Beech Laminated Veneer Lumber Research Articles

Bonding performance and surface characterization of cold-bonded acetylated beech (Fagus sylvatica L.) laminated veneer lumber.

Acetylation of wood with acetic anhydride reduces the wood-moisture interaction, improves the dimensional stability and resistance against biodegradation. However, the adhesive bonding is affected by the modification, which is crucial to manufacture engineered wood products, such as laminated veneer lumber (LVL). In this study we report the bonding of 8-layered acetylated beech (Fagus sylvatica L.) LVL boards to 2-layered LVL beams. The beams were glued together at room temperature adding three common load-bearing construction adhesives: melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF), and one-component polyurethane (PUR). The bonding performance was tested by assessing its dry and wet tensile shear strength(TSS) and wood failure percentage(WF). Also evaluated were the material's density and moisture content(MC). The surface was characterized prior to bonding by its pH, roughness, and contact angle(CA). The adhesive penetration was observed by fluorescence microscopy. Aside from MUF, applying PRF and PUR adhesives achieved good bonding performance on acetylated LVL and references. Acetylated LVL displayed a more hydrophobic behaviour, a higher pH, a somewhat smoother surface, and an increased density.

A new concept for column-to-column connections for multi-storey timber buildings—Numerical and experimental investigations

A new jointing concept for the vertical load transfer between glued laminated timber (GLT) columns in multi-storey timber buildings has been developed. The basic components of the connection are two stepped, pyramid-shaped inserts made from block-glued laminated veneer lumber (LVL), which are bonded oppositely into corresponding cavities of the jointed GLTs. The load transfer between the mating columns relies on an intermediate LVL pin, fitting into pyramid-type cavities. Beech LVL was chosen for both, insert and pin, due to its 2.2 times higher characteristic compressive strength as compared to the employed high-strength softwood GLT (GL32h). The form fit of the stepped components which are bonded together relies on high precision robotic milling and a gap-filling two-component polyurethane adhesive. Manufacturing and mechanical performance of the scalable connection was investigated with full-sized specimens, achieving up to 90% of the characteristic GLT compressive capacity of 1040kN for a cross-section of 180mm×180mm. A parametric model based on the finite element method (FEM) showed a good agreement with empiric strain distributions and provided a stress-spread angle of 35° for the concentrated vertical load for an analytical design approach. The location and magnitude of tensile stresses perpendicular to the grain causing splitting failure was identified by FEM. Reinforcement by self-tapping screws proved a significant load-capacity enhancement. The mean experimental axial compressive load capacity exceeded the analytical prediction by 19%. First investigations on the rotational stiffness delivered a value of 240kNm/rad. The column utilization due to additional directly supported floor slabs was assessed exclusively by FEM. The calculation results predict a significant reduction of the tensile stresses perpendicular to the grain to be verified experimentally.

Properties of heat-treated beech laminated veneer lumber reinforced with carbon fiber fabric

Carbon fiber fabric reinforced laminated veneer lumber (RLVL) pieces were prepared by using heat-treated beech (Fagus orientalis Lipsky) veneers with polyurethane (PU) adhesive as the binder. Carbon fiber fabric was tested in three different locations with solid material and non-reinforced samples (on the bottom adhesive line, upper adhesive line, and upper and bottom adhesive lines-symmetrical). Prior to the manufacture of LVL and RLVL, heat treatment was conducted in a laboratory oven at three temperatures. These temperatures were 150, 170, and 190 °C. Tests were performed on LVL and RLVL to determine their air-dried density, modulus of rupture (MOR), and modulus of elasticity in bending (MOE). Experimental test results showed that reinforcement with carbon fiber increased the air-dried density, MOR, and MOE. In addition, carbon fiber fabric placed symmetrically close to bottom and upper surfaces gave the highest MOR and MOE values. However, locating the carbon fiber fabric closer to the bottom surface tended to give higher mechanical properties for the reinforced LVL.

