Behaviour and modelling of the inelastic response of concrete and steel-concrete infrastructures subjected to low-cycle fatigue

Nowadays, infrastructures are of strategical importance for allowing communication between countries. Owing to its usefulness, the design and the maintenance of bridges, streets and tunnels, which represent the network, become a fundamental issue. In order to investigate the behaviour of infrastructures under different loads, such as gravity, seismic phenomena, thermal differences, and so on, appears essential a comprehensive experimental campaign on scaled and full-scale specimens. In particular, in order to guarantee the safety of citizens, the seismic response of infrastructures under an earthquake requires a careful evaluation of the level of damage of structural elements. In this thesis, typical case studies are considered, such as a concrete tunnel lining and a composite steel-concrete bridge. In the first part of the thesis, a typical concrete tunnel lining is analysed. In order to investigate the inelastic behaviour of a concrete circular tunnel, several tests were performed. In greater detail, the best Fiber Bragg Grating (FBG) package configuration was obtained by means of monotonic and cyclic tests on substructures. Based on these results, the resulting suitable configuration in a full-scale tunnel test was used to measure deformations with high accuracy. Cyclic test on the full-scale tunnel provided data on the damage of reinforcing concrete and the developing of plastic hinges. With the aim of providing information on the structural safety of a tunnel after an earthquake, a damage index was calculated. In this respect, a nonlinear fiber F.E. model in the OpenSEES environmental was developed. This model calculated the stress in terms of bending moment in concrete sections with the use of experimental curvatures measured by FBGs system. Finally, the damage evolution in the concrete tunnel was reported and commented. In the second part of this thesis, a composite steel-concrete short-medium span bridge is treated. The innovation was the application of the PEER Performance-Based Earthquake Engineering (PBEE) to this type of bridge. Moreover, the use of the Hot-rolled (HRS) steel to manufacture I-girder beams has become an innovation in civil infrastructures in Europe, as much as the use of transversal concrete cross-beams (CCBs) to connect spans. With reference to the hazard selected, a suitable case study was chosen. With the aim of understanding the most critical and stressed parts of the case study, preliminary elastic shell and stick models were developed. After the identification of interesting parts, half-scale subassembly specimens were designed and built. Several quasi-static tests, both monotonic and cyclic, were carried out with the objective of exploring global and local mechanisms in the section owing to low-cycle fatigue phenomena. To detect damage in the connection detail, a refined F.E. model in ABAQUS was developed. Fragility curve parameters of the damage's interest quantities were obtained by fitting experimental and numerical data by means of the Maximum Likelihood Estimation method. The results and the numerical model could be ready for the application of the Performance-Based Earthquake Engineering tool, in which decision variables, such as repair costs, downtime, human life loss and lane closures, were taken into consideration in order to increase the confidence in the design for both engineer and owner's viewpoint.