Performance-Based Plastic Design of Steel Concentric Braced Frames for Enhanced Confidence Level

Abstract eng:
Concentrically braced steel frames (CBFs) are very efficient and commonly used steel structures for resisting seismic loads because they provide complete truss action. Based on research performed during the last twenty years or so, current U.S. seismic codes now include provisions to design ductile concentric braced frames called Special Concentrically Braced Frames (SCBFs). When designed by conventional elastic design methods, these structures can undergo excessive story drifts after buckling and yielding of bracing members. That can lead to early fractures of the bracing members, especially in those made of popular rectangular tube sections. Recent analytical studies have indicated that the confidence level to achieve collapse prevention performance objectives for typical SCBF can be extremely and unacceptably low when compared with special moment frames (SMFs). This paper presents results of a study in which a newly developed performance-based based plastic design (PBPD) methodology was applied to CBF with buckling type braces. Originally the method was developed and successfully applied to moment frames and more recently extended to other steel framing systems as well. The design concept uses pre-selected target drifts and yield mechanisms as performance limit states. The design lateral forces are derived by using an energy balance equation where the energy needed to push the structure up to the target drift is calculated as a fraction of elastic input energy which is obtained from the selected elastic design velocity spectra. Plastic design is then performed to detail the frame members and connections in order to achieve the intended yield mechanism and behavior. Results of extensive inelastic dynamic analyses carried out on example frames designed by the PBPD method showed that the frames met all the desired performance objectives, including the intended yield mechanisms and the story drifts. Reliability-based evaluation in accordance with FEMA 351 indicated that the PBPD frames have confidence levels against global collapse much higher than those of corresponding SCBFs designed by current practice.

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