In order to carry out a study of existing structures, for which no data is available on the mechanical characteristics of the materials used or there are reasons for their reassessment, they must be assessed on site (in situ) or in the laboratory (in vitro). The quantitative determination and qualitative assessment are carried out by combining both direct and indirect methods. The results of these investigations constitute the assumptions of the computational model, with which the adequacy, as well as the dimensioning of the interventions, of the existing structure under consideration is checked.