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

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 Desig

Steel4 – A Versatile Uniaxial Material Model for Cyclic Nonlinear Analysis of Steel-Based Elements

Steel4, a new material model for nonlinear dynamic analysis of steel-based anti-seismic solutions has been developed and implemented in the OpenSees environment. The paper highlights the main features of the highly customizable Steel4. It handles nonlinear kinematic and isotropic hardening and their combination; ultimate strength limit can be prescribed to provide more accurate material response at large deformations; it models asymmetric kinematic and isotropic hardening while preserving the Bauschinger effect in the material response; its memory of previous load history allows initial load cycles to influence future behaviour. The material has already been applied to modelling buckling restrained braces and conventional steel braces and showed promising results

Eurocode conforming design of Buckling Restrained Braced Frames – Part II: Design procedure evaluation

Buckling Restrained Braces (BRB) are innovative displacement dependent devices having balanced hysteresis behaviour. Frames equipped with such devices are known as Buckling Restrained Braced Frames (BRBF). The fact that there is no European standardized design procedure available for the BRBF system, seriously limits the application of BRB elements in Europe. The paper proposes a Eurocode conforming design procedure for BRBF, providing the seismic design parameters and capacity design rules, by updating the specifications of Eurocode 8 on steel Concentrically Braced Frames. The suggested modifications are explained and illustrated via brief examples. The design method is illustratively introduced by a numerical example, highlighting the special aspects of the design process. The applicability of the design method is verified with the help of the FEMA P695 methodology. The methodology has been implemented in a software environment that is capable of automatic evaluation of the performance of structural archetypes with the help of the OpenSees finite element code. The seismic performance assessment required extensive experimental studies on BRB behaviour and development of a novel material model capable of complex nonlinear hardening behaviour under irregular cyclic loading. Applicability of the developed design procedure was confirmed through the design and collapse probability evaluation of 24 BRBF archetypes

Effective design measures against soft storey development in Buckling Restrained Braced Frames

Buckling Restrained Braced Frames (BRBF) are concentrically braced steel frames with Buckling Restrained Braces (BRB) in their diagonals. BRBs are special displacement dependent anti-seismic devices. Buckling of the internal steel core of a BRB is prevented by the continuous lateral support of its casing. This configuration ensures balanced hysteretic behavior and large energy dissipation capacity. The advantageous dissipative properties of BRBs can only be utilized with an appropriate design procedure. The objective of the presented paper is to draw attention to the high probability of soft story formation in concentrically braced BRBF under seismic excitation beyond the design level of the frame. Although BRBF had been shown to have sufficiently high performance, a significant reduction in collapse probability could be achieved if premature failure due to soft stories was prevented by appropriate design measures. The issue of soft story formation is tackled using three BRBF archetypes. Structural design is performed with several versions of the BRBF design procedure proposed by the authors for European application. Two dimensional finite element models are created for each scenario (i.e. archetype + design) with the OpenSees finite element code. A custom BRB element developed by the authors and calibrated to experimental results is used to improve the description of nonlinear cyclic hardening behavior of BRBs. An extended version of the probabilistic seismic performance assessment framework presented in FEMA P695 is used for this study. Performance for each scenario is evaluated with nonlinear dynamic analyses in a multi-stripe framework and described by fragility curves after taking several sources of uncertainty into consideration. Results of a sensitivity analyses on hundreds of versions of a two-story BRBF suggest that soft story development can be effectively mitigated by either strengthening the columns of the braced frame or shifting the distribution of lateral stiffness along the height of the BRBF from a modal towards a triangular shape. Detailed probabilistic seismic performance assessment on a few selected structures confirms the advantages of the modifications. These results also highlight that increased interstory drifts shall be expected below the design seismic intensity level due to shifting the BRBF stiffness distribution

Reliability assessment of concentrically braced frames. Risk-based seismic performance assessment of Eurocode conform design

