Long-term and moisture-dependent deformation behaviour of laminated veneer lumber (LVL)

Background and Objective

The long-term and moisture-dependent deformation behaviour of timber components is taken into account in the design (dimensioning and calculation of the structural analysis) using a simplified creep coefficient. Depending on the (wood) building material (e.g., solid wood, laminated veneer lumber, plywood) and the utilisation class (1, 2 or 3; each characterised by the resulting wood moisture due to, among other things, the ambient air humidity), certain factors (e.g., 0.6, 0.8, 2.0) must be applied here. If the factors are unsuitable due to a lack of data and lead to an underestimation of the actual component behaviour (e.g. increased deflection), this can result in aesthetic restrictions and even non-fulfilment of serviceability and load-bearing capacity. This can lead to problems for planners, contractors, building owners and building material manufacturers.

The aim of the project is to answer whether (1) the creep behaviour of LVL differs from that of solid wood in general and (2) that of hardwood LVL differs from that of softwood LVL. If differences are present (working hypothesis), it should be clarified whether this is due to specific structural and manufacturing reasons. Furthermore, it should be determined whether creep curves determined under different climatic conditions (humidity, temperature, alternating climate) and analytical and numerical models created on the basis of these are better suited for design than simplified creep coefficients.

The project "Long-term and moisture-dependent deformation behaviour of laminated veneer lumber (LVL)", LuV-Holz for short, project number 10.08.18.7-24.39, is funded by the Federal Institute for Research on Building, Urban Affairs and Spatial Development (BBSR) on behalf of the Federal Ministry of Housing, Urban Development and Building (BMWSB). Preliminary work was financially supported by the Gesellschaft der Förderer und Freunde der Holzwissenschaft in Hamburg e. V. (GFF).

Target Group

Planners in timber engineering, entrepreneurs in the woodworking industry, timber construction and wood-based materials research

Approach

In cooperation, the Thünen Institute of Wood Research (TI-HF) and the University of Stuttgart (US), Institute for Materials in Construction and Materials Testing Institute, are carrying out research work with the aim of having creep curves and creep figures that can better describe the actual behaviour of laminated veneer lumber made of beech, spruce and pine depending on the situation, e.g., climate and cross-section. The project partners are Brüninghoff GmbH & Co KG, Pollmeier Massivholz GmbH & Co KG and Steico SE. By varying the ambient climate (temperature and relative humidity) and different load levels, the TI-HF records the deflection of a significant number of small and medium-sized test specimens during loading and unloading. Solid wood samples of the respective wood species serve as a reference. The US carries out the modelling and parameter determination as well as the formulation of conclusions for the design in engineering practice and, if necessary, incorporates these into standardisation. Brüninghoff carries out condition analyses of actual deformations on selected structures with FSH components. Pollmeier and Steico provide suitable sample material.