Case Study of the Seismic Behavior of Confined Masonry Structures in the Sarpol-e Zahab Earthquake by Simplifying Finite Element Model based on Failure Modes

Confined masonry buildings, in which all or part of gravity loads and all of lateral loads on both main directions of the building are resisted by unreinforced masonry walls, have been widely used in Mediterranean Europe, Latin America, the Middle East, south Asia, and the Far East. Experiences obtained from past earthquakes and experimental results indicate that confined masonry buildings, if properly built, exhibits an adequate seismic response. Consequently, it represents a good choice in those seismic regions where masonry is widely used due to economical or traditional reasons.
In this paper, first, the functions of tie beam and tie column and the similarity of seismic behavior of the confined masonry walls and the filled frame have been studied. The structural response of confined masonry and infilled frames under in-plan lateral loading is similar, despite the different construction techniques. In both cases, structural separation occurs at the initial stage. After this separation, a diagonal compressive stress field is formed in the masonry. In the following, the field study of five masonry buildings after the Sarpol-e Zahab earthquake have been presented. The buildings have been selected in such a way that all types according to the scope of this research have been investigated. These types include: 1. Two-story masonry building with tie-beam, 2. Two-story masonry building with tie beam and tie column, 3. Two-story masonry building with tie beam and tie column in first story and tie beam in second story, 4. One-story masonry building with tie-beam, 5. One-story masonry building with tie beam and tie column. The results of this study show that the seismic behavior of confined masonry buildings in comparison with a lot of severely damaged buildings with steel and reinforced concrete structures, is much more desirable.
 In the following, the behavior of two main types of two-story masonry building in Sarpol-e Zahab including masonry wall with tie-beam and masonry wall with tie-beam and tie-column is analyzed as finite element models in Abaqus. By using finite element linear analysis, the load path and cracks in the confined masonry walls can be studied with good accuracy. In the condition that there are only tie-beams in the building and gravity load and seismic force are applied to the wall, the maximum stress is formed in the diagonal path in the middle of the first story wall. Consequently, the first cracks will form in the same path when seismic force creates a shear stress more than shear strength of the wall under gravity load. In the condition that there are both tie-beams and tie-columns in the building, in the initial earthquake cycles, when the wall and ties have a good connection and seismic force is applied from left to right, the shear stress at the junction of the left side of the tie-columns to the right side of the walls is maximum and as a result, the walls will separate from the ties in these locations. In the next cycle, in which the direction of the earthquake is from right to left, the shear stress at the junction of the right side of the tie-columns to the left side of the walls is maximum and as a result, the walls will separate from the ties in these locations. These changes in direction of the earthquake will lead to the separation of the walls from the tie-columns in the initial earthquake cycles. Therefore, the behaviour of confined masonry wall will be similar to the infilled frame in subsequent cycles. Accordingly, the maximum shear stress path will be formed in the diagonal of the bays confined by the tie-beams and tie-columns. Consequently, the first cracks in the bearing walls of confined masonry buildings will form in this path. The results of the study show that the failure modes obtained from the finite element analyses are well matched to the actual cases damaged in the Sarpol-e Zahab earthquake.