In this study, the bending performance of a separable cross-laminated timber (CLT)–concrete composite slab for reducing environmental impact was investigated. The slab has consisted of CLT and eco–concrete, and round-notch shape shear connectors resist the shear force between the CLT and eco-concrete. The eco–concrete was composed of a high-sulfated calcium silicate (HSCS) cement, which ensures low energy consumption in the production process. The bending stiffness and load-carrying capacities of the slab were theoretically predicted based on the shear properties of the notch connectors and validated with an experimental test. The shear properties of two types of notch shear connectors (Ø100 mm and Ø200 mm) were measured by planar shear tests. As a result, the stochastically predicted bending stiffness of the slab (with Ø100 mm shear connector) was 0.364 × 1012 N mm2, which was almost similar to test data. The load-carrying capacities of the slab were governed by the shear failure of the notch connectors, and the lower fifth percentile point estimate (5% PE) was 21.9 kN, which was 7.9% higher than the prediction (20.2 kN). In a parameter study, the effect of notch diameter for the CLT-concrete slab span was analyzed depending on the applied loads, and the maximum spans of the slab with Ø100 mm notch or Ø200 mm notch were not significantly different.
The numerical simulation of four-point bending tests on glued laminated timber (GLT) beams requires an adequate description of the material behavior and of relevant failure mechanisms. The wooden lamellas, building up the GLT element, include knots, as a result of the natural tree growth process, which significantly affect the mechanical behavior. The variability of the morphology and arrangement of these knots lead to a large fluctuation, especially of strength properties, along the wooden lamellas. This leads to complex and, in general, quite brittle structural failure mechanisms of the GLT element. Such failure mechanisms can numerically be described with discrete cracks, using the framework of the extended finite element method (XFEM) for cracks without predefined positions or cohesive surfaces for cracks with predefined positions. In this work, a modeling approach to reliably estimate the bending strength and failure mechanisms of GLT beams subjected to four-point bending tests is proposed. Herein, the approach is validated by simulating replications of experimentally tested GLT beams of two beam sizes and strength classes, where each knot group is considered as a section with reduced individual stiffness and strength in exactly the same position as in the real beam. The results show that the application of quasi-brittle material failure may still result in a brittle global failure of GLT beams. The present study exemplarily shows how valuable insight into progressive failure processes can be gained by allowing the formation of continuous crack patterns. Moreover, a refined consideration of the knot geometries with such sophisticated realizations of discrete cracks may be able to simulate the actual failure mechanisms even more precisely.
The modulus of elasticity and bending strength of 45 structural Salzmann pine timber pieces with nominal dimensions of 150x200x5400 mm3 from an existing 18th century structure were estimated by semi-destructive density estimation probing method (drilling chips extraction) and acoustic wave velocity (stress and ultrasound wave). Bending strength, modulus of elasticity and density were obtained according to the EN 408 European standard, and visual grading singularities were recorded. Visual grading methods are highly ineffective for existing timber structures. Sample mechanical properties show a typical profile of material from existing structures, and this was compared with the results of similar works. MOE and MOR predictive models were proposed with determination coefficients r2 of 66–68% and 51–52%, respectively, using dynamic MOE, relative edge knot diameter and slope of grain as independent variables. MOR prediction improved when these grading parameters were included.
Engineered wood products (EWP) such as glulam beams are gaining more and more popularity due to several advantages resulting from the wood itself, as well as the constant search for structural materials of natural origin. However, building materials face some requirements regarding their strength. Thus, the study aimed to assess the static bending strength of structural beams produced with the use of pine wood, after the periodic loading of approximately 80 kN for a year. The manufactured beams differed in the type of facing layers, i.e., pine timber with a high modulus of elasticity and plywood. The produced beams, regardless of their structure, are characterized by a similar static bending strength. Moreover, it has been shown that the loading of beams in the range of about 45% of their immediate capacity does not significantly affect their static bending strength and linear modulus of elasticity.
