1. Theoretical Background
Unreinforced masonry (URM) buildings, historically constructed across numerous European regions like Italy and Turkey, have been vulnerable to seismic forces due to a lack of inherent earthquake-resistant features. With their widespread prevalence, these buildings have become a focal point of seismic retrofitting efforts.
Seismic Vulnerability of URM Structures: The absence of reinforcement and the inherent brittle behavior of masonry materials makes these structures highly susceptible to seismic forces.
Regulatory Contexts and Standards: Eurocode 8 (EN 1998) and national standards like NTC 2018 in Italy provide guidance for seismic evaluation and retrofitting but with nuanced differences in implementation.
Performance-Based Seismic Design (PBSD): This methodology enables the assessment of structural resilience at different performance levels using nonlinear modeling approaches, primarily through pushover analyses.
2. Seismic Performance Evaluation Methods
1D Element-Based Evaluation
Hinge Definition: Masonry walls, including piers and spandrels, are modeled as 1D elements with nonlinear hinge properties as per the design code.
Pushover Analysis: Nonlinear static analysis performed to evaluate the response of structural elements under increasing lateral loads, providing insight into displacement capacity and hinge rotation states.
Design Codes and Critical Deformation: Parameters like yield strength and deformation are set based on Eurocode and NTC 2018 criteria.
2D Plate Element-Based Evaluation
Nonlinear Material Properties: This model approach involves defining detailed material properties (e.g., bricks, bed, and head joints) for a more accurate simulation of masonry behavior.
Stress and Crack Analysis: Evaluation of stress concentrations and crack distributions under seismic loads to determine local damage and the need for retrofitting.
Capacity and Demand Spectrum Evaluation
Capacity Spectrum Method: Generation of capacity curves (base shear vs. roof displacement) and conversion to single-degree-of-freedom (SDOF) representations for comparison with demand spectra.
Performance Levels: Evaluation of structural responses and performance levels (e.g., Damage Limitation, Significant Damage, Near Collapse) as outlined in Eurocode and FEMA 356.
3. Considerations in Seismic Performance Evaluation
Modeling Accuracy: Ensuring accurate representation of masonry material properties, joint conditions, and structural geometries is critical for meaningful results.
Selection of Hinge Properties: Proper assignment of hinge properties for piers and spandrels, considering axial, flexural, and shear forces, is crucial as different structural elements behave uniquely under seismic loads.
Compliance with Local Codes: Adherence to local codes and standards (e.g., NTC 2018) is essential, as they often provide specific criteria for evaluating critical deformations and displacement limits.
Engineering Judgment in 2D Analysis: For plate models lacking explicit evaluation criteria, engineers must rely on professional judgment, referencing stress distributions, crack propagation, and other damage indicators.
Retrofitting Strategies: Effective retrofitting measures, such as external RC reinforcement and internal steel frames, must be evaluated for their impact on structural performance.
4. Conclusion
Evaluating the seismic performance of URM structures is a complex but necessary task to ensure safety and resilience in seismic-prone areas. Performance-based design, informed by standards like Eurocode 8 and NTC 2018, provides a robust framework for assessing and improving the seismic behavior of these buildings. Through accurate modeling, comprehensive analysis, and careful consideration of local guidelines, engineers can develop effective retrofitting strategies that safeguard lives and preserve historical structures. The continuous refinement of seismic evaluation methods and tools will further enhance the resilience of URM buildings in the face of seismic threats.
For more detailed insights, refer to the video linked below.
