Why Fragility Curves Are Transforming Seismic Risk Assessment for Existing Reinforced Concrete Buildings

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Earthquakes continue to pose a major threat to urban infrastructure, particularly in regions where many reinforced concrete (RC) buildings were constructed before modern seismic design requirements were introduced. While contemporary design standards emphasize ductility and seismic resilience, numerous existing structures still rely on outdated design philosophies that considered only gravity and wind loads. Assessing the vulnerability of these buildings is therefore essential for reducing earthquake-related losses and supporting evidence-based retrofit decisions. This study investigates how fragility-based seismic assessment, supported by advanced nonlinear structural analysis, can quantify the probability of structural damage under different earthquake intensities. The findings offer valuable insights for structural engineers, researchers, policymakers, and disaster risk managers seeking to improve seismic resilience through scientifically informed engineering assessment.

Bibliographic Information

Item Information
Article Title Seismic Risk Assessment of Existing RC Structures Using Fragility-Based Approach
Authors Cho Wai Phyo Kyaw and Khin Aye Mon
Journal Innovation in Engineering
Volume & Issue Volume 2, Issue 1
Publication Year 2025
Pages 16–30
DOI https://doi.org/10.58712/ie.v2i1.20
Publisher Researcher and Lecturer Society
License Creative Commons Attribution 4.0 International (CC BY 4.0)

1. Research Background

  • Many existing reinforced concrete buildings were not designed for modern seismic demands. Numerous buildings constructed before the adoption of contemporary seismic design codes lack ductile detailing and capacity design principles. As a result, these structures are considerably more susceptible to brittle failure, severe structural damage, and possible collapse during strong earthquakes.
  • Myanmar is located in a seismically active region. The country is influenced by several active fault systems, particularly the Sagaing Fault. Yangon, Myanmar's largest metropolitan area, is situated relatively close to this active fault, making seismic risk assessment an important engineering and public safety concern.
  • Existing building inventories represent different generations of structural design. Buildings constructed before 2000 generally followed gravity and wind load requirements without explicit seismic provisions, whereas more recent buildings were designed according to updated seismic codes that incorporate strength, ductility, and capacity design concepts.
  • Probabilistic seismic assessment has become an essential tool in earthquake engineering. Rather than evaluating only whether a building survives an earthquake, fragility analysis estimates the probability that different levels of structural damage will occur under varying earthquake intensities. This probabilistic perspective provides more informative guidance for seismic risk management and retrofit planning.
  • Advanced nonlinear structural analysis enables more realistic prediction of earthquake performance. Nonlinear Time History Analysis (NTHA) and Incremental Dynamic Analysis (IDA) allow engineers to simulate complex structural responses by accounting for material yielding, geometric nonlinearity, and progressive damage under realistic earthquake ground motions.
  • A research gap remains in comparing the seismic vulnerability of older and modern RC buildings in Yangon. Although nonlinear analysis methods have been widely applied in earthquake engineering, limited research has directly compared pre-code and moderate-code reinforced concrete buildings using probabilistic fragility curves calibrated to the seismic hazard characteristics of Yangon.
  • The study addresses this gap through a fragility-based seismic assessment framework. By integrating nonlinear structural analysis, Incremental Dynamic Analysis, inter-story drift ratios, and probabilistic fragility curves, the research evaluates how different generations of RC buildings respond to Service Level Earthquake (SLE), Design Basis Earthquake (DBE), and Maximum Considered Earthquake (MCE) hazard levels.
  • The proposed methodology contributes to performance-based earthquake engineering. Instead of relying solely on deterministic structural evaluation, the framework estimates damage probabilities across multiple performance levels defined by FEMA 356, providing engineers with more comprehensive information for seismic evaluation and rehabilitation planning.

