On the Influence of Vertical Loading on the Behavior of Infilled Steel Frames

IntroductionIn the last six decades, several experimental and analytical researches have been carried out to investigate the structural effects of the infill panels, especially in seismic loads. These studies showed that infill panels have considerable effects on the performance of infilled frames that should not be neglected in a safe and realistic design.In most of the buildings that require seismic retrofit, beams are not sufficient for the dead and live loads, and therefore, part of the vertical load is transferred to the ground through the walls. However, in most of the researches on infilled frames, the vertical load is not considered. In other words, very few studies investigate the effect of vertical load and its influence on the behavior of an infilled frame. These researches show that vertical load can have considerable effects on the strength and stiffness of masonry infilled frames.Research ProcedureThispaper presents a numerical study concerning the effects of vertical loads on the behavior of masonry infilled steel frame in seismic events. In this regard, an experimental study is selected that includes two identical infilled frame specimens [1]. One ofthe specimens is only subjected to lateral loading and the other one is subjected to lateral and vertical loading. Finite element method is employed to simulate and analyze the infilled frames. The specimens are modeled and verified based on the corresponding experimental results. Micro modeling method has been used, instead of modeling the mortar, half of its thickness is added to adjacent bricks. Concrete damage plasticity (CDP) approach was used to model the inelastic behavior of the masonry.ResultsThe analysisresults showed that the finite element modeling is well capable of predicting the behavior of the infilled frames. The initial stiffness of the analytical model matches with the experimental stiffness; however, as the loading continues, the strength of the finite element models is greater than that of the experimental specimen. This difference is due to the fact that the experimental specimen is subjected to cyclic loading and therefore experiences more strength degradation than the finite element model that is under monotonic loading. Based on the finite element study, the vertical load applied to the infilled frame is distributed in two ways, part of it (approximately 40%) is transferred through beam to column connection and the other part (approximately 60%) is applied to the wall. When the vertical load raises from zero to 200 kN (in which the vertical loading of 128 kN that is equivalent to 6.7% of compression capacity of the masonry prism is transferred to the masonry wall), strength and stiffness are increased 15% and 50%, respectively. The strength reduces when the vertical loading raises reach from 200 to 300 kN, and the stiffness remains constant approximately after vertical loading of 300 kN. It can be stated that up to a certain point, vertical load results in the increase in the stiffness and strength of the infilled frame, and after that point, the stiffness approximately remains constant and ultimate strength decreases.This occurrence can be justified as until a particular values vertical load increasesthe friction between the bricksandthus increases the strength. However, after this value, the vertical load turns into a destructive factor in combination with the lateral load effects. Ductility of the specimen with vertical loading of 200 kN is less than that of the specimen without the vertical loading.