*2.4. Case-Study: Tapiola Bridge*

The highway overpass in the Tapiola district of Espoo, Finland, was built in the Spring of 2019 and was recently opened to traffic (Figure 8). Two of its three spans are stress-laminated timber decks compressed by steel bars in the transverse direction. This short-/mid-span highway crossing serves local car and bus transportation. In the width direction, the bridge deck is composed of 46 timber beams. The widths of the beams are 0.215 m, and the heights equal to the deck thicknesses are 0.765 m for the 13.45 m span and 1.035 m for the 22.13 m span. The width of the decks is 9.89 m, which is near to its useful width 9.79 m. The basic dimensions of the bridge are listed in Table 1.



Five integrated humidity-temperature sensors, two displacement and two force sensors were installed on the bridge. The displacement sensors monitor vertical and horizontal motion, while the force sensors are measuring the tension force variation in the steel bars. In addition, the monitoring unit cabinet has two thermocouples for tracking its inside and outside temperature. The sensors are described in Table 2. Figure 7 shows the locations of the sensors. More details on the sensor locations are provided in Figures A1 and A2 of Appendix B.


**Table 2.** The installed sensors in Tapiola Bridge deck.

**Figure 7.** Tapiola Bridge. Photos of the sensor locations: (**a**) from the bottom; (**b**): from the lateral side. See the details about the locations of all sensors in Appendix B.

The sensors have wired connections to the monitoring unit. The unit itself is placed in the metal cabinet on the abutment of the bridge and it is connected to the electric grid and the internet. The devices and programmes of the unit are shown in Table 3.

Before the installation, the sensors were tested in the humidity-temperature controlled rooms at VTT Technical Research Centre of Finland Ltd (VTT). Only the temperature sensors (thermocouples), located inside and outside of the measurement enclosure were not calibrated, because they have lower precision requirements.

**Figure 8.** Tapiola Bridge. (**a**) The scheme of the transverse prestressed glulam wooden slabs of the bridge where T2, T3 and T4 indicate the support, reproduced from [30] with permission from VTT publications. Detail A is shown in Appendix B. (**b**) A picture with the view of the bridge.

**Table 3.** The devices and software of the monitoring unit.


The force sensor calibration was performed with the test rig of VTT, and a calibration factor of 0.95 was found. The current measurement system is able to record the relative force shift/change of the pre-tension bars. Displacement sensors were not explicitly calibrated, instead the manufacturer's instructions and precision requirements are followed [29].

The deck is protected with Valtti colour, an oil-based wood strain produced by Tikkurila [35]. According to the producer, this paint exhibits a low vapour resistance.

The FEM model for the deck is a 3D slice having a width of 107.5 mm (half of the lamination), height 1035 mm and thickness 5 mm. The numerical analysis of the Tapiola Bridge deck starts at the end of the construction time (April 2019) until October 2020, see the weather data in Figure 9, while the sensor-based monitoring started later (October 2019) and is on-going. The earlier starting of the numerical analysis demonstrates that the numerical models can assist the monitoring by predicting the hygro-thermal response of the SLTD also in the absence of measurements. The initial relative humidity and temperatures are equal to those of the air (*RH*0 = 65%, *T*0 = 0.85 ◦C) and the initial moisture content in equilibrium with *RH*0 is *MC*0 = 15.3%.

**Figure 9.** Tapiola Bridge. Weather data measured between 2019 and 2020.
