Recent progress in diagnostics of civil structures by laser vibrometry: Fid-Bau Portal

Recent progress in diagnostics of civil structures by laser vibrometry

Esposito, E. / Copparoni, S. / Naticchia, B.

One of the main difficulties of drawing maintenance plans, or choosing interventions necessary to stop the growth of pathological mechanisms regards the determination of an objective and repeatable way of assessing the conservation state of civil structures. At present, we still generally proceed by sight analysis, or perform just some measurements in selected spots to derive information about the static/dynamic behavior of the structure. Also the diagnostic intervention is usually done after that some accident occurs and the situation looks dangerous for building users. Through the employ of a really innovative technique for non invasive diagnostics, scanning laser Doppler vibrometry (SLDV), we propose a testing protocol which can be applied to the entire cycle of construction, starting from accepting material in the building site, to arranging a specific maintenance program, and, generally, for a detailed inspection of pathologies. This technique consists in measuring, using laser instruments, the velocity of small vibrations induced in investigated surfaces by suitable excitation sources. The authors have presented a collection of results derived from measurement session done both in the lab and in situ, aimed at verifying the applicability of the SLDV technique to the diagnostics of civil and historical structures. In all cases, the proposed technique has been able to put in evidence a series of problems typical of examined objects, such as wall cracks and loosening plates, beside demonstrating the capability of monitoring structural components in terms of ambient vibrations. Also the fundamental issue of vibration isolation of the measuring laser device has been dealt with, proposing a first simple effective approach. However, as put in evidence by some authors (Haves, 1999, Adams, 2004) the diagnostic process is quite a complicated issue, divided between 'fault detection and fault diagnosis'. Moreover, to affirm that there is a fault a reference state must be known to be compared with the present situation, and also to predict structural behaviour a detailed structural model should be prepared and fine tuned using experimental data. This issue is rather important also in the planning of 'any treatment that could expose the structure to unusual forces, which have the potential to disrupt its stability'. The contribution of the proposed technique to this procedure is important in the sense of supplying accurate data, gathered in a non-invasive manner that completely avoids structural loading and ease also other issues like sensor placement in hazardous environments, but further studies are still needed to optimize it and to prepare a smoother integration in the other phases of the diagnostic process.