PSI - Issue 78

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

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properties of the structural elements and then compromises the load-bearing capacity under both service and earthquake conditions (Steiger et al. 2017). Moreover, increased moisture can deteriorate the mechanical connections thus exacerbating the seismic vulnerability during the earthquakes. Moisture also creates an environment conducive to fungal attacks, which are another critical threat to timber elements. Fungal decay, categorized as brown rot, white rot, or soft rot, leads to the degradation of wood’s cellular structure. Brown rot causes brittle decay by breaking down cellulose, while white rot attacks lignin, resulting in a stringy or spongy texture. Soft rot targets high-moisture zones and compromises wood integrity without significantly affecting lignin (Tannert et al. 2011). In the context of earthquake safety, such biological degradation can result in localized weakening, particularly around joints and load-bearing zones. Several studies have shown that fungal attacks, exacerbated by high humidity and poor ventilation conditions, can silently reduce the cross-section of structural members in concealed areas, compromising their seismic resistance.

Fig. 1. Typical deterioration phenomena of timber elements: Termite attack (a&c), Fungi attack (b&e), Moisture attack (d) Termite infestations represent another biological threat, causing significant structural damage by consuming cellulose and hemicellulose components of timber and then the resisting cross-section area of the elements (Fig. 1). While termites are often associated with ground-level elements, they can easily migrate through cracks or joinery pathways into roofing structures. The resulting biodeterioration leads to substantial reductions in mechanical strength. In ancient large-span roofs where the (carpentry) joints of the structural elements are designed without redundance such degradation can lead to sudden collapses, especially under seismic loading. Detection of termite activity is challenging, particularly in hidden structural zones. Lastly, weathering due to climatic exposure, primarily ultraviolet radiation, temperature variation and air humidity, leads to photodegradation of timber surfaces. Though superficial, such degradation compromises the aesthetics and the surface integrity of timber. More importantly, weathering increases surface roughness and cracks, which serve as entry points for moisture and pests. Cracking due to weather-induced shrinkage and expansion can compromise continuity of the structural members, causing out of service and making them vulnerable to earthquake-induced actions when stresses concentrate around the defects. Moreover, weathering increases timber’s wettability, further enhancing its susceptibility to decay and biological attack. 3. Sensors, techniques and real applications of SHM in timber structures: recent developments 3.1. Sensors and Techniques SHM in timber structures is expected a systematic approach for detecting and tracking deterioration or damage in real time to ensure the safety, durability and serviceability of the structure. Given the complex characteristics and physical behavior of wood - such as anisotropy, high moisture sensitivity and significant natural variability - developing effective SHM systems for timber demands a tailored and often hybrid strategy that combines multiple measurement and sensing techniques. The core of SHM systems lies in the use of sensors and data acquisition tools that can detect changes in physical, mechanical, or geometrical properties over time. A primary category of SHM techniques includes wave- and vibration-based methods, also referred to as acoustic methods. These are subdivided into two main types: acoustic emission (AE) and ultrasonic testing. AE is a passive monitoring technique that captures stress waves naturally emitted from micro-cracking or other internal damage within the wood. Piezoelectric sensors,