Einfluß der Trägerhöhe auf die Biegefestigkeit von Brettschichtholz

The influence of beam height on the bending strength of glued laminated timber was investigated. Tests showed that with increasing beam height the bending strength decreases for glued laminated timber beams without finger joints (wood failure). Theoretical values confirm calculated values of Eurocode 5. There is not a dependence of strength from the beam height by glued laminated timber beams with finger joints in the outer core. In most cases, glued laminated beams have finger joints in the outer core. Therefore, it is not useful to introduce a height factor in design of glued laminated beams in codes.     

Evaluation of glue line shear strength of laminated timber structures using block and core type specimens

Monitoring and evaluation of glued laminated timber structures in service is essential to warrant its integrity, where non-destructive techniques play an important role. This paper describes the results obtained on glued laminated timber beams of maritime pine, with and without preservative treatment, and of spruce, exposed to natural outdoor environment as well as to artificial weathering. The assessment of glue lines’ shear strength was investigated through the extraction and testing of cores parallel and perpendicular to the glued lines and block shear specimens with 50 mm?×?50 mm and 20 mm?×?20 mm shear area. The results highlight the different performances of the various types of specimens tested, showing that the measured shear strength depends on the specimen size and that the size effect depends on the material strength giving a decreasing modification factor with the increase of strength in opposition to the constant factor suggested by standard EN 14080. They also show that shear testing of cores drilled perpendicular to the glued joints may be a promising tool in the assessment of glulam structures on site.     

Versuche mit großflächig geleimten Stabwerken

For confirmation of results from previous tests with glued lap timber joints six plus two trusses with spans of 10 and 15 m respectively and with differently designed glued joints were produced and tested. The ultimate loads of the investigated trusses varied between 195 and 480 kN depending on the size of the available glued areas and on the different shear strengths of the joints, which in turn related to joint geometry, angle of gluing and type of material used (spruce timber, beech plywood). Maximum deflection ranged from 32 to 46 mm. An effect of density on ultimate load and deflection of the trusses was observed. The ultimate shear strength of the joints varied from 0,68 N/mm² to 2,41 N/mm².     

Determination of characteristic strength and stiffness values in glued laminated beams of Argentinean <Emphasis Type="Italic">Eucalyptus grandis</Emphasis> according to European standards

The present paper reports the results of an investigation regarding the determination of characteristic strength and stiffness values in glued laminated beams of fast-growth Argentinean Eucalyptus grandis according to the criteria established in European standards. An empirical research project with 100 beams in structural sizes and 191 finger-jointed laminations was carried out. Test results allow to analyse the mechanical property values of this glued laminated timber in comparison with those adopted by the strength class system established in the European standard EN 1194 (1999), and they reveal a very high ratio of modulus of elasticity to density. Test results were also compared with values obtained by means of the equations provided by EN 1194 (1999). The effectiveness of the criteria established in this European standard for determining mechanical properties in beams of this deciduous species is analysed through the discussion of test results and those obtained from calculations based on lamination properties.     

Einfluß der Bauteilgröße in der linearen und nichtlinearen (Holz-)Bruchmechanik

Fracture mechanics models for assessment of load bearing capacity of (wood) structures imply an influence of structure size such that larger structures show lower capacities compared to smaller ones of alike shape. The size effect scale depends on whether principles of linear or nonlinear fracture mechanics (LEFM resp. NLFM) are adequate. Following a brief survey of linear fracture mechanics the LEFM size effect law is given and compared to Weibull theory. The necessity of an LEFM application limit along with smaller sizes is demonstrated. next, basics of nonlinear fracture mechanics especially of strain softening materials such as wood are outlined and terms like fracture process zones and characteristic lengths are discussed. It is shown how Bazant's NLFM size effect law enables the link between classic strength approaches and linear fracture mechanics.     

Zur Tragfahigkeit von Nageln im Hirnholz unter Belastung auf Herausziehen and unter kombinierter Belastung (Herausziehen and Abscheren)

This article describes a systematic study of the load bearing capacity of nails in end grain steel plating of laminated wooden beams. Investigated are the bearing capacities under withdrawal load and under combined loading (shear and withdrawal forces).     

