## 7 Hygro-Mechanical Response of Wooden Cultural Heritage
### Historical Buildings  
Heritage wooden buildings and building components might be exposed to outdoor environments, and thus, they are not protected against precipitation, solar radiation, and wind. Most authors, such as [PERSON] et al. [100], do not take into account those parameters to avoid an overly complex analysis, instead supposing the case of a sheltering situation. In that case, the most harmful threat for heritage buildings' durability is humidity. As explained in Section 1, relative humidity variation induces internal stresses and distortions due to restrained swelling and shrinkage. Hence, buildings and structural elements may lose their integrity, as well as experience changes in mechanical properties [137]. Most swelling and shrinkage events occur in the directions perpendicular to the grains, namely radial and tangential. As described by [PERSON] [138], the moisture-induced stresses are more evident in those directions. The same author also explained that the smallest cross-sections are more sensitive to moisture cycling than larger ones, and the surface protective coating represents an effective protection for those cycles. Those assumptions are demonstrated by the work of [PERSON] et al. [100]. As pointed out in their study, in addition to the size of the cross-section and the presence of a coating, the type of climate is also an influencing factor for wooden components exposed to an outdoor environment. Indeed, the higher induced stresses have been detected in climate zones with the highest values of annual relative humidity variations. Figure 16: (**a**) Strain experienced by a wooden structure mimicked a cabinet (i), and (**b**) strain experienced by a wooden structure mimicked a panel painting (ii), owing to a drop of relative humidity. In (**a**), different curves are referred to with different strain gauges on the same mock-up. Strain gauges (SG) 2, 9, 2, and 15 are fixed on the front and rear of the panel near the cleated end. SG 6 and 19 are fixed to the half-length of the panel. In (**b**), thick refers to two samples with a thickness of 6 mm; thin, instead, refers to samples with a thickness of 3 mm. The red square shows the total length of the \(y\) axis of (**b**) compared to (**a**). From [PERSON] et al. (2018) [70] and [PERSON] et al. (2020) [46]. Joining parts play a key role in the force and load transmission; thus, moisture-induced stresses in those areas may compromise the performance of the whole structure. In agreement with the type of joint, forces and stresses are differently distributed. Moisture content changes in this connection part may increase the stresses perpendicular to the grain [139]. As shown in [PERSON] et al. [140, 141] higher moisture content corresponds to higher values and larger areas of stress, when compared to a dried sample. The effect of moisture might consider a further load to be added to the external mechanical load, so their simultaneous effects induce additional deformations and accelerate the reduction in strength with time. Although some works in the literature are not directly referred to as cultural heritage, they might help with the understanding of wooden timber behaviour in the same conditions, i.e., being subjected to natural climate variations while sustaining loads [137, 142, 137, 143, 28]. Research in that direction might be a useful tool to study the mechanosorptive behaviour of wood in building components and structures [142], firstly, due to the lack of these investigations in the heritage field and, also, because useless samples allow for the performance of experiments which may compromise the heritage integrity [143]. For example, [PERSON] and [PERSON] [144] studied the mechanosorptive strains and stress distribution by cutting a timber log, which is the unit element of important heritage buildings. An example is the Heierstad loft, a log house in Tensberg in Norway, shown in Figure 17a [12]; a detailed view of the logs is shown in Figure 17b.