Vibrations in a Seven-Storey Wood Building

Abstract: In 1994 the Swedish Construction Code, BKR, was changed from being based on material to being based on function. This permitted constructing multi-storey buildings with a bearing framework made of wood, which had been prohibited since the end of the 19th century. There is now a growing in- terest of using wood as a construction material. In Växjö four seven-storey buildings, Limnologen, are under construction, which at the moment are the highest wood buildings in Sweden. As wood in many ways is a new construction material there is a lot of research going on in order to build as good as possible. The main problems, when constructing in wood, are sound and vibrations because of the low weight of the material. This thesis is part of a project at Växjö University examining step sounds and vibrations at Limnologen. The objective of our project was to investigate how mechanical vibrations are transmitted through the building at different storeys. The long term goal is to construct high buildings where residents are not disturbed by the noise or motion of their neighbors. A FE-model of six storeys of the house was created with help of the finite element software Abaqus. It would be impossible to handle and perform vibration calculations on a detailed model of the building. Therefore a lot of simplifications were necessary. First of all detailed small scale FE-models of the main building parts (floor structure, ceiling and apartment separating wall) were produced and some analyses were conducted on them to find their normal, bending and torsion stiffness. The full-scale model was then composed of shells with the same stiffness. To variate the stiffness of the connections between walls and floor structures three different models were produced, one with fixed connections and two with weaker ones. Air was not included in the model. In the analyses a harmonic load of 700 N, approximately the weight of one person, was added at the same position on storey 2, 4 and 6, one at a time. A steady-state step was conducted at frequencies from 1 to 200 Hz with steps of 1 Hz. It was found that there is a difference in velocity depending on what storey you are situated at, when looking at the vertical vibration spread. The transmission through the apartment separating wall, diagonally in the house, did not show the same behavior. Moreover it does not seem to matter a lot at which storey the disturbance is acting, it gives the same results at the other storeys anyway. The results from the two weaker models do not differ much from each other, but the velocities in these are higher than in the stiffest model. A lot of simplifications were made in the model, perhaps other ways of doing it would show other results. It is however most important to show what possibilities a full scale model like this can give. A new range of analyses can be made that were not possible before.