Strength Of Timber Research Articles

A Numerical Study of the Stiffness and Strength of Cross-Laminated Timber Wall-to-Floor Connections under Compression Perpendicular to the Grain

The use of cross-laminated timber (CLT) in multi-story buildings is increasing due to the potential of wood to reduce green house gas emissions and the high load-bearing capacity of CLT. Compression perpendicular to the grain (CPG) in CLT is an important design aspect, especially in multi-storied platform-type CLT buildings, where CPG stress develops in CLT floors due to loads from the roof or from upper floors. Here, CPG of CLT wall-to-floor connections are studied by means of finite element modeling with elasto-plastic material behavior based on a previously validated Quadratic multi-surface (QMS) failure criterion. Model predictions were first compared with experiments on CLT connections, before the model was used in a parameter study, to investigate the influence of wall and floor thicknesses, the annual ring pattern of the boards and the number of layers in the CLT elements. The finite element model agreed well with experimental findings. Connection stiffness was overestimated, while the strength was only slightly underestimated. The parameter study revealed that the wall thickness effect on the stiffness and strength of the connection was strongest for the practically most relevant wall thicknesses between 80 and about 160 mm. It also showed that an increasing floor thickness leads to higher stiffness and strength, due to the load dispersion effect. The increase was found to be stronger for smaller wall thicknesses. The influence of the annual ring orientation, or the pith location, was assessed as well and showed that boards cut closer to the pith yielded lower stiffness and strength. The findings of the parameter study were fitted with regression equations. Finally, a dimensionless ratio of the wall-to-floor thickness was used for deriving regression equations for stiffness and strength, as well as for load and stiffness increase factors, which could be used for the engineering design of CLT connections.

Influence of adhesive and layer composition on compressive strength of mixed cross-laminated timber

Different types of wood can be used for making cross-laminated timber (CLT), which is useful as a structural material. Therefore, to assess the viability of mixed cross-laminated timbers prepared with different adhesives, their compressive strength performances were evaluated. Laminae of Japanese larch, red pine, and yellow poplar were used to manufacture eight types of mixed CLTs, which were then tested in a universal testing machine for obtaining the compressive strength. The results were then compared to those obtained from the finite element (FEM) simulation of the CLTs at proportional limit load. The compressive strength of CLTs consisting of Japanese larch laminae, with a high modulus of elasticity, tended to increase. Mixed CLT with polyurethane adhesives showed an average compressive strength that was 14% lower than that of larch CLT, while mixed CLT consisting of red pine and yellow poplar showed an average compressive strength that was 18% lower than that of the larch CLT. The CLT prepared with phenol-resorcinol-formaldehyde adhesive yielded the highest compressive strength among the three adhesives. The FEM-predicted strengths were found to be close to the actual values in all specimens. The obtained results will be useful for selecting material and adhesive for future endeavors.

Prediction of shear strength parallel to grain in clear wood of oak (Quercus robur L.) on the basis of shear plane orientation, density and anatomical traits

Abstract Oak wood is popular for use in construction and as flooring. Evaluating the mechanical strength of oak timber is difficult and time consuming. Therefore, models for predicting mechanical properties, based on easy-to-obtain variables, may be useful. The purpose of the study was to build models for predicting shear strength parallel-to-grain in oak clear wood. With this goal, the shearing resistance was tested on 198 defect-free specimens (target dimensions 50 × 50 mm in tested section) obtained from a sample of 40 oak trees felled in north-western Spain. The mean shear strength of the sampled oak wood provenance was 15 N mm−2, which was almost equal to the highest mean value reported in previous studies. Analysis of the relationships between the variables tested enabled development of a model relating shear strength parallel to grain at 12% moisture content, with air-dry wood density and angle between tangential and shear plane as predictor variables ( R adj 2 ${R}_{\text{adj}}^{2}$ = 0.61, p < 0.01, bias = −0.80%, RMSE = 13.66%, for wood with wavy grain; R adj 2 ${R}_{\text{adj}}^{2}$ = 0.36, p < 0.01, bias = −1.46%, RMSE = 17.22%, for wood without wavy grain). The independence of shear strength relative to the presence/absence of sapwood or the annual growth ring width was also demonstrated.

