A large number of reinforced concrete (RC) buildings collapsed with story failures by 1995 Hyogoken-Nanbu earthquake in Japan. Especially RC columns at the corner post on the failure story collapsed in shear brittly under large compressive axial forces generated by large horizontal and vertical accelerations. In order to prevent happening brittle shear failure of RC columns and occurring the story failure of building structures, it is necessary to make the ductility of columns larger. It can be thought using core steel composite columns is useful as one of the reinforcing RC columns. The beam columns are monolithic concrete encased small depth steel member with longitudinal reinforcing bars. It is thought that the encased core steel are useful for resisting large axial compressive force and little bending moment, and for preventing columns from shear failure. This paper presents the results of an experimental work carried out in order to study elastic-plastic behaviors of the core steel composite columns under a constant axial compressive load and cyclic horizontal load. The experimental parameters were as follows: 1) Cross section (core steel composite column, RC column and usual steel reinforced concrete column), 2) Ratio n of axial compressive load against ultimate compressive strength of cross section, 3) Area ratio of steel section against total concrete cross section (1.5%, 3.0% and 5.0%), and 4) Shape of core steel section (H shaped section and compact circle section). Total 14 specimens were tested. It is the purpose of this study to describe the elastic-plastic behavior of the core steel composite columns under large compressive axial load and cyclic horizontal load, and to show the composite columns have large better earthquake resistant performance than RC columns.
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