PROBABILISTIC TREATMENT OF UNCERTANTIES IN SEISMIC ANALISYS OF STEEL TANKS IUNIO IERVOLINO, GIOVANNI FABBROCINO, GAETANO MANFREDI Department of Structural Analysis and Design University of Naples Federico II ABSTRACT Seismic Risk assessment of industrial facilities requires integrated procedures to probabilistically quantify human, social, environmental and economical losses due to relevant accidents induced by earthquake action. Easy to manage tools for failure probability of equipments evaluation under seismic actions can be used in total risk evaluation of existing hazardous plants, Structural damage state getting probability calculation for each component is the first step for a subsequent consequences evaluation. Since the structural risk is related to site dependent seismic hazard and structural vulnerability, it is worth optimising distinct procedure for those quantities. In the present paper treatment of randomness sources is discussed with specific reference to steel tanks for oil storage in seismic environment. Failure modes and resistance formulation are analysed for this widely present and standardized plant components. Seismic vulnerability is considered as a time-variant reliability problem. Weight of uncertain parameters on seismic response and failure probability is discussed. KEYWORDS: seismic reliability, steel tanks, risk analysis, fragility. INTRODUCTION Risk analysis of industrial facilities consists in evaluation of potential losses related to relevant accidents, investigating short and mid-long term health and environmental consequences of industrial accidents. Pollution, toxic releases, fires and explosions are just few effect of structural collapse of equipment of risk plants. Seismically induced accidents can be very relevant and are triggered by structural failures of single components. Collapse of a plant subsystem can extend the accident involving near components or plants; this mechanism is called “Domino Effect”. Recent national and international rules require detailed quantitative analysis of risk in existing and new plants containing or treating hazardous materials [1]. Total risk evaluation taking into account domino effects and mitigation measures are required, but no detailed procedures or target levels of risk are provided mainly because general purposes suitable techniques have not been already established. Anyway the target of a risk analysis due to a given cause is well known: it is just the probability of a given system to do not survive to all the possible occurrence of the considered source of damage; in other terms it is one minus the probability that the considered system complete its mission successfully. Due to this top-down definition risk have to be related to a time period, that can be arbitrary, but generally is coincident with the design service life of the facility and/or of its main components. It is worth noting that if deterioration mechanism and/or programmed repair intervention are scheduled during the system life they must be taken into account and can consequently affect the failure probability of the system. Since the risk is just quantification of a failure probability, which is basically a non-dimensional quantity, it can include several failure sources (even airplane or meteorites accident). Events algebra allows keeping separate procedures for each considered mechanism and then combining the results. This is why seismic risk is a failure probability also (related to a specific failure source that is earthquake) and can be treated by it. In the simplest way it can be considered as the combination of structural vulnerability and seismic hazard of the location site. These two terms are probabilities also. Seismic hazard depends on ground motion source of the site and it must be considered as a given data in risk analysis since cannot be reduced. On the other hand, structural vulnerability is the core-topic of seismic risk analyses and can be very difficult to address since it depends on the dynamic behaviour of the structure subjected random excitations and on the limit states defined for each component or for the whole system. Vulnerability 1