Role of Infills in Typical Buildings with Seismic Design and Three-Dimensional Modeling

Statistical collection of existing buildings, especially in Tehran, and their classification can be a great help in identifying crisis management in order to reduce earthquake damage. Modern architecture is widely used due to its economical and practical benefits. In this architecture, the infills on the ground floor are generally removed. In Iran, most infills are constructed in such a way that they are connected to the corner of the frame by bricks, mortar, and other elements. For this reason, they are considered structural elements and will affect the lateral stiffness and lateral strength of the structure. Many studies have been conducted on buildings with this style of architecture designed based on gravity loads, which show their poor performance due to the formation of the soft story. In this study, first, statistical information is collected from recently built buildings in regions No. 9 and 11 of Tehran that are designed and constructed based on seismic criteria and are classified based on the reasons for the formation of the soft stories. Most of the buildings in these regions are RC moment-frame structures and have 6-8 stories and 2-3 bays in each direction. The first story in these buildings is used as parking. Therefore, assuming the correct constructions, the factor of removing the infills on the ground floor can cause the formation of a soft story in these structures.Next, a six-story concrete building with similar architecture in accordance with the results of statistical studies, is modeled in a three-dimensional mode in the OpenSees to evaluate the seismic behavior of the structure in different scenarios of infills arrangement on the ground floor and earthquake at different angles. To better cover all angles and reduce analysis costs, through the LHS sampling method, the selected angle for analysis is determined. Due to the lack of laboratory studies in three-dimensional mode and the importance of the axial-flexural interaction in the column in this research, structural modeling in this study was considered through the fiber model. The model of two compression struts placed diagonally in the panel is used to model the infills, and the numerical modeling is verified with experimental research.In this research, the probable formation of the soft story is evaluated based on three criteria: the ratio of the lateral linear stiffness of the stories, mode shapes, and distribution of the nonlinear lateral drift of the stories. The linear analysis results show that because these infills have high initial linear stiffness, the mode shape and lateral linear stiffness of these buildings are like soft-story buildings. However, the results of the nonlinear dynamic analysis show that since these structures are designed based on seismic criteria and the height of the ground floor is smaller than others, the infills have low ductility. Therefore, the removal of infills in this story does not necessarily lead to a soft story and instead can improve the performance of the structure by uniformly distributing damage across all floors. On the other hand, in the case that infills are evenly distributed in all floors, the behavior of the structure is similar to the moment frame and the damage is concentrated in the middle stories. This type of infill distribution increases the lateral stiffness and reduces the capacity of the structure. Therefore, it has even poor performance in comparison with building without infills in the first story. Also, different scenarios of infill arrangements on the ground floor change the behavior of the structure. Applying earthquakes at different angles indicates the building will experience varied behavior at each angle so that for some angles the building even collapses while for others not. Therefore, according to the numerical results, this modeling and analysis method will help show the actual behavior of the structure during an earthquake to prepare capacity curves for design.