The Effect of Infill Walls on the Fundamental Period of Steel Frames by Considering Soil-Structure Interaction

One of the most critical parameters in the process of analysis and design of structures is determination of the fundamental period of vibration. The fundamental period depends on the distribution of the mass and stiffness of the structure. However, the building codes propose some empirical equations based on the observed period of real buildings during an earthquake as well as ambient vibration tests. Furthermore, the majority of these proposals do not take into account the presence of infills walls into the structure despite the fact that infill walls increase the stiffness and mass of structure leading to significant changes in the fundamental period numerical value. These equations are usually a function of type and height of the buildings. The different values between the empirical and analytical periods are due to the elimination of non-structural effects in the analytical methods. For this reason, the presence of non-structural elements such as infill panels should be carefully considered. Another effective parameter on the fundamental period is the effect of Soil-Structure Interaction (SSI). It is obvious that soil flexibility increases the fundamental period of the structure. In most of the seismic building codes, the role of the SSI is usually considered beneficial to the structural system under seismic loading since it increases the fundamental period and leads to higher damping of the system. Recent case studies and post-seismic observations suggest that the SSI can be detrimental and neglecting its effects could lead to the unsafe design for both the superstructure and the foundation, especially for the structures located on soft soil. The current research deals with the effect of infill panels on the fundamental period of steel moment resisting (MRF) and eccentrically braced frames (EBF) considering the influence of soil-structure interaction. In this study, all of the buildings designed under gravity and earthquake loading have been evaluated by utilizing 3-D FE modeling incorporating Eigenvalue-analysis to obtain the elastic periods of vibration. For this purpose, the effect of building height, the infill wall panel stiffness, various percentage of infill openings as well as the effect of soil structure interaction in 3, 6, 9, 12, 15 and 18-story 3-D frames were investigated. The studied frames were modeled and analyzed in SeismoStruct software. The calculated values of the fundamental period were compared with those obtained from proposed equation in the seismic code. The results have shown that the number of stories and the soil type are critical parameters that influence the fundamental period of steel frames. Also, it has been found that the height of structures significantly influences the fundamental period. As it is known, in the absence of infill walls, the numerical fundamental periods have generally higher values than those obtained from the empirical formula recommended in building codes such as Iranian Standard No. 2800 and FEMA450 and the other codes. The presence of infill wall leads to a considerable decrease on the fundamental period of steel frames. This decreasing is strongly dependent on the infill wall panel stiffness. In other words, an increase of the infill wall panel stiffness reduces the fundamental period. Also as the infill opening percentage increases, the fundamental period of the structure almost linearly increases. The soil-structure interaction strongly affects the fundamental period of structures. The fundamental period is higher for more flexible soil types. Furthermore, the influence of soil-structure interaction is higher when the infill wall stiffness is higher. Based on the results, one can conclude that the fundamental period of a building cannot be predicted by only using the height of the building. Finally, new equations, which are a function of the selected parameters (building height, modulus of elasticity and the infill wall thickness, infill wall percentage and soil type), are also proposed for predicting the fundamental period of MRF and EBF buildings.