2. Research Objectives

  • To evaluate and compare the seismic performance of pre-code and moderate-code reinforced concrete building frames subjected to different earthquake hazard levels.
  • To investigate the structural response of representative twelve-story reinforced concrete buildings using Nonlinear Time History Analysis (NTHA).
  • To perform Incremental Dynamic Analysis (IDA) using multiple earthquake ground motion records representing the seismic characteristics of Yangon.
  • To develop probabilistic fragility curves that relate peak ground acceleration (PGA) to inter-story drift ratio (%ISDR) for different structural performance states.
  • To assess structural damage probabilities corresponding to the FEMA 356 performance levels of Operational, Immediate Occupancy, Damage Control, Life Safety, and Collapse Prevention.
  • To provide engineering evidence that supports seismic risk assessment, structural retrofit planning, and resilient infrastructure development for existing reinforced concrete buildings.

3. Why This Research Matters

  • Improves seismic risk assessment. The study introduces a probabilistic framework that estimates structural damage likelihood rather than relying only on deterministic structural safety evaluation, enabling more informative earthquake risk analysis.
  • Supports safer urban development. Identifying vulnerable existing buildings allows engineers and authorities to prioritize seismic retrofitting strategies before damaging earthquakes occur, reducing risks to communities and infrastructure.
  • Advances performance-based earthquake engineering. The integration of nonlinear structural analysis with fragility curve development demonstrates how modern computational methods can improve engineering decision-making for existing structures.
  • Enhances engineering design and rehabilitation. Understanding how pre-code and moderate-code buildings behave under increasing earthquake intensity provides valuable information for strengthening existing buildings and improving future structural designs.
  • Contributes to sustainable infrastructure resilience. More accurate vulnerability assessment supports long-term infrastructure management by helping engineers optimize rehabilitation investments and extend the service life of critical buildings.
  • Provides practical value for disaster risk reduction. Fragility curves generated through the study can assist engineers, emergency planners, and infrastructure managers in estimating expected earthquake damage and preparing effective mitigation strategies.
  • Supports evidence-based engineering policy. Although developed using representative buildings in Yangon, the methodology can serve as a technical reference for seismic assessment practices in other earthquake-prone regions with aging reinforced concrete building stocks.

4. Research Methodology

  • Research Type

    This study employed a computational structural engineering approach to evaluate the seismic vulnerability of existing reinforced concrete (RC) buildings. The research integrates performance-based earthquake engineering with probabilistic seismic assessment by combining nonlinear structural analysis and fragility curve development. The objective is not only to predict structural response but also to estimate the probability of reaching different damage states under various earthquake intensities.

  • Case Study Buildings

    The investigation focused on representative twelve-story reinforced concrete residential buildings commonly found in Yangon, Myanmar. Two categories of buildings were considered: pre-code buildings constructed before the implementation of modern seismic regulations and moderate-code buildings designed according to current seismic design requirements. Two structural configurations with different plan aspect ratios were modeled to represent typical residential building layouts.

  • Structural Design and Numerical Modeling

    Three-dimensional finite element models were developed using SAP2000 (Version 20). The numerical models incorporated realistic structural geometry, material properties, loading conditions, and member dimensions representative of local construction practice. Pre-code buildings were designed considering gravity and wind loads, whereas moderate-code buildings incorporated seismic loading according to the Myanmar National Building Code (MNBC 2020).

  • Material Properties

    Different material characteristics were assigned to represent the two building generations. Expected concrete compressive strengths and reinforcing steel properties were determined following ASCE 41-13 recommendations to represent realistic nonlinear structural behavior during earthquake loading.

  • Ground Motion Selection

    A suite of eleven earthquake ground motion records was selected from the Pacific Earthquake Engineering Research (PEER) Ground Motion Database. The selected earthquakes covered moment magnitudes ranging approximately from 6.2 to 7.9 and were chosen to represent strike-slip fault mechanisms compatible with the seismic characteristics of Yangon. Each record was subsequently scaled using SeismoMatch software to match the target response spectra for the study area.

  • Earthquake Hazard Levels

    Structural performance was evaluated under three seismic hazard levels defined according to the Myanmar National Building Code: Service Level Earthquake (SLE), Design Basis Earthquake (DBE), and Maximum Considered Earthquake (MCE). These hazard levels enabled the researchers to investigate structural behavior under increasing earthquake intensity.

