Robustness assessment in reliability terms of reinforced concrete structures in seismic zone

In the past decades structural robustness has become an important issue among civil engineering studies, in the light of what has happened in terms of accidental events like the World Trade Center terrorist attack (New York, 11th September 2001). Normally, buildings are designed to withstand to normal actions, whose types and intensities are defined according to the building’s importance. Nevertheless, when an action is unlike to occur, or when it occurs whit a larger intensity than in normal scenarios, the building cannot resist to this event and a collapse can occur. When the facility is able to avoid disproportionate collapse, if a local damage occurs after an accidental event, the structure is judged robust from a structural point of view. In this work of thesis, a probabilistic approach will be applied to evaluate the robustness of a reinforced concrete building, since a reliability evaluation of the problem can be useful in the contest of robustness analysis, in order to take into account the uncertainties affecting structural performance. The frame under analysis is located in a highly seismic area of Italy, in L’Aquila city. Previous works of thesis have demonstrated that the structure, as it has been designed according to code rules, is not adequate, from a robustness point of view, to resist to an accidental event, like an explosion or an impact, which causes the removal of the central column. This means that different approaches with respect to the ones prescribed by code rules should be followed to enhance structural robustness. The starting point of this thesis is the same frame previously described, with the structural details that have been judged the most appropriate from a robustness point of view. A probabilistic approach has been applied, by sampling materials and loads parameters with a Latin Hypercube Sampling (LHS), one-hundred times. A FEM software, called ATENA 2D, has been used. Two different analysis have been performed, for all the one-hundred scenarios: a pushdown analysis and a static-equivalent non-linear analysis where the removal of the column is simulated. The former was necessary to compute the dynamic amplification coefficients, to be applied to the loads in the second type of analysis, after the column is removed. The latter is needed for the reliability evaluation of the structural robustness. These dynamic amplification coefficients, different for all the one-hundred scenarios, have been evaluated using an approach proposed by Izzudin et al., which consists on performing a pushdown analysis, obtaining the capacity curves and computing the dynamic displacement. This corresponds to the intersection point between the internal and external works. If the equilibrium is not found (i.e. if the curves do not intersect), the structure reaches collapse, which has happened for almost the 50% of the cases. After that, the static-equivalent non-linear analysis was performed for all the one-hundred scenarios, so that it was possible to verify the method proposed by Izzudin; all the scenarios where the equilibrium was not found, did not effectively reach convergence criteria in the probabilistic analysis, which confirmed the initial results. In the end, the strains at different points of the frame have been monitored, both for the concrete and the reinforcement parts, so to evaluate the local probability of failure. This has resulted in a global reliability evaluation.