Compressive behaviour of hardwood dowel reinforced CLT loaded perpendicular-to-Grain

Cross-laminated timber (CLT) elements in structures may be exposed to high perpendicular-to-grain compressive stresses, which often require reinforcement to improve the inherently low strength and stiffness of timber perpendicular-to-grain. This experimental study evaluates the compressive behaviour of CLT perpendicular-to-grain when reinforced by hardwood dowels. Groups of 3 and 5-ply CLT specimens with dimensions of 400 mm × 400 mm, with and without dowel reinforcement, were tested. The specimen with the most reinforcement, 5-ply Douglas Fir CLT reinforced with 4-ф25mm American White Ash dowels, achieved a mean strength and stiffness increase of 120% and 240%, respectively. Finite element models were created in ABAQUS to predict the stiffness of tested CLT specimens and to evaluate the distribution of compressive stresses. Finally, a simple design equation is presented and provides reasonable strength predictions for the reinforced CLT.

Tensile shear strength of steel plate-reinforced larch timber as affected by further reinforcement of the wood with carbon fiber reinforced polymer (CFRP)

To improve the connecting strength of larch timbers, tensile shear test specimens were fabricated, and their connecting shear strength performance was examined. The control specimens consisted of larch timber reinforced with steel plate. These were compared with similar specimens in which the wood had been reinforced with carbon fiber reinforced polymer (CFRP). The reinforced specimens were fabricated in three types depending on the position of the CFRP reinforcement in the wooden part. All specimens were fabricated in two end distance types, depending on the bolt insertion position. The end distances examined were 60 mm (5D) and 84 mm (7D). The maximum connecting strength and the yield shear strength of each type were not different according to the CFRP reinforcement position. The reinforced specimens had an average connecting strength and yield shear strength that was 24% to 29% higher than the control specimens. The CFRP-reinforced specimens with an end distance of 5D had an average connecting strength and an average yield shear strength that that was 70% and 26% higher, respectively, than non-reinforced 7D specimens. The yield shear strength was predicted by measuring the bearing strengths of the larch timber samples and CFRP-reinforced timber samples. The predicted yield shear strength matched the measured yield shear strength.

Influence of Age on the Strength of Different Species of Timber

Timbers are used as structural members in various areas of construction works. Hence the knowledge of properties of wood especially strength and factors affecting the strength of timber is very important. This study was conducted to determine the effect of age on the structural strength of timber. The wood species studied are Iroko (Chlorophora excelsa), Nigerian Mahogany (Khaya anthotheca) and Ita (Celtis mildbraedii). The age of timber were determined by counting the number of annual growth rings, which is the combination of early wood and late wood and samples of different ages of timber, were obtained by personal visit to the forest where fresh timber was cut. The samples obtained were machined and trimmed to standard size with respect to BS 373 1957 (imprint 1999), 20mm X 20mm X 60mm for the determination of maximum compressive strength parallel to grain (MCS//), 20mm X 20mm X 20mm for static shear strength and 20mm X 20mm X 300mm to determine the bending strength. The samples obtained were carried to the laboratory at Forestry Research Institutes of Nigeria (FRIN) in Ibadan for testing using Instron Tensiometer machine. SPSS was used to analyze the relationship between strength and age of timber. The result obtained showed that the maximum compressive and Shear strength for Iroko, were at 120years, whereas it has maximum bending strength at 70years. Nigeria Mahogany and Ita has maximum compressive, bending and shear strength at 80 years, and 70 years respectively and among the three timber samples Ita is better in compression and shear strength and iroko is better in bending strength. It was concluded that age of timber has significant influence on timber strength

Predicting nail withdrawal resistance and bearing strength of cross-laminated timbers from mixed species

