Design of frames with Buckling Restrained Braces - FEMA P695 based Evaluation of a Eurocode 8 Conforming Design Procedure

Zsarnóczay, Ádám and Balogh, Tamás and Vigh, László Gergely (2014) Design of frames with Buckling Restrained Braces - FEMA P695 based Evaluation of a Eurocode 8 Conforming Design Procedure. In: EUROSTEEL 2014, 10-12. September 2014, Naples, Italy.

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Abstract

Buckling Restrained Braced Frames (BRBF) discussed in this paper are concentrically braced steel frames using continuous columns with rigid supports and BRBs as diagonal members [1]. The BRB is an innovative brace characterised by significant energy dissipation capability under cyclic loading. Because of their beneficial energy dissipation capability, BRBF are often more economical alternatives to conventional steel frame solutions. However, their application in Europe is hindered by the absence of a standardised design procedure. This paper is based on a research effort that aims to propose such a procedure that is compatible with the concepts and methodology of the Eurocode 8 standard (EC8) [2]. Evaluation of a design procedure is challenging, because of the vast number of possible design scenarios required to consider the variability in structural geometry and the seismic hazard. Until recent years the lack of computational resources impeded direct consideration of demand and capacity variability and led to simplified studies using only a small set of scenarios and crude models. The authors believe that besides being economical, the primary target of a good design procedure shall be assurance of sufficiently low failure probability with high confidence. Therefore, merits of a design procedure can only be judged by robust and reliable evaluation of collapse probability. This requires a framework for probabilistic assessment of the performance of a large number of typical structural solutions under various seismic hazard scenarios. The authors developed an extended version of the framework proposed in FEMA P695 [3] to assess the seismic performance of an EC8 conform BRBF design procedure. The paper briefly presents the design procedure, the extended framework and the results on a set of 24 BRBF archetypes. The archetypes are arranged into 8 Performance Groups; each group collects buildings with similar characteristics. Performance of each structure is evaluated using nonlinear dynamic analyses with a set of characteristic ground motion records. Detailed results for each archetype are available in [4]. All performance groups fulfilled the requirements of FEMA P695, namely that the conditional probability of failure of their structures at the design seismic intensity is less than 10%. The majority of individual collapse probabilities of the archetypes are below 3%. In spite of the good performance as per FEMA P695 the probabilities of collapse over the lifetime of structures and the corresponding reliability indices do not fulfil the limits in Eurocode 0 (EC0) for Ultimate Limit State design (P C < 0.01%). Such a stringent regulation can only be fulfilled by structures that resist extremely rare ground motions. The authors believe that it is not economical to design a structure to resist such rare effects. This observation has been made by other researchers as well [5] and it draws attention to the need for further research on this topic and an assessment of relaxed EC0 limits for seismic performance evaluation. The experienced advantageous behaviour of BRBF does not only stem from the high ductility and energy dissipation capacity of the braces, but also from application of an appropriate design procedure. The conservative approach applied in both numerical modelling and uncertainty estimation provides high confidence in the collapse assessment results. Based on the performance of BRBF archetypes, the proposed design procedure is considered appropriate for BRBF design for frames that are within the scope of the presented research. Therefore, applicability currently is limited to concentrically braced frames with chevron-type brace topology, continuous columns with rigid supports and a maximum of 6 stories. Investigation of additional archetypes in the future will lead to a better understanding of BRBF behaviour and allow relaxation of the above limits. ACKNOWLEDGMENT The work reported in the paper has been developed in the framework of the “Talent care and cultivation in the scientific workshops of BME” project. This project is supported by the grant TÁMOP-4.2.2.B-10/1-2010-0009. This paper was also supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. REFERENCES [1] López W.A., Sabelli R. 2004. “Seismic Design of Buckling-Restrained Braced Frames”. Steel Tips, Vol. 78 [2] EN 1998-1:2008 2008. Eurocode 8: Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings, CEN [3] FEMA P695. 2009. Quantification of building seismic performance factors, Federal Emergency Management Agency (FEMA), Washington, D.C. [4] Zsarnóczay Á., 2013. Experimental and Numerical Investigation of Buckling Restrained Braced Frames for Eurocode Conform Design Procedure Development, PhD Dissertation, Department of Structural Engineering, Budapest University of Technology and Economics. [5] Joint Committee on Structural Safety (JCSS) 2001 Probabilistic Model Code Part 1 – Basis of Design

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