Glulam Timber Research Articles

Analytical Approach to Predict Pre–Camber Deflection of the Pre-stressed Glulam Beams

A new approach to reinforce glulam timber beams using compressed wood (CW) has been developed by the Author. The compressed wood blocks are inserted into pre-cut holes on the top of glulam beams to produce pre-camber and to generate initial tensile and compressive stresses on the top and on the bottom extreme fiber of the glulam beam, respectively. A simple analytical approach has been developed to predict pre-camber deflections of the beams reinforced with three CW blocks. Assuming that moisture-dependent expansion of the CW block creates an interactive linear pressure between the glulam beam and the CW block, this would generate a pair of eccentric force away from the neutral axis of the beam. It is shown that the pre-camber deflection predicted at midspan of the short beams with various reinforcing arrangements give good agreement with the measured pre-camber from previous experimental results.

Experimental Study of the Lateral Resistance of Bolted Glulam Timber Post and Beam Structural Systems

AbstractTo study the lateral resistance of bolted glued-laminated (glulam) timber post and beam structures, two monotonic tests and eight cyclic tests were performed on full-scale, one-story, one-bay timber post and beam construction specimens. Four laterally strengthened structural systems were considered: a frame with an X-brace, a frame with a K-brace, a frame with a knee-brace, and a frame filled with light wood shear walls. Seismic performances were evaluated according to the experimental phenomena, hysteretic characteristics, stiffness degradation, strength degradation, and energy dissipation capacity. The results showed that the frame with an X-brace system and the frame with a K-brace system provided markedly better elastic stiffness but lower ductility. The frame with a knee-brace system showed increased elastic stiffness compared with the simple frame system and provided higher ductility compared with both the frame with the X-brace system and the frame with the K-brace system. Additionally, the...

Post-tensioning of glulam timber with steel tendons

This paper describes a series of four-point bending tests that were conducted, under service loads and to failure, on unreinforced, reinforced and post-tensioned glulam timber beams, where the reinforcing tendon used was 12mm diameter toughened steel bar. The research was designed to evaluate the benefits offered by including an active reinforcement in contrast to the passive reinforcement typically used within timber strengthening works, in addition to establishing the effect that bonding the reinforcing tendon has on the materials performance.The laboratory investigations established that the flexural strength and stiffness increased for both the reinforced and post-tensioned timbers compared to the unreinforced beams. The flexural strength of the reinforced timber increased by 29.4%, while the stiffness increased by 28.1%. Timber that was post-tensioned with an unbonded steel tendon showed a flexural strength increase of 17.6% and an increase in stiffness of 8.1%. Post-tensioned beams with a bonded steel tendon showed increases in flexural strength and stiffness of 40.1% and 30% respectively.

Special Issue on Performance of Timber and Hybrid Structures

Special Issue on Performance of Timber and Hybrid Structures

Long-Term Behavior of Posttensioned Timber Connections

AbstractTests regarding the long-term behavior of posttensioned timber connections have been performed at the ETH in Zurich. The aim of these tests is to estimate the loss in tendon force that can be expected during the lifetime of a structure. Several posttensioned beam and beam-column timber specimens exposed to controlled and uncontrolled environmental conditions and loaded only with the axial prestressing force have been monitored for more than one year. The force in the tendon was measured together with the strains in the specimens and temperature and relative humidity in the testing environment. Creep tests performed on small glulam timber blocks loaded in compression have been started as well. The estimated creep coefficients parallel and perpendicular to the grain are used to calculate the losses with an analytical model for the larger posttensioned specimens. A prestressing loss in the range of 5–10%, depending on the geometrical properties and environmental conditions, has been found at the end ...

Numerical simulation and analytical study of glulam timber beams

Glulam beams or glued-laminated beams consist of sawn lumber laminations (timber) bonded with an adhesive material. This paper, through the mathematical description of the contact conditions that apply at the interfaces of glulam beams and the development of two three-dimensional finite element models by the use of the ANSYS software package, studies the flexural properties of unreinforced (UGB) and reinforced (RGB) glulam beams. The first computational model presents an unreinforced glulam beam that has been produced by three wood laminations of dimensions 6 by 3.6 by 176 cm. The latter one describes a reinforced glulam beam, which has been produced by gluing a 0.15 cm thick steel plate at the bottom edge of the previously described beam. The computational analysis indicates that the two glulam beams have significantly different bearing capacities under the same load and support conditions. The failure mode of the UGB is brittle whereas the one of the RGB is ductile. The numerical results of both models are in close agreement with experimental ones from the international literature. Keywords: Glulam Timber Beams, Numerical Simulation, Contact.