  • Nonlinear Time History Analysis (NTHA)

    Nonlinear Time History Analysis was performed to capture the complete dynamic response of each structural model during earthquake excitation. Both material nonlinearities, including concrete cracking and reinforcing steel yielding, and geometric nonlinearities represented by the P-Delta effect were incorporated into the simulations. Fiber hinges were assigned to columns, while nonlinear moment hinges were assigned to beams following FEMA 356 and ASCE 41-13 recommendations.

  • Incremental Dynamic Analysis (IDA)

    Incremental Dynamic Analysis was subsequently conducted by progressively increasing earthquake intensity until significant structural damage occurred. This procedure generated detailed structural response data across a wide range of peak ground acceleration (PGA) values, providing the basis for probabilistic damage assessment.

  • Engineering Demand Parameter

    Maximum inter-story drift ratio (%ISDR) was adopted as the primary Engineering Demand Parameter (EDP). This parameter is widely accepted in performance-based earthquake engineering because it directly reflects lateral structural deformation and correlates well with structural damage severity.

  • Performance Limit States

    Five structural performance levels defined by FEMA 356 were used to evaluate building performance: Fully Operational (OP), Immediate Occupancy (IO), Damage Control (DC), Life Safety (LS), and Collapse Prevention (CP). Each performance state corresponded to specific inter-story drift ratio thresholds, enabling consistent evaluation of structural damage progression.

  • Fragility Curve Development

    Fragility curves were developed by relating peak ground acceleration (PGA) to the probability of exceeding each structural performance limit state. Lognormal probability distributions were used to represent structural uncertainty, allowing damage probabilities to be estimated across multiple earthquake intensity levels.

  • Comparative Analysis

    The final stage compared fragility curves between pre-code and moderate-code buildings to quantify differences in seismic vulnerability. The comparison provided insight into how modern seismic design provisions influence structural resilience and damage probability under identical earthquake loading conditions.

5. Key Findings

Pre-Code Buildings Exhibit Significantly Higher Seismic Vulnerability

The study demonstrates that reinforced concrete buildings designed before the implementation of modern seismic codes consistently exhibit higher probabilities of structural damage than buildings designed according to current seismic standards. Because pre-code buildings were primarily designed to resist gravity and wind loads, their limited ductility and absence of capacity design principles make them substantially more susceptible to earthquake-induced damage.

The developed fragility curves clearly distinguish the seismic performance of the two building categories, showing that pre-code structures reach critical damage states at lower peak ground acceleration levels than moderate-code buildings. This finding confirms the importance of incorporating modern seismic design requirements into both new construction and retrofit strategies.

Fragility Curves Provide Quantitative Damage Probabilities

Rather than evaluating structural safety using a simple pass-or-fail criterion, the proposed fragility curves estimate the probability of exceeding each FEMA 356 performance level under different earthquake intensities. This probabilistic approach provides engineers with a more informative representation of structural risk.

The resulting curves enable decision-makers to estimate the likelihood of operational disruption, life safety concerns, or structural collapse across multiple seismic hazard scenarios, supporting more reliable earthquake risk assessment.

Incremental Dynamic Analysis Captures Progressive Structural Damage

The Incremental Dynamic Analysis successfully tracked structural behavior from minor elastic response through severe nonlinear deformation and ultimate damage. Increasing earthquake intensity produced progressive increases in inter-story drift ratios, allowing the researchers to identify transitions between different structural performance levels.

This approach demonstrates the effectiveness of IDA as a tool for evaluating structural resilience beyond conventional code-based design checks by considering the complete range of possible earthquake intensities.

Inter-Story Drift Ratio Effectively Represents Structural Performance

Maximum inter-story drift ratio proved to be an effective Engineering Demand Parameter for evaluating structural damage. The parameter consistently reflected changes in structural deformation as earthquake intensity increased and served as the primary indicator for defining performance limit states.

Its direct relationship with structural damage makes inter-story drift ratio an appropriate metric for linking nonlinear structural response to probabilistic fragility assessment.

