Modal Analysis, Dynamic Properties and Horizontal Stabilisation of Timber Buildings

Sammanfattning: Engineers face new challenges as taller timber buildings are constructed. According to Eurocode 1-4, both horizontal deformations from static wind and acceleration levels shall be limited. Due to the low self-weight of wood, dynamic vibrations and acceleration levels cancause problems. The current knowledge in the field is limited and there is a need for increasing the understanding of dynamic properties in tall timber buildings. This research project has been a collaboration between Luleå Technical University and Sweco Structures AB, where the author has gained practical experience as a designer in parallel to the research studies.The purpose of this research is to understand and describe the dynamic behaviour of tall timber buildings using FE-simulations , studying their dynamic properties, and comparing acceleration levels to comfort criteria. By varying different parameters, dynamic properties have been studied and compared with assumptions and recommendations in Eurocode 1-4.In this study, buildings with cross-laminated timber panels (CLT) have been studied but also post-and-beam systems with trusses. Depending on the shape, layout and materials of the building, the dynamic properties of the building will vary: natural frequency, mode shape, modal mass, and modal stiffness. To assess the comfort of the building, the standard ISO 10137 has been used evaluating the natural frequency of the building and its peak acceleration. Simulations have been performed using finite element (FE) software where modal analyses have been performed. Over 250 simulations have been performed in this study.Adding mass reduces the natural frequency and the acceleration level of the building, which is an appropriate measure if the building has a frequency below 1 Hz. Increased stiffness increases the natural frequency and reduces the acceleration level, which is suitable for buildings with a natural frequency over 2 Hz.The empirical expression f = 46 / h should be used with caution as it is based onmeasurements of concrete and steel buildings. The recommendation is to perform FEsimulations until the empirical knowledge base is sufficient for timber buildings.The placement of the stabilizing system is important for creating a balanced (symmetrical) system resulting in pure translation modes. Eurocode 1-4 presupposes 2D modes in the plane while asymmetry can create diagonal and even torsional modes, which Eurocode 1-4 cannot handle. Openings and asymmetry in the floor plan affect the dynamic properties of the building. The assumption that the building can be modelled as a homogeneous beam where the mass is evenly distributed can result in an over- or underestimation of the equivalent mass, which in turn can lead to an underestimation of the acceleration level, around 20% - 30%. It is recommended that the equivalent mass is calculated from FE generated modal mass and mode shapes.Acceleration levels vary over the building height depending on the mode shape. Timber buildings with a slenderness <3.9 have more or less a pure shear mode and with increasing height it shifts to a linear mode. For timber buildings, it is recommended to use the generated mode shape from FE simulations, and not those prescribed in Eurocode 1-4 as these can underestimate the acceleration levels, around 30 %.