关于指接集成材力学性能及其分析模型的研究

概述目前研究集成材力学性能的两大方法,重点阐述影响集成材力学性能的因素:层积效应、尺寸效应、应力分布效应和指接。分析了指接集成材力学模型优缺点,总结了建模的关键点。     

Zur Tragfähigkeit von Nägeln im Hirnholz unter Abscherbelastung

Systematic studies were carried out to investigate the load bearing capacity of nails in end grain steel plating of laminated wooden beams. From the results suggestions for minimum nail distances and for dimensions are discussed. According to these results, the load bearing capacity of nails in end grain appear to be satisfactory.     

Moment resisting connections composed of friction-welded spruce boards: experimental investigations and numerical strength prediction

Research on friction welded wood-to-wood connections has shown high potential for further development regarding the industrial application of timber construction. This paper addresses the question to which extent welding of wood can be applied to fabrication of cross-laminated timber panels (CLT). Those panels can be used as wall elements, in which load transfer occurs by transmission of moments and shear stresses. For this purpose, experimental investigations on friction welded L-shaped specimens were carried out. The welded bond between the boards, arranged perpendicularly to their fibre directions, represents points of intersection of those CLT panels. The results allowed for (a) the determination of a fracture mechanism of welded timber connections under torsion, (b) the definition of a set of design parameters influencing the resulting joint strength and (c) the comparison between the performance of welded timber joints and those of commonly glued alternatives. The experimental investigations were compared to results obtained by numerical modelling, thus allowing a probabilistic strength prediction algorithm for predicting the joint capacity as a function of the surface size.     

Experimental study on glued laminated timber beams with well-known knot morphology

Nowadays, the impact of knots on the failure behaviour of glued laminated timber (GLT) beams is considered by subjecting the single lamellas to a strength grading process, where, i.a., tracheid effect-based laser scanning is used to obtain information about knot properties. This approach single-handedly defines the beam’s final strength properties according to current standards. At the same time, advanced production processes of such beams would allow an easy tracking of a scanned board’s location, but, at this point, previously obtained detailed information is already disregarded. Therefore, the scanning data is used to virtually reconstruct knot geometries and group them into sections within GLT beams. For this study, a sample of 50 GLT beams of five different configuration types was produced and tested under static four-point-bending until failure. As for each assembled lamella the orientation and position within the corresponding GLT beam is known, several parameters derived from the reconstructed knots can be correlated to effective GLT properties. Furthermore, the crack patterns of the tested beams are manually recorded and used to obtain measures of cracks. A detailed analysis of the generated data and their statistical evaluation show that, in the future, dedicated mechanical models for such timber elements must be developed to realistically predict their strength properties. A potential approach, using fluctuating section-wise effective material properties, is proposed.     

Verklebung von Buchenholz für tragende Holzbauteile

The load bearing capacity of glulam could be enhanced by using lamella of European beech (Fagus sylvatica??L.). Essential requirements for the production of those beams are adhesives which allow for a durable und reliable bonding of lamella. The paper shows to what extent commercially available MUF and PU adhesives and adapted bonding parameters ensure gluing of European beech timber for load-bearing timber structures. Both beech wood containing heartwood and beech wood without heartwood were included in the study. With regard to technical approval, the chosen test methods were resistance to delamination according to EN??302-2 (2002) and shear test of glue line according to EN??392 (1995). From the two tested MUF systems it was evident, that delamination decreased by increasing the closed assembly time. The requirements of EN??301 (2006) were fulfilled for adhesive type??I and??II. Without any limitations beech wood containing heartwood could be glued using MUF-1. On the other hand, the results of the PU adhesive were insufficient. Light-optical micrograph examinations show that long assembly times secure the formation of a glue line. It was assumed that the curing process is retarded by beech wood. No relationship was found between the results of the shear strength test and the delamination test. For quality control of glulam the delamination test is recommended. Based on the positive results an application for technical approval of glulam consisting of beech was made. For this reason beech-glulam can be generally utilized in service class??1.     

