Clarification of Unstable Behavior of Building Steel Frames Subjected to Repeated Loading and Development of Estimation Method of Safety

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A series of theoretical and numerical studies have been carried out to clarify the unstable behavior of framed structure, which can not be captured with any simplified method of numerical analysis used in design practices, and to develop an estimation method of safety. The results obtained by these studies are summarized as follows :1. A new type of critical behavior in a H-shaped simple unit frame has been found and its appearance condition has been clarified by performing a symmetry limit analysis.2. A new type of critical behavior of multi-story multi-span planar frames, which is characterized by cyclic growth of a bow-shaped column deflection mode spanning a certain number of stories. This critical behavior is possible to occur, if the column force is in excess of a definite limit, which can be predicted by the symmetry limit theory. The mechanisms of the appearance and cyclic exponential growth of the column deflection mode have also been clarified theoretically.3. Dynamic response analyzes have been performed on some multi-story frames subjected to harmonic ground motions and recorded seismic ground motions. It has been shown that a similar type of column deflection mode to that observed in the static behavior can appear also in the dynamic response to the repeated ground motions.4. Both the appearance limit condition and the deflection mode of this critical behavior in the responses to the dynamic ground motions well coincides with those of the critical behavior under statically repeated loadings.5. Since the limit condition is to be determined by the parameters of the frame itself only, then it should be useful for the estimation of seismic stability and practical seismic design of framed structures.