The purpose of this paper is to demonstrate the properties of glued laminated beams made in diverse configurations of timber quality classes, reinforced using a new technique that is cheaper and easy to apply. The aim of the experimental investigations was to enhance reinforcement effectiveness and rigidity of glued laminated beams. The tests consisted of four-point bending of large-scale specimens reinforced with basalt fibres (BFRP). The tests were meant to obtain images of failure, the load–displacement relation and load carrying capacity of basalt fibres depending on the reinforcement ratio. The tests, which concerned low and average quality timber beams, were conducted in a few stages. The aim of the study was to popularize and increase the use of low-quality timber harvested from reafforested areas for structural applications. In the study, theoretical and numerical analysis was carried out for reinforced and unreinforced elements in various configurations of wood quality classes. The aim was to compare the results with the findings of experimental tests. Based on the tests, it was found that the load carrying capacity of beams reinforced with basalt fibre was higher by, respectively, 13% and 20% than that of reference beams, while their rigidity improved by, respectively, 9.99% and 17.13%. The experimental tests confirmed that basalt fibres are an effective structural reinforcement of structural timber with reduced mechanical properties.
Portuguese forests have changed in recent years. These changes were mainly boosted by the wildfires that affected a significant percentage of the softwood area. Data from 2015, conveyed by the Portuguese Institute for Nature Conservation and Forests, indicates that hardwoods occupy 70% of the Portuguese forest area. This paper presents the Blackwood (Acacia melanoxylon R. Br.) species potential, focusing on construction applications, based on recent studies performed at the University of Coimbra and SerQ—Forest Innovation and Competences Center. The valuation of Blackwood for structural applications has been considered through the non-destructive and destructive assessment of their mechanical properties as sawn wood. Their potential was also assessed for a more technologically engineered wood product, the glulam. The dynamic modulus of elasticity (MOE) was estimated through the Longitudinal Vibration Method (LVM) and the Transformed Section Method (TSM); the static MOE and bending strength were assessed through a four-point bending test. Agreement was obtained between both approaches. Sawn Portuguese Blackwood showed a density of 647 kg/m3, 13,900 MPa of MOE and a bending strength of 65 MPa (mean values). The glulam beams fabricated with this raw material had improved properties relative to sawn wood, most obviously concerning the bending strength, with an improvement of 29%. This proves the significant ability and potential of these species to be used in construction products with structural purposes like sawn wood and glulam.
To develop a high-performance, lightweight cross-laminated timber (CLT) floor, this study tested the delamination performance between carbon fiber-reinforced plastic (CFRP) and CLT and the bending performance of a CFRP composite CLT that was differently reinforced according to the shape of CFRP. The test results showed that the soaking and boiling delamination between CLT and CFRP of the CFRP composite CLT produced by spreading a polyurethane (PUR) adhesive at 300g/m2 were both less than 5%, satisfying the Korean standard. Furthermore, the composite CLT (3ply) of which the entire outer surface of the tension laminae was reinforced with a CFRP plate (thickness: 1.2 mm) showed a mean MOE and a mean MOR higher by 27% and 48%, respectively, than those of the unreinforced CLT (3ply). Furthermore, even though the weight of this CFRP composite CLT was smaller than that of 5ply CLT by approximately 40%, its bending moment was measured higher by 14% than that of the 5ply CLT (thickness: 175 mm) fabricated by limited state design (LSD) as specified in PRG-320.