The durability of acetylated beech (Fagus sylvatica L.) laminated veneer lumber (LVL) against wood-destroying basidiomycetes

The overall aim of this study was to investigate the durability of acetylated beech (Fagus sylvatica L.) laminated veneer lumber (LVL) against wood-destroying basidiomycetes. The secondary objective was to test whether the specimen size affects the mass loss and durability assessment of wood-material under test. The durability test was based on the pre-standard prEN 113-3. Six materials (acetylated beech LVL, untreated beech LVL, beech solid wood, pine sapwood (Pinus sylvestris L.), larch (Larix decidua Mill.) 3-layer slab, larch solid wood) were tested using three specimen geometry designs (50 × 25 × 15 mm3 as well as 50 × 50 × 19 mm3 with and without sealed edges) against Coniophora puteana, Rhodonia placenta, Gloeophyllum trabeum, Trametes versicolor, and Pleurotus ostreatus. The durability assessment was made using the arithmetic mean and median percentage mass loss (ML), the relative ML (x-values), and the decay susceptibility index (DSI). It was found that mass loss was affected by the test fungus, the material, and the specimen size and design, with the latter being the most essential factor in this study. In addition, the assessment parameter had a significant effect on the durability classification. Furthermore, small differences in ML resulted in different durability classes (DC) in some cases, whereas large differences in ML did not. However, acetylated beech LVL was always considerably durable (DC 1) against all tested fungi independent of the specimen design and durability assessment method.

Development and investigation of an innovative, light-weight, two-way spanning timber-concrete composite slab

Timber-concrete composite (TCC) slabs play an essential role in modern timber construction. Consisting of a timber member in the tension zone, a concrete layer in the compression zone and a shear connection between the two parts, TCC slabs feature a number of advantages that make them ideally suitable for use in both office and residential buildings. However, currently used TCC slab systems carry loads only in one direction. In the frame of a large research project in cooperation with the industry, an innovative, light-weight, two-way spanning timber-concrete composite slab using beech laminated veneer lumber with steel tubes as connectors was developed. This paper presents the main results of the research project. First, the structural behaviour of the connection is characterised and discussed based on extensive experimental and analytical analysis. Second, the load-bearing behaviour of the TCC slab investigated in uniaxial bending tests is presented. The uniaxial bending tests showed a remarkably ductile global load-bearing behaviour due to the ductile failure mode in the connectors. A developed analytical model can describe the observed load-bearing behaviour of the tested TCC members accurately. Finally, the biaxial load-bearing behaviour of the TCC slab is characterised and discussed based on a large-scale experiment that allowed to perform static and dynamic tests on the same specimen in different support conditions. A substantial increase in stiffness and fundamental frequency in biaxial versus uniaxial support conditions was observed and accurately modelled, showing the great potential of the novel TCC slab.

Guidelines for a Finite Element Based Design of Timber Structures and Their Exemplary Application on Modelling of Beech LVL

Design verifications of buildings are usually carried out supported by a finite element analysis (FEA), for which, however, there are only a few and almost exclusively non-binding application rules. Within the Cluster of Excellence Integrative Computational Design and Construction for Architecture (IntCDC) at the University of Stuttgart, Guidelines for a Finite Element-Based Design of Timber Structures have been developed. The scope of the guidelines is daily engineering practice, expert engineering applications and product development and certification. Essential parts of the guidelines are design procedures, modelling (including geometrical, material and imperfection modelling), analysis, model verification and validation and design. The content and application of the guidelines are described and illustrated in this paper using two benchmarks. These two benchmarks, which are based on experimental investigations, deal with the elastic material modelling of glulam made of beech laminated veneer lumber (beech LVL) and dowel-type connections for beech LVL members. The experimental basis of the benchmarks is described. With the experiments for the benchmarks, all Poisson’s ratios and the complete elastic material stiffness matrix of beech LVL are determined by means of an optical measuring system. The experimentally determined stiffnesses of the investigated dowel-type connections in beech LVL are compared with normative values. Based on the experiments, a numerical model is developed in RFEM (Dlubal).

