Wood preservation is an important issue for agricultural buildings with timber structure. This is among others due to their halfway opened construction, high level of moisture release from livestock breeding or storing goods. However, regarding the possibly high moisture content in the building structure and the potential threat caused by wood-destroying organisms, there is still a substantial need for research. The latest results of the research work carried out by Technical University of Munich, in cooperation with the Bavarian State Research Center for Agriculture, show that, for the most agricultural buildings built from spruce, no preventative chemical wood preservation is necessary to ensure a durable construction.
Through long-term measurements of climate data (temperature, relative humidity) and timber moisture content on large-span timber structures in buildings of typical construction type and use, data sets were generated which deliver information on the sequence and magnitude of seasonal variations. The measurement of moisture in different depths of the cross-section is of particular interest to draw conclusions on the size and speed of adjustment of the moisture distribution to changing climatic conditions. The moisture gradient has direct influence on the size of the internal stresses and possible damage potential. Similarly, the results provide a review and extension of the previous classification of buildings into use classes. They allow for a more precise indication of range of resulting equilibrium moisture content for the specific use, enabling the installation of timber elements with adjusted moisture content. The results of the research project also support the development of appropriate monitoring systems, which could be used in the form of early warning systems based on climate measurements.
Concentrated loads on Cross Laminated Timber elements (CLT) in areas of point supports or load applications cause high local shear stresses. Inclined self-tapping screws with continuous threads have turned out to be an effective reinforcement. As neither the German design standard DIN 1052 nor technical approvals cover this construction method a research project funded by the AiF was conducted to gather basic information for its application. These basics include the determination of shear stresses next to concentrated loads, the interaction of compression perpendicular to the grain and rolling shear stresses as well as theoretical and experimental examinations of the load bearing behaviour of reinforced CLT-elements. This paper presents the main research results. A design concept validated by means of the test results is proposed.
This state-of-the-art report has been prepared within COST Action FP1402 Basis of structural timber design from research to standards, Working Group 3 Connections. The Action was established to create an expert network that is able to develop and establish the specific information needed for standardization committee decisions. Its main objective is to overcome the gap between broadly available scientific results and the specific information needed by standardization committees. This necessitates an expert network that links practice with research, i.e. technological developments with scientific background. COST presents the ideal basis to foster this type of joint effort. Chapter 8 Connections presents an integral part of Eurocode 5 and is in need of revision. This state-of-the-art report shall provide code writers with background information necessary for the development of the so-called Second Generation of the Eurocodes, now aimed to be produced in 2022.
The use of glulam beams with changing depth offers the possibility to adapt the section modulus to the bending moment. In the case of single-span beams under uniformly distributed load, however, a change in beam depth will lead to a contrary effect for the shear stresses. Curved and pitched cambered beams feature not only high utilization rates in bending but also areas of high tension stresses perpendicular to the grain and shear parallel to the grain stresses, two stress components for which timber features only small capacities as well as brittle failure modes. Out of 245 cases of damaged or failed large-span timber structures, several failures document the possibility of a shear fracture (full separation) developing in grain direction from the curved part towards the supports, partly followed by a failure of the beam in flexural tension due to a change in stress distribution resulting from the change in section modulus. Reinforcements against tension stresses perpendicular to the grain in form of fully threaded screws or threaded rods can be considered state of the art. With respect to their application as shear reinforcement, not many research results are yet available, resulting in a lack of experimentally validated design approaches.
Within this paper, approaches to design shear reinforcement for glulam beams in the unfractured and the fractured state are presented, validated and discussed. The moment of failure, i.e. the transition from the unfractured to the fractured state is characterized by dynamic effects. This situation is not covered in this paper. The same applies to the subject of moisture induced stresses, resulting from the reinforcement restricting the free shrinkage or swelling of the glulam beam.
Reinforcement in glulam beams in form of screws or rods can restrict the free shrinkage or swelling of the wood material. The objective of the project presented was to evaluate the influence of such reinforcement on the magnitude of moisture induced stresses. For this purpose, experimental studies were carried out in combination with analytical considerations on the basis of the finite-element method. Taking into account the influence of relaxation processes, the results indicate that a reduction of timber moisture content of 3 - 4 % around threaded rods, positioned perpendicular to the grain, can lead to critical stresses with respect to moisture induced cracks. In addition, a substantial mutual influence of adjacent reinforcing elements has been identified. A reduction of the distance between the reinforcement thus results in a lower tolerable reduction of timber moisture content around the reinforcement.
International Conference on Structural Health Assessment of Timber Structures
Numerous large-span, load-bearing structures, e.g. for public venues, sports halls or industrial facilities, utilize glued-laminated timber (glulam) due to its versatility and aesthetic appearance. Since glulam is an organic and composite material consisting of wooden lamellas joined with glue, mistakes during planning, fabrication and use of the structure can lead to deficiencies or even damages. Following a visual inspection suitable to detect surface deterioration, further holistic investigation of the glulam material is necessary to appraise structural safety or estimate the need for restoration. Besides the integrity of the wooden lamellas, the glue bond of the lamellas is vital for the operational reliability of glulam. A currently practiced, semi-destructive assessment method to revalue the condition of the glue lines consults a shear test on drill core samples with included glue line. The presented paper links this method to medium-scale shear tests and large-scale 3-Point bending-shear tests representing a practical loading situation. To provoke shear failure during bending, reinforcements of the specimens at critical tension and compression zones included glued-on beech veneer lamellas and self-tapping screws, respectively. Executed on the same sample material, shear resistance was determined for all three testing formats. The sampling included aged and new glulam. Published values derived from comparable test programs augmented the database. Based on the evaluated test results of the drill cores under shear loading, suggestions regarding the drill core extraction and the implementation of the valuation method of EN 14080 are outlined. After incorporating a size effect to account for varying dimensions of the bendingshear specimen, their shear resistance values correlated well with the obtained values from the drill core tests. This adumbrates the possibility to derive the shear resistance of structural members from shear values of drill core samples taken from an existing glulam structure.
The evaluation of damages in large-span timber structures indicates that the predominantly observed damage pattern is pronounced cracking in the lamellas of glued-laminated timber elements. A significant proportion of these cracks is attributed to the seasonal and use-related variations of the internal climate within large buildings and the associated inhomogeneous shrinkage and swelling processes in the timber elements. To evaluate the significance of these phenomena, long-term measurements of climatic conditions and timber moisture content were taken within large-span timber structures in buildings of typical construction type and use. These measurements were then used to draw conclusions on the magnitude and time necessary for adjustment of the moisture distribution to changing climatic conditions. A comparison of the results for different types of building use confirms the expected large range of possible climatic conditions in buildings with timber structures. Ranges of equilibrium moisture content representative of the type and use of building were obtained. These ranges can be used in design to condition the timber to the right value of moisture content, in this way reducing the crack formation due to moisture variations. The results of this research also support the development of suitable monitoring systems which could be applied in form of early warning systems on the basis of climate measurements. Based on the results obtained, proposals for the practical implementation of the results are given.
Experimental and numerical investigations on round holes in glulam beams are presented. These were conducted in order to extend the field of practical application, to study the structural behaviour of holes arranged eccentrically or in groups and to generate basic results for deriving a design format. Within these investigations the influence of parameters like eccentricity, clear distance between holes or effect of reinforcement by fully threaded selftapping screws was considered. A comparison of estimated load-bearing capacities on the basis of the Weibull theory and test results shows good agreement. Strain gauge measurements in reinforcing elements confirm the validity of the chosen methods.