Deteriorating behavior due to local buckling and fracture of H-shaped steel beam with axial deformation restriction

1. Introduction: Against quite huge earthquake, steel moment frames may collapse completely because strength deteriorates due to local buckling, lateral-torsional buckling or fracture of their members. In order to represent with high accuracy the collapse behavior of the frame, past studies have been conducted by using analytical models which can consider deteriorating behavior of steel members. Many of them applied rotational spring models for beams, based on the idea that the axial force does not act on the beams. On the other hand, steel beams in moment frames are restricted in the axial deformation by columns, other beams and floor slab. Although it is considered that the deteriorating behavior of beams is affected by such axial deformation restriction, there are previous studies about deteriorating behavior of beams after occurring only lateral-torsional buckling. In this paper, cyclic loading test and finite element analysis (FEA) were carried out to reveal effect of axial deformation restriction on deteriorating behavior of H-shaped steel beams when local buckling or fracture occurs. 2. Cyclic loading test of H-shaped beam with/without axial deformation restriction: Cyclic loading test was conducted to verify hysteresis characteristics of H-shaped steel beams with/without axial deformation restriction when local buckling and/or fracture occur. Test specimen, as illustrated in Fig. 1, consists of a welded H-shaped steel beam and a square hollow section column, and they are connected by through-diaphragms. Test parameters are width-thickness ratio of web and flange, beam-to-column connection type and the presence or absence of axial deformation restriction. As an experimental result, it is clarified that according to axial deformation restriction, out-of-plane deformation of flange due to local buckling becomes smaller and deterioration of strength after occurring local buckling becomes moderate. Furthermore, it is revealed magnitude of axial deformation restriction does not affect timing of crack initiation if deterioration by local buckling is not remarkable. 3. Finite element analysis: In order to examine range of the axial deformation restriction which influences deteriorating behavior due to local buckling of beams, FEA was carried out with varying magnitude of axial deformation restriction. First, the validity of the analytical model was confirmed by comparing between analysis results and test results, which are shown in Chapter 2. Next, effect of rigidity of restriction on deteriorating behavior is investigated, based on FEA results. As shown in Fig. 13, the axial deformation restriction is classified into three parts, considered with characteristic of the deteriorating behavior. When the axial restriction is small (kf/kb, < 0.01), the deteriorating behavior due to local buckling shows the same tendency as when it is not restricted. In this part, it is considered that a rotational spring model can be applied for beams in numerical analysis of frames. When the axial restriction is large (1 < kf/kb), the behavior shows the same as when it is completely restricted. When the axial restriction is moderate (0.01 < kf/kb < 1), the behavior is intermediate between these. In these two parts, axial deformation of beams should be considered in numerical analysis of frames. 4. Conclusions: This paper studied the effect of axial deformation restriction on deteriorating behavior of H-shaped steel beams due to local buckling or fracture, by means of both cyclic loading test and finite element analysis. From FEA results, the relationships between the rigidity of the axial deformation restriction and the deteriorating behavior by local buckling of beams is examined. Furthermore, A method to verify criterion where axial deformation restriction should be considered in numerical analysis of frames is proposed. © 2018 Architectural Institute of Japan. All rights reserved.