BMe Research Grant Vásárhelyi Pál Doctoral School Faculty of Civil Engineering, Department of Structural Engineering Supervisor: László Gergely Vigh PhD. Buckling Restrained Braced Frame design procedure evaluation through experimental and numerical analyses Introducing the research area My research focuses on the behaviour of Buckling Restrained Braces(BRB). The primary objective is to verify that the design procedure proposed by our research group is appropriate for European application and its use results in economical earthquake resistant Buckling Restrained Braced Frames. A special framework for evaluation of the design of anti-seismic systems is adopted to achieve this goal. The framework is configured with the help of experimental and numerical analysis results.
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8/16/2019 Buckling Restrained Braced Frame Design Procedure Evaluation Through Experimental and Numerical Analyses (Zs…
My research focuses on the behaviour of Buckling Restrained Braces (BRB). Theprimary objective is to verify that the design procedure proposed by our researchgroup is appropriate for European application and its use results in economicalearthquake resistant Buckling Restrained Braced Frames. A special framework
for evaluation of the design of anti-seismic systems is adopted to achieve thisgoal. The framework is configured with the help of experimental and numericalanalysis results.
Figure 1. Buckling Restrained Braced Frames (source: Star Seismic)
Brief introduction of the research place
I carry out my research at the Department of Structural Engineering. Our department has been actively participating in state-of-the-art research in the fieldof Structural Engineering and Bridge Design. We are also members of severalhigh priority projects as consultant, co-designer or independent inspector (e.g.Pentele bridge, Hárosi bridge). Experiments performed at our accreditedStructural Laboratory provide valuable information on structural behaviour.Earthquakes and earthquake resistant design became one of the main research
interests in the department during the past decade. Currently a research group isdevoted to this subject. László Gergely Vigh, the leader of the group is also amember of the TC13 committee of ECCS, thus actively participates in theimprovement of the European seismic design standard (Eurocode 8 [1]).
History and context of the research
Earthquake engineers have recognised the inefficiency of conventional, elasticstructural design at regions of high seismicity by the second half of the 20thcentury. When subjected to loading, linearly elastic structural members suffer deformations that are proportional to the load intensity. These deformations areonly temporary; the members regain their original shape after unloading.Dissipative structural members are typically characterized by plastic behaviour
beyond a certain load level. Plastic deformations are irreversible, they remain asresidual deformations even after unloading. An important consequence of plasticbehaviour is the significant amount of dissipated energy through plasticdeformations.
Figure 2. Comparison of the behaviour of linearly elastic and dissipative structural members
Provided that there are dissipative members designed at key points in a structure,it will be able to dissipate the majority of seismic energy and significantly reducethe internal forces resulting from earthquakes. Design of such structures is acomplex procedure (capacity design) that was established by Tamás Paulay.Structural steel is frequently used in dissipative structural members because of itsadvantageous inelastic behaviour. However, capacity of conventional steelmembers is significantly limited under compression, because they typicallybuckle before reaching the load level that corresponds to plastic behaviour.Consequently, these members are taken into account in design as tension-only
braces and not used to their full capacity. Buckling Restrained Braces weredeveloped in the 1980s in Japan [2] to improve steel brace performance bypreventing the occurrence of buckling. Figure 4 displays the components of theelement. A central steel core is continuously supported by a concrete-filled steelhollow section. The continuous lateral support prevents buckling of the core. Thecore and the casing are decoupled, so that axial loads are resisted by the steelcore only. BRB elements are capable of producing the theoretical steel materialbehaviour at an element level, thus their energy dissipation capability is superior to conventional steel braces.
Figure 3. Comparison of cyclic behaviour of conventional steel braces and theBRB
Figure 4. Primary components of a buckling restrained brace
The research goal, open questions
BRBs have been actively used and researched in both Japan and the UnitedStates by the year 2000 [3,4]. Their investigation in Europe began at the end of the last decade, while practical use is hindered by the lack of a standardizedEuropean design procedure. Our primary objective is the evaluation of a designprocedure based on the principles of capacity design that can be included in thenext revision of the European seismic design standard. The proposed procedureshall be simple enough to be applicable as a part of everyday engineeringpractice.The performed research shall analyse the introduced simplifications to BRB
design and verify the appropriate behaviour of structures designed with theproposed procedure. This requires detailed and accurate understanding of BRBbehaviour, especially its energy dissipation capability under cyclic loading and itsexpected failure modes. Although there are experimental results in the literature(e.g. [5,6]), they do not fully answer to some of our questions. BRB failurebecause of low cycle fatigue and the resulting dependence of BRB performanceon the load history for instance is generally accepted, but there are only a limitednumber of results in literature concerning this phenomenon. Collecting moreexperimental data on this topic promises more accurate estimation of the capacityof BRB-based structural solutions.Besides facilitating the application of BRB frames in Europe, our results also leadto a more detailed understanding of BRB behaviour, thus they are also applicableto research outside of Europe. Our research group cooperates with one of theworld’s leading BRB manufacturers, Star Seismic, and we established jointresearch projects with several European universities.
