## 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 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, [PERSON] 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 \({}^{\circ}\)C to temperatures below zero ranging between \(-12.2\)\({}^{\circ}\)C and \(-34.4\)\({}^{\circ}\)C, strongly depending on the _MC_ of the wood. Therefore, [PERSON] [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 long-term 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, Dalerasen, Flisa, Serliveien and Maser [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.