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The majority of building population in Iran and other developing countries consists of unreinforced masonry buildings and sometimes confined masonry (CM) buildings. In such buildings, masonry shear walls are the main earthquake resistant components. The Iranian seismic standard IS2800 provides some specifications for seismic design and construction of confined and reinforced masonry buildings which all are based on the observed behavior of them during the past destructive earthquakes. In other words, the specifications are merely qualitative. This shows the necessity of assessment of masonry buildings behavior both experimentally and numerically. Despite the extensive numerical studies available in the literature, it seems that the lateral load behavior of masonry buildings cannot be properly investigated by continuum mechanics based methods such as traditional finite element method. As an alternative to the available finite element methods, a distinct/discrete element method (DEM) can be used to investigate the nonlinear lateral load behavior of masonry buildings. Distinct element method has the capability to con-sider large displacements, shear sliding and complete joints openings between bricks as well as automatic detection of new contacts during the analysis process. In this paper a two-dimensional numerical model is developed using distinct element method using the specialized distinct element software UDEC (Itasca, 2004) for the nonlinear static analysis of unreinforced masonry buildings subjected to in-plane monotonic loading. The Univer-sal Distinct Element Code (UDEC) is a 2D program based on the DEM to simulate the behavior of jointed materials subjected to either static or dynamic loading. The developed DEM model is validated using the results of a two-story unreinforced masonry building designed and tested based on the Iranian seismic standard IS2800 regulations at the Building and Housing Research Center (BHRC). Due to low intensity of gravitational normall stresses in conventional masonry buildings, the bricks were built using an elastic material model. In order to develope a DEM micro-model based on interface elements with zero thickness, the size of the bricks was expanded by the mortar thickness in both directions and the elastic properties of the expanded brick were assumed to be the same as that of the real brick. Howevr, For the joints, simulating the characteristics of the mortar, a Mohr–Coulomb slip model was employed. It was found that the model can be used confidently to simulate nonlinear behavior of unreinforced masonry buildings for parametric studies. The Iranian seismic standard IS2800 specifications pertain mainly to the masonry shear walls percentage need in each direction. In other words, the perpendicular shear walls are not taken into account in masonry buildings’ lateral load capacity calculations. However, unreinforced masonry buildings resist lateral loads through box action behavior of all constituent components (i.e. walls, foundation and diaphragms). Therefore, a parametric study was conducted to investigate the contribution of perpendicular masonry shear walls on buildings’ lateral load capacity. Parametric study showed that perpendicular masonry shear walls contribute considerably to the shear capacity of the masonry building.

Keywords: Unreinforced masonry building, Distinct Element Method (DEM), Nonlinear analysis, Seismic behavior, Shear capacity

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