A Methodological Approach To Assess Seismic Resilience of City Ecosystems Through the Complex Networks Theory

Abstract eng:
Earthquakes have always been representing a sleeper risk affecting local and global communities, as they are lowprobability-high-risk events. In addition, being contemporary urban societies threatened by increasing exposure along with growing urbanization, resilience represents a key issue for modern societies, as the capability to withstand and recover from disasters. Understanding resilience as an engineering issue in the sense of ecosystems, cities can be modelled as complex networks, made of coexisting and mutually interacting physical and social components. The proposed methodology aims at quantifying seismic resilience of city ecosystems according to a multidisciplinary approach. Resilience is evaluated while also ensuring an adequate level of sustainability, according to a social and humancentric perspective. A multi-scale approach is then performed in order to measure urban efficiency and systemic structural damage through the assessment of specific engineering measures. Hence, to effectively do this, cities are modelled as hybrid social-physical networks (HSPNs), merging the infrastructural and human urban components. The efficiency of different hybrid network typologies is estimated, as nodes are modelled as residential buildings (citizen-citizen efficiency) or schools (service-citizen efficiency). A real case study for the inner city Naples, namely the Quartieri Spagnoli area, is developed to validate the robustness of the proposed metrics. Synthetic HSPNs are also modelled accounting for different geometric shapes, according to the most common topologies in Europe and USA. Moreover, for each city shape, simulations are ran accounting for increasing city size, to study changes in resilience with the HSPN scale. As a point in matter, it is fundamental to account for specific urban dynamics in time and space. According to diverse studies in the literature, urban dynamics grow proportionally with a city size, becoming denser with the increasing scale, until a certain boundary, out of which such dynamics become unpredictable. Seismic scenarios are simulated assuming for the earthquake’s intensity to be deterministic in terms of the attained peak ground acceleration (PGA). The building portfolio for each modelled city is assumed to be constituted by reinforced concrete (RC) frame buildings, non-seismically designed and the vulnerability is quantified, by averaging fragility curves selected from the literature. Mathematical and engineering measures are performed before and after the seismic scenario and during the recovery phases with a time-discrete approach, enabling to quantify the level of urban functionality and happiness of city inhabitants and environmental sustainability, as the urban resilience. Hence, structural damage is evaluated in a systemic manner over the entire urban territory soon after an earthquake occurs, as a measure of the city efficiency drop. Two diverse recovery strategies are modelled and simulated to study the efficiency recovery and progress with a step-by-step procedure. The proposed framework is a high-potential means of seismic risk mitigation, which can help local communities and support disaster managers to know the pre-event urban capacity and to face the post-event reconstruction. This can be even more effective when arguing at the local level, where recovery actions and financial sources are more easily manageable.

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