Measuring-up in timber: a critical perspective on mid- and high-rise timber building design

Architects, engineers and researchers alike often cite practical reasons for building with wood. Since the development of curved glulam beams and columns over a century ago, the widespread use of massive structural timber elements has allowed architects and engineers to design and build in wood with unprecedented speed and scale. Moreover, rising concerns of climate change and the carbon-dioxide emissions associated with construction encourage the use of wood as a viable alternative to steel and concrete, due to CO2 sequestration in trees.In mid- and high-rise buildings, the current shift from steel and concrete towards massive structural timber elements like glulam, laminated-veneer lumber (LVL) and cross-laminated timber (CLT) is evident in a number of recently completed timber buildings in Europe, ranging from seven to nine storeys. Several speculative design proposals have also been made for ‘timber towers’ of thirty, fortytwo and even sixty-five storeys, recognising that designing with massive structural timber elements in high-rise buildings is still in its infancy. This paper offers a new perspective on building with wood at this scale, beyond carbon sequestrationand construction.

Numerical Modelling of the Initial Stress and Upward Deflection of Glulam Beams Pre-Stresseed by Compressed Wood

A new approach to reinforce glulam timber beams has been developed by using compressed wood (CW) which is made of a lower grade wood through densification processes. In the reinforcing practice, compressed wood blocks are inserted into pre-cut holes on the top of glulam beams to produce pre-camber and to generate initial tensile and compressive stresses on the top and the bottom extreme fibre of the glulam beam. In order to optimize the size, the number and the location of CW blocks, 3-D finite element models have been developed. 3D non-linear finite element models have been developed to simulate the pre-camber of Glulam beams locally reinforced by compressed wood blocks. The models developed have also produced the initial tensile and compressive stresses at the top and bottom extreme fibres with building-up moisture-dependent swelling on the CW blocks. With the pre-camber and the initial stress state that cancel out proportions of working deflection and stresses.

Untersuchungen zur Festigkeit der Klebefugen von historischem Brettschichtholz

AbstractDer Beitrag befasst sich mit Untersuchungen zur Festigkeit von Kaseinverleimungen von Brettschichtholz in Hetzer‐Bauweise. Kaseinverleimungen treten heutzutage nur noch im Bereich von Instandsetzungsarbeiten an historischen Brettschichtholzkonstruktionen in Erscheinung.Eine Bewertung der Ergebnisse nach der DIN EN 392:1996 erwies sich als ungeeignet, da sich diese Norm auf moderne, auf Kunstharzen basierende Klebstoffsysteme bezieht. Um dennoch eine Bewertung vornehmen zu können, wurde auf frühere Veröffentlichungen zur Festigkeit von Kaseinverleimungen bzw. Regelwerke des Holzflugzeugbaus der 1950er‐Jahre zurückgegriffen.Die Auswertung eigener Untersuchungen ergab, dass Kaseinverleimungen auch nach 100 Jahren Nutzungsdauer, insbesondere unter Wechselwirkung von Feuchtigkeit und chemisch‐aggressiven Medien, eine relativ hohe Tragfähigkeit und Dauerhaftigkeit aufweisen.Studies on the strength of the adhesive joints of historic glued laminated timberThis paper presents a study on the strength of casein glue layers of glulam timber in Hetzer's construction method. Nowadays casein adhesive joints are only appearing in maintenance of historic glulam structures.An evaluation of the results according to DIN EN 392:1996 proved unsuitable, as this standard relates to modern, synthetic resins based adhesive systems. Therefore earlier publications on the strength of casein glue respectively regulations for wooden aircrafts from the 1950s were utilized in order to be able to assess these kind of adhesive joints.The results of this study revealed, that the casein adhesive joint has an relatively high loading capacity and durability even after 100 years especially under interaction of moisture and chemically aggressive media.

12 Years of Structural Health Assessment and Maintenance for the Glulam Timber Structure of the Mormanno Traffic Bridge in Calabria

The Mormanno traffic Bridge, built at the end of the year 2000 in South Italy, with its glulam exposed structure spanning around 70 m, is one of the most impressive traffic timber bridges in the world. The paper illustrates the various analysis executed for the assessment of the health of the structure and for the following maintenance actions during the first twelve years of service of the Bridge (2001-2013). In the year 2004 a first visit for the assessment of the health of the timber bridge was decided. A team of experts inspected the bridge in order to check the conditions of the timber structures and register all the damages produced by weathering, fungi and insects on the structural elements. The experts found the decay still limited but recommended a maintenance action capable to stop the initial degenerative process mainly due to a moisture content higher than normal. In the year 2008 [ a new inspection team visited the bridge for the second time in order to check the effectiveness of the measures of protection adopted four years before. Non-destructive testing were used to investigate the health of the main glulam arches. The decay in the members object of the previous maintenance in general had stopped or slowed down, but in some other parts the moisture content was still higher than acceptable and more protections were added. In the following years the maintenance was completed giving the bridge the possibility to extend its service with a sufficient level of safety. The frequent inspections of the bridge structure provided useful information for the maintenance of the Mormanno bridge but also in general for the design of the durability and for the maintenance planning of exposed timber structures. Fig. 1 the Mormanno Bridge in service