The increasing demand for sustainable architecture has led to a growing interest in wood structures. Hence, ensuring their structural stability and strength performance is an imperative. This study investigated the nail bearing strength and withdrawal resistance of mixed cross-laminated timber (CLT) using Japanese larch and yellow poplar layers. The mixed CLT was composed of three larch laminas (major) and two yellow poplar laminas (minor). The bearing strength of the mixed CLT decreased as the ratio of the thickness of the minor lamina to nail depth increased. The nail withdrawal resistance differed in the penetration and axial directions of the laminas. In the direction perpendicular to the grain, the withdrawal resistance load of the yellow poplar lamina was measured to be 1.45-times that of the larch lamina. The withdrawal resistance of the mixed CLT with the yellow poplar layer was 17% higher than that with larch. Therefore, the length of the nail used for the mixed CLT should be selected based on the thickness of the minor lamina to achieve efficient bearing and withdrawal resistance of the nail connection.

Investigation of the high temperature performance of a polyurethane adhesive used for structural wood composites

Fire resistance and thermal stability are among the most important criteria for engineered wood product applications in mass-timber buildings. This project investigated the shear strength of mass timber adhesively bonded joints at high temperatures. Shear tests were conduct on specimens made of Douglas-Fir Glued-laminated Timber and a steel plate glued in with a one component polyurethane adhesive. The results indicate that the shear strength of the adhesive was affected by temperature. Between 20 °C and 75 °C, the shear strength of the joints decreased with increasing temperature. Above 75 °C, the strength of the adhesive became constant at less than half that at 20 °C. Therefore, it is recommended that if the polyurethane adhesive is used for glulam-steel plate structures above 75 °C, the design strength should be only about 40% of the design strength at room temperature.

Shear strength properties of hybrid (hinoki cypress and Japanese cedar) cross-laminated timber

In this study, the out-of-plane shear strength of hybrid cross-laminated timber (CLT) with outer layers of hinoki (hinoki cypress, Chamaecyparis obtusa) and inner layers of sugi (Japanese cedar, Cryptomeria japonica) is investigated for four different layer configurations. To investigate the influence from rolling shear properties of cross layers on the shear strength of CLT, stress analysis was conducted using the shear analogy method. The nominal shear strength, the maximum shear force divided by the cross-section of CLT, was in the 1.0–2.1 MPa range. Using the shear analogy method, the rolling shear modulus in the cross layer was determined as 72.9 MPa, which was comparable with the value obtained for laminae in previous study as well as the value confirmed by strain measurements in the present study. The magnitude of rolling shear stress in the cross layer was 0.9–1.1 times the average shear stress, which was negatively correlated with the nominal shear strength. From the regression line between the nominal shear strength and the magnitude of the shear stress in the cross layer, the mean shear strength of the cross layer was estimated to be 1.33 MPa.

Finite element modelling of timber infilled steel tubular short columns under axial compression

Timber infilled steel tubular (TIST) columns can be a promising alternative to the concrete infilled steel tubular (CIST) columns, particularly for the lightly loaded prefabricated structures, as timber is lighter and more sustainable material than the conventional concrete. However, limited research studies were dedicated to investigate the structural behaviour of TIST columns. Therefore, in this study an attempt has been made to investigate the parameters that influence the compressive behaviour and to develop appropriate design guidelines through finite element (FE) analyses of TIST columns. A three-dimensional FE modelling technique of TIST has been developed and validated with available experimental results. The developed model considered the steel tube induced confinement on the strength and orthotropic characteristics of timber as well as contact interaction between steel and timber. The validated modelling technique has been extended to parametrically (120 FE models) study the compressive behaviour of TIST columns varying parameters such as strength properties of steel and timber, thicknesses of steel tube, and cross-sectional shapes. The analyses reveal that the TIST columns can be used on par with CIST columns. The calculated strength and ductility indices reveal that the maximum strength and ductility enhancement up to 60% and 65% can be achieved with TIST. Further, the parametric analyses results have been used to verify suitable design formulations for TIST column, subsequently the design formations that are similar to CIST columns can consciously be extended to design the TIST columns.