Methodology
Investigation of BRB behaviour BRB behaviour under cyclic loading has been investigated by uni-axial cyclic loadtests on a total of 10 specimens. A custom loading frame has been built at theStructural Laboratory for these experiments.
Figure 5. Custom loading frame designed for the experiments
Three load protocols (load history functions) have been developed that complywith the requirements of EN15129 [7], the European standard for anti-seismic
devices. Each protocol focuses on a specific BRB property (e.g. energydissipation capability, load history influence). Loading was displacement controlledin all cases; the prescribed displacement levels were reached by applying load onthe BRB specimen through hydraulic jacks in the loading frame.
8/16/2019 Buckling Restrained Braced Frame Design Procedure Evaluation Through Experimental and Numerical Analyses (Zs…
Figure 6. Load protocol for the analysis of load history influence - the initial cycleswith large amplitude are expected to affect the behaviour in later cycles
The experienced behaviour is best described by force-displacement diagrams.The diagram in Figure 7 shows the initial elastic behaviour, followed by theyielding of the steel core. The effect of the two types of hardening (kinematic andisotropic) is also visible on the curves. Note that BRB behaviour is asymmetric;the elements have increased capacity under compression.
Figure 7. Typical experimental force-displacement diagram Numerical BRB model Laboratory experiments require significant financial resources and time, thereforevirtual experiments are often used in current civil engineering research projects.These are experiments performed on computer models (typically in a finiteelement modelling environment) calibrated by the available results fromlaboratory tests. Our research group has such a model for BRB that providesdeeper understanding to element behaviour [Z1, 8].Virtual experiments require a detailed model made of three dimensional finite
elements that follow the geometry of the specimen and the changes in materialproperties with high accuracy. This complex model is an effective tool for simulated experiments, but it is inefficient for the global analysis of framestructures, because it uses a the large amount of computational resources. Globalanalysis requires a simplified, faster model that still approximates BRB behaviour with sufficient accuracy. I developed this simple model by using a simple prismaticbeam element with a custom made material model [Z2]. Since element geometry
cannot be followed by such a simple finite element, all BRB characteristics had tobe modelled by the custom material.
Figure 8. Configuration of the simplified numerical BRB model Design procedure evaluation The proposed design procedure is verified with a framework based onrecommendations in the FEMA P695 document [9]. The suggested methodologyevaluates design procedure through the investigation of a large number (30-50)of typical structures, so-called archetypes. These are buildings that are expectedto use the proposed system in the region under consideration (Europe in our case). Performance of each archetype is evaluated by detailed nonlinear dynamicanalysis [10].The seismic effect and the structural response (e.g. displacements, internalforces) are considered probabilistic variables that are approximated by a finitenumber of samples during the nonlinear analyses. Seismic excitation is describedby 44 acceleration records from recent earthquakes that include some of thedevastating ground motions in the past four decades (e.g. Kobe, Northridge, Chi-Chi etc.) Structural response is evaluated separately for the 44 records; thisprovides 44 samples for the structural response variable. The set of records is
scaled to increasing levels of seismic intensity and the response of the structure isevaluated at each level. This provides a detailed understanding of the seismicperformance of the structure that is summarized in fragility curves. Fragility curvesdescribe the collapse probability of a structure at different seismic intensity levels.Design procedure evaluation is based on the fragility curves corresponding toeach archetype.
Figure 9 Result of nonlinear dynamic analyses and the corresponding fragility curves
Results
Laboratory experiments Experimental tests with custom load protocols have verified the advantageousenergy dissipation capabilities of BRB elements [Z3, Z4]. Sensitivity of BRBbehaviour to the geometric proportions of the steel core has been highlightedthrough the results of two specimens. Disadvantageous core geometry leads tostrong axis buckling near the transition zone of the core, and this eventuallyresults in a failure mode that significantly reduces the energy dissipation capabilityof the element [Z5].