Post-Tensioned Glulam Beam-Column Joints with Advanced Damping Systems: Testing and Numerical Analysis

This article describes tests investigating a feasible source of passive damping for post-tensioned glue-laminated (glulam) timber structures. This innovative structural system adapts precast concrete PRESSS technology [Priestley et al., 1999] to engineered wood products combining the use of post-tensioned tendons with large timber members. Current testing is aimed at further improvement of the system through additional energy dissipation. Testing has favorably compared glue-laminated timber (not previously implemented in this way) with laminated veneer lumber (LVL) used in New Zealand. After initial benchmark testing with post-tensioning only, a simple, minimally invasive and replaceable type of hysteretic damper was added.

Comparison of Various Glulam in Physical Properties and Flexural Behaviors

The glulam is determined by, and therefore a representation of, a new kind of ecological structural materials. The aim of this study was to summarize the mechanical performance especially the flexural behavior of various kinds of glulam and the physical properties of their relevant original timbers including pseudotsuga menziesii, larch, Yi poplar, poplar, China fir, mongolian scotch pine and camphor. And then it established and analyzed the relationship between the two to contrast those timber species so as to provide engineers with some reference in selecting timber glulam.

A Creep Non-Linear FEM Analysis of Glulam Timber

This work is focused on a creep formulation in bending for timber beams, of glue laminated (glulam) type, which is suited to be implemented in both geometrical and material non-linear finite element method (FEM) models. It studies the performance of timber beams by accounting the deformation under increments of time and stress in the long term. The proposed creep formulation uses a power-law function expressing the creep strain by means of one coefficient and two exponents. This function needs a fewer number of material coefficients than a classical mathematical analysis of non-linear viscoelasticity, so design applications, such as FEM codes, may more simply describe the deformations in the long term. The formulation has been developed for cantilever beams made of glulam timber using experimental tests in bending and accounting the relations between strains, and both time and tension progresses. Also, the timber creep law has been numerically tested by FEM code runs, with large displacements and large strains assumptions, and further compared with a Navier analytical model. The scatters between experimental and analytical values, and FEM numerical values have reached a standard deviation of 4.06 and of 7.66%, respectively. The acceptable agreement in the comparison between experimental, analytical, and FEM numerical results have demonstrated the capability of the creep formulation to model the behavior in bending of a glulam timber. Also, the proposed creep model has shown an interesting ability to be employed in a FEM code to analyze a timber beam in bending, along time and load evolutions.

Pull-out strength of glued-in FRP plates bonded in glulam

The use of FRP laminates as connectors in glulam timber members is considered to be particularly promising since the durability of the system is improved and the application is easier and faster due to the lightweight properties of FRPs. In this paper an investigation aimed at characterizing the bond performance of FRP strips glued into glulam elements is presented. Several pull-out tests were carried out to assess the influence of various parameters: the bond length, the type of fibers for the reinforcement, the direction of the fibers in the FRP with respect to the timber grain. The elastic behaviour of the joint was also studied analytically and numerically. The ultimate strength of the experiments was compared with predictive expressions available from the literature.

Engineered Wood in Cold Climate - Application to Monitoring of a New Swedish Suspension Bridge

Engineered wood is increasingly used in large structures in Europe, though little is known of its behavior in cold climate. This paper presents the structural health monitoring (SHM) system of a newly built suspension bridge with a deck of glulam timber as well as a bond stability study regarding cold climate performance of engineered wood. The bridge is located in Skellefteå in northern Sweden, and it connects two parts of the city situated on opposite shores of the Skellefteå river. In this ongoing study of the timber-bridge, a structural health monitoring system is employed to verify structural design and long-term performance. This 130m-span bridge is monitored using GNSS receivers, MEMS accelerometers, laser positioning systems, wireless moisture content sensors, strain gauges and weather stations. Data from the monitoring systems is analyzed regarding accuracy, complexity, costs and reliability for long time use. Engineered wood application in bridges, sports centers and timber buildings are discussed. Bond stability of glulam structures in cold climate is also examined in a range of experiments ranging from small glued wood joints to full size glulam bridge performance over time. From an engineered wood material point of view, the study is relevant to cold regions such as Scandinavia, Canada, Alaska, Russia, and the northern parts of China and Japan etc. The engineered wood constructions in these areas will be exposed to low temperature in a quite long period each year. The goal is to determine how engineered wood behaves when exposed to temperatures between 20 °C to -60 °C.