Load-bearing behaviour of partially threaded screws in hardwood

ABSTRACT Partially threaded screws are essential fasteners in timber structures, and their use in combination with hardwoods is promising in order to obtain high-performance connections. Depending on the used wood species, the load-displacement behaviour and failure mechanisms of connections change, and, in particular for high-density species, brittle failures may occur more frequently with rather low ductility of the connection. Consequently, a good understanding of the mechanical behaviour of connections at different displacement levels helps to assess where previous rules reach their limits for hardwoods. The determination of all input parameters serves as a basis for a subsequent test programme on single-shear timber-to-timber connections using beech laminated veneer lumber, birch, spruce glulam and predrilled and non-predrilled partially threaded screws. Tests on connections with one screw and groups of screws are conducted, stopped at predefined displacement stages of 5, 10 and 25 mm, and opened. A comparison of the screw deformation at the different displacement stages reveals that non-linear connection behaviour is quite different in softwood and hardwood. Essential aspects of the analysis are particularities of the load-displacement behaviour, fastener spacing, ductility of connections and the progressive plasticising of the fasteners.

Threaded rods grouted in beech laminated veneer lumber

AbstractGlued‐in rods are a class of adhesively bonded joints for timber engineering applications resulting in high‐strength and stiffness connections. However, the use of polymeric adhesives may lead to issues related if the temperatures exceed their glass transition temperature, restricting their performance under quasi‐static, or more critically, sustained loads. To overcome these, the substitution of polymeric adhesives by mineral high‐performance grout was investigated. It was found that primers have neither a significant effect on strength nor on the failure mode; threated wood surfaces, however, resulted in a significant improvement of the latter. Based thereupon, grouted‐in rods were manufactured. The best performance was achieved with a threaded wood surface, which achieved roughly 50% of the strength comparable adhesively bonded glued‐in rod's strength. While the obtained strength may seem quite low, it is important to remind that the latter will largely remain unaffected by temperature; accordingly, made at room temperature, the comparison between grouted and glued rods is in favor of adhesive bonding, it may well be different at elevated ones.

Experimental and numerical determination of the screw deformation in laminated beech veneer lumber as a result of moisture‐induced swelling

The strong hygroscopic behavior of wood can lead to severe moisture-induced swelling deformations, especially perpendicular to the grain direction. Fully threaded screws arranged in this direction can be used as reinforcement and to constrain those deformations. Depending on the wood moisture content increase, tensile stresses in the screws and compression stresses in the surrounding solid wood evolve. A sufficiently accurate prediction of the maximum stresses occurring is important for reliable structural designs. For this reason, moisture-induced swelling experiments for two different configurations of screw diameter and specimen dimensions were performed within this work. Due to its homogeneity and its high swelling rate perpendicular to the grain, beech laminated veneer lumber (LVL) was used. A moisture content increase from oven-dry to about 7% led to screw tensile stresses in the magnitude of the yield strength and to highly nonlinear effects in the surrounding wood material. By means of a two-dimensional finite-element-based numerical simulation model, taking into account orthotropic plasticity and moisture-dependent material properties, the experiments could be simulated with high accuracy. The numerical results agree well with experimental observations. Based on a small parameter study, new insights into the complex interaction behavior between beech LVL and fully threaded screws could be gained.

