normal-strength ECC, since it matches the stiffness of the masonry wall resulting in more ductile behavior. This study aims to investigate the cyclic response of unreinforced masonry walls subjected to cyclic seismic loadings. This thesis presents a numerical study on in-plane cyclic behaviors of the unreinforced masonry wall retrofitted with the low-strength ECC. The numerical study was conducted on nonlinear two-dimensional models in ABAQUS software using a simplified micro-modeling approach for the masonry wall and the concrete damage plasticity for the ECC. The dynamic analysis using explicit analysis technique was adopted due to the non-linear behaviors of the masonry wall and ECC. The model was verified from an existing experimental study. The study parameters include the strength of ECC, wall aspect ratios, and vertical pressures, which represent typical wall geometry and gravity loading conditions of unreinforced masonry buildings in Almaty. This study concentrates on 0.5, 0.75, and 1.0 wall aspect ratios and walls subjected to 0.43, 0.60, and 0.78 MPa vertical pressures, or in axial force ratio 0.10, 0.15 and 0.19, respectively. Moreover, the proposed material, Engineered Cementitious Composite (ECC), with different compressive strengths (10, 20, 30, 40, and 50 MPa) was evaluated. The behavior of the retrofitted wall under the cyclic loading was compared to that under the pushover analysis. The numerical study results concluded that with the increase of ECC compressive strength on the retrofitted masonry wall, deformation capacity and ductility decrease, while wall strength and stiffness increases simultaneously. However, as the ECC becomes stronger, energy dissipation decreases. The effect of ECC for different wall aspect ratios on wall strength decreases with the increasing aspect ratio, but increases with the increase of vertical pressure. This is because the friction between the brick units and mortar increases. Regardless of the aspect ratio and vertical pressure, low strength ECC with compressive strength 10 and 20 MPa improve the lateral deformation capacity of the masonry wall. Moreover, the pushover analysis shows a slower strength reduction than the cyclic loading analysis due to the cumulative damage from the cyclic loading.