Post-construction vertical deformation in post-tensioned wooden structures – Lighthouse Joensuu
This article examines post-construction vertical deformation in the post-tensioned wooden structures of Lighthouse Joensuu. This article is part II of a series, the first part of which appeared in Puu-lehti 3/19 and dealt with the vertical deformations that occurred during construction and finishing work . The world’s tallest wooden residential apartment building with tendon steel stiffening was completed in 2019 in the Penttilä district of Joensuu. Commissioned by the student housing organisation Opiskelija-asunnot Oy Joensuun Elli, the 14-storey Lighthouse Joensuu houses 117 student apartments and is made entirely of wood except for the ground floor.
Text: Riku Hirvonen
Read the article in Finnish here.
The combined total vertical deformation at Lighthouse Joensuu is 20.9 mm. Since the lowest floor of the building is concrete, the vertical deformation per floor is 1.60 mm. The total vertical deformation after construction is 5.9 mm when the vertical deformation during construction is subtracted from the total. This means the post-construction vertical deformation per storey is 0.45 mm.
The structural system and tie rods
The structural system selected for the project was a large element system, where LVL-X wall elements and CLT horizontal elements serve as the load-bearing structures. These solid wood elements were prepared for installation on site under a shelter set up specifically to provide protection from the weather. Once weather conditions permitted, the prepared elements were lifted up and installed. During construction, the building was protected from the weather by means of movable roof elements. Moisture management was handled using the Kuivaketju10 (Dry Chain 10) operating model.
The building has relatively small stabilising loads, which is why the building is stiffened with post-tensioned structural components. Horizontal loads in Finland are mainly caused by the wind. In areas prone to earthquakes, buildings must be designed for both seismic and wind loads.
The selected system was originally developed to strengthen concrete buildings in areas with earthquakes. However, this system has since been adopted and developed for wooden buildings as well . Lighthouse Joensuu uses a post-tensioned system based on the PressLam and Simpson Strong-Tie Strong Rod systems [3, 4]. For more information about the post-tensioned system used, see the publication listed in the sources .
Lighthouse Joensuu uses corner angle plates as connectors for the elements. These plates mainly serve to transfer shear forces between the elements of each storey. They also help prevent the risk of progressive collapse and make it possible to distribute tie rod capacity to the intersecting walls.
Measuring vertical deformation and creep
Vertical deformation in the building was measured with a custom- designed potentiometer style displacement sensor. Measurements are being taken in two phases with the first lasting about four months and the second still ongoing.
Displacement sensors installed across three storeys were used for measurements in the first stage. The exception was the 10th floor, which was measured separately. The combined results from the sensors indicate the total vertical deformation in the building. The sensors were installed in the middle of wall elements in the elevator shaft to take measurements in the first stage.
One sensor was installed in an outlet pipe in a stairwell wall to collect measurements over the long term.
Factors affecting vertical deformation
Long-term vertical deformation mostly stems from changes in the moisture content of large elements, compression caused by the load, and creep in the wood materials. Research indicates that variation in moisture content is the single biggest factor in the long-term vertical deformation of wooden structures . However, the size of this moisture effect varies depending on the material and structural system used. The deformation coefficient for moisture is significantly higher perpendicular to the wood grain when compared to the deformation coefficient in the direction of the grain. If load-bearing wood structures have a grain perpendicular to their orientation, which is the practice in the Platform structural system, for example, the deformation caused by moisture is relatively large. The vertical loads in Lighthouse Joensuu are transferred along the LVL-X element walls as so-called hard joints without vibration isolation.
Improper moisture management results in greater vertical deformation when the heat in the building is first turned on as the moisture content of the wood seeks to match the surrounding conditions. These dimensional changes caused by shrinkage and swelling are apparent in the measurements taken at Lighthouse Joensuu after construction. However, these changes are relatively small because the moisture of the LVL-X elements is only in the range of 8-12% upon delivery and because moisture protection during construction was planned pre-emptively .
The construction phase was still on-going during the first measurement phase (2018-2019), which meant the building was not completed and was missing some of its eventual total mass. The deformation in this initial phase mostly stemmed from the tightening of the tie rods, an increase in the total load on the building, and the element deformation caused by these factors . The total vertical deformation measured in this phase was about 15 mm.
The second measurement phase began in spring 2020 and is still on-going. Diagram 2 illustrates the variation in the total vertical deformation for this period. The maximum vertical deformation is -5.9 mm. Diagrams 2 and 3 also illustrate that there are large changes in the vertical deformation measurements over the seasons. The vertical deformation was about -2 mm in June 2020. However, this value was up to +0.9 mm in October-November of the same year. This means that the overall dimensions of the building increased between June and November. The same pattern can also been seen in 2021. In May of that year, the total vertical deformation was around -4.3 mm, while the equivalent number for September-October was about -1.2 mm. The measurement results find a similar cycle in longitudinal deformation measurements in the building throughout the measurement period.
The changes in shape are probably mainly caused by changes in the wood’s moisture levels. Diagram 3 has an added line illustrating the wood’s moisture balance, calculated using the relative humidity and temperature measured by the sensor located in the outer wall structure. The sensor has been positioned on the inner surface of an LVL element of the outer wall. in other words, the diagram shows the moisture conditions on the inner surface of the element and should therefore only be considered indicative.
Seasonal effects on wooden structures can be seen in the measurement results. When structures dry, vertical deformation in the building increases. On the other hand, wooden structures swell when the moisture increases. While there is a delay in the effect, ambient conditions cause changes in the lengths of structures. When November and December bring drier air, structures dry out and the vertical deformation in the building increases. In contrast, the same structures swell in June-July when absolute humidity increases.
The Karelia University of Applied Sciences has supplied project staff and students to the Lighthouse Joensuu project with the aim of producing and publishing information among other things about the technical functioning of these structures. Riku Hirvonen works at the Karelia University of Applied Sciences as a project coordinator. The R&D activities focus on the themes of wood construction, the environmental effects of construction, and digitalization of the construction industry.
 Keskisalo, Matveinen, 2019. Jälkijännitettyjen rakenteiden painumat, Lighthouse Joensuu. https://puuinfo.fi/ wp-content/uploads/2020/06/PUU_3_kokonaan_low.pdf
 PRESSS (Precast Seismic Stuctural [Systems) https://www.pci.org/PCI_Docs/Design_Resources/Guides_ and_manuals/references/PRESSS/PRESSS-Phase-3_The-Five-Story-Precast-Test-Building_Vol-3-9_Design- Guidelines-For-Precast-Concrete-Seismic-Structural-Systems.pdf]
 Keskisalo, M 2020. Jälkijännitettyjen seinämäisten puurakenteiden suunnittelu: systemoitu kirjallisuuskatsaus. Opinnäytetyö, ylempi AMK. Tampereen ammattikorkeakoulu, Rakentamisen ylempi tutkinto-ohjelma. https://urn.fi/ URN:NBN:fi:amk-2020120426164
 Keskisalo, M. 2018. Use of tension rods in wood construction – 14 storeys with lami-nated veneer lumber as shear walls: Lighthouse Joensuu. Internationales Holzbau-Forum IHF 2018, Garmisch-Partenkirchen, Germany, December 6-7.
 Leivo, Virpi. 2015. Puukerrostalon painuman arviointi.