Calibration and comparison of two moisture content measurement methods for in situ monitoring of beech laminated veneer lumber

ABSTRACT A timber bridge constructed in 1815 was renovated in 2018, wherein the load-bearing system was reinforced with beech laminated veneer lumber (LVL) elements. Beech LVL is primarily found in dry and heated structures (Service Class 1). Although the material of this bridge would not be directly exposed to weather, it would be exposed to outdoor relative humidity ranging between 50% in spring and 90% or higher in autumn (Service Class 2). Thus, the moisture content development in the reinforcements was monitored to observe the response of the material under such conditions. The moisture content was measured using both sorptive and electrical resistance methods. The relationship between the electrical resistance of the material to moisture content and sorption isotherms was determined through laboratory experiments. Thereafter, the derived relationships were used to determine the moisture content measured on the bridge over a period of two years. Consequently, the two measurement methods resulted in overlapping of moisture content values, which remained within safe levels and are expected to be maintained in future. The overlapping moisture content measurements validated that either of the monitoring methods could be independently deployed to obtain reliable results.

Calibration of Electrical Resistance to Moisture Content for Beech Laminated Veneer Lumber “BauBuche S” and “BauBuche Q”

Electrical resistance measurements are often employed for the purpose of nondestructive long-term monitoring of wood moisture content (MC) in timber structures. As a structural material for high-performance load-bearing applications in such structures, beech laminated veneer lumber (LVL) enjoys a growing popularity. However, due to the processing of beech LVL affecting physical properties, calibration curves for bulk beech wood cannot be used. In this study, resistance was measured on 160 beech LVL samples equilibrated in four different relative humidity (RH) climates. The results show a difference not only between the beech LVL products “BauBuche S” and “BauBuche Q”, but also between measurements at two different depths. For each data set, parameters for calibration models using two and using three model parameters were determined by regression analysis to MC determined by the gravimetric method.

Wood-cement-composite behaviour of beech circular hollow sections

Beech laminated veneer lumber (LVL) circular hollow sections for use as nails in temporary geotechnical soil nailing systems were developed within this research. For the soil wall stability, beside a sufficient load carrying capacity of the nail cross section, a load carrying connection between the cement annulus and the nail is necessary. As cement grout ensures the bond between the nails and soil for the mobilisation of a soil reinforcement effect, a reliable connection between cement and wood surface is required. Occurring shear forces must be transferred from the nail into the soil through the wood-cement interface similar to reinforced concrete. Pull-off tests on clear LVL specimens with different cement and wood surface modifications as well as the results of large-scale push-out tests are presented. The results reveal a sufficient short-term bonding strength between the cement annulus and the wooden member to transfer the geotechnical loads, but also highlight the necessity of further research on long-term behaviour.

Recycleshell: Wood-only Shell Structures Made From Cross-Laminated Timber (CLT) Production Waste

The article describes a new construction system for shell structures made from door- and window cut-offs resulting from cross laminated timber (CLT) production lines. For the first time, form-fitting fasteners made of beech laminated veneer lumber are used in a shell made of engineered wood plates, enabling a particularly simple, fast and precise construction. The efficient planning and production of a multitude of differently shaped elements is made possible by a CAD plug-in developed by the authors. The article presents the general application possibilities and limitations of the system, as well as the case study of a shell demonstrator with a span of 12 meters, built from 229 hexagonal plates. A detailed structural analysis is presented for the demonstrator, including an experimental load test to confirm the FEM results.

Experimental testing of pre-stressed timber-to-timber composite (TTC) floors

The experimental out-of-plane performance of timber-to-timber composite floor structures was experimentally investigated. Specifically, 6-point bending tests were performed on seventeen full-scale floor specimens (6.4 m span) assembled with inclined screw fasteners (both single threaded and double threaded screws) and by using combinations of different timber products (e.g. spruce cross laminated panels, beech laminated veneer lumber beams/panels). For most of the specimens, a cambering and pre-stressing technique (CP procedure) was utilised by exploiting the compression force produced by screw fasteners. The experimental observations indicated that it is possible to obtain camber values that go from approximately 1/900 to 1/250 of the floor span, according to the design needs. As an indicator of the wide possible range of pre-stress forces, the vertical force required to eliminate the camber was monitored during testing, resulting in measured values ranging from 2.4 kN/m2 to 13.8 kN/m2 (0.6 m joist spacing). The applicability of the proposed solutions in real design scenarios was evaluated by comparing the experimental results with the predictions obtained by using a proposed analytical approach. A good correlation was observed between the test data and the theoretical predictions.

