INVERSE TECHNIQUES FOR MODEL IDENTIFICATION OF MASONRY STRUCTURES
Many old masonry structures still in use need to be assessed considering the safety requirements proposed by current building codes. To this aim, the use of detailed numerical models, allowing the description of complex mechanical phenomena such as anisotropy, softening and crack propagation, could become tremendously beneficial. Although a large number of material models with different degrees of complexity have been recently proposed in the literature, the calibration of the model material parameters, which is critical for an accurate response prediction, has received remarkably less attention. This has resulted in a gap between the potential of these models and their actual use. This thesis is aimed at developing a consistent approach for the calibration of numerical model parameters for masonry structures by means of inverse techniques. After an introduction and a literature review discussing current numerical modelling strategies for masonry structures and the Inverse Problems theory, the optimisation tool utilised in this thesis is described. It makes use of Genetic Algorithms, and it has been previously applied to a number of different problems, from optimal design to the identification of base-isolated bridges. One of the major challenges of Inverse Problems is handling the noisy data and assessing the uncertainty of the results given by the optimisation algorithm. A numerical strategy for choosing the optimal sensor layout is proposed, aimed at minimising the influence of the noise in the calibration. Uncertainty in the calibration has been considered in all numerical and experimental examples studied, leading to an approach based on the use of a set of numerical validation models. This allows a realistic evaluation of the field of applicability of the estimated parameters. The approach has been applied to a novel in-situ test for the calibration of the material parameters for a mesoscale masonry representation. The proposed setup has been designed for low-invasive in-situ experimental static tests on existing structures. In the test, flat-jacks are utilised to apply a specific stress state within a masonry wall. The results, in terms of displacements recorded by LVDTs or optical devices are then analysed by means of inverse techniques in order to estimate the basic parameters of the interface elements simulating the mortar joints. The effectiveness of the test set-up has been studied applying analytical formulations and using the results of numerical simulations. The results of an experimental campaign based on the use of the proposed experimental procedure are then reported and critically discussed. The thesis ends with the conclusions, where it is confirmed that the proposed calibration approach can be effective adopted in the characterization of masonry in existing structures. Finally some directions for the future research related to this work are proposed.