The in-plane permeability was measured for thick, unidirectional oriented strand lumber made from aspen (Populus tremuloides) strands and pressed to five different densities. The press cycle was such that the vertical density profile of the panels was uniform. Specimens were cut from the boards and sealed inside a specially designed specimen holder; this jig was connected to a permeability measurement apparatus and in-plane permeability measured parallel, perpendicular, and 45° to the strand orientation. Permeability decreased markedly with increasing board density. The highest permeability was in the strand alignment direction and lowest perpendicular to it. The permeability in the 45° direction fell between those in parallel and perpendicular to strand alignment. A polynomial equation was fit to the results of each direction with r2 of 0.938 and 0.993. The in-plane distribution of permeability as a function of flow direction was obtained and its vector diagram was lenticular in shape.
At the Institute of Structural Engineering at the ETH Zurich numerous of investigations are conducted to analyse the load bearing capacity of glued laminated timber beams. The investigations are part of the research project ’Influence of varying material properties on the load bearing capacity of glued laminated timber (glulam)’. The investigations are taking place on 24 glulam beams with well-known material properties. The glulam beams are fabricated out of 400 timber boards. From those boards the material properties are investigated non-destructively within a former research project. During the glulam fabrication it is particularly focused to keep the information of the timber boards; i.e. after the glulam fabrication the position of each particular timber board within the glulam beam and thus the position of each particular knot is still known. The glulam beams are investigated during a 4-point bending test. On the glulam members the load bearing capacity, the bending stiffness and the density is measured. Furthermore local strains within the glulam beams are investigated using an optical coordinate-measurement device. Following the test the failure is investigated in detail. Hereby the type of failure (knot cluster, finger joint, clear wood) and the amount of failure (number of damaged lamellas) is documented. Afterwards the failed glulam beams are loaded again to analyse the remaining bending strength and the corresponding remaining bending stiffness. The major aim of the experimental analysis is the investigation of the load bearing capacity of glulam beams with well-known local material properties. The gained results can be used for an investigation of the influence of local weak zones, such as knot clusters or finger joints, on the load bearing capacity of glulam. In addition a data basis is produced to develop a new model (or to evaluate existing models) for the estimation of the load bearing capacity of glulam.
For enhancing productivity of glulam, high frequency (HF) curing technique was researched in this study. Heat energy is generated by electromagnetic energy dissipation when HF wave is applied to a dielectric material. Because both lamina and adhesives have dielectric property, internal heat generation would be occurred when HF wave is applied to glulam. Most room temperature setting adhesives such as phenol-resorcinol-formaldehyde (PRF) resin, which is popularly used for manufacturing glulam, can be cured more quickly as temperature of adhesives increases. In this study, dielectric properties of larch wood and PRF adhesives were experimentally evaluated, and the mechanism of HF heating, which induced the fast curing of glue layer in glulam, was theoretically analyzed. Result of our experiments showed relative loss factor of PRF resin, which leads temperature increase, was higher than that of larch wood. Also, it showed density and specific heat of PRF, which are resistance factors of temperature increase, were higher than those of wood. It was expected that the heat generation in PRF resin by HF heating would occur greater than in larch wood, because the ratio of relative loss factor to density and specific heat of PRF resin was greater than that of larch wood. Through theoretical approach with the experimental results, the relative strengths of ISM band HF electric fields to achieve a target heating rate were estimated.
Fire-resisting wood structural elements for building were developed. It is composed of three layers made of glued laminated timber, “a load-bearing part, a self-charring-stop and a surface layer”. Tree species of load-bearing part is limited to the only kind. In order to enable a species different, this research was carried out. Fire resistance test was performed to demonstrate the effect on fire resistance of the wood density. The relationship of fire resistance and wood density of the load-bearing part was clear. Usage of high wood density of the load-bearing part was obtained a conclusion that is advantageous to fire performance.
In timber construction, curved timber components have been used repeatedly. Yet the use of curved CLT elements is a relatively recent phenomenon. To obtain a European Technical Approval (ETA) for so-called radius timber (single curved CLT elements), Holzbau Unterrainer GmbH commissioned the accredited testing institution TVFA – Innsbruck to carry out the tests required for this purpose. To this end, overall 158 tests were performed in building component dimensions from December 2013 to May 2014, and several laboratory tests were carried out to monitor adhesive joint quality. Due to the single curved shape of radius timber elements, it is key to particularly focus on possible implications on load bearing capacity due to pre-stress of the slats and to the tensile stress perpendicular to grain resulting from deflection forces. To comply with the criteria laid down in the semi-probabilistic safety concept used in Eurocode 5, the impact caused by these pre-curvatures on strength, rigidity and gross density must be known.