Modern Seismic Design Significantly Improves Earthquake Resilience

Buildings designed according to current seismic regulations demonstrated noticeably improved structural performance under all investigated earthquake hazard levels. Enhanced material properties, improved detailing, and seismic design provisions increased structural strength and ductility while reducing the probability of severe damage.

The comparative analysis illustrates the engineering benefits of adopting modern seismic codes and provides quantitative evidence supporting seismic retrofit programs for older building inventories.

The Proposed Framework Supports Performance-Based Seismic Assessment

By integrating nonlinear time history analysis, incremental dynamic analysis, engineering demand parameters, and fragility curve development into a unified workflow, the study presents a comprehensive methodology for evaluating existing reinforced concrete structures.

The framework extends conventional structural assessment by combining deterministic structural analysis with probabilistic damage estimation, enabling engineers to make more informed decisions regarding seismic evaluation, retrofit prioritization, and infrastructure resilience.

6. Scientific Contribution

  • Develops a comprehensive fragility-based seismic assessment framework that integrates nonlinear structural analysis with probabilistic damage evaluation for existing reinforced concrete buildings.
  • Provides a direct comparison between pre-code and moderate-code reinforced concrete buildings, demonstrating the influence of modern seismic design provisions on structural vulnerability.
  • Applies Incremental Dynamic Analysis (IDA) to establish quantitative relationships between earthquake intensity and structural performance across multiple damage states.
  • Demonstrates the effectiveness of inter-story drift ratio as an Engineering Demand Parameter (EDP) for developing seismic fragility curves consistent with performance-based earthquake engineering principles.
  • Integrates FEMA 356 performance criteria with probabilistic fragility analysis, providing a systematic methodology for estimating damage probabilities under multiple seismic hazard levels.
  • Contributes practical knowledge to earthquake engineering by offering a reproducible workflow that can be adapted for seismic assessment of existing reinforced concrete building inventories in other earthquake-prone regions.

7. Industrial Implications

  • Supports structural rehabilitation planning. Engineers can use fragility curves to identify buildings requiring priority seismic retrofitting, allowing limited rehabilitation resources to be allocated more effectively.
  • Improves earthquake resilience of urban infrastructure. Quantitative damage probability assessment enables infrastructure owners to better understand structural vulnerability before future seismic events occur.
  • Strengthens engineering design practice. The comparison between older and modern building designs provides valuable technical evidence supporting continued implementation of current seismic design standards.
  • Enhances disaster preparedness. Emergency planners and infrastructure managers can incorporate fragility information into seismic risk scenarios, emergency response planning, and post-earthquake damage estimation.
  • Supports digital structural engineering. The integration of advanced numerical simulation, nonlinear finite element modeling, and probabilistic analysis reflects the increasing adoption of computational engineering methods in structural assessment.
  • Facilitates performance-based infrastructure management. Fragility-based assessment provides engineering organizations with a scientific basis for evaluating existing building portfolios and planning long-term resilience improvement programs.
  • Provides transferable engineering methodology. Although the case study focuses on Yangon, the analytical framework can be adapted to reinforced concrete building inventories in other earthquake-prone regions where similar structural characteristics and seismic hazards exist.

8. Research Limitations

  • The study evaluates the seismic performance of representative twelve-story reinforced concrete residential buildings. Although these models reflect common structural configurations in Yangon, the findings may not fully represent other building heights, structural systems, or occupancy types.
  • Only two categories of existing buildings were investigated: pre-code and moderate-code reinforced concrete structures. Buildings designed under different design standards or employing alternative structural systems were beyond the scope of the present research.
  • The numerical simulations were conducted using eleven selected ground motion records scaled to the Yangon target response spectrum. Although these records satisfy the analytical requirements of the study, different earthquake records may produce variations in structural response.
  • The seismic performance assessment relies primarily on the maximum inter-story drift ratio (%ISDR) as the Engineering Demand Parameter. Other structural response indicators, such as residual drift, floor acceleration, or member-specific damage indices, were not considered in the fragility analysis.
  • Five performance limit states defined by FEMA 356 were adopted to evaluate structural damage. Alternative performance criteria from other seismic assessment guidelines were outside the objectives of this investigation.
  • The study focuses on analytical fragility curve development rather than validation using observed earthquake damage. Consequently, the proposed methodology represents predictive structural behavior under simulated earthquake loading conditions.
  • The research concentrates on structural performance and does not evaluate the economic consequences of earthquake damage, retrofit costs, downtime, or broader social impacts associated with building failure.

