Structural Modeling Parameter Uncertainties and their Influence on Seismic Reliability Assessment of an Existing RC Structure

The effect of structural modeling uncertainty in reliability assessment of existing reinforced concrete structures can be more significant compared to that of new structures. This is due to uncertainties in determining both the structural details (e.g., number and position of re-bars, number of stirrups in joints and other members) and material parameters (e.g., strength of concrete in each story, yield stress in the bars, and strain-hardening in the steel). This lack of knowledge could be mainly caused by the costs and limitations of in-situ testing prescribed by the codes as well as unavailability of complete original blue-prints for existing RC buildings. These uncertainties are not explicitly accounted for in current seismic codes such as the European Code [1] and the Italian Seismic Normative [2]; these only provide the so-called confidence factors that should be applied to the measured mean strength of materials based on the level of knowledge of the structural system. This work aims to gain insight into relative importance of uncertain parameters, related to structural modeling, on the seismic reliability of an existing RC structure. The relative importance of these parameters is measured in terms of the corresponding level of confidence in the estimation of structural reliability [3]; where structural reliability is calculated by employing simulation-based reliability methods. For each realization, the structural capacity is obtained based on the results of non-linear incremental static analysis (pushover) of the structural finite element model and the structural demand is derived from the code spectrum [2]. The structure selected for this application is the central frame of a 3-story RC plane-frame school located in Avellino (Campania, Southern Italy) designed for gravity loads without moderncode resistance and ductility provisions. The building falls in category II according to the Italian Seismic Normative [2] classifications for the level knowledge about the building, in which, complete knowledge of the geometric configuration and partial knowledge about the detailing of structural members based on original construction details are assumed. In this particular case, various confidence levels associated with the presence of uncertainties in a set of structural modeling parameters, are compared against the current code recommendations and the relative importance of the uncertain parameters are discussed.