Effect of Vertical Load, Number of Bay and Frame Connection Rigidity on the Modeling of Infill Panel in Infilled Steel Frames

Infill panel significantly affect the behavior of surrounding frame. The infill walls are usually considered as non-structural element in analysis and designing process of the structures which is due to inherent complexity and uncertainty behavior of the infill wall and its materials. However, the seismic codes and guidelines are recommended to consider the effect of infill walls on strength and stiffness of the frame structures by using some simple macro models as well as changing the fundamental period of the structures. Among many model proposed by researchers in the literatures, the equivalent diagonal compression strut model is more prevalent, which is recommended by seismic guideline codes. Due to dead and live loads, the presence of vertical loads applied on the beams and columns of the frames is unavoidable. Moreover, the rigidity of beam to column connections of the frame is different and changed depending on the connection types. The previous studies have shown that the presence of vertical load on the frame or the rigidity of frame joints affect the behavior of infilled frames. However, these effects are not considered to estimate the mechanical and geometric characteristics of equivalent strut in seismic codes. In other words, the previous researches have not presented an obvious and explicit conclusion to take into account the vertical load and connection rigidity effect on the modeling of equivalent strut. Moreover, it is assumed that the equivalent strut of multi-bay infilled frame have the same characteristics of that in single-bay ones, which is doubtful based on results of previous researches in the literature. In this paper, the effect of vertical load, connection rigidity and number of bays on the seismic lateral behavior of infilled frames are investigated. For this purpose, an experimental program has been carried out to investigate the lateral behavior of infilled steel frames. Seven specimens included one bare frame and six infilled frames were tested under cyclic loading. The infilled frames were containing single-bay, double-bay frames, rigid frames and pinned frames. Also two infill specimens were tested under combined lateral and vertical loadings. Afterward, an extensive parametric finite element analyses were carried out to achieve more accurate results. The results also show that the stiffness and strength of infilled frames are increased by applying vertical load, but do not affect the properties of equivalent strut. Moreover, it is found that the contribution of infill panel on global behavior of infilled frames is decreased in specimens with pinned connections in comparison with the infill panels in rigid frames. It also concluded that using the struts with the same properties in multi-bay infilled frame are accurately acceptable. In other words, the properties of equivalent struts do not vary with the increase in the bay numbers in the infilled frames.