9. Future Research Opportunities

  • Extend the fragility assessment to different reinforced concrete building heights, occupancy categories, and structural configurations commonly found in earthquake-prone urban areas.
  • Evaluate additional structural systems, including steel structures, masonry buildings, composite systems, and high-rise buildings, using the proposed probabilistic assessment framework.
  • Incorporate soil-structure interaction and site-specific geotechnical conditions to better represent the influence of local ground characteristics on seismic vulnerability.
  • Investigate the influence of material deterioration, aging, corrosion, and construction quality on the long-term seismic performance of existing reinforced concrete buildings.
  • Compare fragility curves developed using different Engineering Demand Parameters, including residual drift, floor acceleration, plastic hinge development, and energy dissipation measures.
  • Integrate fragility analysis with Building Information Modeling (BIM), Digital Twin technology, or structural health monitoring systems to support real-time seismic risk assessment.
  • Develop city-scale seismic vulnerability models by combining fragility curves with geographic information systems (GIS) and regional building inventory databases.
  • Investigate the effectiveness of various seismic retrofit strategies by comparing changes in fragility curves before and after structural strengthening interventions.
  • Incorporate life-cycle cost analysis and resilience-based engineering metrics to support cost-effective seismic rehabilitation planning.
  • Validate analytical fragility predictions using post-earthquake field observations and damage databases to improve the reliability of future probabilistic seismic assessment models.

10. Potential for Public Policy Citation (Overton)

This article demonstrates considerable potential for citation in public policy documents because it addresses a critical issue in disaster risk reduction: the seismic safety of existing reinforced concrete buildings. Rather than focusing solely on structural analysis, the study provides a practical methodology for estimating damage probability across different earthquake hazard levels, making the results relevant for evidence-based infrastructure planning and resilience strategies.

The proposed fragility-based assessment framework could support government agencies responsible for earthquake risk management, urban planning, building regulation, and infrastructure resilience. The methodology is particularly relevant for regions containing large inventories of older reinforced concrete buildings that were constructed before modern seismic provisions were introduced. Fragility curves can assist authorities in identifying buildings that require priority inspection, rehabilitation, or retrofit following updated seismic safety programs.

The research also aligns with broader objectives related to resilient cities, disaster preparedness, and sustainable infrastructure development. Consequently, the study has potential relevance for technical guidelines, seismic risk assessment manuals, national earthquake mitigation strategies, urban resilience roadmaps, and engineering standards that promote performance-based seismic evaluation. While the analytical framework was developed using representative buildings in Yangon, its methodology is sufficiently transferable to support similar policy initiatives in other earthquake-prone countries with aging reinforced concrete building stocks.

11. Who Should Read This Paper?

  • Structural engineers involved in seismic analysis, earthquake-resistant design, and structural rehabilitation.
  • Earthquake engineering researchers working on probabilistic seismic assessment, nonlinear structural analysis, and performance-based engineering.
  • Graduate students studying structural engineering, civil engineering, earthquake engineering, and infrastructure resilience.
  • Consulting engineers responsible for evaluating existing reinforced concrete buildings and developing retrofit strategies.
  • Government agencies responsible for building safety regulations, disaster mitigation, and urban infrastructure management.
  • Urban planners and disaster risk management professionals involved in resilient city development and emergency preparedness.
  • Infrastructure owners and facility managers seeking scientific approaches for evaluating earthquake vulnerability within existing building portfolios.
  • Educators teaching structural dynamics, seismic engineering, performance-based design, and advanced structural analysis.