Accelerated curing of glued-in threaded rods by means of inductive heating – Part I: experiments

ABSTRACT Adhesive bonding is nowadays an integral part of timber engineering and sets the basis for efficient joint typologies, as well as for most Engineered Wood Products (EWP). Regarding the manufacturing of joints, in particular under on-site conditions, two-component (2K) adhesives, in particular epoxies (EPX) and polyurethanes (PUR), are preferred over one component (1K) adhesives, because of their ability to cure under ambient conditions. If compared to mechanical fastening, however, the relatively slow curing of most 2K-adhesives used in practice remains a serious disadvantage. This paper summarises investigations on induction heating as a way to speed up significantly the curing process of Glued-in Rods (GiR), a joint type used in timber engineering. Firstly, four commercially available adhesives were investigated with regard to their potential for accelerated curing, resulting in the determination of maximum curing temperatures, and heating rates. Based thereupon, a series of large-scale GiR, involving threaded rods and beech Laminated Veneer Lumber (LVL), were manufactured, and inductively cured. The curing time was dramatically shortened from days to 5 min. Resulting joint capacities depended upon the time elapsed since induction. The resulting findings served as a basis for subsequent investigations to be presented in the second part of this series Figure 16.

Influence of imperfections on the load capacity and stiffness of glued-in rod connections

Glued-in rods (GiR), which consist of rods glued into structural elements of wood, or engineered wood products, share the manufacturing quality concerns associated to adhesively bonded joints in general. Because of the difficulties associated with the bonding process, it is often argued within the timber engineering community that under the specific conditions prevailing in construction sites, it is not possible to meet the high-quality requirements for bonded joints typically encountered in other industries. The present study presents experimental evidence gathered on 240 individual probes in which the effect of several different types of defects (excessive wood moisture content, wood chips not removed from the borehole, simulated air inclusions by mixing polystyrene spheres, oil contaminated rods, and corroded rod) was investigated with threaded rods glued into beech spruce glued laminated timber (GLT) and beech laminated veneer lumber (LVL) using four adhesives (including 2K-epoxies and a 2K-polyurethane). Results were analysed by means of an analysis of variance (ANOVA). The findings showed that glued-in rods represent a relatively robust class of adhesively bonded joints, as the majority of defects types could not be distinguished on statistical ground from the reference sets, with the notable exception of air voids leading to a significant decrease of joint capacity.

Study on a newly developed diagonal connection for hybrid timber trusses made of spruce glulam and beech laminated veneer lumber

ABSTRACT The paper presents a truss concept characterised by structural simplicity and economic efficiency. The intended materials are spruce glulam for chords as well as posts and beech LVL (laminated veneer lumber) for diagonals, which are stressed in tension. Connections between the truss members are preferably be realised using self-tapping screws. The main focus of the paper is on a newly developed diagonal connection where the diagonals are screwed on to both outsides of the chords with countersunk screws and special washers. For this diagonal connection, a calculation model of the load-bearing capacity and the connection stiffness is presented. Tests were performed which encompass local and global stiffness examinations between screwed-on beech LVL diagonals and chords even for technically different screw connections. The corresponding test results agree with analytical values according to the proposed calculation model. A corresponding small-scale truss prototype was designed and tested. Computer-aided analyses of the truss deformations give first evidence for the actual feasibility of the truss concept employing the newly developed connection.