As timber is being used for several millennia as construction material, glued laminated timber (glulam), a highly engineered timber product, exists for about hundred and fifty years. In Europe, it is nowadays common practise to make glulam from softwood species, though in the last few decades glulam made from different kinds of hardwoods emerged. Iroko glulam is part of this development, as iroko is a hardwood species from the African tropical regions. The aim of this thesis is to investigate the bending strength of iroko glulam, as well as strength influencing features. From literature it is expected that the following features are of influence: density, modulus of elasticity, tension strength of the lamellas, finger joint strength and size. Several researches conducted in the past experiments to determine these mechanical and physical properties, focusing mainly on iroko sawn timber. Only few investigated iroko glulam, and none of those focused on finger jointed iroko glulam. In this lies the originality of this work: determining bending strength values of finger jointed iroko glulam, as well as density, modulus of elasticity and investigating mechanical and physical properties of the base material: iroko sawn timber and iroko finger joints. The laboratory experiments included the following: tension tests on 38 unjointed and 38 finger jointed lamellas, and four point bending tests on 12 glulam beams. Also density, modulus of elasticity and moisture content were determined. The experimental results yield the following characteristic values: a lamella tension strength of 17 N/mm2, a finger joint tension strength of 29 N/mm2, and a glulam bending strength of 42 N/mm2 (including size effect according to NEN-EN 1995, 2011). The experimentally determined characteristic lamella tension strength is a little lower than values found in literature. This is due to a large scatter in the test results: a coefficient of variance equal to 0.37 was found. However, if the grain angle is equal or smaller than 5°, a higher lamella tension strength of 27 N/mm2 is feasible. Grain angle is as expected a significant strength influencing parameter for iroko sawn timber. And it would suggest that the strength class is as expected D40 if the lamella bending strength equals 0.6 divided by the lamella tension strength. The ratio of finger joint bending strength (30 N/mm2) and tension strength (29 N/mm2) on the characteristic level was found to be equal to 1.06. This is smaller than expected from theory: apparently the 1.4 ratio commonly assumed for softwood finger joint strength values does not hold for iroko finger joint strength values. The investigated iroko glulam beams with depth 108 mm yielded a mean bending strength of 66 N/mm2 and a characteristic bending strength of 42 N/mm2. Due to the size effect and quasi-brittle failure this figures lie lower for full scale glulam beams, however, strength class GL24h is indeed a safe assumption for iroko glulam beams. These aspects explain the higher mean glulam bending strength compared to the mean finger joint tension strength of 40 N/mm2. A strong mathematical relationship between characteristic glulam bending strength and both lamella tension strength and finger joint strength was not found; however lamella and finger joint tension strength do influence the glulam bending strength. Furthermore, density does not influence any strength or stiffness property for both iroko sawn timber, finger joints, and glulam beams. Although there is a slight positive correlation with both dynamic and local modulus of elasticity of lamellas and its tension strength.
IOP Conference Series: Earth and Environmental Science
Summary
An efficient butt-joint bonding technology allows to build new types of timber structures. Under the name Timber Structures 3.0 a connection has been developed which connects timber elements with an end-grain to end-grain butt joint bonding. Therefore, it is now possible to build continuous, point supported flat slabs in cross laminated timber (CLT). Multiple CLT slabs are connected rigidly together and are only supported by columns. Some major challenges had to be solved in terms of bending strength of the glued connection and shear resistance of the part of the slab above the column. The research in both topics is successful and more projects were built in the last two years using this technology. Starting point was a real scale structure at ETH Zurich, followed by a working platform for a timber construction company and finally four three storey residential buildings. The research team is continuing to optimize the different elements of this innovative technology and will soon provide engineers with guidelines to design their own biaxial, point supported timber flat slabs.
This paper summarises parts of the research outcomes of a university-government collaborative project aiming at determining the capacity and reliability of veneer-based structural products manufactured from early to midrotation (juvenile) hardwood plantations logs. Two species planted for solid timber end-products (Eucalyptus cloeziana and Corymbia citriodora) and one species traditionally grown for pulpwood (Eucalyptus globulus) were studied for the manufacture of the new products. Focus of this paper is on LVL beams. To cost-effectively determine the nominal design bending strengths of the new beams, a numerical model was developed. The model was found to accurately predict the strength of LVL beams with an average predicted to experimental ratio of 1.00 with a low coefficient of variation of 0.10. Using an established probabilistic database of the material properties of the veneered resources as model input, Monte-Carlo simulations were then performed. The design strength of the new LVL beams was established and found to be comparable to, and in some cases up to 2.5 times higher than, the ones of commercially available softwood products. Recommendations are also made in the paper on the appropriate capacity factors to be used for various service categories of structures. The proposed capacity factors were found to be 5% to 12% lower than the ones currently used in Australia for beams manufactured from mature softwood logs