Figure 10. Significant deformation from strong axis buckling prior to failure near the transition zone of the steel core (left); no deformation experienced when
proper core geometry is used(right) Based on experimental results, I defined parameters required for practical BRBdesign and quality control in accordance with European standards. Theseparameters describe a bilinear stress-strain relationship and its acceptablevariance [Z6].
8/16/2019 Buckling Restrained Braced Frame Design Procedure Evaluation Through Experimental and Numerical Analyses (Zs…
Figure 11. Simplified bilinear stress-strain relationship for practical design of BRBelements
Numerical model I developed a custom BRB material model [Z7] in the OpenSEES open sourcefinite element code [11]. When combined with a prismatic beam element, the newmaterial model provides an effective and accurate representation of BRBbehaviour that does not require large amounts of computational resources. Themodel is based on the Menegotto-Pinto steel material [12]. The kinematichardening of the original material is extended with isotropic hardening. Theoriginal stress function is modified to include three asymptotes instead of two.With this modification, a stress limit can be introduced in the model that effectivelydescribes the ultimate stress of the material. Load history dependence is alsoincluded in the new model, thus material behaviour is influenced by the previouslytaken path in the stress-strain plain. These characteristics are controlled
independently under tension and compression, therefore the material model canhandle the asymmetric BRB behaviour. I am not aware of any other steel materialmodel with such small computational resource requirement and such largeflexibility in application in the currently available finite element codes.
Figure 12 Characteristic stress-strain curves of the developed material model and the stress function that describes them
Design procedure evaluation framework I developed a custom software for design procedure evaluation based on theOpenSEES environment. The software automatically produces the fragility curveof any archetype according to the complex methodology described in FEMA P695[Z8, Z9]. I proposed modifications to the original methodology to reduce theuncertainty in the results. I noted that the original procedure is based on detailedinvestigation of reinforced concrete and wooden structures performed inCalifornia, thus its applicability to other structural systems (e.g. steel frames) andother regions shall be verified [Z10].Our joint research with the University of Porto focuses on the extension of theoriginal procedure so that it becomes applicable to any structural solution in any
seismic region. We suggest using groups of record sets instead of the single setof 44 records. The record sets shall correspond to the seismic intensity and thevibrational properties of the structure under investigation [Z11]. Figure 13 showsthe significant difference between the spectra that correspond to increasing levelsof seismic intensity and structures with various natural periods. Record selectionis facilitated by a web-based application that automatically provides theappropriate set for the given design scenario from a database of more than10,000 earthquake records [13]. I developed a procedure that combines theresults from several record sets and produces a single fragility curve, thus theextended methodology can be integrated in the original framework.
Figure 13. Comparison of so-called response spectra from earthquakes withincreasing intensities at the same location (left); governing response spectra for
structures with different vibrational properties (right).
Expected impact and further research
The design parameters defined from experimental results and the evaluateddesign procedure is expected to be included in a future revision of the Eurocode 8standard through the work of the ECCS TC13 committee. Therefore, theinvestigated procedure is expected to define the design of BRB frames in EuropeThe accuracy and efficiency of the developed numerical BRB model is expected
to facilitate its integration in the official OpenSEES release. This would lead to itsworldwide use in BRB related state-of-the-art research.The extension of the original framework for design procedure evaluation is animportant step towards a general solution for this problem. The proposedframework is expected to be capable of evaluating arbitrary anti-seismic solutionsand lead to more economical design and a better understanding of structuralbehaviour under seismic excitation.Besides the University of Porto, we are establishing joint research projects withseveral other European universities (Instituto Superior Técnico of Lisbon,University of Naples “Federico II”, Politechnica University of Timisoara, University
of Oxford) to improve existing BRB designs and extend the application of BRBs tonew areas.
