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.     

Bonding performance of heat treated wood with structural adhesives

Bonding of untreated, intermediate (hydro-thermolysed) and heat treated wood with melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF) and polyurethane (PUR) adhesives was studied. An industrial heat treatment process (Plato ^®) was used, which included two separate heat treatment stages and a drying stage in-between. Laminated beams having four lamellas were prepared from untreated and treated timber for mechanical testing of the bond lines. The results of the tests showed that heat treatment affected the shear strength and the delamination of the laminated wood depending on the adhesive system used for bonding. The PUR and MUF adhesives performed in a rather similar way, and better than the PRF adhesive. The shear strength of laminated wood bonded with the waterborne MUF and PRF adhesives decreased for the specimens made of hydro-thermolysed timber and decreased further for the specimens made of fully heat treated timber. The difference in adhesive bond shear strength between untreated, intermediate and fully treated wood was less obvious in the case of the PUR adhesive. Delamination of the PRF bond line decreased drastically for all the specimens made of heat treated timber.     

Influence of low and moderate temperature kiln drying schedules on specific mechanical properties of Norway spruce wood

The knowledge of the influence of different temperature ranges on mechanical properties of kiln dried timber is very important in structural applications. The current paper deals with an investigation of the influence of low and moderate temperature drying schedules on the selected mechanical properties of Norway spruce wood (Picea abies?L.). The drying experiments were performed at four different temperatures (45, 55, 65, and 80°C) and clear wood specimens were prepared for the mechanical tests, three-point bending test, impact bending test, and tensile test perpendicular to the grain, respectively. The results of this systematic investigation were statistically analysed and as a conclusion, it can be stated that the mechanical properties investigated were not influenced by the drying temperatures applied.     

Load carrying capacity of curved glulam beams reinforced with self-tapping screws

The aim of this research was to determine the load carrying capacity of curved glulam beams reinforced perpendicular to grain and subjected to climate induced internal stresses. The self-tapping screws used as reinforcement prevent the glulam to split along the grain causing failure perpendicular to grain. Different tests were performed; specimens seasoned in dry and moist climate, specimens exposed to single climate change and reinforced specimens seasoned in a dry and moist climate. Specimens without reinforcement where tested to failure and then reinforced and tested again. From the tests it can be concluded that reinforcement improves the capacity of the beam in some cases up to 50% compared to unreinforced beams. The effect of moisture gradients on the capacity is significant and the worst case is when the beams are in a moistening phase.     

人工林杉木胶合工艺优化研究

采用SAS全因子试验设计,研究了胶粘剂类型、涂胶量、单位压力、加压时间和胶合面纹理五个因子对人工林杉木木材胶合性能的影响,并对其胶合工艺进行了优化。研究结果表明:胶粘剂类型对杉木木材常态胶合剪切强度和木破率的影响不显著,但是老化处理后API和PF的胶合剪切强度和木破率比PVAc和UF的要高;在以API为胶粘剂时,涂胶量、单位压力、加压时间和胶合面纹理对杉木胶合剪切强度和木破率的影响显著,优化的胶合工艺为涂胶量250g/cm2,单位压力1.5 MPa,加压时间50 min和胶合面纹理为弦切面-弦切面。     

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.

     

Load carrying capacity of curved glulam beams reinforced with self-tapping screws

The aim of this research was to determine the load carrying capacity of curved glulam beams reinforced perpendicular to grain and subjected to climate induced internal stresses. The self-tapping screws used as reinforcement prevent the glulam to split along the grain causing failure perpendicular to grain. Different tests were performed; specimens seasoned in dry and moist climate, specimens exposed to single climate change and reinforced specimens seasoned in a dry and moist climate. Specimens without reinforcement where tested to failure and then reinforced and tested again. From the tests it can be concluded that reinforcement improves the capacity of the beam in some cases up to 50% compared to unreinforced beams. The effect of moisture gradients on the capacity is significant and the worst case is when the beams are in a moistening phase.     

Lignin-based cold setting wood adhesives structural fingerjoints and glulam

A traditional-type cold-setting wood adhesive as well as separate applications fast-set wood adhesives for exterior-grade structural fingerjoints and glulam based on soda bagasse lignin were developed and tested. Their results satisfy the requirements of the relevant international specifications. The soda bagasse lignin used composed approximately 75% of the total adhesive solids while resorcinol content as low as 13.6% on liquid resin was used, with good results. On top of their novelty and of their excellent performance these adhesives appear to be economically very attractive.     

