Experimental and Numerical Investigation of Cyclic Behavior and Seismic Performance of Stainless Steel （STS304） Buckling-Restrained Braces = 스테인리스강（STS304）을 적용한 좌굴방지가새의 반복하중에 대한 이력 거동 및 내진성능에 관한 실험 및 해석적 연구

국문 초록 (Abstract)

좌굴방지가새（Buckling-Restrained Brace, BRB）는 지진하중을 받는 건축물 및 구조물에 적용되는 대표적인 횡력저항시스템 중 하나이다. BRB는 금속 코어와 좌굴을 억제하는 구속 메커니즘으로 구성되며, 코어는 압축 시 좌굴이 방지되는 동시에 인장 및 압축 영역에서 안정적이고 거의 대칭적인 이력거동을 나타내어 우수한 에너지 소산 성능을 발휘한다. 이러한 에너지 소산 성능의 크기는 코어의 횡방향 변형을 효과적으로 억제하는 구속부의 성능에 크게 좌우된다.
본 연구의 주요 목적은 신뢰성 있는 내진성능을 확보할 수 있는 지속가능형 좌굴방지가새를 개발하는 데 있다. 이를 위해 신속한 설치 및 해체가 가능한 조립식 구속부를 적용한 스테인리스강 조립형 좌굴방지가새（STS304-BRB）를 제안하였다. 제안된 시스템은 코어, 구속판 및 체결요소를 포함한 모든 구성요소를 스테인리스강으로 제작하였으며, 스테인리스강의 우수한 내식성과 현저한 반복경화 특성을 활용하여 향상된 반복거동, 장수명 성능 및 우수한 항복 후 강성을 확보하고자 하였다.
제안된 BRB 시스템의 설계 및 성능평가를 위하여 종합적인 실험 및 해석 연구를 수행하였다. 먼저, STS304 스테인리스강 및 SS275 탄소강의 시편에 대하여 단조 및 반복하중 실험을 수행하여 재료특성 평가를 실시하였다. 이를 통해 반복경화 거동 및 저주기 피로성능과 같은 주요 재료 특성을 분석하였으며, 수치해석에 적용하기 위한 구성방정식 모델을 보정 및 검증하였다.
이후, STS304 및 SS275 재료를 사용하여 동일 형상의 축소형 BRB 시험체 총 6개를 제작하고 반복가력 실험을 수행하여 두 재료군의 반복거동 특성을 비교·분석하였다. 실험 결과는 이력거동, 파괴모드, 에너지 소산능력, 강도보정계수（strength-adjustment factor） 및 항복 후 강성 측면에서 평가되었다. 또한, ABAQUS를 이용하여 구축한 유한요소해석 모델은 실험 결과와 전반적으로 유사한 결과를 나타내어 제안된 해석기법의 정확성과 신뢰성을 확인하였다.
부재 수준의 성능평가 이후에는 BRB가 적용된 골조 시스템에 대하여 대표 지진파를 이용한 비선형 시간이력해석을 수행하여 구조 시스템 수준에서의 내진성능을 평가하였다. 해석 결과를 통해 제안된 BRB시스템이 구조물의 지진응답 저감 및 구조적 회복탄성（resilience） 향상에 효과적임을 확인하였다. 마지막으로, 실험 결과 및 기존 문헌 데이터를 기반으로 머신러닝 모델을 구축하여 주요 영향인자를 이용한 BRB의 누적소성변형능력（Cumulative Plastic Deformation, CPD） 예측기법을 제안하였다.
본 연구에서 개발된 스테인리스강을 적용한 좌굴방지가새는 향후 구조물의 내진성능 향상뿐만 아니라, 우수한 내구성과 내식성을 갖춘 차세대 지진방재시스템 개발에 기여할 것으로 기대된다.

