**1. Introduction**

Timber and engineered wood have increased their popularity as structural materials thank to their outstanding environmental performance, competitive price, mechanical properties, and relatively easy handling. However, the use of wood in unsheltered bridges is rather limited because of the exposure to the harsh climate conditions. Designers and structural engineers are mostly worried about the service life of the load-carrying structures which is recommended to be one hundred years in Europe [1].

Although evidence exists that structural wood can retain its strength through many centuries [2], it is very sensitive to the variable temperature (*T*) and moisture content (*MC*) which may lead to the material degradation and loss of its structural performance [3]. In some cases, the biotic damage can grow from inside out, and therefore the proper monitoring of internal material condition is essential in wooden bridges.

Stress-laminated timber decks (SLTDs) are composed of wood lamellas placed longitudinally between the supports of the bridge and compressed together with preloaded steel bars in the transverse direction (see [4] and the related references). This technology was developed in Canada in 1976 to replace nail-laminated wooden decks, which delaminated under cyclic loading and moisture variation. The first stress-laminated bridges were built in North America in 1980. The technology was adapted in Europe in mid 1980s and it was introduced in Australia, Japan and other countries since 1990. The greatest advantage of

**Citation:** Fortino, S.; Hradil, P.; Koski,K.; Korkealaakso, A.; Fülöp, L.; Burkart, H.; Tirkkonen, T. Health Monitoring of Stress-Laminated Timber Bridges Assisted by a Hygro-Thermal Model for Wood Material. *Appl. Sci.* **2021**, *11*, 98. https://doi.org/10.3390/ app11010098

Received: 29 November 2020 Accepted: 22 December 2020 Published: 24 December 2020

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laminated decks is that they form a stiff and solid base for the pavement, and therefore can redistribute the external loads to their supports. This effect is due to the prestressing action of the high-strength steel bars that squeeze the wooden lamellas together. The bar force, measured by using load cells, is typically from 89 to 356 kN [4].

Even though many of the originally built stress-laminated decks are performing well over three decades, it is essential to avoid errors during the construction and maintenance of the bridge. For instance, Scharmacher et al. [5] reported that blistering between wood and asphalt surface may occur, because of high *MC* of the deck and elevated asphalt temperature. This will affect the performance of the shear connection, but may also create conditions for water accumulation or ice formation under the asphalt surface.

Since the stress-laminating technology was developed in Canada and the northern parts of the United States, the effect of freezing temperatures has been thoroughly examined [4]. Laboratory tests revealed significant decrease of the bar forces of deck sections placed from a temperature of 21.1 ◦C to temperatures below zero ranging between −12.2 ◦C and −34.4 ◦C, strongly depending on the *MC* of the wood. Therefore, Wacker [4] recommends thermal design considerations in cold climates such as Alaska and Canada. This recommendation should also be applicable to the Nordic countries with similar weather conditions. Apparently, the simplest thermal design consideration is to keep the *MC* low in winter months to prevent the loss of pre-loading forces in the high-strength steel bars.

During the last decades, the development of timber bridges in European Nordic countries has been promoted by the joint effort of road authorities, timber industries and research organizations. A result of this cooperation was the Nordic Timber Bridges Programme [6]. Part of the activities under this programme was monitoring the longterm behaviour of wooden bridges in Norway financed by the Norwegian Public Roads Administration. Five of the monitored bridges in Norway have SLTDs and are located in Evenstad, Daleråsen, Flisa, Sørliveien and Måsør [7]. The bridges are built between 1996 and 2005 and are typically multi-span structures with glue laminated arches or trusses as the main load-carrying system. All of them have similar deck composed of 48 × 233 mm lamellas treated with creosote excepting the footbridge in Sørliveien (Figure 1), which has a deck of untreated spruce and a deck height of 333 mm, made of vertically sawn glulam beams. The SLT deck protecting the whole bridge structure is shown for Sørliveien Bridge (Norway) in Figure 1a. In addition, Figure 1b shows a detail of the SLT deck with the view of the wood lamellas and the steel bars for the same bridge.

**Figure 1.** Sørliveien Bridge. (**a**) Side view of the whole bridge structure. (**b**) Detail of the stress-laminated timber (SLT) deck protecting the bridge.

The efforts to promote timber bridges continued also after the end of Nordic Timber Bridges Programme. For instance, the Wood Building Programme (2016–2021) was launched in Finland as a governmen<sup>t</sup> undertaking to increase the use of wood in urban development, public buildings, bridges and halls [8]. The programme is also seen as an efficient way of attaining the energy and climate targets to reduce Finland's carbon footprint by 2030. However, the number of laminated wooden deck bridges for vehicle traffic in Finland is still relatively small. One such bridge, carrying significant vehicle traffic, is the highway crossing recently erected in the Tapiola district of the city of Espoo. The Tapiola Bridge is now being permanently monitored under the supervision of the Finnish Transport Infrastructure Agency.

In addition to the durability problems, a common effect of moisture variation in SLTDs is the cupping deformation, which is usually measured as the uplift at the corner in the bottom surface of the deck [9]. A sharp increase of cupping is usually observed during wetting and only a partial decrease during drying. For the details, the reader is referred to Section 4.3 of the Durable Timber Bridges report [9].

