Dynamic Instability of Steel Frames under Combined Stressduring Earthquake

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Steel beam-columns were tested under biaxial and uniaxial bending and varying, axial load as well. Three kinds of loading, types were applied : (1) monotonic loading tests, (2) cyclic loading tests, and (3) computer on-line earthquake response tests. Observations in these tests can be used to check the design formulae for the resistance as well as the deformability of steel members, and also to study an appropriate limitation of axial forces in the beam-column design practice.Additionally, an inelastic portion of a steel beam-column is modeled as an assembly of spring elements, and a simple hysteresis rule is proposed to simulate its inelastic behavior in including the strain-hardening and the Bauschinge's effects. Analyses based on such a model were carried out for each loading cases, and compared with the test results. The results of the analyses proved to agree fairly well with the test results. Especially in the case of the cyclic loading test, very large deformation was observed i … More n the direction where displacement was not constrained, and even such an unstable phenomenon was well simulated by the present analysis.The results of this research project are summarized as follows :1. A computer-controlled testing system was developed to provide varying combined stress states to a beam-column specimen, which would be produced in a specified member of a fictitious complicated structural system such as a high rise building. The effectiveness and the accuracy of such an intelligent loading system were confirmed during all the test procedures.2. An existing design formula to evaluate the resistance of steel beam-columns under biaxial bending and axial load was compared with the test results, and proved to give a conservative evaluation for the axial load smaller than 60 percent of the yield axial load. However, more accurate evaluation of the resistance may be needed in the ultimate limit state design of the beam-columns, and this shall be studied by more extensive sets of experiments and analyses.3. In some of the cyclic loading tests, considerable amounts of plastic deformation were cumulated in the out-of-plane direction. Such an out-of-plane instability shall be investigated more carefully.4. The analysis method based on an inelastic joint model proved to provide a good prediction of the resistance and the deformability of steel beam-columns under combined stress states. Such a modeling can be applied, because of its simplicity, to the analysis on large-scale 3-dimensional steel frames. Less