Abstract
A developmental study of Concrete Encase Steel (CES) composite structure system has been continuously conducted toward its practical use. CES structure is composed of steel and fiber reinforced concrete. In previous studies, it was found that the CES structure has good seismic performance from the experimental study of columns, shear walls and a two story two span frame. As for CES beam-column joints, it was confirmed that failure behavior, strengths and restoring force characteristics, CES beam-column joints also possess good seismic performance. It was shown that the flexural strength of the beam could be evaluated by superposition strength theory and the shear strength of the joint panel could be evaluated by a formula in AIJ SRC standard conservatively. The shear strength evaluation method in the SRC standard calculates shear strength as a product of the effective area of concrete and the shear stress of the concrete at its ultimate state. On the other hand, three-dimensional FEM analysis was conducted to understand the stress transfer mechanism of the joint panel. As a result, it was found that the stress state was different in the area which was surrounded by steel flange and the area which was not. However, it is thought that the structural performance of the joint panel is affected by many structural factors and it is still the case that experiments of CES beam-column joints with shear failure of the joint panel are low in number.
Therefore, in the present study, static loading tests of CES beam-column joints with different cross sectional configurations of joint panels and length of columns were conducted to grasp the structural performance and stress state of the joint panel and examined by simulation with FEM analysis. Furthermore, parametric analysis was conducted in order to understand the effect of structural valuables on the stress state of concrete in the joint panel, and the validity of an effective area of concrete in the ultimate shear strength evaluation was examined.
Firstly, in comparison between specimens with different cross sectional configuration of joint panel, the damage condition of concrete was different and it could be thought that the stress state of concrete was different depending on the peripheral configuration to the joint panel. FEM analysis of test specimens was conducted, where the analytical relationship of shear force and drift angle was in approximate agreement with that of test results and the validity of the analytical modeling method could be confirmed.
Secondly, parametric analysis was conducted with a verified modeling method and the effect of axial force ratio, concrete strength, and width of steel flange in joint panel on shear strength of the joint panel was examined. From these analyses, the difference in shear strength of the joint panel was within 10% depending on axial force, the effect of the width of the steel flange was small although the width of the joint panel steel flange on shear strength affects to the shear stress state depending on the area inside or outside of the steel flange. In addition, the effect of the compressive strength of concrete on shear strength was dominant and shear strength increases with increasing the compressive strength of concrete.
Finally, a calculation formula of the shear strength of the joint panel in a CES beam-column joint was proposed based on knowledge obtained from analysis and experimental results. The shear strength of the concrete panel was expressed with the effective area of concrete and the average shear stress of concrete at the ultimate state in the joint panel. Shear strength, calculated with the proposed formula, was in good agreement with the experimental data.
