Structural diagnostic

This report describes the results obtained by applying an extensive NDTs experimental investigation in order to evaluate the extent of structural damage and their causes. A correct preservation design of a historic structure should start from an accurate diagnosis of the state of damage based on: experimental on-site and laboratory investigacions, and a structural analysis using the result of said invesùgations. These operations will enhance knowledge of the building and help respect its authenticity through an appropriate choice of intervention techniques. The lack of knowledge about the originai materials and structures has frequently been the main cause of unsuccessful interventions. In this context a further problem was found, namely the modality of past intervention on the building with inappropriate techniques. The questions to be answered are: What actually caused the failures? How dìd they develop? and, How serious were theìr consequences to the overall response of the building? Masonry structures in seismic areas are subjected to severa!earthquakes during their lifetime, often followed by partia1 reconstruccion. They can also suffer for lack of maintenance, so that synergetic effects can cause their damage. In order to overcome the gap between our poor knowledge and the complexity of these structures an experimental on-site investigation is required as well as recommended by the Codes and Standards of severa! countries. Non-Destructive Techniques (NDTs) and/or Minor Destructive Techniques (MDTs) can be helpful in detecting hidden features (wall texture, internal voids and Aaws and characteriscics of the wall section), which are crucial to define the structural behaviors under dead loads and environmental action. Before choosing the appropriate NDTs, it is therefore important to know which type of problem has to be solved, i.e. what is to be found by NDT. In the case of ND tests, a correlation between the measured parameters and the mechanical ones is usually difficult, but it can give an overall qualitative indication of the masonry. At present the most diffused ND techniques are sonic (or ultrasonic), radar and thermography tests. Sonic and radar tests seem to be very promising when a diagnosis has to be clone on the state of damage present in inhomogeneous materials; therefore the designer is asked to interpret the results and use them at least as comparative values betwecn different parts of the same structure'smasonry or by using different ND techniqucs.