Design for efficient assembly is essential to further enhance the competitiveness of cross-laminated timber building systems for multi-story timber structures. This requires a holistic view from the design of the load bearing structures by structural engineers, over the production, pre-fabrication, and transport to the assembly of the structural elements on-site, which often is done by different companies with input from different stakeholders in the construction process. Especially the design of connections between CLT elements, and CLT and other construction materials and products, as well as the size of CLT elements and possibilities for pre-fabrication are crucial for an efficient assembly process. The paper summarizes findings from expert interviews with a focus on Sweden along the before-mentioned value chain, with the aim to identify current practice and potentials for further improvements. Design for efficient assembly starts at the early-stage design and involves all stakeholders in the design construction process. The reduction of uncertainties in the design and assembly process of multi-storey CLT structures as well as knowledge and experience transfer could lead to more efficient design. The identified requirements for efficient assembly should be combined with a life cycle analysis to quantify the potential for a reduction of the carbon footprint of CLT-based building systems, which is the aim of the ongoing research project ‘Improving the competitive advantage of CLT-based building systems through engineering design and reduced carbon footprint’.

Wood and wood-based products interact with the surrounding environment. The interaction with moisture is particularly interesting since it influences the structural properties and may lead to degradation. A structural health monitoring system was established in House Charlie, a four-story office building in Växjö, Sweden, made from timber. It has been running since summer 2018, collecting vibration data and information on temperature and humidity at multiple positions within the façade and the slab. The present work shows that the moisture content within a slab of the building varies throughout the years, attributed to an ongoing dry-out and seasonal changes. Furthermore, the variation is directly coupled with the weather data from a public weather station in Växjö.

Wood is widely used in the construction sector and gaining increased market share. It isinteracting with the surrounding so that its mechanical and geometrical properties (stiffness,strength, swelling, density, …) change with temperature and humidity levels. In a full-scalebuilding, the eigenfrequencies are hence also varying with the climate. In the current paper,results from a preliminary experimental study are presented. A beam made from cross-laminated timber was hanging freely supported inside a climate chamber. Enforced vibrationsfrom a controlled shaker were taken to obtain the eigenfrequencies. With decreasing moisturecontent, the first and third eigenfrequencies were increasing (bending modes) while the secondeigenfrequency was decreasing (torsional mode). A finite element study allowed for checkingwhich parameters is influencing to which degree so that individual changes can be combined.

Korslimmat trä (KL-trä) tillverkas av brädor som limmas ihop korsvis i flera skikt. Skivorna som erhållsanvänds som byggelement, mestadels för väggar och bjälklagselement. Utgångsmaterialet trä tarstår i jämvikt med det omgivande klimatet och kommer därför ändra fuktkvoten. I studien undersöktesen skiva av KL-trä under varierande fuktförhållanden i en klimatkammare hos Linnéuniversitetet.Egenfrekvenser samt fuktkvoten följdes upp och sambandet med klimatet studerades. Det visade sigatt första och tredje uppmätta egenfrekvensen (böjning) visade negativ korrelation med omgivandefukten, den gick upp när fukten minskades (och tvärtom). För andra egenfrekvensen (torsion)däremot visade sig ett mer komplicerat samband. Ett flertal möjliga orsaker presenteras som förklaring. Medverkande organisationer var Linnéuniversitetet som huvudpart samt SödraSkogsägarna och Saab som bidragit som stödfunktionen och bollplank.

Hybridkonstruktioner i trä och betong är en innovativ lösning för att öka andelen trämaterial som används i dagens byggnader. På grund av träets fördelaktiga klimategenskaper i jämförelse mot traditionella byggmaterial som t.ex. stål och betong, har både utbudet av träprodukter och efterfrågan på dessa ökat. Då trä är en naturlig råvara, finns det vissa egenskaper som måste beaktas när det används för byggproduktion. Som exempel påverkas styvhet och hållfasthet av variabler som fiberriktning och fuktkvot. Dessutom är trä ett material med förhållandevis låg densitet, som påverkar den dynamiska prestandan i byggnader som använder bärande träelement. För att möta de krav som ställs på dagens byggnader, kombineras i flera fall bärande element i trä med bärande element i betong. Denna typ av byggnad kallas hybridkonstruktioner i trä och betong. Arbetet syftar på att öka andelen hybridkonstruktioner inom byggsektorn. Målet med denna studie är att utvärdera hur den här typen av byggnader beter sig för att kunna ge konstruktörer ökad kunskap om säker och robust dimensionering. Det här arbetet presenterar fyra olika typer av hybridkonstruktioner i trä och betong som används i Sverige. I den här typen av projekt används fler konstruktörer än i traditionella byggnadsprojekt, detta då kompetensen inom träkonstruktion är bristfällig hos de traditionella konstruktörerna. Dessutom använder konstruktörer olika antaganden och statiska modeller för sina beräkningar, även om de gäller samma byggnad. Ett mobilt mätsystem har utvecklats för att kunna genomföra dynamiska mätningar på plats. Under byggtiden av ett 9-vånings flerbostadshus har det mobila mätsystemet använts för att fånga in dess dynamiska egenskaper och för att validera beräkningsmodeller. En jämförande studie visade vilka parametrar som i hög grad påverkar resultaten i dessa modeller. De avgörande parametrarna var främst skjuvstyvheten i KL-träväggarna, grundläggningen, samt icke-bärande innerväggar. Långtidsmätningar av de dynamiska egenskaperna hos en 4-vånings kontorsbyggnad presenteras. Dessa resultat visar på en tydlig säsongsvariation i egenfrekvenserna hos byggnaden som korrelerar väl med den uppmätta fuktkvoten inuti ett KL-träbjälklag över mätperioden på tre år. Resultaten visar tydligt på att det omgivande klimatet bör beaktas vid jämförandestudier med en finita elementmodell. Utöver detta presenteras en modell som predikterar egenskaperna för implementering i ett system för tillståndsövervakning för skadedetektering så att den naturliga variationen kan filtreras bort.