다국어 초록 (Multilingual Abstract)

Buckling-restrained braces （BRBs） is one of the lateral force-resisting systems for buildings and other structures subjected to seismic loads. The brace configuration consists of a metallic core and a restraining mechanism, where the core is prevented from buckling in compression while exhibiting stable and nearly symmetrical hysteretic behavior in both tension and compression, thereby providing high energy dissipation capacity. The magnitude of this energy dissipation is strongly governed by the effectiveness of the restraining units in confining the core against lateral deformation. The primary objective of this study is to advance sustainable buckling- restrained braces for reliable seismic protection systems. An all-stainless-steel assembled buckling-restrained brace （STS304-BRB） is investigated, featuring rapidly installable and removable restraining units. All components of the system, including the core, restraining plates, and fastening elements, are fabricated from stainless steel. This design utilizes the inherent corrosion resistance and pronounced cyclic-hardening characteristics of stainless steel to achieve enhanced cyclic performance, extended service life, and improved post-yield stiffness. To support the design and evaluation of the proposed BRB system, a comprehensive experimental and analytical program was conducted. Initially, a detailed material characterization study was performed in which small-scale coupons of STS304 stainless steel and SS275 carbon steel were tested under monotonic and cyclic loading. Key material properties, including cyclic hardening behavior and low-cycle fatigue performance, were evaluated, and corresponding constitutive models were calibrated and validated for numerical simulations. Subsequently, reduced-scale BRB component tests were carried out on six specimens, comprising two groups fabricated from STS304 and SS275 with identical geometries, to investigate and compare their cyclic performance. The experimental results were analyzed in terms of hysteretic response, failure modes, energy dissipation capacity, strength-adjustment factors, and post-yield stiffness. Finite element models developed in ABAQUS demonstrated strong agreement with the experimental observations, confirming the accuracy and reliability of the adopted modeling approach. Following the component-level investigation, the seismic performance of BRB-equipped frames was evaluated through nonlinear dynamic analysis under representative earthquake ground motions. The results provided insight into the system-level behavior and effectiveness of the proposed BRB in enhancing structural resilience. Finally, machine learning models were formulated using experimental and literature-based datasets to predict the cumulative plastic deformation （CPD） capacity of BRBs based on key influencing parameters. The developed stainless-steel BRBs are expected to contribute significantly to the advancement of resilient, durable, and corrosion-resistant seismic protection systems for built structures. Keywords: Buckling-restrained braces, lateral force-resisting systems, stainless- steel, low-cycle fatigue performance, hysteretic response, post-yield stiffness, finite element models, machine learning models

목차 (Table of Contents)

• I. Introduction 1
• 1.1. Research Background and Motivation 1
• 1.2. Main Objectives 6
• 1.3. Research Methodology 7
• 1.4. Thesis Outline 9
• II. Literature Review 11
• 2.1. General 11
• 2.2. Buckling-restrained Braces （BRBs） 11
• 2.3. Systematic Literature Review 12
• 2.3.1. Search Strategy 13
• 2.3.2. Search Results 15
• 2.4. Synthesis and Classification of BRB Systems 18
• 2.4.1. Studies Based on Core Component Materials 20
• 2.4.2. Studies Based on Core Component Shapes 26
