Lifetime probabilistic seismic assessment of multistory precast buildings

Evaluation of seismic performance of structures during their lifetime is nowadays an emerging issue in the research community. In particular, a key aspect concerns the possibility to include in the seismic assessment the effects of environmental hazards, because current seismic codes and capacity design criteria are time-invariant and do not take into account such problem. Actually, considering the lifetime of a generic structure, the energy dissipating collapse mode may vary over time due to a reduction of both strength and ductility of the sections where plastic hinges are expected to occur during an earthquake. Such an interaction could finally bring to undesired failure mechanisms like weak column-strong beam, unpredicted during the design phase. Regarding environmental hazards, this investigation focuses on the role of a chloride attack, evaluating the loss of mechanical properties of the structural elements. Among different structures, precast buildings are particularly subjected to the effect of corrosion, because most of their structural members can be directly exposed to the atmosphere. In such conditions, the diffusive attack from external aggressive agents, like sulphates and chlorides, can take place and lead to a deterioration of concrete and steel. Damage induced by corrosion can significantly reduce local strength and ductility, modifying in this way the failure mechanism and the corresponding seismic performance during structural lifetime. Despite of the extensive research on seismic behavior of precast structures, few studies focused on the lifetime behavior of precast buildings subjected to environmental hazards. In such conditions, the diffusive attack of aggressive agents can lead to a deterioration of mechanical properties of structural members and to a decrease of the overall response. As a consequence, capacity design criteria should be properly calibrated to consider the severity of environmental exposure and the required structural lifetime. In particular, in this investigation the corrosion of reinforcement due to chloride attack is considered, assuming a contamination by chlorides the most significant source of environmental hazard for reinforced concrete structures. With respect to this problem, in recent years different mitigation strategies emerged, with the purpose to extend the lifetime of reinforced concrete structures or to reduce the potential damage due to strong ground motions. In particular, new advanced materials have been proposed in order to improve the seismic performance of structures and to extend their durability characteristics. Among them, the use of the so called Engineered Cementitious Composite (ECC) in place of normal concrete allows, by an appropriate mix design and adding a minimum quantity of polimeric fibers (about $2\%$), the formation of multiple narrow cracking in structural elements. This leads to a localized damage and a more uniform distribution of energy dissipation. Nevertheless, the reduction of the crack width tends to decrease the diffusion process of aggressive agents such as chlorides, with significant benefits in terms of durability. Finally, considering that all the phenomena involved in this investigation have an inherent variability, the rational approach to take into account their randomness is based on a probabilistic assessment. Moreover, since the nature of the problem is highly non linear, numerical simulations provide the only practical and effective method. In particular, if random variables are included, the numerical process with repeated simulations can be based on Monte Carlo sampling technique, which is particularly effective in treatment of aleatory and epistemic uncertainties. In order to reduce the computational cost involved in the simulation analysis based on plain Monte Carlo method, advanced tools are also needed to have reliable results. A stratified sampling called Latin Hypercube sampling is implemented, since such technique requires a relative small number of simulations to have reliable information on the performance of structural systems. Based on the considerations above, it is expected a significant influence of environmental hazard on the seismic performance of precast structures during their lifetime, and such influence can be quantified in an effective way using the tools developed in this work. In fact, one of the most relevant contributions of the present investigation is the possibility to show and evaluate how same structures, placing at sites with the same seismic hazard, can have a different seismic reliability depending on the environmental conditions. Such results should lead to improve the current seismic design criteria included in design codes and recommendations to properly take into account the potential coupling among seismic and environmental hazards.

Negli ultimi anni la valutazione delle prestazioni sismiche di un qualunque sistema strutturale durante la vita utile di servizio è diventato un tema di primaria importanza nella comunità scientifica. In particolare, nell'ambito del Life Cycle Assessment, una corretta stima dell'influenza della pericolosità ambientale sulle performance sismiche di una struttura riveste un ruolo fondamentale, poiché l'insorgere di fenomeni di degrado, agendo sulle caratteristiche di resistenza e duttilità degli elementi strutturali, può indurre meccanismi di collasso fragili (del tipo weak column-strong beam) non considerati in fase di progettazione. Nel presente studio si analizzano le prestazioni sismiche di strutture prefabbricate multipiano soggette nel tempo ad una corrosione indotta da cloruri; l'approccio seguito rientra nell'ambito del Performance-based assessment, con particolare riferimento alla metodologia PEER, la quale offre un efficace strumento per condurre una completa analisi di rischio su di una qualsivoglia struttura. Poiché i fenomeni coinvolti in tale processo presentano una variabilità intrinseca, una corretta stima dei loro effetti non può prescindere da un approccio di tipo probabilistico. In letteratura esistono diversi metodi di indagine, e tra questi la simulazione numerica basata su analisi Monte Carlo è sicuramente la metodologia più corretta, sebbene la più onerosa da un punto di vista computazionale. Al fine di ridurre la dimensionse del campione da analizzare, si adotta quindi una tecnica di campionamento avanzata, che prende il nome di Latin Hypercube. Infine, si presenta una possibile strategia di riduzione del rischio sismico, che prevede la sostituzione del calcestruzzo standard con un composto cementizio ingegnerizzato, denominato ECC, nelle zone dove è prevista la formazione delle cerniere plastiche. La scelta di tale materiale dipende dalla sue caratteristiche peculiari, che lo rendono particolarmente "attraente" nelle applicazioni sismiche ed in presenza di ambienti aggressivi.