In recent years, there has been a rapid development of new timber products, such as cross-laminated timber, leading to an increase in buildings using timber as a structural material. House Charlie, a 4-story office building in Växjö, Sweden, is a typical example of a hybrid building that uses glulam timber for the post-beam system combined with slabs in cross-laminated timber and shear walls in concrete. A structural health monitoring system has been installed, collecting data ever since its completion in 2018. This work presents the building's modal performance collected by geophones under ambient vibrations over three years, which are used to calibrate a finite element model. The effects of changes in different material properties and model assumptions on the overall dynamic behavior of the building are shown. The aim is to establish a structural model that captures the actual behavior of the built structure that uses both timber and concrete as structural materials.

With the increased availability of timber materials, such as cross-laminated timber, the number of buildings using timber as a structural material has been rapidly increasing. As these buildings are new to the market, limited data and research on their long-term structural modal performance are available. This is particularly important in timber buildings since the material properties of wood are highly affected by environmental factors, especially the moisture content. Over time, the evolution of the dynamic properties is essential for damage indication in structural health monitoring systems since natural changes can mask the influence of damage. This work presents three years of observations from a structural monitoring system collecting data ever since completing a four-story timber-concrete hybrid building in Sweden. Ambient vibrations of the building were measured using geophones, resulting in 3,100 datasets. The temperature and relative humidity were measured both externally using a weather station and internally using sensors embedded in several walls and a slab in the building. The observed natural frequencies of the building vary with +/- 0.2 Hz around the mean value over time. Linear regression analysis shows a significant correlation between the moisture content of a cross-laminated timber slab and the natural frequencies (coefficient of determination R2 up to 0.84). A predictive model for the natural frequencies is presented, taking seasonal variations and a dry-out of the structure into account. Variations from the expected values are +/- 0.1 Hz at most. The model clearly narrows the error margins for damage indication in a structural health monitoring system.

Timber-concrete hybrid buildings are an innovative solution to increase the amount of timber materials used in modern buildings. This study presents a dynamic evaluation of a nine-story timber-concrete hybrid residential building during construction. The building consists of a seven-story structure in cross-laminated timber (CLT) on top of two stories in concrete. Ambient vibration tests were conducted seven times during the 13-month construction period, including tests with only the structural elements in place and tests of the finished building with the façade, non-structural walls, and other internal finishing. The results show a clear decrease in the natural frequencies of the building as the building gets higher and more elements are installed. However, a slight increase in the natural frequency was observed following the installation of the non-structural walls in the final construction stage. A corresponding finite element analysis is presented for each test, providing additional insights into the parameters typically used in the structural design process. The study demonstrates the importance of properly selecting reduction factors for CLT elements in a dynamic finite element analysis. It also shows the importance of considering non-structural walls, both regarding weight and stiffness, even in buildings where the number of non-structural walls is relatively small compared to structural walls.

There is a growing interest in timber buildings in Sweden and increased availability of Glulam (GLT) andparticularly Cross Laminated Timber (CLT) products. Timber buildings, though, often have difficulties in fulfilling theperformance requirements of a building project. The use of concrete elements in addition to timber elements in the load-bearing structure is a widespread solution, introducing timber-concrete hybrid buildings. The study presents responsesfrom interviews regarding ten different timber-concrete hybrid building projects in Sweden with a load-bearing structureabove the foundation level in both timber and concrete. Four main types of timber-concrete hybrids were found: a CLTstructure on top of a concrete structure, a post-beam system in GLT with CLT slabs and concrete walls, a post-beamsystem in GLT with concrete hollow core slabs, and a timber structure with some slabs in concrete. The results show thattimber-concrete hybrid buildings are flexible and suitable for various construction types. The reasons for using concretein timber construction were primarily to increase self-weight, obtain longer span lengths, and overcome shear wallcapacity issues. There is still a lack of competence in the design of structural timber projects, and at most, five differentstructural designers were involved in the load-bearing design of a single building. This highlights issues regarding projectmanagement of the design process within timber-concrete hybrid buildings

Högre byggnader i KL-trä kräver ett ökat fokus på dynamiska egenskaperså att brukaren inte upplever obehag vid hög vindlast. Utvärderingarhar gjorts på tre byggnader där resultaten stämmer in påbefintlig forskning: akustikisolering placerad under väggar i KL-träreducerar den horisontella skjuvstyvheten och kan leda till signifikantaminskningar av egenfrekvenserna. Effekten bör beaktas vid framtagningav förutsättningar för dimensionering av höga trähus.

The dynamic action of footbridge users in different forms of activity (especially during walking andrunning) may cause an excessive vibration of the footbridge deck and may disturb the comfort of use of the structure. Thedynamic susceptibility of the footbridges varies depending on the construction material used to build the footbridge andthe typical construction solutions (construction details) resulting from the construction material used. The paper presentsthe basic dynamic characteristics of timber footbridges of various structural solutions, collected during dynamic field testsof these structures. The obtained results indicate relatively high dynamic resistance of timber footbridges to the dynamicloads generated by users under normal conditions of use. In addition, the results show that in the case of timberfootbridges, it is possible to consider changing the requirements of international standards defining the range of naturalfrequencies sensitive to the dynamic impact of users. In the case of timber footbridges, characterized by a relatively highself-weight (compared to steel footbridges), high stiffness and high damping, it is possible to consider changing therequirements for performing forced vibration analyses only for structures with fundamental vertical vibration frequency f_v <= 3.0 Hz instead of the currently defined f_v <= 5.0 Hz.