Modern testing machines for investigation of wood and timber-based composite materials

A detailed analysis of test machines and presses operating in both soft (increasing loads) and hard test mode (displacement increase) has been performed. The results of tests of deciduous, coniferous and tropical timber species on modern test machines of foreign production under a strict test regime were analyzed. The results of tests of timber and concrete on advanced presses of old production with the help of additional equipment are also given and analyzed. With the help of such equipment it is possible to build a "stress-strain" diagram completely on the ascending branch and partially on the descending branch. And also allows you to more widely establish the strength and deformation properties of concrete and timber. It was found that the influence of age on the main strength and deformation parameters has not been studied. In recent decades, testing machines and presses have begun to appear, which allow to investigate these or those materials from the beginning of loading and to its full destruction. That is, to analyze the work of different materials (metal, timber, concrete, etc.) on the ascending and descending branches, and in particular in the so-called supercritical stage of work.
 At the end of the 20th century, scientists developed electromechanical and servo-hydraulic universal testing machines, which make it possible to test samples from the beginning of loading until complete destruction (rigid test mode for incremental displacements), while fixing all the necessary strength and deformation indicators on the ascending and descending branches of the diagram "stress - strain", setting the necessary modes and loading speeds. Measuring instruments determine the forces and deformations at different stages of testing and transmit the measurement results to the recording device or means of information accumulation. The characteristics of modern testing machines, in particular electromechanical and servo-hydraulic, are given: WDW (Time Group Inc.), INSTRON (USA), REM (Russia), LFM (Switzerland).
 The possibilities of the STM-100 servo-hydraulic testing machine for the study of timber and composite materials based on it are described in detail..

Orchard biomass residues: Chemical composition, biological activity and wood characterization of apricot tree (Prunus armeniaca L.)

AbstractRecent EU directives have encouraged member states to follow circular economy guidelines on waste reuse, especially for waste originating from wood agro‐forest biomass. Delineating the antioxidant and chemical activity of biomass is a starting point to find new ways to utilize such materials. Here, an 8‐year‐old apricot (Prunus armeniaca L.) tree stand located in southern Italy was evaluated. First, dendrometric parameters (total volume, diameter, and height of trunk below the branches) and physical‐mechanical properties (density, total, tangential and radial shrinkage, moisture content, and compression strength) of timber collected from five experimental plots was evaluated. Bark and wood from apricot pruning were then subjected to different extraction techniques, in vitro antioxidant assays, and gas chromatographic studies to determine their antioxidant activity and their phytochemical composition. Bark had the highest antioxidant activity compared to wood. Several antioxidant compounds (e.g., scopoletin and naringenin) were detected in the samples, explaining the measured antioxidant activity. In conclusion, this study demonstrates the potential of using apricot extracts in nutraceutical, cosmeceutical, and industrial fields. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd

The Effect of Direction of Load on Bending Strength of <i>Melia compositae</i>

In this paper, empirical study of stiffness (modulus of elasticity-MoE) and Bending strength/flexural strength (modulus of rupture-MoR) in wood, mainly on radial and Tangential surface of <i>Melia compositae</i> wood were evaluated. Here loading in the radial direction means that load is applied to the tangential surface and loading in the tangential direction means that load is applied to the radial surface. The strength properties vary with species to species and also application of direction of load. Loading direction appreciably affects the bending properties remarkably due to the anisotropic /orthotropic nature of timber. It was observed that always MoE and MoR have greater value in Radial surface. The direction of application of load has an appreciable effect on strength properties of wood. While this is generally attributable to the presence of medullary rays in the radial direction. The bending strength of timber when loaded parallel to the direction of load is greater than that of timber loaded perpendicular to the direction of load. The ratio of flexural strength values varied from13% to 14% for <i>Melia composita</i>.