常温固化大豆蛋白胶黏剂制备非结构集成材研究

研究利用豆粉、环氧类固化剂、水性弹性体乳液制备了一种常温固化大豆蛋白胶黏剂,并用于制备非结构集成材.结果表明,胶黏剂最优配方为18 g豆粉、82 g水、2.52 g环氧固化剂、36 g水性弹性体乳液,胶黏剂固体含量27.8％,适用期可达36 h;集成材优化制备工艺为涂胶量190～210 g/m2、加压时间3 h、加压压...     

Analytical cracking load estimation of Laminated Veneer Lumber (LVL) beams with holes

Predicting the load-carrying capacity of timber beams with holes cannot be performed by usual analysis methods if the failure of the beam is governed by the crack initiation and propagation around the hole at low load levels. Predicting the cracking load is an important design issue because it corresponds to the load-carrying capacity of the timber beam before the crack propagation. One of the models that can be used for the fracture formulation is that of a beam on elastic foundation. In this model a part of the beam is assumed to follow the conditions of beam with elastic foundation which has spring stiffness equal to the fracture properties of the material in the crack surface. Based on beam on elastic foundation model, the cracking load prediction is the target of the paper. Some closed form solutions for the beam with hole are presented. The formulation has been derived for two cases of pure shear and pure bending moment. Finally a semi-empirical formulation for combination of shear and bending moment in the section is presented. The model predictions are compared with the results of an experimental program showing good correlation. The analytical model can therefore be proposed for future revisions of codes of practice such as the Eurocode 5.     

Prefabricated timber walls anchored with glued-in rod connections: racking tests and preliminary design

A new beam and post system for multi-storey timber buildings has been developed in Sweden. The building is braced with timber walls constructed from two Kerto-Q LVL boards glued and screwed onto a glulam frame. The walls are prefabricated off-site and can be connected to the foundation using either glued-in steel rods with metric thread or nail plates. Introductory racking tests of full scale walls anchored with glued-in threaded rods were performed. The paper presents the results of the experiments and discusses the use of the transformed section method to predict racking capacity of the anchored wall. To evaluate the strength of the glued-in rods, a newly proposed model was employed. An analytical study was conducted to investigate the role of the sheathing and the contribution of the axial force on the racking capacity of the walls. The wall panels tested in this experimental programme showed high strength and stiffness under racking load. The anchoring joints with glued-in steel rods with metric thread demonstrated a high load-carrying capacity with, however, large scatter and a brittle failure mode characterized by pull-out from the timber member. The transformed section method was successfully used to predict the racking capacity of timber walls anchored with glued-in steel rods.     

Mechanical performance curves for the strength grading of maritime pine

Finite element models (FEM) were used to simulate beam sections in bending with a knot. They aim at evaluating the mechanical behaviour of timber considering the effect of this singularity on resulting bending strength. By using linear elastic theory and an adapted failure criterion (Tsaï–Hill), an equivalent state can be deduced, giving information about the influence of knot on the load bearing capacity of the element. Several 2D-FEM were then developed using the free software Castem^® by considering plane strain. The beam section was considered like a composite material with two materials; the wood and the knot. The slope of grain around the defect was implemented by using a simple exponential equation parameterized as a function of the radius of the singularity. The location and the size of the defect and the longitudinal modulus of elasticity of wood were investigated in this study. Results exhibit performance curves specific to bending strength. These numerical performance curves were not used to predict the bending strength but to identify some thresholds, function of the size and the location of a critical knot and the stiffness of structural beams. 188 beams with a length of 4.5 m and a cross-section of 200?×?100 mm² were tested to study the efficiency of numerical thresholds. This study shows that it is possible, by identifying only the geometrical properties of the critical defect and the stiffness of the structural beam, to improve the prediction of the bending strength of beams with a large cross-section in Maritime pine up to C40.     