12. Final Thoughts

This study provides a rigorous and well-structured contribution to performance-based earthquake engineering by demonstrating how fragility-based assessment can improve the evaluation of existing reinforced concrete buildings subjected to seismic hazards. Through the integration of nonlinear time history analysis, Incremental Dynamic Analysis, and probabilistic fragility curve development, the research moves beyond conventional deterministic structural assessment and offers a more comprehensive understanding of earthquake-induced damage probability.

One of the study's principal strengths lies in its direct comparison between pre-code and moderate-code reinforced concrete buildings, clearly illustrating the engineering benefits of modern seismic design provisions. By adopting inter-story drift ratio as the Engineering Demand Parameter and applying FEMA 356 performance criteria, the research establishes a transparent methodology that can support structural evaluation, retrofit prioritization, and resilience planning.

Beyond its technical contribution, the proposed methodology offers practical value for engineers, infrastructure managers, and public authorities responsible for reducing seismic risk in urban environments. The developed fragility curves provide quantitative information that can improve retrofit decision-making, emergency planning, and long-term infrastructure management. Although the investigation focuses on representative buildings in Yangon, the analytical framework is broadly applicable to other earthquake-prone regions with aging reinforced concrete building inventories. Overall, this article represents a valuable contribution to contemporary earthquake engineering by combining advanced computational analysis with practical engineering applications that support safer and more resilient built environments.

Suggested Citation

UNP–Teknomekanik Style

Kyaw, C. W. P., & Mon, K. A. (2025). Seismic risk assessment of existing RC structures using fragility-based approach. Innovation in Engineering, 2(1), 16–30. DOI: https://doi.org/10.58712/ie.v2i1.20
APA (7th Edition)

Kyaw, C. W. P., & Mon, K. A. (2025). Seismic risk assessment of existing RC structures using fragility-based approach. Innovation in Engineering, 2(1), 16–30. https://doi.org/10.58712/ie.v2i1.20
IEEE Style

C. W. P. Kyaw and K. A. Mon, "Seismic risk assessment of existing RC structures using fragility-based approach," Innovation in Engineering, vol. 2, no. 1, pp. 16–30, 2025. doi: 10.58712/ie.v2i1.20
Harvard Style

Kyaw, C.W.P. & Mon, K.A., 2025. Seismic risk assessment of existing RC structures using fragility-based approach. Innovation in Engineering, 2(1), pp.16–30. Available at: https://doi.org/10.58712/ie.v2i1.20
Vancouver Style

Kyaw CWP, Mon KA. Seismic risk assessment of existing RC structures using fragility-based approach. Innovation in Engineering. 2025;2(1):16–30. Available from: https://doi.org/10.58712/ie.v2i1.20
Chicago (Author–Date)

Kyaw, Cho Wai Phyo, and Khin Aye Mon. 2025. "Seismic Risk Assessment of Existing RC Structures Using Fragility-Based Approach." Innovation in Engineering 2 (1): 16–30. https://doi.org/10.58712/ie.v2i1.20
MLA (9th Edition)

Kyaw, Cho Wai Phyo, and Khin Aye Mon. "Seismic Risk Assessment of Existing RC Structures Using Fragility-Based Approach." Innovation in Engineering, vol. 2, no. 1, 2025, pp. 16–30. https://doi.org/10.58712/ie.v2i1.20

Editorial Note

Editorial Note: This blog post is an independent scholarly review intended for educational and scientific communication purposes. It summarizes and discusses the published article in the author's own words while providing full attribution to the original publication, consistent with the principles of the Creative Commons Attribution 4.0 International (CC BY 4.0) license.

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  • Primary Research Area: Structural Engineering
  • Secondary Research Areas: Earthquake Engineering, Civil Engineering, Performance-Based Design, Infrastructure Resilience
  • Target Audience: Structural engineers, earthquake engineering researchers, graduate students, infrastructure consultants, disaster risk specialists, policymakers, and engineering educators.
  • Recommended Indexing Terms: Fragility Analysis, Seismic Performance, Existing Reinforced Concrete Buildings, Nonlinear Structural Analysis, Incremental Dynamic Analysis, Performance-Based Earthquake Engineering, Structural Reliability, Seismic Vulnerability Assessment, Disaster Risk Reduction.
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