Investigation strategy for structural assessment of historic towers

Historical masonry towers are relevant architectural heritage often in a strategic position within city centres. Their height and position require specific controls in order to define the state of preservation. The paper describes the investigation procedures developed by the authors in selected case studies. According to the timing and to the complexity of the structure, the approach requires preliminary visual inspections, geometric, crack pattern survey supplemented by historical research and stratigraphic survey. Operational modal testing evaluates the overall structural behaviour, indicating eventual local (or global) problems to study in depth by monitoring or further local tests. Emergency operations, such as controls after earthquakes, could require prompt procedures. In this case, the combination of visual inspection, geometric and damage survey with dynamic testing is a reliable procedure for structural assessment. Additional investigation increases the knowledge of local problems or gives information for further activity such as structural modelling. For instance, relevant data are the evaluation of the masonry quality or the control of the local state of stress to estimate through non-destructive or minor destructive testing in selected positions. Nevertheless, such activities require accurate projects of the investigation too, planning and localising several tests in order to solve the problems detected in the preliminary steps of the diagnosis process.