Seismic Performance Evaluation of Concentrically Braced Frames
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Concentrically braced frames (CBFs) are broadly used as lateral-load resisting systems in buildings throughout the US. In high seismic regions, special concentrically braced frames (SCBFs) where ductility under seismic loading is necessary. Their large elastic stiffness and strength efficiently sustains the seismic demands during smaller, more frequent earthquakes. During large, infrequent earthquakes, SCBFs exhibit highly nonlinear behavior due to brace buckling and yielding and the inelastic behavior induced by secondary deformation of the framing system. These response modes reduce the system demands relative to an elastic system without supplemental damping. In design the re reduced demands are estimated using a response modification coefficient, commonly termed the R factor. The R factor values are important to the seismic performance of a building. Procedures put forth in FEMAP695 developed to R factors through a formalized procedure with the objective of consistent level of collapse potential for all building types. The primary objective of the research was to evaluate the seismic performance of SCBFs. To achieve this goal, an improved model including a proposed gusset plate connection model for SCBFs that permits accurate simulation of inelastic deformations of the brace, gusset plate connections, beams and columns and brace fracture was developed and validated using a large number of experiments. Response history analyses were conducted using the validated model. A series of different story-height SCBF buildings were designed and evaluated. The FEMAP695 method and an alternate procedure were applied to SCBFs and NCBFs. NCBFs are designed without ductile detailing. The evaluation using P695 method shows contrary results to the alternate evaluation procedure and the current knowledge in which short-story SCBF structures are more venerable than taller counterparts and NCBFs are more vulnerable than SCBFs.
- Civil engineering