The above review about performance of bridges shows that control of the *MC* in wooden parts is not only essential for the durability of the material, but for the whole superstructure as well. Variation of the *MC* directly affects structural integrity, serviceability and loading capacity of the bridge. Therefore, the monitoring techniques have a fundamental role in controlling the health of large structures exposed to outdoor climates, such as timber bridges. However, measurements obtained by the usual monitoring techniques based, e.g., on integrated humidity-temperature sensors, provide hygro-thermal measurements only in specific locations of the wood components.

As shown in the recent literature [10–12], advanced multi-phase models are an effective tool to assist the hygro-thermal monitoring of timber bridge components such as glulam beams. Compared to the single-phase (or single-Fickian) models for transient moisture transport in wood [13–15], where the *MC* is the only variable of a Fick's second law equation, the multi-phase models below the fibre saturation point (FSP) analyse two different water phases, i.e., the water vapour in lumens and the bound water in wood-cell walls. Starting from the seminal works of Krabbenhøft [16] and Frandsen [17], there was a strong effort to develop a multi-phase theory (often called multi-Fickian) for moisture transport in wood that includes the conversion rates between the different water phases. The multi-Fickian theory below the FSP is based on the identification of three phenomena occurring in cellular wood during moisture transfer, i.e., the diffusion of water vapour in the lumens, the sorption of bound water and the diffusion of bound water in the cell walls. In the multi-phase models available in the current literature, the two water phases are separated and the coupling between them is defined through a sorption rate [10–12,17–20]. Recently, Autengruber et al. [21], developed a whole multi-Fickian model including also the transport of free water in the lumens above the FSP. Therefore, in addition to the sorption rate between the two phases of water vapour and bound water, also the sorption rate between the free water and bound water phases, as well as the evaporation/condensation rate between the free water and the water vapour phases, need to be defined. These phenomena are schematized in Figure 2. For a complete description of the moisture transfer in wood, a sorption hysteresis characterized by two isotherms of adsorption and desorption was originally introduced in the multi-Fickian model by Frandsen [17]. In the present work, only case-studies with moisture states below the FSP are studied.

**Figure 2.** Scheme of the water phases and sorption phenomena in wood. (**a**) Below the fibre saturation point (FSP): bound water in the wood cell walls, water vapour in the lumens and sorption rate between bound water and water vapour phases (.*cbv*). (**b**) Above the FSP: Bound water in the wood cell walls, water vapour and free water in the lumens, sorption rates between bound water and water vapour (.*cbv*) and between water vapour and bound water (.*cwb*), and evaporation/condensation rate between free water and water vapour (.*cwv*).

As discussed by Svensson et al. [22] and Fragiacomo et al. [13], high values of moisture gradients due to high yearly and daily variations of relative humidities are the main causes of moisture induced stresses (*MIS*) perpendicular to the grains in wooden members. Larger yearly variations of *RH* (average values above 80%) and larger moisture gradients and *MIS* in timber cross sections were found under Northern European climates when compared to Southern European climates [13]. In [10,11] it was observed that, under Norther climates, high gradients in the vicinity of surfaces of bridge glulam beams during drying periods of the year are caused by high peaks of *RH* (above 85%) in conjunction with high daily variations of *RH* (above 50%). Knowledge on moisture gradients is therefore important to identify the zones prone to crack risk in wooden components, as shown in [12] for the case of a bridge glulam beam where the *MIS* were also calculated and discussed in relation to the moisture gradients. In [12] it was found that the most critical *MIS* are the tensile stresses perpendicular to the grain that can be also greater than the limits prescribed by the Eurocodes. Due to this, the uncoated bridge wooden beams may be exposed to the formation of moisture induced cracks and delamination. The structural significance of cracks in timber bridges under outdoor environments is discussed also in [23] where the asymmetric damage (longitudinal splitting cracks) is especially investigated.

Models for moisture transport, coupled with mechanical models, can be used to calculate the moisture induced cupping in stress laminated timber decks. The models can also allow the evaluation of the bar force losses during time, as shown in Section 4 of [9], were a single-phase model for moisture transport was used.

The novelty of the present paper is the use of a recent multi-phase model, proposed by some of the authors in [11], to assist the monitoring of SLTDs of bridges under Nordic European climates carried out by integrated humidity-temperature sensors. In particular, the hygro-thermal monitored data are collected from a previous study of the untreated deck of Sørliveien Bridge in Norway [24,25] and from the on-going monitoring of the painted and thick deck of Tapiola Bridge in the city of Espoo, Finland. Untreated and painted bridges are interesting cases to study in terms of their hygro-thermal performance. In this paper, the monitoring systems of the Sørliveien and Tapiola bridges are presented, and selected measurements are used for simulation by the finite element method (FEM). While the monitoring provides the *RH* and *T* in some locations of the analysed decks, the numerical model completes the health monitoring providing the overall hygro-thermal response of a representative volume of the deck in terms of distribution of *MC*, vapour

pressure and *T*. In particular, the hygro-thermal response of the bottom deck, which is more affected by the external climate, is investigated.
