勾配曲げモーメントと圧縮軸力を受ける上フランジ連続補剛H形鋼梁の横座屈荷重と連続補剛材の水平・回転拘束効果

Abstract

When steel moment frames are subjected to seismic forces, H-shaped beams carry the gradient flexural moment. Large cross-section beams are used as main beams to design structural members effectively in real large space structures. Long span beams may not possess the plastic strength due to lateral buckling, so that many lateral braces along the beams should be set up to prevent the lateral bucking deformation (AIJ 1998). Most oy beams in frames are connected by continuous braces such as folded-roof plates, which are effective to prevent the lateral buckling oy beams. However, in the Japanese design code, non-structural members are not considered as the braces. On the other hands, there have been many moment resisting frames with dampers to prevent damages oy main frames recently, and then H-shaped beams connected with dampers are subjected to compressive axial forces in addition to gradient flexural moment. If the beam carries the large axial force induced by the damper during earthquake, the lateral-torsional buckling behavior oy H-shaped beam becomes more unstable than that under flexural moment only. The axial force which the beams carry is possible to exceed 30% oy yield load oy the beam. Therefore, in this paper, the lateral buckling behavior for H-shaped beams with continuous braces under gradient flexural moment and compressive axial force is clarified and elasto-plastic lateral buckling stress oy the beams or lateral stiffing force and rotational stiffing moment oy continuous braces are evaluated by the energy method and numerical analyses. In this study, for H-shaped beams with continuous braces subjected to gradient flexural moment and compressive axial force, two types oy loading conditions and two types oy bracing rigidities are considered. One oy the loading conditions, which is called as Type A, is the case that the upper flange's compressive load is larger than the lower flange's one in the left side oy beam as shown in Fig. 3(a), and another, which is called as Type B, is that the upper flange's compressive load is smaller than the lower flange's one in the left side oy beam as shown in Fig. 3(b). Furthermore, the ratio oy the compressive axial force, P2 to P1 is represented as an axial force ratio ‘p’ in equation oy elastic lateral buckling load. The continuous braces are divided into the lateral braces, ku, and rotational braces, kβ, as shown in Fig. 1. In the case oy TypeA, ku is effective for preventing lateral deformation oy compressive upper flange, whereas in the case oy TypeB, kβ, is effective for preventing torsional deformation. This study is conducted by the following procedures: 1. The equations oy lateral buckling loads oy H-shaped beams with continuous braces under gradient flexural moment and compressive axial force are derived by energy method. To simplify the equations by energy method, the new equations oy lateral buckling loads are suggested with reference to the terms oy the flexural and torsional rigidities oy beams, the rigidities oy braces, and condition oy the loads. 2. The elasto-plastic buckling behaviors oy the beams are simulated by elasto-plastic large deformation analysis. The lower bound oy the elasto-plastic buckling stress oy the beams can be evaluated with the interpolated buckling curve between those oy bending member and compression member, which are provided by Recommendation for Limit State Design oy Steel Structure (AIJ), depending on the axial force ratio. 3. The upper bound oy moment and lateral force which continuous braces carry at the lateral buckling oy beams is evaluated based on the ratio oy the flexural and torsional rigidities oy beams, the rigidities oy braces, the axial force ratio, and the gradient oy flexural moment.