Civil Engineering Association

Performance-Based Seismic Demand Assessment of Concentrically Braced Steel Frame Buildings

The special concentrically steel braced frame (SCBF) system is one of the most effective structural systems to resist lateral forces. Because of its effectiveness and straightforward design,
many SCBFs are incorporated in structures throughout the world. However, the highly nonlinear behavior associated with buckling and non-ductile fracture of braces reduces the ability of the system to dissipate energy resulting in undesirable modes of behavior. While many studies have investigated the cyclic behavior of individual braces or the behavior of subassemblies, the dynamic demands on the structural system under various seismic hazard levels needs additional
study for performance-based earthquake engineering.
Archetype buildings of SCBFs and buckling restrained braced frames (BRBFs) were analyzed using the computer program OpenSees (the Open System for Earthquake Engineering Simulation) to improve the understanding of the seismic behavior of braced frame systems, and to assess
seismic demands for performance-based design. Numerical models were calibrated using test data determined from testing of conventional buckling braces, buckling restrained braces, and the braced frame specimens. In addition, fiber-based OpenSees models were constructed and compared with results of a sophisticated finite-element model that realistically captured local
buckling and local fracture of structural elements. Because the OpenSees models are reasonably accurate and efficient, they were chosen to perform set of parametric computer simulations.
The seismic demands of the system and structural elements were computed and interpreted for 3-,
6-, and 16-story SCBFs and BRBFs under various hazard levels. The analysis results show large seismic demands for the 3-story SCBF, which may result in unexpected damage of structural
and non-structural elements. The median expected probability of a brace buckling at one or more levels in a 3-story SCBF is more than 50% for an earthquake having a 50% probability
of exceedance in 50 years (the service-level event). The possible need to replace braces following such frequent events due to brace buckling should be considered in performance-based earthquake engineering assessments. In addition, brace fracture in SCBFs is likely for an
earthquake having a 2% probability of exceedance in 50 years (the MCE-level event). Analyses show that in general, BRBF models had larger drift demands and residual drifts compared to SCBF systems, because of the BRBF’s longer fundamental period. However, the tendency to
form a weak story in BRBFs is less than that in SCBFs.