Numerical Modeling Approach for Progressive Collapse Analysis of Infilled RC Frames

Most design codes treat masonry infill walls as non - structural elements, and in the analysis of steel or reinforced concrete (RC) frame structures, their presence is often neglected. However, numerous studies have shown that infill walls significantly inf luence the strength, stiffness and ductility of the structure. Therefore, it is crucial to consider the contribution of infill walls to the building's behavior. Accurate modeling of the infill walls requires knowledge of the mechanical properties of masonry, various interacting parameters, and the contact conditions along the interface between the infill and the surrounding frame. Two prim ary techniques used in the analysis of infilled frame structures are macro - modeling and micro - modeling. Micro - modeling utilizes Finite Element (FE) software to represent the masonry panel as composed of many elements that simulate the bricks and mortar. In contrast, macro - modeling treats the masonry panel as a few elements, typically modeled as one or more compressive diagonal struts. Both techniques aim to capture the nonlinear behavior of the materials. The objective of this study is to assess the influence of the infill walls on the progressive collapse resistance of RC framed structures. To achieve this, an RC frame experimentally tested by Li et al. (2016) is modeled in Abaqus/Explicit software in two configurations: bare frame (without infill walls) an d infilled frame (with infill walls). The frame consists of four bays and two stories, and progressive collapse is triggered by the failure of the middle column from the first story. This failure is simulated through a step - by - step unloading process in a displacement - controlled manner. To evaluate the progressive collapse behavior of the two numerical models, nonlinear explicit dynamic analysis is adopted to simulate the quasi - static loading scheme. The Concrete Damaged Plasticity (CDP) model is used for both concrete and masonry , with the compressive stress - strain relationship following the Kent - Scott - Park constitutive model. The steel reinfor cement bars material properties are specified based on a bilinear stress - strain relationship. Tie - type connections are employed to model the interaction between the RC frame and masonry elements, while general contact is used to simulate th e interaction between masonry elements. The resistance force (applied vertical load) versus the vertical displacement of the middle column is monitored up to a displacement of 500 mm. The progressive collapse behavior of the frame is divided into four stages. The numerical results obtained for both models show good agreement with the experimental ones. It was found that during the first deformation phase (when adjacent and exterior columns moved outward), the infilled frame model resisted a vertical force approximately 4.8 times greater than the bare frame model. In conclusion, completely neglecting the infill walls in the progressive collapse analysis of RC framed structures leads to unrealistic results.