Embedding strength of fully-threaded dowel-type fasteners in cross-laminated timber: an experimental study

The purpose of this paper is to experimentally evaluate the embedding strength of dowel-type fasteners in Cross-Laminated Timber (CLT), observing the effect of lamellae orientation and the dowel diameters. Thereby, CLT specimens – with three layers of lamellae – and solid timber specimens were produced and tested under loads at 0o and 90o with the direction of the grain, considering five dowel diameters. The results showed that CLT embedding strength depends on the dowel diameter in the two investigated directions. The CLT embedding strength parallel to the grain was up to 8% lower than those for solid timber in the same direction, and up to 26% higher than the solid timber strength in the perpendicular direction. The analysis of variance (ANOVA) showed no significant difference between the mean values of the CLT and solid timber embedding strengths, except for the embedding strength perpendicular to the grain found for 16 mm dowel diameter. When comparing the results obtained experimentally with those determined analytically, the equation proposed by KENNEDY et al. (2014) led to a better agreement, for both the loads applied at 0o and 90o to the grain direction. Keywords: Cross-laminated Timber; Dowelled connections; Embedment strength; Half-hole embedment tests; Design approaches.

Predicting the thickness of zero-strength layer in timber beam exposed to parametric fires

In the design of timber structures, the mechanical resistance in fire conditions has to be ensured among others. In the European standards, Eurocodes, the reduced cross-section method is given to determine mechanical resistance of timber structural elements in fire conditions. The reduced cross-section method is based on an effective cross-section that is determined by two key parameters, namely the charring depth and the thickness of zero-strength layer where the latter accounts for the reductions of the stiffness and the strength of timber due to elevated temperatures. Although, the thickness of zero-strength layer of 7 mm is only prescribed for the ISO 834 standard fire exposure longer than 20 min in the Eurocodes, the same value is often used for non-standard fire exposures as well, which might not always be correct. Thus, in the present paper the thickness of zero-strength layer is investigated for a timber beam exposed to 44 different parametric fires by means of advanced numerical models and a simple design method. A hygro-thermal model and a mechanical model are applied to determine the temperature field over a timber beam cross-section and the mechanical resistance of the beam during fire exposure, respectively. The simple design method is based on the reduced cross-section method. The obtained results show that the thickness of zero-strength layer varies between 8.4 mm and 30.5 mm, which are substantially higher values than the value of 7 mm suggested in the Eurocodes for the standard fire exposure. The results also indicate that the thickness of zero-strength layer is not a constant value and should be written as a function of the parameters defining a parametric fire curve. Alternatively, the effective cross-section could be simply determined by finding the combined thickness of zero-strength layer and charring depth at temperature of about 90 °C.

Effect of wood species, clamping pressure and glue spread rate on the bonding properties of cross-laminated timber (CLT) manufactured from tropical hardwoods

The effects of wood species, clamping pressure and glue spread rates on the joint strength of cross-laminated timber (CLT) manufactured from four tropical hardwood species were studied. CLT boards were fabricated from batai (Paraserianthes falcataria), sesendok (Endospermum malaccensis), rubberwood (Hevea brasiliensis) and kedondong (Canarium sp.). These species have densities ranged from 380 to 750 kg/m3. Three layers CLT were prepared by applying pressure of 0.7 and 1.4 N/mm2 (100 and 200 psi respectively) and glue spread rates of 200, 250 and 300 g/m2, single glue-line. Block shear bond strength, wood failure percentage and delamination percentage of the CLT specimens were determined as function of clamping pressure and glue spread rate. The results revealed that glue spread rate significantly influenced both the shear bond strength and wood failure percentage of the CLT. Clamping pressure, on the other hand, had no effect on bonding properties of the CLT despite various range of wood density. Delamination behavior of the samples also was not influenced by both clamping pressure and glue spread rate. Overall, properties determined in this study met the minimum values required in BS EN 16351 standards.

The effect of adhesive type and ply number on the compressive strength retention of CLT at elevated temperatures

This paper describes novel experiments to identify the reduction in compressive strength of cross-laminated timber (CLT) at elevated temperatures. The adhesive type and number of timber plies was varied between CLT samples to allow an assessment of the CLT ‘system’, rather than timber only. Samples were subjected to steady state and transient heating conditions and their ultimate load bearing capacities were measured. At ambient reference temperatures, a statistically significant difference in ultimate compressive strength was observed between ply numbers but not between adhesive types. For transiently heated samples, capacity was reduced at 100 °C for CLT bonded with polyurethane adhesive type as compared to samples bonded with melamine formaldehyde. The presented results indicate that the composition of CLT can influence the whole system structural response, both at ambient temperature and under fire conditions.