Pull-out behaviour of axially loaded Basalt Fibre Reinforced Polymer (BFRP) rods bonded perpendicular to the grain of glulam elements

The behaviour of Basalt Fibre Reinforced Polymer (BFRP) loaded perpendicular to glulam timber elements was investigated. It was found that pull-out load increased approximately linearly with the bonded length up to maximum which occurred at a bonded length of 250mm (∼15times the hole diameter) and did not increase beyond this bonded length. Failure mode of the samples was mostly shear fracture which was located at the cylindrical zone at the timber/adhesive interface. Increased bonded lengths resulted in corresponding decrease in interfacial bond stress. At 250mm bonded length, the pull-out capacity of the proposed design model was about 2% lower than that of the tests. The results also showed that the bond stress of the theoretical model (at the ascending and descending branches) of the stress–slip curve was approximately 5–10% of that of the experiment.

Prestressed glulam timbers reinforced with steel bars

This paper discusses a series of four-point bending tests that were conducted to failure on unreinforced, reinforced and reinforced-prestressed glue laminated (glulam) timber beams with steel-bars in a simply-supported scheme to determine their flexural behavior. The cross sectional ratio between the steel and the wood was 0.82%. To increase the level of reinforcement, some reinforced beams were prestressed by applying a moderate force to the lower bar. The experimental and numerical data provided the load–deflection, load–strain relationships and strain profiles of the tested beams. The results of the reinforced beams showed that the mechanical strength, the load-carrying capacity and the stiffness for both the simple and prestressed beams were enhanced compared to the unreinforced beams. For the simply reinforced beams, the stiffness increased by 25.9%, the ultimate load increased by 48.1% and the ductility increased by 43.8%. For the reinforced and prestressed beams, the stiffness increased by 37.9%, the ultimate load increased by 40.2%, and the ductility increased by 79.1%.

Intralaminar and interlaminar fracture characterization in glued-laminated timber members using image analysis

This paper is concerned with the characterization of Mode I and Mode II fracture present in glued-laminated (glulam) timber members. In a typical glulam timber member, fracture can occur within each lamina (i.e., intralaminar fracture) or at the interface of two laminae (i.e., interlaminar fracture). Both the intralaminar and interlaminar fracture was investigated through an experimental study, where the double cantilever beam test for Mode I fracture and the end-notched flexure test for Mode II fracture were performed to a series of specimens. A salient feature of the intralaminar fracture in glulam timber members lies in the direction normal to the crack propagation plane. It is often the case that this direction neither coincides with the radial direction nor the tangential direction of the wood. Consequently for each intralaminar specimen tested, an image analysis was carried out to identify the relevant growth ring orientations, which were in turn represented by an angle. Analogous procedures were also utilized to associate three angles indicating growth ring orientations to each interlaminar specimen. After the specimen failure, crack tip morphology was analyzed by scanning electron microscopy micrographs. The critical loads used in the computation of the fracture energies were identified from the recorded load-versus-displacement curves. Further, statistical analyses were implemented to examine the obtained intralaminar Mode I and Mode II fracture energies with regard to different growth ring orientations.

Comparative assessment of life cycle impacts of curtain wall mullions

Glass curtain wall (CW) systems have been inevitable elements of commercial buildings for over a century. The systems mainly consist of mullion materials and glazing units that are selected and designed to achieve the desired structural, thermal and daylighting performances as well as to meet cost and aesthetic concerns. The health and environmental life cycle impacts of CW systems, however, are not usually considered in design. The main objective of this paper is to study how change of mullion materials would affect the health and environmental impacts associated with a typical CW system over its life cycle. The mullion materials studied for the purpose of this paper include extruded aluminum, carbon steel and glulam timber. Also, the health and environmental impact categories of interest include global warming, acidification, eutrophication and human toxicity. To achieve the objective, a process-based cradle-to-gate attributional Life Cycle Assessment (LCA) method was applied. Results indicate that CW system with glulam timber mullions causes the least and CW system with extruded aluminum mullions causes the most damage to the environment and human health over their life cycle. A CW system with carbon steel mullions falls in-between.

Direct Displacement-Based Design of Glulam Timber Frame Buildings

We introduce a direct Displacement-Based Design methodology for glued laminated timber portal frames with moment-resisting doweled joints. We propose practical expressions to estimate ultimate target displacement and equivalent viscous damping, and we demonstrate that these expressions provide prior values that are close to those obtained a posteriori using a more refined model. Applied to case studies, the method yields base-shear forces lower than those obtained using the force-based approach of Eurocode 8. This is due to the high dissipation capacity of the specific connection technology, which apparently is conservatively accounted for in the q-factor of Eurocode 8.