The influence of veneer thickness and knot proportion on the mechanical properties of laminated veneer lumber (LVL) made from secondary quality hardwood

The first objective of this work was to study the influence of veneer quality on the mechanical properties of laminated veneer lumber (LVL) made of secondary quality hardwood. The second objective was to propose an adapted veneer thickness that provides the optimum mechanical properties of LVL, taking the veneer properties into account. Forty-eight LVL panels glued together using polyvinyl acetate (PVAc) were prepared. The quality of fresh veneers was assessed by measuring veneer knot proportion, lathe check depth and lathe check interval. The static modulus of elasticity (MOE), dynamic MOE, modulus of rupture (MOR) and shear modulus were measured using destructive and non-destructive methods. The 3 mm thick veneer provided the optimum mechanical properties for LVL for both species. The test direction did not have any significant influence on the mechanical properties. In the flatwise direction, the average MOE values obtained were 13.2 GPa for beech LVL and 13.3 GPa for oak LVL, whereas the MOR was 72.0 MPa and 63.4 MPa, respectively. Increasing knot proportion in veneers results in a decrease in LVL MOE and MOR. Moreover, deeper lathe checks and higher lathe check intervals on veneer surfaces provide lower LVL shear modulus in the edgewise direction for both species. Internal veneer provides LVL with a higher density but weaker mechanical properties due to a higher knot proportion in the internal veneer.

Fracture mechanics based joint capacity prediction of glued-in rods with beech laminated veneer lumber

ABSTRACTGlued-in rods (GiR) represent a very successful type of adhesively bonded joints in timber engineering. Despite their apparent geometrical simplicity, their dimensioning still challenges practitioners. A major source of mechanical complexity resides in the orthotropic nature of the wood, or engineered wood products, as laminated veneer lumber (LVL). This paper presents a relatively simple design approach based upon fracture mechanics (FM) and associated double-cantilever beam (DCB) tests that complemented tensile tests for material characterisation. In comparison with the state-of-the-art related to FM in timber engineering, the paper presents a practitioner oriented approach of a yet complex set of GiR geometries involving beech LVL (M16-8.8 threaded rods embedded in cross sections of 120 × 120 mm2 and embedment lengths of 96mm, 128mm and 160mm). The developed numerical model resulted in a good description of the load-displacement of a series of full scale, including very good estimates of their load capacities. Additionally, it allowed for significant insights regarding the complicated relationship between geometry, orthotropy, strength of the component and failure of the GiR, as for examples the complex fracture process, and the importance of transverse tensile stresses, which play a preponderant role, equal in importance to shear stresses.

Adhesive properties of laminated veneer lumber poles for use in temporary soil nailing: bonding performance of curved structures in a high-alkaline environment

Circular hollow sections of beech-laminated veneer lumber (LVL) for the use as temporary geotechnical soil nailing systems are currently being developed. Due to their permanent subsoil cement embedment, investigations of the bond line quality of the timber sections are essential. This paper presents the bonding properties of flat and curved beech LVL after cyclic conditioning in a water–cement grout solution aimed at inducing short- and long-term alkaline attack of the timber. In total, 409 and 69 samples were tested in tensile shear tests after short-term and long-term conditioning, respectively. Three different adhesive systems, a one-component polyurethane adhesive, a melamine–urea–formaldehyde adhesive and a melamine–urea–formaldehyde adhesive modified by means of (polyvinyl)-acetate adhesive were investigated and compared. Short-term conditioning by submersion in boiling cement suspension was found to be a reliable method for testing the bonding performance rather than long-term conditioning. In the case of tensile shear tests of samples subjected to long-term treatment, wood material strength was the decisive criterion. Generally, tensile shear test samples of all investigated adhesives achieved reliable bonding for a pressing force up to 1.0 MPa. No relationship was recognised between the determined bonding failure and the wood properties tensile shear strength, wood failure percentage, fracture pattern and bulk density of veneers adjacent to the bond line. For the determination of the bond line integrity of curved veneer poles, it was necessary to test bonding quality in a combined test using curved and flat samples and to compare tensile shear strength with data determined on reference samples in the same veneer population without bond line.