# Numerical Modeling Technique of Damage Behavior of
## 1 Introduction  
Masonry-infilled RC frame structures are widely built in seismically active areas around the world [1]. The observation of post-earthquake damage has shown that the interaction between the infill and the bounding frame has a significant impact on the seismic performance of these structures that may or may not be beneficial [2, 3]. Therefore, it is always important to consider the behavior of the frame-infill wall interaction in seismic regions. A large number of numerical methods have been proposed in the literature to consider the nonlinear behavior of masonry-infilled frames. Generally, two main modeling approaches are used for the cracking and damage behavior of the masonry material, these being the continuum approach and the discretization approach. In the continuum approach, the infill is usually modeled as a homogenized continuum and the weakness of mortar joints is smeared out in the entire wall. It has been widely adopted in the past due to its efficiency of calculation and convenience of implementation [4, 5, 6, 7, 8, 9, 10, 11, 12], which allows it to be used for the seismic performance analysis of actual large-scale structures [13, 14]. In this approach, the crack and damage propagation path is represented by a zone in which the load-bearing capacity is gradually lost. The best-known model of this approach is the smeared cracking model, which was pioneered with the fixed smeared crackingmodel [15; 16], then extended to the rotating smeared cracking model [17; 18; 19]. Furthermore, in order to consider the anisotropy and post-peak softening behavior caused by the presence of mortar in masonry walls, some heterogeneous continuum models with the features of orthotropic elastic-plastic behavior with softening have been proposed [20; 21; 22]. In the discretization approach, continuum elements are combined with interface elements to separately represent masonry units, mortar joints and unit-mortar interfaces. In fact, mortar joints and unit-mortar interfaces are usually reduced to interfaces for computational efficiency. With this approach, different failure modes can be captured in an accurate manner. Therefore, it is also widely used in a lot of literature to explore the detailed structural behavior of masonry-infilled RC frames [23; 24; 25; 26; 27; 28; 29; 30]. In addition, some novel simulation methods have been proposed for the comprehensive consideration of computational efficiency and simulation accuracy. [PERSON] [31] proposed a homogenization model of masonry walls at a structural level by uniformly discretizing the wall into rigid triangular elements and nonlinear interface elements. [PERSON] et al. [32; 33] proposed a novel multi-pier (MP) method to analyze the in-plane behavior of masonry walls, especially for walls with openings. With this MP method, [PERSON] et al. [34; 35] predicted the load-bearing capacity and stiffness of masonry walls by the application of machine learning, which greatly improves calculation efficiency. In the analysis of masonry-infilled RC frames, the choice of numerical method depends on the accuracy and detail required. The uncertainties of the frame-infill wall interaction may result in different failure modes, which leads to complexity in the numerical analysis. The discretization approach is usually preferred to describe the failure behavior of masonry-infilled RC frames in detail. In this approach, however, interface elements are required to be preset in the modeling process before the analysis to represent the potential cracking behavior. [PERSON] [36; 37] tried to add interface elements into the modeling of columns to model the diagonal shear failure in the columns according to the experimental observation of cracking behavior. However, in frame members and masonry blocks, the diagonal cracks and many distributed cracks may develop in an arbitrary manner; it is therefore difficult to preset interface elements in the modeling procedure.