A semi-automatic method to determine the wood failure percentage on shear test specimens

An alternative method to quantitatively determine the wood failure percentage on shear test specimens was developed, experimentally tested and successfully applied to specimens with typical combinations of adhesive types and wood species used by the glulam industry in Switzerland. The method consists of a staining technique for a better differentiation of wood fibers and adhesive, and an image processing procedure for a standardized estimation of the fractured surface ratio covered by wood fibers and adhesive, respectively. The semi-automatic method allows for a more objective determination of wood failure percentage and is suitable for quality control in the glulam industry.     

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.     

Wechselwirkung zwischen der Anisotropie der Schwind- und?Quellma?e sowie Holzfeuchte-?nderungen in der Ebene von?gekr��mmtem Brettschichtholz

Findings on the differential shrinkage in the cross-section of solid timber as cylindrically orthotropic material are applied to the plane of curved softwood glulam. From an engineering point of view, the differential shrinkage in the plane of curved glulam is characterised by a ratio of longitudinal shrinkage to averaged shrinkage perpendicular to the grain of 1:24. For solid timber, the ratio tangential to radial is about 2:1. By means of modelling glulam as part of a circular ring it is shown that curvature variations are caused by both pronounced differential shrinkage in the plane of curved glulam and moisture content variations in the hygroscopic range of wood. This can result in constraining and internal stresses in structural systems.     

Festigkeiten von Hirnholzverbindungen mit dicken Epoxidharzfugen

The tensile strength of epoxi resin-filled end to end joints of spruce were tested on specimens of 8×50 mm and 60×80 mm. The bond strength was influenced by the viscosity of the resin at the time of specimen preparation. The tensile strength of the small specimen was about 1/4 of the strength of small clear solid wood specimens. The MOE decreased with increasing glue lines thickness from 8.000 N/mm² to 4.000 N/mm². According to the analysis of fracture morphology the causes for the relative low strength of the end to end joints are due to weak mechanical and probable also chemical adhesion between wood surface and resin as well as in notch tension at the contact surface. The bending strength of scarf joints was about double that of butt joints. The maximum bending strength, however, reached only 70% of the solid timber. The tests have indicated that epoxi resin-filled end to end joints cannot substitute solid timber for structural purposes because of insufficient mechanical strength. Further tests with other fillers such as polymer concrete, and other timber joints show more encouraging results.     

Integrity check of structural softwood glue lines: correspondence between delamination and block shear tests

In factory production control (FPC) of glued-laminated timber the integrity of glue lines according to product standards has to be checked by carrying out either delamination or block shear tests. The chosen test method should provide reliable and reproducible results, and from a practical point of view should be as cheap and easy to perform as possible. An extensive experimental study aiming at systematically comparing the outcomes of delamination and shear tests on structural softwood glue lines using different wood species and adhesives was carried out. In the delamination tests, the length of open glue lines was assessed after subjecting the test specimens to vacuum- and pressure-soaking and to subsequent drying. In the block shear tests, the shear strength and wood failure percentage were determined. Only if the benchmark values in the product standards were met or exceeded, the respective specimen passed the test and the glue lines were regarded as sound. As a main overall result it turned out that the outcomes of both test methods generally agree well, but will not necessarily lead to the same pass/fail conclusion in any case. Therefore, for FPC the choice of their application should be defined more specifically.     

Production of glued laminated timber with copper azole treated maritime pine

A study was conducted to evaluate the performance of glued laminated timber (glulam) of maritime pine treated with a copper azole preservative product. Shear strength of glue lines met the requirements of EN 386 (2001) with no influence of clamping pressure and cure temperature. According to the same standard, delamination was satisfactory for higher cure temperatures applied with a clamping pressure of 0.6?N/mm². Finger joints made with treated wood gave satisfactory bending strength. The preservative treatment did not influence the modulus of elasticity of the beams. According to a Monte Carlo simulation and following the requirements of EN 1194 (2002), glulam of class GL 28c can be manufactured if visually graded maritime pine of classes E and EE (NP 4305 1995) is used in the inner and outer lamellas, respectively, and class GL 24h when using both grades in equal proportions randomly distributed through the glulam element.     

Technologische Eigenschaften von armierten Polymerbetonen und Untersuchungen zu deren Eignung als Holzersatzelemente

Polymer bonded concretes and mortars with mineralic or organic additives have a wide range of use today within the building trade, specially to redevelop buildings and to procedure special structural members. To investigate the possible use of these concrete within the timber construction trade, tests were carried out with reinforced structural elements of polymer bonded concrete, today often used as wood substitute elements. It was proved, that the rules of design and reinforcement of concrete are also applicable for wood substitute elements. To reduce the great creep deformations of the polymeric concretes a reinforcement in the pressed area of bending beams must also be inserted.     