• 2.4.3. Studies Based on Debonding Mechanism 30
• 2.4.4. Studies Based on Restraining Units 32
• 2.4.4.1. Conventional-Concrete BRBs 32
• 2.4.4.2. All-Steel BRBs 35
• 2.4.4.3. Timber BRBs 39
• 2.4.5. Self-Centering BRBs 40
• 2.4.6. Corrosion-Resistant BRBs 43
• 2.5. Summary and Identified Research Gaps 46
• III. Low Cyclic Fatigue Properties and Cyclic Constitutive Modeling of Korean Steel Grades STS304 and SS275 for Seismic Applications 49
• 3.1. Introduction 49
• 3.2. Past Relevant Studies 50
• 3.3. Experimental Program 53
• 3.4. Test Results and Analyses 54
• 3.4.1. Tensile Material Properties 54
• 3.4.2. Cyclic Stress–Strain Response 56
• 3.4.3. Strain–life Relationships 62
• 3.4.4. Calibration of Nonlinear Constitutive Model Parameters 64
• 3.4.5. Numerical Simulation of the Coupon’s Cyclic Behavior 67
• 3.5. Summary 71
• IV. Experimental and Numerical Study on Cyclic Performance of Stainless steel （STS304） and Carbon Steel （SS275） Buckling-restrained Braces 73
• 4.1. Introduction 73
• 4.2. Configuration and Design of BRBs 73
• 4.2.1. Assembly Details 73
• 4.2.2. Basic Design Parameters 75
• 4.2.3. Restraint Mechanism for Global Buckling 75
• 4.2.4. Higher-mode Buckling of the Core Component 76
• 4.2.5. Forces Acting on Restraining Units and Bolts Design 77
• 4.3. Experimental Program 78
• 4.3.1. Test Specimens 78
• 4.3.2. Material Properties 81
• 4.3.3. Loading Protocol and Experimental Setup 84
• 4.4. Results and Discussion 86
• 4.4.1. Failure Patterns and Strain Response 86
• 4.4.2. Hysteretic Response 90
• 4.4.3. Mechanical Performance Indexes 94
• 4.4.4. Strength Factors 96
• 4.4.5. Energy Dissipation Capacity 97
• 4.4.6. Cumulative Plastic Deformation （CPD） 99
• 4.4.7. Post-yield Stiffness 102
• 4.5. Numerical Simulation 105
• 4.5.1. Finite Element Modeling of BRBs 105
• 4.5.2. FE Models Validation 107
• 4.6. Material Cost Comparison 109
• 4.7. Summary 111
• V. Seismic Performance Evaluation of Steel Frame Structures Equipped with STS304 and
• SS275 Buckling-restrained Braces 114
• 5.1. Introduction 114
• 5.2. Structural Design of Prototype Building 115
• 5.3. Validation of BRBs Hysteretic Response in OpenSees 118
• 5.4. Ground Motion Records 123
• 5.5. OpenSees Modeling of 2D Frames 125
• 5.6. Seismic Performance Results 128
• 5.6.1. Inter-Story Drift Ratio （IDR） 129
• 5.6.2. Residual Inter-Story Drift Ratio （RIDR） 131
• 5.7. Summary 133
• VI. Data-driven Prediction of Cumulative Plastic Deformation for Buckling-restrained Braces Using Generative Adversarial Networks （GAN） Synthetic Data 135
• 6.1. Introduction 135
• 6.2. Database and Augmentation 137
• 6.3. Models Formulation and Evaluation 143
• 6.3.1. Formulation of ML models 143
• 6.3.2. Evaluation Metrics 145
• 6.4. Results and Discussion 147
• 6.4.1. Performance of ML Models 147
• 6.4.2. SHAP Interpretation 150
• 6.5. Summary 152
• VII. Conclusions and Future Work 154
• 7.1. Conclusions 154
• 7.2. Limitations 157
• 7.3. Future Recommendations 158
• 7.3.1. Frame-level Shaking Table Experiments 158
• 7.3.2. Investigation of Corroded BRBs under Aggressive Environments 158
• 7.3.3. Exploration of Alternative Stainless-steel Grades 159
• 7.3.4. Lifecycle Cost Analysis 159
• References 161
• Appendix 181
• 국문초록 185
• Acknowledgements 187
• Table of Figures
• Figure 1.1 Past earthquakes in Korea 1
• Figure 1.2 （a） RC Building Retrofitted with BRBs （b） Steel braces frame structure 2
• Figure 1.3 Components of conventional BRB 3
• Figure 1.4 Corrosion phenomenon of carbon steel 4
• Figure 1.5 Applications of stainless-steel: （a） Millennium Bridge, York, UK （b）
• stainless-steel shear wall system （c） stainless-steel slit damper （d） stainless-
• steel beam-column connections 5
• Figure 1.6 Sequential framework of study 7
