Seismic assessment and retrofit strategies for URM buildings

Unreinforced masonry (URM) buildings constitute a substantial share of the existing building stock in Europe and North America and remain highly vulnerable to seismic actions due to their limited tensile strength, heterogeneous construction, and limited seismic detailing. Improving their performance requires retrofit strategies that can enhance structural response without compromising constructability, reversibility, or architectural value. This thesis develops an integrated framework to evaluate and compare seismic retrofitting strategies for URM buildings, combining nonlinear static assessment, nonlinear dynamic analysis, and mechanically consistent multi-scale modeling. A structured classification of retrofit interventions is first established and applied to a representative archetype through Equivalent-Frame nonlinear static analyses. The results highlight the governing role of diaphragm action in achieving box-type behavior and show that combined interventions provide more effective performance improvements than isolated techniques. The assessment is then extended to a probabilistic level through multi-stripe nonlinear dynamic analyses and fragility-curve derivation. Two URM case studies are used to quantify the probability of exceeding different performance limit states under varying seismic intensities and soil conditions. The results indicate that retrofit effectiveness depends on both the target performance objective and the characteristics of seismic demand, with wall-level strengthening primarily influencing collapse resistance and diaphragm interventions affecting drift control and force redistribution. A multi-scale mechanical modeling strategy is further developed to investigate an innovative lightweight timber cladding system. Deformable-block discrete element simulations, calibrated against experimental evidence, are used to characterize panel-level behavior under monotonic and cyclic loading. The resulting mechanical parameters are incorporated into macro-element formulations within an Equivalent-Frame model, enabling consistent representation of stiffness, deformation capacity, and post-cracking response at the building scale. Application to a representative URM structure shows that the retrofit increases displacement capacity and stabilizes damage progression without significantly altering initial stiffness or peak strength. In summary, the results demonstrate that a reliable comparison of retrofit strategies requires the combined use of deterministic assessment, probabilistic evaluation, and multi-scale mechanical calibration. The proposed framework provides a coherent basis for performance-oriented retrofit design and contributes to the development of efficient and compatible strengthening solutions for existing URM buildings.