Multi-stage structural identification of a tall timber building during construction: The role of connections and non-structural elements

Understanding the dynamic behaviour of timber buildings is essential for accurate vibration serviceability 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. • Ambient vibration tests were performed seven times during the construction of a 51 m tall timber building. • The slip modulus of the timber connections increased during construction. • The glass façade had minimal structural impact, likely due to its flexible mounting system. • Foundation stiffness evolved from flexible to rigid-like as construction progressed.