The demand for multi-story timber buildings is rapidly increasing due to key factors such as lower costs of realization, high level of prefabrication, and growing environmental awareness. This work focuses on evaluating the dynamic properties of two multi-story Cross-Laminated Timber (CLT) buildings. The selected buildings are two identically realized eight-story residential buildings, part of the Kvarter Limnologen in Växjö (Sweden). At approximately 15 years old, Kvarter Limnologen is among the oldest CLT buildings and is entering a new stage in its life cycle.

To evaluate the modal parameters, three sensor layouts were used during ambient vibration tests. In the first configuration, four sensors were placed on the top story to capture preliminary modal parameters. The second layout increased the number of sensors to twelve, alternating across floors to improve the spatial resolution. Finally, a multi-setup approach was adopted to achieve more accurate mode shapes, using a roving accelerometer configuration while keeping the sensors on the top floor as reference.

The results demonstrate that an optimized sensor placement strategy for multi-setup modal testing allows the evaluation of detailed information about the dynamic properties of the two buildings. The current modal properties—natural frequencies, damping ratios, and mode shapes—of the two building were compared with the results obtained a decade ago. The findings provide valuable insights into the long-term behavior and performance of CLT modular structures, supporting the development of effective structural health monitoring methods.

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

This study presents a comprehensive investigation of the dynamic behavior of a six-story lightweight timber frame building through periodic and continuous ambient vibration tests. Seasonal changes in temperature and relative humidity can influence the dynamic response of timber buildings, by affecting stiffness, strength, and connection properties due to changes in moisture content. Systematic tests were performed from October 2022 until May 2024 to identify natural frequencies, damping ratios, and mode shapes of the building using five battery-driven data acquisition units equipped with 15 uni-axialaccelerometers. Two different only-output frequency and time domain Operational Modal Analysis (OMA) methods were used to evaluate the dynamic properties of the building. Additionally, the building has been continuously monitored with temperature and humidity sensors since its construction. Results obtained from the periodic measurements highlighted the need for a permanent system to capture the transient changes in the modal parameters. A complementary permanent system to record the accelerations at the roof level was installed in May 2024. The modal parameters from in situ measurements were compared with those obtained from the Finite Element (FE) model of the structure. The FE model, calibrated through a multi-stage approach, incorporating non-structural elements, and varying imposed loads, showed good agreement with experimental data. Comparative analysis of the results obtained under different temperature and humidity conditions showed the effect of the environmental conditions on the building dynamics properties, for both periodic and permanent measurements. This study highlights the importance of continuous monitoring and detailed FE modeling in understanding the dynamic properties and long-term structural behavior under varying conditions. This information can be used to ensure that the building meets safety and performance requirements and to identify potential issues that may need to be addressed during the design and maintenance phases.

The material and mechanical properties of wood are influenced by the changes in environmental conditions where moisture content and temperature are particularly important. This study presents a detailed analysis of the evolution of the dynamic properties of a six-story lightweight timber frame building in Sweden over a nine-month monitoring period. A permanent monitoring system, consisting of a data acquisition system and three accelerometers, was installed at the roof level in May 2024. The system is recording the building’s accelerations four times per day for a duration of fifteen minutes each session, resulting in approximately one thousand datasets over the monitoring period. Additionally, a monitoring system for environmental data was installed, measuring temperature and humidity at 18 different measurement point within walls and slabs since construction.

A comparative analysis of the results obtained under varying temperature and humidity conditions was able to show the impact of environmental conditions on the building's dynamic properties. Recorded data show that, on the one hand, a decrease in temperature can lead to an increase in natural frequencies, indicating a structural stiffening, while a decrease in humidity and moisture content generally results in a decrease in the natural frequency’s values.

The importance of incorporating environmental factors into broader structural health monitoring schemes of timber structures is crucial, particularly since changes of environmental conditions could mask the influence of structural damage in the building.

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.

The current standards need more specific serviceability criteria for Cross-Laminated Timber (CLT) floors since in some cases discrepancies can be observed between the floor in-service performance in reality and those predicted in standards and design codes. This paper presents a series of analyses of a case study in Sweden, which presented higher fundamental frequency in the CLT floors than predicted in Eurocode 5 (EC5). It aims to investigate the effect of non-structural walls on the floor modal parameters through experimental measurements. The authors compared the analytical predictions of the fundamental frequency using the new draft version of EC5 against the corresponding experimental estimations. Using a sensor-roving approach, a dense sensor configuration was adopted to identify the three floors’ modal parameters. Then, different human-induced excitation scenarios were performed to analyze the floor dynamic responses. The results and analysis in this study shed light on the significance of non-structural internal walls on the vibration performance of the CLT floors, a factor not considered in the practical design for serviceability verifications. The study shows that the presence of partition walls can provide a higher stiffness leading to a significant increase in the first fundamental frequency.