Concrete Encased Steel (CES) structural buildings are expected to adopt the structural design by the Calculation of Response and Limit Strength and time history response analysis, hence it is necessary to confirm the validity of both restoring force characteristics models for the CES members and the accuracy of response evaluation for the CES building. However, a restoring force characteristics model proposed by Seki et al for CES columns cannot be considered the interaction between axial force and flexural moment. Outlines and results of analytical model using a Multi-Spring (MS) model for CES columns with H-shaped steel are described in this paper.
Eight CES column specimens with H-Shaped steel were analyzed. The variable investigated were column height, axial force ratio and amount of inner steel.
The MS model composed of two MS elements, one elastic beam element and one components element for shear were used for static analysis for the CES columns with H-shaped steel. Axial length of the MS elements was assumed to be 0.1 times the column height by using Ishii’s study as reference. The concrete and steel in the MS elements was replaced by six elements in the width direction, respectively. The stress versus strain relationship for concrete in the MS elements was expressed as Hoshikuma model. On the other hand, that for steel in the MS elements was expressed as modified Ramberg-Osgood model. Moreover, the backbone curves of shear force versus shear strain relationship was expressed as tri-linear model. Takeda model was used for hysteresis properties. The first point of tri-linear model was assumed as a shear crack point. The second point of that assumed as an ultimate shear strength point.
The stiffness after a flexural crack of shear force versus drift angle relationships in analytical results was higher than that in the test result for all specimen. Because the flexural deformation by sinking and pullout in the embedded part and occurrence of cracks near the center in the longitudinal direction were observed in the experimental results, while those cannot be considered in the MS elements. Therefore, it was indicated from test and analytical results that it is necessary to increase the strain at the maximum stress of the MS elements by 2.5 times in order to correspond the drift angle at the yielding of the steel. The analytical results of the skeleton curves, flexural crack point, and ultimate strength showed good agreement with test results for all specimen. However, strength deteriorations confirmed in the test results for Specimen B3L cannot be simulate, because local buckling of steel elements was not assumed. In addition, it was confirmed that the hysteresis loops in the analytical results were smaller than that in the test result after maximum capacities. These are the study task from now on.
The conclusions in this study are shown as follows
1) It is necessary to increase the strain at the maximum stress of the MS elements by 2.5 times when the height of the MS elements for CES column specimens was assumed to be 0.1 times the column height according to consider the effects of flexural deformation by sinking and pullout in the embedded part.
2) The MS elements assumed stress versus strain relationships of concrete and steel shown in Section 3.2 can evaluated the test results of flexural crack strength and ultimate flexural strength.
3) The analytical for CES columns with H-shaped steel using the MS model proposed in this paper showed good agreements with test result of skeleton curves and hysteresis characteristics up to the maximum capacities.
