PSI - Issue 78

Florence More et al. / Procedia Structural Integrity 78 (2026) 944–951

947

typically used for their sensitivity to high-frequency signals (above 50 kHz), detect these emissions. The AE data can help in localizing damage sources such as micro-cracking, degradation due to moisture fluctuations, fungal attack by comparing the arrival times of signals at various sensors (Tu et al. 2022). Ultrasonic methods, on the other hand, are active techniques involving the artificial generation of stress waves (often through mechanical impact or piezoelectric actuators) and the measurement of their propagation characteristics, such as speed and amplitude, across timber components. These methods can help to detect internal defects like cracks or delamination and estimate material properties such as density or the modulus of elasticity. For accurate readings, proper sensor coupling with the timber surface is critical, often requiring the use of gels or springs to maintain pressure. Advanced forms like ultrasonic tomography can generate internal images of structural elements but are less compatible with continuous SHM systems due to their complexity and high data demands. Another important SHM method is Operational Modal Analysis (vibration-based monitoring) , which evaluates changes in a structure’s dynamic properties, particularly modal frequencies and mode shapes, caused by damage or deterioration of the structural elements, thus affecting the service behavior (Rainieri et al. 2013), whose explanatory application to wooden beams can be found in Choi et al. 2007. Sensors like accelerometers (including piezoelectric, piezoresistive and capacitive types) are attached to the structure to measure responses to ambient or forced vibrations. When damage alters the stiffness or mass distribution of the structure, it leads to detectable shifts in these modal parameters (Cieri et al. 2024). Vibration-based systems are especially useful for global monitoring, where damage location is not known in advance, although higher accuracy in localization requires dense sensor networks and different type of measures and data (Rainieri et al 2019). Optical and photogrammetric techniques are non-contact methods used to monitor surface deformations, displacements and crack development. These include Terrestrial Laser Scanning (TLS), Digital Image Correlation (DIC) and photogrammetry, which generate 3D surface models and track geometric changes over time. These techniques are particularly effective for detecting moisture-induced deformation, surface cracking, creep, delamination and other geometrical distortions caused by mechanical or environmental stress. Such methods are highly effective in assessing large areas and external deterioration without physically interfacing with the structure, although their utility for internal damage detection is limited (Sieffert et al. (2016). Fibre-optic-based methods, especially those using Fibre Bragg Grating (FBG) sensors (Hamann et al. 2013), represent another promising technology. These systems are highly suited for embedding directly into timber elements during production (especially engineered wood products like glulam and CLT), allowing for real time monitoring of strain, temperature and deformation. A key requirement in successful SHM deployment is the careful selection of the sensor types, placement, measurement intervals and data processing strategies. The system must be robust enough to endure environmental conditions and provide reliable long-term measurements. Advanced SHM systems may include actuators, real-time data transmission modules and even energy harvesting capabilities, though most current systems are either battery-powered or require direct power sources. Finally, while SHM offers powerful tools for ensuring timber structure safety, the report emphasizes that it should complement, not replace, robust structural design and regular maintenance. SHM cannot correct poor design or misuse but can effectively track their consequences and alert engineers to potential risks before catastrophic failure occurs. 3.2. Real-world applications Either ancient and modern timber structures are challenging regarding durability, environmental sensitivity and the demands of long-term maintenance (Arda et al. 2019). These challenges are significant under dynamic loading conditions and varying climatic influences. In this Section a brief overview on recent developments in SHM of timber is discussed, and some relevant activities quoted in Table 1. First studies on SHM applied to timber structures arises in the field of the heritage timber buildings aimed at understanding their dynamic behaviour, especially under seismic and environmental loads. A pivotal study conducted by Hayashi et al. 2010 examined the Goei-do Hall in Kyoto through ambient vibration and seismic monitoring. Their findings revealed that natural frequencies in timber elements are highly sensitive to humidity, with seasonal variations of up to 5%, while torsional behaviour emerged as a critical mode of response during seismic activity. These observations underscored the need for multi- directional monitoring and validated the material’s elastic recovery post excitation, key considerations for the preservation of historic timber architecture. Studies focusing on engineered timber systems are also available in the technical literature. Leyder et al. 2015 conducted a long-term monitoring of a hybrid timber structure, combining Laminated Veneer Lumber (LVL), Cross-Laminated Timber (CLT) and post-