Nonlinear Model of RC Columns Considering Axial Collapse

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During the 1995 Kobe earthquake, about thirty concrete buildings suffered complete collapse at an intermediate story. To study such collapse, seismic tests of RC columns subjected to rather low axial load corresponding to the intermediate story were conducted. The loading path was a test parameter. The ultimate limit state, as was defined as state of the loss of axial load carrying capacity or collapse, was mainly discussed. The major findings from the test are, 1) the lateral deformation and axial deformation at the ultimate limit state vary depending on the deformation path imposed on each specimen, and 2) on the contrary, the deformation increment ratio, a ratio of vertical deformation increment to lateral deformation increment at the ultimate limit state does not vary depending on the deformation path, the reason for which can be explained by plastic theory in such that the collapse occurs when the failure surface is reduced to a certain size.The fiber model analysis was applied to simulate the test results, where the hysteresis deterioration of steel due to buckling was considered. Although the computed results did not agree fully well with the observations, they were much better than the results derived from the analysis without considering the buckling.