Rolling shear modulus and strength of cross-laminated timber treated with micronized copper azole type C (MCA-C)

The effect of micronized copper azole type C (MCA-C) treatment on rolling shear (RS) strength and RS modulus of cross-laminated timber (CLT) was evaluated. The CLT test specimens were either constructed with untreated 2 × 6 (38 mm × 140 mm) No. 2. southern yellow pine (the United States grown) laminations or MCA-C treated laminations. The shear-free modulus of elasticity (Esf) and longitudinal-radial shear modulus (GLR) of the laminations were non-destructively measured prior to CLT manufacturing. The average Esf and GLR of the untreated lumber were 11.08GPa and 231.42 MPa, respectively, while those of the treated lumber were 9.60GPa and 236.01 MPa, respectively. Four-point bending test described in EN 16,351 standard along with the Shear analogy method was adopted to measure the RS properties. The preservative treatment decreased the mean RS strength, while it increased the mean RS modulus. However, the differences in the means were not significant based on one-way analysis of variance and Kruskal-Wallis H test, respectively. The experimentally obtained bending stiffness of the CLT test specimens and the RS strength estimated using the simplified method described in the CLT Handbook were compared against the values obtained based on the Shear analogy. The agreement between these two approaches indicates that the short-span test setup implemented in this study and the simplified method can be used for measuring bending stiffness and estimating RS strength of 3-ply CLT.

Fluence of age factor on main strength and deformative properties of timber

The analysis of literature sources on this topic is carried out. It was found that the influence of age on the main strength and deformation parameters has not been studied. The method of experimental researches of coniferous and deciduous species of solid wood of different age at one-time short-term loading along fibers is resulted. To solve the problems, a series of samples of 1 sort of solid wood in the form of prisms with a cross section of 30x30x120 mm were made. The following species of wood were tested: coniferous species - larch, pine, spruce; deciduous - birch, alder, ash. The age of wood was 20, 40, 60 years. At the time of testing, the moisture was 12%. The humidity of the wood was controlled using a moisture meter MD-814. Experimental studies were performed on a servo-hydraulic test machine STM-100 under a rigid load regime (by increasing the displacements of the press plate of the test machine). Based on the obtained results, complete diagrams of deformation of deciduous (birch, alder, ash) and coniferous (larch, pine, spruce) species of solid wood at different ages (60, 40, 20 years) were constructed. The number of twin samples that were tested was 126 pieces. The main strength and deformation parameters were determined experimentally, in particular the average critical deformations of all studied wood species and the corresponding average maximum stresses. It is established that in the case of a rigid test regime, the deformation diagram of solid wood necessarily has two branches - ascending and descending. That is, the samples work in the so-called supercritical stage of work. It was found that the age of wood directly affects the strength and deformability of solid wood. In particular, with increasing age from 20 to 60 years, the strength and deformability of deciduous and coniferous species of solid wood increases significantly. Based on the test results, conclusions were drawn. Further research on this topic is highlighted.

Influence of slope of grain on the mechanical properties of tropical hardwoods and the consequences for grading

The main parameter that influences the bending strength of timber from tropical hardwood is the slope of grain. Although, in the grading rules, a specific threshold value is given, in these hardwoods, the global slope of grain is very difficult to quantify by visual assessment. The slope of grain measured after testing gives a better indication, but still it can only poorly describe the Hankinson relations. By rewriting the Hankinson relations, the slope of grain can be determined from the bending strength test values and from the MOE test value, both in combination with the density values and constants derived by non-linear regression analysis. These two values correlate very well, and the average value is designated as the theoretical slope of grain. With the theoretical slope of grain, five test samples of the tropical wood species okan were evaluated, and slope of grain values of 0.3 were observed, where 0.1 is the limit value. Because all pieces passed the visual grading method as applied in practice, slope of grain values should be incorporated in the strength class assignment test program, when these qualities cannot be ruled out for coming on the market. The (dynamic) modulus of elasticity can be used to evaluate the occurrence of the desired range of slope of grain values in the test samples.