• Figure 2.1 Protocol of SLR 13
• Figure 2.2 Distribution of articles per year 16
• Figure 2.3 List of journals featuring selected articles 17
• Figure 2.4 Geographical distribution of publications 18
• Figure 2.5 Classification of BRBs based on components 19
• Figure 2.6 Shapes of cores 26
• Figure 2.7 BRBs with different unbonding materials 30
• Figure 2.8 RC Restraining unit BRB 33
• Figure 2.9 Fibers for restraining units of BRBs 34
• Figure 2.10 Polymer filled BRB 34
• Figure 2.11 Tire-derived aggregate concrete BRB 35
• Figure 2.12 All-steel BRB with replaceable steel angle fuses 36
• Figure 2.13 Assembly of angle restraining units all-steel BRB 36
• Figure 2.14 All-steel buttoned BRB 37
• Figure 2.15 Tripple-truss-confined all-steel BRB 38
• Figure 2.16 Configuration of Shuttle-shaped all-steel BRB 38
• Figure 2.17 Timber BRB 39
• Figure 2.18 Self-centering BRB 41
• Figure 2.19 Self-centering spring BRB 42
• Figure 2.20 GRFP tube concrete-filled BRB 43
• Figure 2.21 GFRP foam-filled BRB 44
• Figure 2.22 Cold-formed stainless steel sandwich tubular brace 45
• Figure 2.23 BRBs distribution w.r.t core and restraining system 47
• Figure 3.1 Dimensions of coupons （a） Monotonic （b） Cyclic 52
• Figure 3.2 Cyclic loading protocol for testing coupons 53
• Figure 3.3 Testing setup for cyclic tests 54
• Figure 3.4 Stress-strain curves from monotonic tensile tests （a） SS275 （b） STS304 55
• Figure 3.5 Stress-strain curves of STS304 cyclic coupons 57
• Figure 3.6 Stress-strain curves of SS275 cyclic coupons 58
• Figure 3.7 Half-life stabilized loops （a） SS275 （b） STS304 59
• Figure 3.8 Cyclic stress-strain curves （a） SS275 （b） STS304 62
• Figure 3.9 Fatigue-life curves （a） SS275 （b） STS304 63
• Figure 3.10 Chaboche model calibration （a） Isotropic （b） Kinematic 65
• Figure 3.11 FE model of coupon 68
• Figure 3.12 FEA validation of STS304 coupons 69
• Figure 3.13 FEA validation of SS275 coupons 70
• Figure 4.1 Details of BRBs （a） Components （b） Assembly 74
• Figure 4.2 Core buckling （a） Strong axis （b） Weak axis 76
• Figure 4.3 Dimensions of BRBs 80
• Figure 4.4 Sample of core coupons for monotonic tensile tests 81
• Figure 4.5 Stress-strain curves 82
• Figure 4.6 BCJ loading protocol 84
• Figure 4.7 Testing setup and data acquisition system 85
• Figure 4.8 Peak strains at restraining plates 89
• Figure 4.9 Hysteretic curves 91
• Figure 4.10 Comparison of hysteretic loops between STS304 and SS275 BRBs at 3% 92
• Figure 4.11 Skeleton curves 93
• Figure 4.12 （a） Elastic stiffness computation （b） ECCS method 94
• Figure 4.13 （a） Energy dissipation per cycle - Ehi （b） Cumulative energy dissipation - Eh 98
• Figure 4.14 Cumulative plastic deformation （CPD） 100
• Figure 4.15 Comparison on stiffness degradation of STS304 and SS275 BRBs . 102
• Figure 4.16 Two-stage post-yield stiffness behavior 103
• Figure 4.17 Finite element model of BRBs. 106
• Figure 4.18 Comparison of BRBs experimental and FEM hysteretic curves. 108
• Figure 4.19 Stress contours （a） STS304BRB1 （b） SS275BRB1 109
• Figure 5.1 Layout of considered BRB-frame building 116
• Figure 5.2 4-story BRB frame building 117
• Figure 5.3 Comparison between experimental and Opensees hysteretic curves （a）
• SS275BRB （b）STS304BRB 122
• Figure 5.4 Selected earthquake ground motions 125
• Figure 5.5 Schematic illustration of the modeling approach of BRBF in Open sees 126
• Figure 5.6 STS304-BRB frames IDR comparison 129
• Figure 5.7 SS275-BRB frames IDR comparison 130
• Figure 5.8 STS304-BRB frames RIDR comparison 131
• Figure 5.9 SS275-BRB frames RIDR comparison 132
• Figure 6.1 Framework for data-driven prediction of BRBs’ CPD 136
• Figure 6.2 Frequency distribution normal curve of variables 138
• Figure 6.3 Pearson’s correlation between variables 139
• Figure 6.4 Structure of GAN model 141