The concept and methodology of structural reliability analysis is controlled by the Eurocode 0 (EC0, [1]) standard in Europe for several common scenarios: basic rules and requirements including reliability differentiation of structures and the target probability of failure (or reliability index) are prescribed by the code. Eurocode 8-1 (EC8-1, [2]) requires design checks of buildings at two performance objective levels: a) no-collapse (ultimate limit state); b) damage limitation (serviceability limit state); for which states, however, there is no custom target value of the reliability index defined. Furthermore, no comprehensive study is available on the reliability of the simplified capacity design procedures in EC8-1. This paper examines a common structural solution – Concentrically Braced Frames (CBF) – and assesses the collapse probability of a set of typical representations of such structures to provide information on the reliability of Eurocode conform structural design. The seismic performance assessment methodology is based on the FEMA P695 [3] framework using incremental dynamic analysis (IDA), and extended with the calculation of failure probability. A total of 6 archetype CBF structures with X-bracing were designed varying the storey number (2-4-6 stories) and the dissipation capacity (quasi-elastic or dissipative structure with q = 1.5 or 4.0, respectively). A peak ground acceleration agR = 0.3 g is considered; with ground type D and Type I response spectrum. The considered dead load is 6.5 and 4.5 kN/m2, while the live load is 3.0 and 1.0 kN/m2 on the intermediate and roof floors, respectively. Combination factor of the live load is 0.3. The floor area is 30x24 m. Design of the buildings is completed by using modal response spectrum analysis and capacity design rules in accordance with EC8-1. For the non-linear dynamic analysis, numerical models for the structures are developed in the OpenSees [4]. Simplified planar models with non-linear material models and equivalent geometric imperfections are applied. The time-history analysis results in IDA curves. Fragility curve reflecting the conditional probability of failure with given seismic intensity is determined from the IDA results, and modified to handle additional sources of uncertainties. Site specific hazard curve describing the relationship between seismic intensity and the probability of occurrence of ground motions with such intensity is generated using EFEHR [5]. The collapse probability density function is obtained as the product of the hazard probability density function and the fragility curve. The total probability of collapse is calculated by numerically integrating the collapse PDF over the entire spectral acceleration domain. The obtained reliability index values fall within the range of 2.25 ~ 3.06 and 3.14 ~ 3.79 in case of the dissipative and the quasi-elastic structures, respectively. These values are lower than the target values prescribed by EC0 for ultimate limit state (RC2, BETAtarget = 3.8, meaning 0.01% probability of failure over lifetime, [1]). This observation may be interpreted as the design provisions of EC8-1 conflict EC0 rules and the minimum safety level is not assured. Note that, however, it is only possible to achieve the 0.01% probability of failure if the designed structure does not collapse from a ground motion with 0.01% probability of occurrence over the lifetime of the structure. Such a ground motion is extremely rare and the authors believe that it is not economical to design a structure to resist such rare effects. Accordingly, one may conclude to that seismic specific target values of reliability index shall be introduced. By accepting the compliance of EC8-1, the target value should be lower than 2.25, i.e. higher risk can be associated to seismic events. Such distinction is also made by JCSS [6]. For further details on the research refer to [7]. ACKNOWLEDGMENT This paper was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. REFERENCES [1] EN 1990:2005 Eurocode: Basis of structural design. Brussels: CEN. [2] EN 1998-1:2008 Eurocode 8: Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings. Brussels: CEN. [3] FEMA P695. 2009. Quantification of building seismic performance factors, Federal Emergency Management Agency (FEMA), Washington, D.C. [4] McKenna F., Feneves G.L. 2012. Open system for earthquake engineering simulation, Pacific Earthquake Engineering Research Center [5] European Facility for Earthquake Hazard and Risk (EFEHR), Available at: www.efehr.org [6] Joint Committee on Structural Safety (JCSS): Probabilistic Model Code, 2000. [7] Gulyás Gy. 2013. Reliability analysis of steel building structures in high seismicity zones – Analysis of quasi-elastic and dissipative concentrically braced frames. MSc thesis. BME Dept. Struct. Eng

Eurocode conforming design of Buckling Restrained Braced Frames with Chevron bracing – Part II: Evaluation of the design method

