تحلیلی بر رفتار لرزه ای قابهای مهاربندی شده معکوس Analytical Investigation on Seismic Behavior of Inverted V-braced Frames

1. Introduction Until the Northridge earthquake in 1994 and the Kobe earthquake in 1995, building construction experts used to think that “steel structures are safe from earthquake because of the ductility capacity of steel materials themselves.” However, the two earthquakes caused huge damage to buildings with steel structures, and it was found to be mostly attributable to connection failure caused by brittle fracture of welded structures. That is, unless a balance between connection designs and the strength of frame members is secured to ensure the inherent ductility capacity of structural materials is well exhibited, structures can never be safe. A braced frame is an earthquake-resistant design system of steel-frame structures, and thanks to its high rigidity and strength, as well as economic feasibility, it is widely used in strong motion generation areas and frequent earthquake areas. Braces of concentrically braced frames (CBFs) that are used in capacity design methods are subject to deformations caused by repeated tension and compression after buckling, and through this behavior, seismic energy is dissipated. To achieve this behavior, restraint-free plastic rotations should be allowed for a gusset plate to have flexible brace end conditions, which requires sufficient free length (clearance distance) between the end of the brace member and the restrained line of the gusset plate, but short enough to preclude the occurrence of buckling in the gusset plate before brace buckling. CBFs are divided, depending on the shapes of braces, into brace frame, X-braced frame, and V-braced and inverted V-braced frames. Inverted V-braced frames are widely used owing to many advantages in constructional design. In addition to the beam-column connection, another gusset plate should be placed at the middle of a beam due to its geometric characteristics. Under earthquake, compression and tension brace forces coexist in inverted V-braced frames. Compression brace force starts to decrease after buckling, while tension brace force continues to increase until it reaches full tension yield, which results in unbalanced forces between the two braces. Since this creates the vertical unbalanced force in the beam, and in turn imposes additional loads, this should be considered in designing members. Thus, in this study, the failure mode, yield mechanism and overall behavior of inverted V-braced frames were evaluated using finite element analysis methods. As mentioned above, axial force and post-buckling strength imposed on each of tension and compression braces were analyzed, and the vertical unbalanced force acquired from the equation of the current standard (AISC, 2010a) was compared with the vertical unbalanced force that is actually created in the analysis model. It was analyzed how the size of cross sections affects earthquake-resistant performances of the frame by varying the cross section of beams to which additional bending and shear force are applied by vertical unbalanced force. Earthquake-resistant performances were also evaluated by comparing the difference of earthquake-resistant behaviors of inverted V-braced frames depending on the methods of calculating the clearance distance of gusset plate connections.