Structural health assessment of historical timber structures combining non-destructive techniques: The roof of Giotto's bell tower in Florence

Examining the timber structure supporting the roof of Giotto's bell tower in Florence, this paper discusses the combination of non-destructive (NDT), wave- based methods for the characterization of timber structural elements. In particular, it analyses the on-site applicability of selected wave-based techniques for the identi- fication of decay, damage, defects, and moist areas in structural timber. The applied NDT techniques are IR termography, microwave reflectometry, time-of-flight tomography, and mapping by means of elastic waves. Experimental results are compared with data obtained by means of consolidated NDT techniques, such as the electric resistance method for moisture content estimation and the drill-resistance test for decay characterization. These can be considered non-destructive, although they are invasive. The wave-based NDT methods are found to be a value-added complement to routine methodologies for a holistic diagnosis of timber members. Normal practice based on visual inspection, decay detection through point measure- ments of drill resistance, and moisture content estimation through local readings of electric resistance, can be substantially improved through full-field, multi-sensor, multi-resolution imaging. Nevertheless, while the diverse NDT methods illustrated here can be useful for screening large areas in a completely non-invasive way, local measurements (i.e., the drill resistance and the electric resistance measurements) are still necessary.