Publications, references, links
Publications[Z1] Budaházy V., Zsarnóczay Á., Vigh L.G., Dunai L., Numerical modeldevelopment for cyclic hardening investigation of steel-yield based displacementdependent devices. Proc. 15th World Conference on Earthquake Engineering (15WCEE), Lisbon, Portugal, pp. 1-10. paper 5222. (2012) [Z2] Zsarnóczay Á., Vigh L.G., Kihajlásbiztos merevítőrúd ciklikus viselkedésénekelemszintű modellezése - in Hungarian. XI. Magyar Mechanikai Konferencia,Miskolc, Hungary, 9 p. (2011) [Z3] Zsarnóczay Á., Vigh L.G., Experimental analysis of buckling restrained bracebehaviour under cyclic loading. 28th Danubia – Adria – Symposium on Advancesin Experimental Mechanics, Siófok, Hungary, pp. 297-298. (2011) [Z4] Zsarnóczay Á., Vigh L.G., Kihajlásbiztos merevítőrudak kísérleti vizsgálata -in Hungarian. Magyar Építőipar LXII:(6) pp. 222-230. (2012) [Z5] Zsarnóczay Á., Vigh L.G., Experimental analysis of buckling restrainedbraces: Performance evaluation under cyclic loading. Proceedings of EUROSTEEL 2011 – 6th European Conference on Steel and CompositeStructures. Budapest, Hungary, pp. 945-950. (2011) [Z6] Zsarnóczay Á., Budaházy V., Vigh L.G., Dunai L., Cyclic hardening criteria inEN 15129 for steel dissipative braces. Journal of Constructional Steel Research83, pp. 1-9. (2013) [Z7] Zsarnóczay Á., Budaházy V., Uniaxial Material Model Development for Nonlinear Response History Analysis of Steel Frames. Proc Second Conferenceof Junior Researchers in Civil Engineering, Budapest, Hungary, pp. 307-317(2013) [Z8] Zsarnóczay Á, Seismic Performance evaluation of buckling restrained bracesand frame structures. Proc 9th fib International PhD Symposium in CivilEngineering, Karlsruhe, Germany, pp. 195-200 (2012)
[Z9] Zsarnóczay Á., Influence of Plastic Mechanism Development on the SeismicPerformance of Buckling Restrained Braced Frames – case study. Proc.Conference of Junior Researchers in Civil Engineering, Budapest, Hungary, pp.289-297 (2012) [Z10] Zsarnóczay Á., Vigh L.G., Capacity design procedure evaluation for buckling restrained braced frames with incremental dynamic analysis. Proc. 15thWorld Conference on Earthquake Engineering (15 WCEE), Lisbon, Portugal, pp.1-10. paper 3533. (2012) [Z11] Zsarnóczay Á., Macedo L., Castro J.M., Vigh L.G., A novel ground motionrecord selection strategy for Incremental Dynamic Analysis. Proc. Vienna
Congress on Recent Advances in Earthquake Engineering and StructuralDynamics, (2013) (submitted) Links:Buckling restrained bracesCapacity designOpenSEES
FEMA P695Star Seismic Europe References:[1] EN 1998-1:2008, Eurocode 8: design of structures for earthquake resistance –part 1: general rules, seismic actions and rules for buildings. CEN (2008) [2] Watanabe A., Hitomi Y., Saeki E., Wada A., Fujimoto M., Properties of BraceEncased in Buckling-Restraining Concrete and Steel Tube. Proc. Ninth WorldConference on Earthquake Engineering IV. pp. 719-724 (1988)
[3] López WA, Sabelli R, Seismic design of buckling restrained braced frames.Steel Tips (2004) [4] ANSI/AISC 341-10. Seismic provisions for structural steel buildings. AISC(2010) [5] Romero P., Reaveley L.D., Miller P.J., Okahashi T.O., Full scale testing of WCseries buckling-restrained braces – test report. Salt Lake City: Department of Civil& Environmental Engineering, The University of Utah (2007) [6] Merritt S., Uang Ch.M., Benzoni G., Subassemblage testing of star seismicbuckling restrained braces – test report. San Diego: Department of Structural
Engineering, University of California (2003) [7] EN 15129, Anti-seismic devices. CEN (2010) [8] Budaházy V., Modelling of the hysteretic behaviour of buckling restrainedbraces. Proc Conference of Junior Researchers in Civil Engineering, Budapest,Hungary, pp. 34-41 (2012) [9] FEMA P695, Quantification of building seismic performance factors. FederalEmergency Management Agency, Washington, D.C. (2009) [10] Vamvatsikos, D. and Cornell, C.A., Incremental dynamic analysis.Earthquake Engineering and Structural Dynamics. 31: 491-514. (2002)
[11] McKenna F., Feneves G.L., Open system for earthquake engineeringsimulation. Pacific Earthquake Engineering Research Center, (2012) [12] Menegotto, M., Pinto, P., Method of Analysis for Cyclically Loaded ReinforcedConcrete Plane Frames Including Changes in Geometry and Nonelastic Behavior of Elements under Combined Normal Force and Bending. IABSE Symposium onResistance and Ultimate Deformability of Structures Acted on by Well-DefinedRepeated Loads, Final Report, Lisbon (1973) [13] PEER NGA Database:http://peer.berkeley.edu/peer_ground_motion_database
