Instability and Complete Failure of Steel Columns Subjected to Cyclic Loading

In a testing system design for large deformations, structural columns were loaded to complete failure, defined as either complete separation of the column or inability to sustain the prescribed axial load. The test system consists of very large stroke quasistatic jacks, digital displacement transducers that can ensure accurate measurement of large deformations, hydraulic pump units capable of controlling the oil flow, controllers that control the jack motion, and separate personal computers for operating the jack controllers and for supervising and measuring data. These components are connected on-line for data and signal operations, which enables automatic and accurate load control for tests that lead specimens to complete failure. Six columns having a square tube cross section are tested in cyclic loading condition, with the axial load and column length as major parameters. The load–deformation relationships obtained from the tests are presented in detail, and relationships among the deformation capacity, failure mode, slenderness, and axial load are discussed. Intermediate axial load (30% of the yield axial load) is effective in retarding the occurrence and growth of cracks, resulting in larger deformation capacity to complete failure. Finite element analysis accurately duplicates the experimental behavior up to a large inelastic range including material yielding, strain hardening, and local buckling. It fails to simulate the experimental behavior in a very large deformation range where the column surfaces crashed and contacted each other. More experimental data is strongly needed for the behavior of structural systems and elements at and near complete failure.