Einfluß keilgezinkter Lamellen auf die Biegefestigkeit von Brettschichtholzträgern

The reliability of a computer model was verified by bending tests. Eight test series with three replications were performed with glulam beams having certain characteristic attributes in the tension lamellae. A satisfactory agreement was stated between the calculated and the tested ultimate loads. The computer model presented may be used for further problems to be treated in glulam research.     

Shear strength of glued laminated timber made from European beech timber

This paper describes an investigation into the shear strength of glued laminated timber (GLT) made from European beech. Special consideration was paid to the possible strength influence of a frequently occurring discolouration of the timber, termed red heartwood, which is inherent to the species. The beech lamellae were visually graded according to German hardwood grading standard DIN?4074-5. Grade LS13, conforming to European hardwood strength class D40, was split into two sub-grades LS13? and LS13+. Additionally, modulus of elasticity (MOE) was determined by ultrasound pulse, longitudinal vibration and static tension tests. Sub-grade LS13+ showed a mean density and MOE of 690 kg/m³ and 14,800 N/mm², respectively. The GLT shear strength was evaluated by means of four-point bending tests on structural sized I-shaped beams with a depth of 0.6?m and a span to depth ratio of 5:1. The slightly inhomogeneous build-up of the cross-section conformed to glulam strength class GL42c. Two beam samples were investigated, each with seven specimens, where one grouping had no red heartwood and the other with a high red heartwood in the web laminations. Additionally block shear tests on bond line strength were performed with standardized small specimens according to EN?392. Neither the beam shear capacity tests nor the bond line block shear tests revealed an influence of the red heartwood discolouration on strength. The fifth-percentile value of shear strength of all beams was 3.5?N/mm². The results of the block shear tests indicate that the present requirements on minimum block shear strength are set too low in the European standard EN 386 with regard to beech GLT.     

Bondline shear strength and wood failure of European and tropical hardwood glulams

The study reports on block shear investigations with bondlines of face-glued laminations and matched solid wood specimens from hardwood glulam (GLT) beams produced industrially from eight technically and stand volume-wise important species. The European hardwoods comprised oak, beech, sweet chestnut and ash and the tropical species were teak, keruing, melangangai and light red meranti. The adhesives were phenol-resorcinol and melamine-urea. When combining all species in one sample, a rather strong linear relationship of bond and wood shear strength was observed. The ratio of bond vs. wood shear strength was for all species on the mean value level?≥?0.9, and likewise (with one exception) for the respective strengths’ 5%-quantiles. Consistent with literature, the test results showed no significant correlations between bond shear strength and density, wood shear strength and wood failure percentage of individual species, respectively. The investigations render the methodological basics of some international standards on bond quality verification as being inappropriate. New, empirically validated hardwood GLT bond requirements are proposed for discussion and implementation at the CEN and ISO levels. The strength ratio specifications reflect respective ANSI provisions, yet the reference quantity wood shear strength is now determined in an unbiased manner from matched GLT specimens. The wood failure verification proposal is based on the 10%-quantile and mean level for initial type testing and factory production control. The requirements further account for the pronounced difference observed in scatter of wood failure between European and tropical species.     

The effect of climate variations on glulam—an experimental study

Climate variations affect timber structures by causing moisture induced stresses which may lead to cracks in timber members. The paper presents experimentally determined moisture induced stresses that arise perpendicular to the grain of glulam specimens during exposure to 1-dimensional drying and wetting. Although to a certain extent reduced by creep effects, the determined tensile stresses can significantly exceed the characteristic tensile strength of glulam. Additional measurements of moduli of elasticity (E) and hygroexpansion coefficients (α) revealed that these parameters are strongly affected by the geometrical configuration (pith locations) of the specimens. By means of all the parameters determined here, it can be avoided that parameters have to be selected from different literature sources, which introduces large uncertainties into the numerical simulations.     

Verifikation von Festigkeitsmodellen für die Brettschichtholz-Biegefestigkeit

The aim of the study was to validate predictive values of new strength models for the characteristic glulam bending strength. Softwood glulam beams were tested in bending. In order to exactly assess the structural and mechanical properties of the laminations used for the production of glulam beams, representative samples were taken from the boards and the finger joints and tested in tension. The experimental investigation was conducted on two different strength levels in order to obtain two independent comparisons between predictive and empirical values. The experimental values of the board and finger joints were used as input data for the strength models to calculate the predictive values of the glulam beams. The predictive values for the characteristic glulam bending strength and the experimental ones differed not more than??6%. This indicates that the new strength models are suitable to reliably determine the characteristic glulam bending strength.