Timber buildings, including hybrids, are increasingly popular due to their beneficial environmental aspects. During 2023-2024, the authors had a unique opportunity to perform ambient vibration testing of two six-story buildings with identical timber structures placed on top of different types of concrete substructures and with varying soil conditions. Ambient vibration tests were performed twice during the construction of each building. Finite element models were created for each vibration test, and their parameters were updated simultaneously to accurately simulate the building's natural frequencies and corresponding mode shapes. The model updating process was facilitated by training four surrogate models to represent the four finite element models corresponding to the two buildings and the two construction stages. The updated models were used to investigate the effects of the in-plane shear stiffness of cross-laminated timber (CLT) walls, soil-structure interaction, moisture content, and non-structural walls on the dynamic properties of hybrid timber-concrete buildings. The results showed nearly identical dynamic performance of the two buildings, suggesting that the differences in sub-structure and soil conditions do not affect the natural frequencies and mode shapes. A sensitivity analysis revealed that the in-plane shear stiffness of the CLT walls is the most significant factor affecting the modal properties of the two buildings. 

Timber buildings, including hybrids, are increasingly popular due to their beneficial environmental aspects. During 2023-2024, the authors had a unique opportunity to perform ambient vibration testing of two six-story buildings with identical timber structures placed on top of different types of concrete substructures and with varying soil conditions. Ambient vibration tests were performed twice during the construction of each building. Finite element models were created for each vibration test, and their parameters were updated simultaneously to accurately simulate the buildings' natural frequencies and the corresponding mode shapes. The model updating process was facilitated by training four surrogate models to represent the four finite element models corresponding to the two buildings and the two construction stages, which were then verified against the finite element models. The updated models were used to investigate the effects of the in-plane shear stiffness of cross-laminated timber walls, soil-structure interaction, moisture content, and non-structural walls on the dynamic properties of hybrid timber-concrete buildings. The results showed nearly identical dynamic performance of the two buildings, suggesting that the differences in substructure and soil conditions do not affect the natural frequencies and mode shapes. A sensitivity analysis revealed that the in-plane shear stiffness of the CLT walls is the most significant factor affecting the modal properties of the two buildings.

Fyrtornet, a pioneering 51-meter-tall mass timber office tower in Malmö, Sweden, is thefirst and tallest building of the Embassy of Sharing project. Fyrtornet’s lightweight naturemade wind-induced accelerations for occupant comfort a critical design factor. The designincorporated analytical assessments validated by wind tunnel testing, with mass distribu-tion and CLT core wall shear connections identified as key factors. Iterations led to the finaldesign with an optimised brazing system and added mass in the concrete roof slab so thatcomfort criteria were met.

Extensive ambient vibration testing at seven construction stages in combination with FE-model updating revealed that non-structural elements (screed and internal walls) signifi-cantly decreased natural frequencies, as the added mass outweighed the stiffness in-creases. The foundation exhibited rigid behaviour, and GLT connection stiffness ap-proached rigidity in the final stages, likely due to screed and partition walls. A permanentmonitoring system is now installed to track dynamic performance and environmental ef-fects. With this, Fyrtornet serves as an essential case study for the dynamics of tall timberbuildings

Cross-laminated timber (CLT) is increasingly proposed as a low-carbon alternative to steel or concrete in mid-rise multi-storey structures. In this study, the embodied carbon footprint of a CLT multi-storey building is analysed in a life cycle perspective, and strategies to optimise the carbon footprint are explored, based on CLT panel thickness optimisation and insulation material substitution. The analysis shows that the product and construction stage account for 72% of the life cycle embodied carbon footprint, and the effective post-use management of the materials arising from the end-of-life stage can give a significant climate benefit. A reduction of up to 5% of the life cycle carbon footprint can be achieved when employing the material optimisation and substitution strategies. This, together with effective post-use management of the building materials, results in a reduction of up to 20% of the life cycle carbon footprint of the studied CLT building. The study also emphasises the significance of service life in achieving a low carbon footprint for CLT buildings. Overall, this study shows that the life cycle embodied carbon footprint of a CLT building can be further reduced through material-related strategies.

Understanding the dynamic behaviour of timber buildings is essential for accurate vibration ser-viceability modelling, yet remains underexplored due to complexities in connection behaviour and contributions of non-structural elements. The study investigates these aspects through the case of Fyrtornet, a newly constructed 51 m tall 11-storey timber building in Sweden, featuring a glued-laminated timber (GLT) truss system and cross-laminated timber (CLT) slabs, as well as a CLT core. Ambient vibration testing was conducted at seven construction stages. For each stage, a finite element model was developed and updated based on the measured modal properties. The research identifies the progression of stiffness contributions of GLT and CLT connections, the foundation, and the glass façade. Results show that while the GLT connection stiffness changes during construction, it reaches a value consistent with a rigid behaviour. This observation was attributed to the installation of screed and partition walls before the final construction stage. In the earlier stages, the slip modulus equation provided by EC 1995-1-1 appears to offer a suitable estimate for GLT connection stiffness. Another important observation was that the foundation exhibited characteristics of a rigid boundary condition. Finally, the stiffness contribution of the glass façade was minimal.

Structural identification and structural health monitoring often require the development of surrogate models. Either for their computational efficiency or the possibility of offline computation of the surrogate model. The latter is beneficial in monitoring where access to the licensed software may be limited. The basis for fitting a surrogate model are the evaluation points, which are sampled over the domain of the selected parameters and evaluated by the parametrised finite element model. When the quantities of interest are the modal properties, additional challenges linked to the mode classification arise. Common approaches to classifying the computed modes into fitting groups include frequency-based ordering, mode-assurance-criterion-based ordering, and mode tracking. However, none of these approaches offers a reliable method for splitting the modes into well-distinguished fitting groups in case of mode degeneration phenomena such as mode crossing and mode veering. The MOSAIC approach to surrogate modelling has been recently proposed to combat the issues connected to mode degeneration. This paper provides a case study evaluating the accuracy of the surrogate model trained according to the MOSAIC approach and comparing it to a benchmark model using artificial neural networks. The surrogate model is trained on the evaluation points obtained from a detailed finite element model of a tall timber building. The results of the comparison show a promising potential for improved surrogate modelling accuracy adopting the MOSAIC approach.

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.

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