Risk-Based Seismic Assessment of Existing Unreinforced Masonry Buildings in Switzerland

Abstract

Earthquakes in Switzerland possess the potential for catastrophic impacts, with estimated damages of around 7 billion Swiss francs for a magnitude 5.5 to 6.0 earthquake and 40 billion Swiss francs for a magnitude 6.0 to 6.5 earthquake (Duvernay and Lateltin). While seismic risk mitigation has gained attention in the last decades, the vast majority of Swiss building stock either does not conform with current seismic provisions or their performance in the event of an earthquake is unknown. The codified approaches, though suitable for practice, often rely on simplified methods that may not accurately achieve the desired or perceived level of safety. Therefore, exploring sophisticated and reliable seismic assessment methodologies becomes essential to safeguard both human lives and economic assets in the face of seismic hazards. The requirement for existing structures to meet the same standards as new ones can be impractical and challenging, necessitating the adoption of different performance objectives in the assessment process. This is primarily due to the shorter lifespans and use of older construction practices in existing structures, making it difficult to achieve the same performance level as new ones designed with modern seismic provisions. Consequently, retrofitting existing structures to align with current design codes can be financially prohibitive. In light of these considerations and to meet a growing need for the seismic assessment of existing structures, SIA 269/8, the code for the seismic assessment of existing structures in Switzerland, adopts a risk-based approach based on capacity/demand ratios, denoted as the compliance factor $\alpha_{eff}$. The compliance factor represents the ratio between the capacity of an existing structure and the code demand of a hypothetically identical new structure, serving as an indicator of its conformity with the design code. According to SIA 269/8, existing structures are permitted to have capacity/demand ratios lower than 1.0 when assessed under the design-level seismic hazard. Consequently, the code prescribes only those retrofit measures that yield a greater benefit compared to their implementation cost while maintaining minimum capacity/demand ratio targets to ensure a certain level of life safety. To this end, a risk curve proposed in SIA 269/8 associates capacity/demand ratios with the annual probability of death of an individual, known as the unit casualty risk, which aids in assessing the feasibility of seismic retrofit measures to mitigate risk. In this matter, SIA 269/8 treats the assessment from a cost/benefit point of view and allows existing structures to have a lower capacity if the assessment is performed under the design-level seismic hazard (reduced capacity approach). Acknowledging the shorter lifespan of existing structures, another assessment approach, not considered by SIA 269/8, focuses on reducing the seismic hazard under which the assessment is performed and dictating a minimum capacity/demand ratio of 1.0 under reduced seismic hazard levels (reduced hazard approach). Irrespective of the selected approach and the practicality of the SIA 269/8 proposal to link capacity/demand ratios to unit casualty risks, the simplistic nature of capacity/demand ratios raises concerns regarding their accuracy and adequacy as a reliable performance metric. The primary objective of this study is to undertake a comprehensive investigation into the seismic assessment techniques employed in Switzerland, with a specific emphasis on unreinforced masonry buildings, given their dominance in the built inventory of Switzerland. The study focuses on evaluating the reliability of the capacity/demand ratio-based performance metrics utilized for assessing and communicating seismic risks and seeks to enhance their performance through the definition of extended capacity/demand ratios. Furthermore, the research explores the implied collapse risks related to the hazard and capacity-controlled performance relaxation philosophies. In addition to these objectives, the study seeks to thoroughly examine the influence of both epistemic and aleatory uncertainty on the structural response, highlighting the significance of uncertainty quantification in seismic risk assessment. The findings of this study reveal that the use of capacity/demand ratios as a means to represent collapse risks is generally not reliable, and particularly inadequate for estimating fatality risks. Apart from the lack of a precise correlation between capacity/demand ratios and risks, the SIA 269/8 approach does not necessarily yield conservative risk estimates, in contrast to the conservatism tendency in codified methods. The reduced hazard approach emerges as a promising alternative, employing capacity/demand ratios not as direct indicators of associated risks, but rather as a means to express relaxed performance objectives. Its goal is to attain comparable risk levels between existing and new structures, as indicated by the findings of this study. Therefore, it is crucial to acknowledge that translating capacity/demand ratio-based performance objectives into seismic risks, both in terms of collapse and fatality risks, remains challenging. Furthermore, the study observes that aleatory uncertainty, stemming from the random characteristics of ground motions, holds greater significance than epistemic uncertainty, emphasizing the need to focus on addressing the former in seismic risk assessment. In response to the limitations observed in using capacity/demand ratios to assess risks, particularly in estimating fatality risks, the final part of this dissertation proposes a novel and user-friendly graphical framework for seismic fatality risk assessment for existing buildings. Starting from a seismic hazard model, probabilistic seismic demand, damage, and loss models are introduced, taking the uncertainty into account at each step of the assessment process. The key advantage of this graphical framework lies in its ability to handle probabilistic models and associated uncertainties in a visually intuitive manner without requiring an in-depth understanding of probability theory. This transparency in risk assessment is achieved through a clear separation of risk components. Moreover, the proposed framework has been validated and shown to deliver accurate results through the demonstration of increasingly complex analysis routes, which range from the widely adopted nonlinear static pushover analysis to more sophisticated nonlinear dynamic analysis, or a combination of both. Such adaptability allows analysts to employ the assessment tools that align best with their expertise, ensuring a comprehensive and reliable seismic fatality risk assessment for existing buildings.