Performance assessment of concentrically braced frames with modified braces depending on the applied beam-column joints

Abstract

Concentrically braced frames with crossed diagonals (X-CBFs) are a popular and traditional system in seismic resistant design. Their energy dissipation capacity is primarily provided by yielding of braces in tension and buckling in compression. An improved CBF configuration by introducing bracings with modified sections, resulting in low cycle fatigue endurance and enhanced ductility of the critical member, has been developed during an experimental and numerical research program. Apart from improved dissipative behaviour, proper structural seismic performance implies control over residual drifts and self-centering features of the seismic resistant systems. In that direction proper collaboration between the braces in CBF and contour frame has to be established and improvement of structural performance might be achieved by the engineer at a very affordable level. The beam-to-column joints and the column bases in the contour frame in CBF's could be designed as rigid, semi-rigid or pinned. The connection behaviour has a major influence on the residual drifts of the system. That effect was investigated in series of braced frames with different types of joints. Numerical simulation by nonlinear time history analysis and a detailed comparison is presented. Recommendations for the joints stiffness in CBF with modified braces and assessment of their seismic performance are presented. Different techniques for modelling of the braces as Multi-linear Plastic Link with Pivot hysteresis type and three-hinge model are compared.