9.1 PAPER I
Experimental Evaluations of Material Damping in Timber Beams Labonnote N., Rønnquist A., Malo K.A.
Background
Excessive vibrations in buildings are usually not a safety concern, rather a serviceability issue due to annoyance and other discomforts. Damping has a large beneficial influence on the structural response close to resonance since it decreases both the amplitude of steady-state oscillations as well as the duration of transient oscillations. Despite its substantial effects, damping is rarely prescribed in design codes or standards, mainly because of lack of knowledge. For timber structures, damping evaluations are generally considered to depend too much on the engineer’s judgment, because of the lack of reliability in the experimental methods. Due to the scarce quantification of damping properties for structural use, the intention of the present study is to improve knowledge on material damping in timber structures by developing a reliable experimental method to evaluate damping in timber beams, and to provide new and reliable values for the material damping of timber beams that are typical for common floor structures.
Main findings
Material damping evaluations are performed on 11 solid wood beams and 11 glulam beams, through the impact test method. Damping evaluations are performed for various configurations, which include different spans as well as orientations (edgewise and flatwise). A total of 420 material damping evaluations are performed out of which 14 evaluations are found inconsistent and thus discarded. Statistical indicators to the damping evaluations are provided together with their mean values for each configuration. The reliability of the results is assessed by carefully evaluating the consistency of the data, and by concluding that no significant differences are due to the operator and/or his/her skills. General trends are that the evaluated damping ratio increases with higher modes, shorter spans, and the edgewise orientations as compared to the flatwise orientation. The shear deformation is found to be the governing factor to explain the variation of the damping ratio from one configuration to another. Shear deformation is finally conveniently evaluated by mode shape characteristics.
RESEARCH -Main Results and Discussion
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9.2 PAPER II
New Model of Hysteretic Damping in Timoshenko Timber Beams Labonnote N., Rønnquist A., Malo K.A.
Background
Nowadays, more extensive use of wood in buildings is of sustainable interest. Feasibility of high-rise timber buildings has been demonstrated by the erection of an 8-storey timber building in London in 2007, but serviceability issues are still the main limitation to the complete development of tall timber buildings. Improved design criteria and better solutions to serviceability issues must be based on an enhanced knowledge of damping mechanisms in general, and on reliable prediction models for damping quantities in particular. A first step towards prediction of damping in buildings requires the understanding of the transmission of material damping from single material properties to vibrating systems. The main motivation of the present study is therefore to derive a model that predicts material damping in timber beams.
Main findings
The system hysteretic damping of Timoshenko beams is derived by introducing complex elastic moduli and complex global stiffness into the equation of motion. The system hysteretic damping is derived as the sum of the longitudinal loss factor and the shear loss factor, respectively weighted by a bending contribution and a shear contribution. System damping due to bending is defined as the product of the longitudinal loss factor with the bending contribution, while the system damping due to shear is defined as the product of the shear loss factor with the shear contribution. Contributions are shown to be mechanically related to either bending or shear deformation.
Experimental damping evaluations on overhanging timber beams are used to fit the loss factors. Fitting is performed with fairly good agreement. The system damping due to bending is observed to have a nearly constant value for each type of wood. However, the system damping due to shear is observed to express most of the inherent variation of the total system damping. Furthermore, glulam beams are found to have a reduced system damping compared to solid wood beams.
RESEARCH -Main Results and Discussion
131 9.3 PAPER III
Semi-Analytical Prediction and Experimental Evaluation of Material Damping in Timber Panels
Labonnote N., Rønnquist A., Malo K.A.
Background
A better understanding of material damping in timber elements would enable the development of more adapted standards or design codes, and could contribute to the development of higher timber buildings in fine. The main motivation for this study is to describe a method for predicting material damping in timber panels. The method is derived from the strain energy approach, and input is based on loss factors, which are intrinsic properties of the considered materials, together with material properties and mode shape integrals, whose calculation can easily be implemented in most finite element codes.
Main findings
An analytical model is first derived to predict the material damping of a vibrating structure (a timber panel), which depends on material properties - more precisely loss factors - , and system properties such as mode shapes. Mode shapes can easily be calculated by means of finite element analyses. Three specific prediction models are derived: one for thin isotropic plates, one for thin orthotropic plates with three independent loss factors, one for thin orthotropic plates with five loss factors, among which four are independent. Damping values of three types of timber panels: particleboards, OSB panels and LVL panels, are experimentally evaluated. The particleboards damping is best described by the thin isotropic plate prediction model. The results related to OSB panels and LVL panels suggest that the five-loss factor thin orthotropic plate prediction model is more appropriate for transversely isotropic materials, and that the three-loss factor orthotropic plate prediction model is more appropriate for orthotropic materials. The obtained fitted loss factors are observed to be consistent with previous studies. Good agreement between semi- analytical predictions and experimental evaluations shows that the prediction models are efficient tools for predicting global material damping of a structure, knowing only its loss factors and its mode shapes.
RESEARCH -Main Results and Discussion
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9.4 PAPER IV
Prediction Model of Material Damping Used to Evaluate Structural Damping in Timber Floors
Labonnote N., Rønnquist A., Malo K.A.
Background
At present the omission of damping in design criteria originates from the difficulty for the engineer to predict the damping characteristics of a floor during the design process. This is especially relevant for wood structures, where the damping characteristics to a large degree will depend on the workmanship and construction techniques. The total damping is commonly divided into at least two categories: the material damping which refers to internal friction, and the structural damping which may arise from other sources such as friction in-between components and/or friction due to connectors. A first, yet conservative, step towards better prediction of damping in timber structures can therefore be accomplished by considering material damping as a lower boundary for total damping. The great advantage in this formulation lies in the availability of prediction models for material damping. Their use may also result in possible estimations of the structural damping as the difference between the measurable total damping and the predicted material damping.
Main findings
The prediction method for material damping is derived from the strain energy approach, and a procedure has been written using the commercial finite element software Abaqus. The different contributions to material damping from each floor member are precisely estimated. In particular it is observed that top and bottom plates introduce larger material damping than joists or edge joists.
Results show equivalent share of structural damping compared to material damping. Insignificant influence of the type of connectors is observed, and this is probably due to the very low level of induced motion. In addition, the contribution from structural damping to total damping is observed to increase with the mode number. In particular structural damping is larger than material damping for the third mode considered. The proposed procedure to predict material damping is dependent on the accuracy and the availability of loss factors, but is convenient to implement and efficient to use.
RESEARCH - Conclusion and Further Work
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