444 O. Flodén et al. the vibrating structure by some elastic material acting as a spring. The TMD is tuned to have a certain natural frequency and provides an effective way of cancelling the vibrations caused by a single vibration mode. Another example is the semi-active absorber, which can change its damping properties rapidly to adjust for changes in the excitation. An even more sophisticated measure is the active control system, which drives the system through feedback loops, an effective but costly procedure due to the energy required to drive the system. Moreover, an experimental investigation is presented in [7], studying the use of viscoelastic materials as vibration isolators in the junctions of a floor-ceiling structure in steel. It was found to be an effective method for reducing the vibrations caused by modes in which the floor and the ceiling move out of phase, and it was concluded that viscoelastic materials function well for wider frequency ranges, as compared to TMDs. Many of the newly constructed multi-storey wood buildings in Sweden are composed of timber volume elements (TVEs), such buildings being described in more detail in Sect. 45.1.2. TVE buildings are constructed by stacking volume modules which are separated by layers of viscoelastic elastomer materials. In the studies presented here, the TVE buildings will serve as example case for studying the effect of different design choices for the elastomer layers. The use of elastomer materials in junctions is common in different types of lightweight buildings. In [8], measurements on full-scale structures were carried out in order to investigate the effect of a number of measures for reducing the impact sound in TVE buildings. Measures that were found to be effective in reducing the vibrations were, for example, an extra layer of plaster board on the floor structure and the use of a floating floor, in which mineral wool is placed between the beams and the boards of a floor structure. The use of a heavier mineral wool as insulation, or a larger cavity between floors and ceilings, were found to be less effective measures for reducing the vibrations. Elastomer layers in the junctions were also tested, leading to ambiguous results from two different measurement setups; one resulting in reduced sound pressure levels, compared to having no elastomer material in the junctions, the other in increased sound pressure levels. The majority of the tested measures were found to change the impact sound less than 2 dB when evaluated according to the ISO standards. This should be compared with the measured variance of 1.1 dB between theoretically identical constructions due to the varying quality of the workmanship. An experimental investigation of the effect of using elastomer layers in junctions of wooden constructions is reported in [9]. A mock-up consisting of a floor structure, supported by three walls, was used to study the vibration transmission from the floor to the walls of the storey below. Marked differences in the behaviour was found for certain vibration modes when inserting elastomer materials in the junctions, as compared to a setup with the floor mounted directly on the walls. For example, the damping is larger for mode shapes where large deformations occur in the elastomer layers. It was also concluded that the step sound insulation can decrease for low frequencies (20–70 Hz), possibly due to shear resonances in the elastomer layers. This points out the need for a careful design of the elastomer layers with respect to the structure, the vibration sources and the requirements in question. 45.1.2 Timber Volume Element Buildings The lightweight properties of wood simplify the use of prefabrication in the construction process compared to conventional concrete buildings. The TVE buildings are prefabricated multi-storey buildings, increasing in popularity in Sweden. A TVE is a module consisting of floor-, roof- and wall elements completed with, for example, electrical installations, flooring, cabinets and wardrobes. Each TVE typically constitutes a small apartment, one room or part of a larger room. Prefabricated TVEs are transported to a construction site where they are stacked to form the complete building. In Fig. 45.1, the conceptual layout of a TVE building is illustrated, and in Fig. 45.2, drawings of junctions between a floor-ceiling structure and an apartment separating wall (to the left) and a facade wall (to the right) are shown. An advantage regarding vibrations and acoustic performance is that a floor is structurally separated from the ceiling of the storey below; the upper volume contains the floor whereas the lower volume comprises the ceiling. Elastomer blocks are placed on the flanks in between the TVEs in order to reduce the vibration transmission through the junctions. The major structural connection between adjacent volumes is by means of the elastomers, the only additional connection being through a few studs and tie plates, used to position and fixate theTVEs. 45.1.3 Objective In this paper, a numerical investigation is presented, aiming at an increased understanding of how the design of elastomer layers in junctions of multi-storey wood buildings affects the vibration transmission between storeys and rooms.
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