Experimental investigation of end-notched glulam beams reinforced with GFRP bars

End-notched timber beams can have a significant decrease in load-carrying capacity as a consequence of stress concentration due to height reduction at the supports. To prevent crack opening and its propagation at the notches, these beams are usually reinforced. In this paper, extensive experimental research on end-notched glulam beams reinforcement with glass fibre reinforced polymer - GFRP bars is presented. Bending tests on unreinforced and reinforced end-notched glulam beams were carried out until the point of failure. Behaviour of tested beams is described through load–deflection diagrams, failure modes and ultimate loads. In addition, different reinforcement schemes of notches were analysed in order to determine an optimal configuration under the aspect of load-carrying capacity improvement in relation to the unreinforced beams. The effectiveness of GFRP bars as a reinforcement of end-notched glulam beams was evident from significantly increased ultimate load and deformability. The design procedures for unreinforced end-notched beams according to Eurocode 5 and reinforced end-notched beams according to German national annex of Eurocode 5 were analysed through comparison with the experimental results. Recommendations for the design of GFRP reinforcement are given.

     

Stabwerkträger mit geleimten Knotenpunkten

The strength behaviour of 27 glued node joints for plane frameworks was determined in short-time failure tests. The different types of nodes and varying types of joints per node were tested. The longitudinal changes of the members at the nodes were measured, using glue-fixed strain gauge strips and novel strain gauge extensometers. After evaluating the test results from single nodes, not only shorttime failure tests, but also tests under dynamic long-duration load (about 10⁶ cycles) with concluding failure tests were carried out on twelve glued frameworks. The results confirm the reliable bearing behaviour of these types of structure. A reduction in bearing capacity due to dynamic long-duration load could not be detected.     

Stabwerkträger mit geleimten Knotenpunkten

The strength behaviour of 27 glued node joints for plane frameworks was determined in short-time failure tests. The different types of nodes and varying types of joints per node were tested. The longitudinal changes of the members at the nodes were measured, using glue-fixed strain gauge strips and novel strain gauge extensometers. After evaluating the test results from single nodes, not only shorttime failure tests, but also tests under dynamic long-duration load (about 10⁶ cycles) with concluding failure tests were carried out on twelve glued frameworks. The results confirm the reliable bearing behaviour of these types of structure. A reduction in bearing capacity due to dynamic long-duration load could not be detected.     

Festigkeit großflächig geleimter Knotenverbindungen

The load bearing behaviour of joints with large overlapped glued areas was tested experimentally in 203 specimens. The following parameters were varied: The angle of gluing (0°, 30°, 60°, 90°), size of overlapped glued area (50, 100, 200, 400, 600 cm²), clamping pressure (0.2 and 0.6 MPa) and pressure technique (conventional clamping pressure, pressure by nailing), wood quality (knots in the overlapped glued area) and the kind of load transmission in the overlapped glued area (shear by loading in tension-compression, shear by loading in pure tension). Corresponding to the joint design the shear strength values obtained ranged from 1.4 MPa to 3.8 MPa. The angle of gluing, the kind of load transmission and, in small overlapped glued areas, the tension stresses perpendicular to the grain as a result of excentric chord junction were found to be the decisive parameters. Nailed-glued joints proved equivalent to conventional glued joints.     

Einfluß des querzugbeanspruchten Volumens auf die Tragfähigkeit gekrümmter Träger und Satteldachträger aus Brettschichtholz

To study the influence of volume on the load carrying capacity of glued-laminated curved beams and curved pitched beams tests with full-scale specimens have been carried out. The distribution of the tension stresses perpendicular to the grain was analysed using a finite element computer programme. The relationships between the tensile strength and the stressed volume, the shape of the beams and the load conditions is evaluated on the basis of Weibull's theory of brittle fracture. A comparison of the results with the current design proposals in the draft of EUROCODE 5 “Structural timber design code (1987)” indicated higher characteristic ultimate tensile stresses and for some cases new stress distribution factorsk _dis.