## 4 Discussion  
The combination of the sensor-based monitoring and numerical model presented in the previous sections, allowed analysing the hygro-thermal response of the uncoated SLTD of Sorliveien Bridge in Norway and the thick painted SLTD of Tapiola Bridge in Finland. In addition to the temperature \(T\) and vapour pressure \(p_{\
u}\), the numerical model was able to provide quantitative values of the moisture content \(MC\) and the moisture gradients trends close to the surface that could be responsible of surface cracks, as shown in earlier works for large gulam beams of timber bridges [10; 11; 12] and in a research on the monitoring of large span timber structures [36]. In particular, we analysed the bottom part of decks which are sheltered from rain but subjected to both the continuously variable air humidity and temperature. The uncoated deck of Sorliveien Bridge in Norway was analysed as a first case-study, and showed a relatively stable moisture behaviour after one year from the bridge erection. The main findings are listed below:  
* Referring to Table 4, the \(MC\) values on the bottom surface span in a range between 4% (reached at the beginning of summer 2006) and 25.8% (autumn 2005), are highest during the first year of the analysis and more stable during the successive years (Figure 12). The minimum and maximum \(MC\) values at 20 mm from the bottom surface are 12.5% at the beginning of the summer 2006, and 19% at the end of winter 2010. The results show that no significant changes of the average moisture content would be expected after the first year of the bridge service life, i.e., from June 2006 to January 2010. * High levels of \(MC\) over 20% were found only on the exposed surface at the bottom deck and in locations very close to this surface (Figure 12). These \(MC\) levels could be also critical for the wood durability [3], however the decay is a major problem mainly in the presence of liquid water due e.g., to rain and eventual water traps, and these cases were not investigated in the present work. The painted and thick deck of Tapiola Bridge was analysed as second case-study, starting from an earlier stage after construction. The main findings are the following:  
* Referring to Table 5, the moisture contents on the bottom surface varies between 10.8%, reached at the beginning of the summer, and 23% at the beginning of the winter. In the internal locations, the maximum and minimum moisture contents are reached earlier depending on the maximum moisture penetration depth, which is about 200 mm (see Table 5). The maximum and minimum \(MC\) values in the location of the humidity-temperature sensor at 60 mm from the bottom surface are 16.7%, at the beginning of spring, and 14.9% at the end of summer. * High levels of \(MC\) (i.e., \(>\)20%) were found only on the exposed bottom surface and in locations very close to this surface. Compared to the \(M\)Cs of Sorliveien Bridge, the peaks remained below 23% (Figure 15). This is because Tapiola Bridge is protected, even if the used paint has low vapour resistance (see Table A3 of Appendix A). The following observations are based on the comparisons between the two case-studies:
* Considering the \(MC\) results of Sorliveien Bridge, it could be estimated that also for Tapiola Bridge the average values of \(MC\) will not change significantly during the successive years. * The envelope curves shown in Figure 15b indicate similar levels of moisture gradients close to the surface as those of the Sorliveien Bridge's deck (Figure 12b). The average \(M\)Cs are around 16% up to 20 mm depth, and remain at an almost constant level up until 60 mm depth in Tapiola Bridge's deck. Previous hygro-thermal models of SLTDs, based on single-phase moisture transport, found cupping deformations of around 16 mm and steel bar force losses of around 33% at these \(MC\) levels after 15 months [9].