Buckling Restrained Braces (BRB) are innovative displacement dependent devices having balanced hysteresis behaviour. Frames equipped with such devices are known as Buckling Restrained Braced Frames (BRBF). The fact that there is no European standardized design procedure available for the BRBF system, seriously limits the application of BRB elements in Europe. The paper proposes a Eurocode conforming design procedure for BRBF, providing the seismic design parameters and capacity design rules, by updating the specifications of Eurocode 8 on steel Concentrically Braced Frames. The suggested modifications are explained and illustrated via brief examples. The design method is illustratively introduced by a numerical example, highlighting the special aspects of the design process. The applicability of the design method is verified with the help of the FEMA P695 methodology. The methodology has been implemented in a software environment that is capable of automatic evaluation of the performance of structural archetypes with the help of the OpenSees finite element code. The seismic performance assessment required extensive experimental studies on BRB behaviour and development of a novel material model capable of complex nonlinear hardening behaviour under irregular cyclic loading. Applicability of the developed design procedure was confirmed through the design and collapse probability evaluation of 24 BRBF archetypes

Kihajlásbiztos merevítőrudak tervezési eljárásának kidolgozása és ellenőrzése

A cikkben bemutatott kutatás keretében kihajlásbiztos merevítőrudak (Buckling Restrained Brace, röviden BRB) viselkedését vizsgáljuk. Kísérleti és numerikus eredmények alapján célunk azt igazolni, hogy a kutatócsoportunk által javasolt tervezési eljárással az európai szeizmikus viszonyokhoz igazodó, az elvárt teherbírási követelményeknek megfelelő és gazdaságos BRB-vel merevített keretek tervezhetőek. Ehhez egy olyan keretrendszert dolgoztunk ki, mely alkalmas különböző földrengésvédelmi megoldások tervezési eljárásainak értékelésére. A cikkben a kutatás kísérleti és numerikus hátterét, a kidolgozott keretrendszert és a főbb eredményeket mutatjuk be

Eurocode conforming design of Buckling Restrained Braced Frames with Chevron bracing – Part I: Proposal for codification

Buckling Restrained Braces (BRB) are innovative displacement dependent devices having balanced hysteresis behaviour. Frames equipped with such devices are known as Buckling Restrained Braced Frames (BRBF). The fact that there is no European standardized design procedure available for the BRBF system, seriously limits the application of BRB elements in Europe. The paper proposes a Eurocode conforming design procedure for BRBF, providing the seismic design parameters and capacity design rules, by updating the specifications of Eurocode 8 on steel Concentrically Braced Frames. The suggested modifications are explained and illustrated via brief examples. The design method is illustratively introduced by numerical example of BRBF system, highlighting the special aspects of the design process. The design method is justified by the help of the FEMA P695 methodology. The methodology has been implemented in a software environment that is capable of automatic evaluation of the performance of structural archetypes with the help of the OpenSees finite element code. The seismic performance assessment required extensive experimental studies on BRB behaviour and development of novel material model capable of complex nonlinear hardening behaviour under irregular cyclic loading. Applicability of the developed design procedure was confirmed through the design and collapse probability evaluation of 24 BRBF archetypes

On the European norms of design of Buckling Restrained Braced Frames

The application of buckling restrained braced frames is hindered in Europe by the absence of a standardized design procedure in Eurocode 8, the European seismic design standard. The presented research aims to develop a robust design procedure for buckling restrained braced frames. A design procedure is proposed by the authors. Its performance has been evaluated for buckling restrained braced frames with chevron type brace configurations using a state-of-the-art methodology based on the recommendations in the FEMA P695 document. A special numerical material model was developed within the scope of this research to represent the behavior of buckling restrained braces more appropriately in a numerical environment. A total of 24 archetype designs were prepared and their nonlinear dynamic response was calculated using real ground motion records in incremental dynamic analyses. Evaluation of archetype collapse probabilities confirm that the proposed design procedure can utilize the advantageous behavior of buckling restrained braces. Resulting reliability indices suggest a need for additional regulations in the Eurocodes that introduce reasonable structural reliability index limits for seismic design