One of the recent additions to the panoply of engineered wood products is cross-laminated timber (CLT). CLT is a prefabricated, large-scale, solid wood panel that consists of multiple layers of lumbers stacked together, with each layer arranged perpendicular to the next layer, glued with structural grade adhesives, and pressed. The use of massive CLT panels in wood construction provides several advantages over the traditional wood frame systems, making it particularly attractive for tall wood building construction. These main advantages are satisfactory distribution of defects, adequate seismic performance, ability to carry large loads, improved strength and stiffness, adequate acceptable fire performance, acceptable acoustic performance, and improved pre-fabrication.It is expected that as the CLT market will continue to mature, more diversified grades and special CLT products will be introduced into the markets. One special CLT product developed in at Oregon State University has been designated as hybrid CLT. Hybrid CLT refers to CLT panels manufactured with layers of high- and low-grade and low-density species, which aims at improving the economic efficiency and sustainability of the CLT industry with focus on the North America market.
One of the potential issues with hybrid CLT panel application is related to the unknown performance of the connection systems which are highly dependent on the density of the wood in which the fasteners embed. Most of the existing models that have been developed for estimation of the fasteners capacities in withdrawal and lateral loading scenarios are developed based on the assumption of uniform density profile across the layers to which fasteners penetrate. In a hybrid CLT panel, there is a possibility of a variation in density profile along the panel thickness so that the fasteners can be driven into wood of different densities and driven in directions parallel and perpendicular to grain. Because of the potential variation in density profile in the hybrid CLT, the connection system performance cannot be predicted using design models used for uniform density profile applications similar to the models in National Design Specification (NDS). Therefore, there is a need for evaluation of connections performance in hybrid layup.
The main objective of this work is to characterize the performance of connection systems for hybrid CLT. This is achieved through testing and modeling of single fastener connections and then testing and modeling of the typical connection systems. So, the specific objectives are: (1) evaluate the single fasteners performance to account for density variation and compare the results to a proposed modified model, (2) perform an experimental program to test different connection systems with different hybrid CLT panel layups, (3) develop a numerical algorithm based on the use of meta-heuristics tools to fit the optimal parameters for constitutive models to match the experimental data for the connection systems, (4) obtain the optimal parameters for constitutive models of the connection systems tested.
The effects of veneer orientation and loading direction on the mechanical properties of bamboo-bundle/poplar veneer laminated veneer lumber (BWLVL) were investigated by a statistical analysis method. Eight types of laminated structure were designed for the BWLVL aiming to explore the feasibility of manufacturing high-performance bamboo-based composites. A specific type of bamboo species named Cizhu bamboo (Neosinocalamus affinis) with a thickness of 6 mm and diameter of 65 mm was used. The wood veneers were from fast-growing poplar tree (Populus ussuriensis Kom.) in China. The bamboo bundles were obtained by a mechanical process. They were then formed into uniform veneers using a onepiece veneer technology. Bamboo bundle and poplar veneer were immersed in water-soluble phenol formaldehyde (PF) resin with low molecular weight for 7 min and dried to MC of 8–12 % under the ambient environment. All specimens were prepared through hand lay-up using compressing molding method. The density and mechanical properties including modulus of elasticity (MOE), modulus of rupture (MOR), and shearing strength (SS) of samples were characterized under loading parallel and perpendicular to the glue line. The results indicated that as the contribution of bamboo bundle increased in laminated structure, especially laminated on the surface layers, the MOE, MOR and SS increased. A lay-up BBPBPBB (Bbamboo, P-poplar) had the highest properties due to the cooperation of bamboo bundle and poplar veneer. A higher value of MOE and MOR was found for the perpendicular loading test than that for the parallel test, while a slightly higher SS was observed parallel to the glue line compared with perpendicular loading. Any lay-up within the homogeneous group can be used to replace others for obtaining the same mechanical properties in applications. These findings suggested that the laminated structure with high stiffness laid-up on the surface layers could improve the performance of natural fiber reinforced composites.
The aim of this project was to validate the assumption used within the Australian Standard AS1720.1 for calculating compression perpendicular to grain for common timber species and develop practical methods to reduce compression deformation.
The project was successful in demonstrating that perpendicular to grain displacement of wall plates by studs can be reduced. It found that stud on stud connection was the best method as it removed the wall plates out of the load path altogether. The study also found where this is not possible, replacement of the wall plate with a stiffer timber such as a high density hardwood, softwood or cross laminated timber reduces this deformation.
The investigation also found that the method used in AS1720.1 to assign perpendicular to grain bearing capacities for various timber species by “strength group” or stress grade over predicted low to medium density timber species whilst under predicting high density timber species. Accordingly, it is recommended that perpendicular to grain bearing capacity be assigned by the timber species’ density and that the characteristic values of commonly used timber species be re-established.
Cross-Laminated Timber (CLT) structures exhibit satisfactory performance under seismic conditions. This ispossible because of the high strength-to-weight ratio and in-plane stiffness of the CLT panels, and the capacity ofconnections to resist the loads with ductile deformations and limited impairment of strength. This study sum-marises a part of the activities conducted by the Working Group 2 of COST Action FP1402, by presenting an in-depth review of the research works that have analysed the seismic behaviour of CLT structural systems. Thefirstpart of the paper discusses the outcomes of the testing programmes carried out in the lastfifteen years anddescribes the modelling strategies recommended in the literature. The second part of the paper introduces theq-behaviour factor of CLT structures and provides capacity-based principles for their seismic design.
