Evaluating rolling shear strength properties of cross-laminated timber by short-span bending tests and modified planar shear tests

This paper presents an experimental study on rolling shear (RS) strength properties of non-edge-glued cross-laminated timber (CLT) made out of New Zealand Radiata pine (Pinus radiata) structural timber. CLT specimens with 35 and 20 mm thick laminations were studied to evaluate the influence of lamination thickness on the RS strength of CLT. Short-span three-point bending tests were used to introduce high RS stresses in cross layers of CLT specimens and facilitate the RS failure mechanism. Modified planar shear tests from the conventional two-plate planar shear tests were also used to evaluate the RS strength properties. It was found that two test methods yielded comparable RS strength properties and the lamination thickness significantly affected RS strength of the CLT specimens. The test results also indicated that the recommended characteristic RS strength values of CLT products in Europe and Canada might be over conservative. Also, it might be more efficient to specify different RS strength values for CLT with different lamination thickness given the minimum width-to-depth ratio of laminations is satisfied.     

Strength properties of glued laminated timber made from edge-glued laminae II: bending,tensile, and compressive strength of glued laminated timber

The purpose of this study was to investigate the strength properties of glued laminated timber composed of edge-glued laminae and to investigate the influence of edge gluing on the strength properties. Glued laminated timber composed of multiple-grade laminae (symmetrical composition, strength grade E95-F270, 10 laminations) was produced from karamatsu (Larix kaempferi) edge-glued laminae according to the Japanese Agricultural Standard. The bending, tensile, and compressive strengths of the glued laminated timber were measured. The average bending, tensile, and compressive strengths were 33.4, 24.5, and 35.9 MPa, respectively, and these values are almost equal to those of glued laminated timber composed of karamatsu single-piece laminae. It was determined that finger-joints and knots in the edge-glued laminae played a significant role in the failure of specimens. However, the use of glued edge-joints did not appear to affect the failure of specimens.     

Numerical simulation of embedding strength of glued laminated timber for dowel-type fasteners

A non-linear material model is proposed to describe the timber behavior. Anisotropic elasto-plastic constitutive law with hardening according to Hill yield criterion was used for the compressive behavior. Brittle behavior in tension and shear was modeled using the modified Hill failure criterion. The material model was implemented in a finite element code to simulate embedding strength of glued laminated timber for dowel-type fasteners. Reasonable agreement is found between numerical simulations and experimental measurements.     

Simultaneous determination of shear strength and shear modulus in glued-laminated timber using a full-scale shear block specimen

A newly developed full-scale shear block specimen was used to simultaneously determine the shear strength and shear modulus of glued-laminated timber. The shear modulus was calculated using the shear strain distribution measured by means of digital image correlation. To obtain the exact relationship between shear modulus and shear strength, the shear strain in the intended shear plane was measured. A relatively high correlation coefficient of 0.75 was found between the shear modulus and nominal shear strength in two types of glued-laminated timber tested: sugi (Cryptomeria japonica) and Douglas-fir (Pseudotsuga menziesii). Good correlation was also found between the density of middle lamina and shear strength. However, the relationships to density showed species dependence, which was not observed in the relationship between shear modulus and shear strength.     

Failure analysis of laminated timber beams reinforced with glass fibre composites

Summary A qualitative analysis is presented of failure, perpendicular to the grain, in laminated timber reinforced with a glass fibre composite. The study is focused on beams with holes of different shape. The stress by corners, infinitesimal cracks and finite cracks are investigated. An initial crack model is suggested that brings about some of the phenomena observed in earlier performed experiments. A crack appears to propagate in the wood but is retarded in the reinforced beams. Eventually, the composite will fracture and failure of the beam follows. Finite element computations suggest that the reinforcement decreases the stress intensity at cracks in the wood and acts as a crack stopper. The reinforcing effect increases with the crack length. A point stress criterion is used to predict failure in the fibre composite.     

Evaluation of rolling shear strength of plywood by flexural vibration method

The rolling shear strength of plywood was evaluated using a flexural vibration test. Test specimens were lauan and Douglas fir three-ply plywoods made from thick veneers. The dynamic shear and Young's moduli were determined using the flexural vibration method, which involved in-plane and out-of-plane flexural vibration. The rolling shear strength was determined using the static destructive method, which is dependent on the direction of the lathe check in the core veneer. Before and after accelerated aging treatments were conducted, there were relations between out-of-plane dynamic properties (out-of-plane shear and Young's moduli) and its rolling shear strength. It was concluded that the rolling shear strength is related not only to the shear property of the core but the flexural stiffness of two faces when the deformation of out-of-plane plywood was not restrained.Part of this work was presented at the 47th Annual Meeting of The Japan Wood Research Society, Kochi, April 1997     

Fracture mechanics analysis of row shear failure in dowelled timber connections

A quasi-nonlinear fracture mechanics model is presented for the analysis of row shear failure in timber connections with multiple fasteners in a row. A prerequisite for use of the model is a known distribution on the fasteners of the total applied load. It is shown that the ideal plastic and linear elastic fracture mechanics solutions appear as special cases of the quasi-nonlinear model. The model offers strength predictions that include the effect of among others the number of fasteners in a row, fastener spacing, row spacing, end-distance, edge-distance, fastener diameter, and material properties, such as shear strength, fracture energy, and modulus of elasticity. Simple explicit expressions are obtained from the analysis for any known fastener load distribution. For bolted connections, where bolt-hole gaps cause high loads on certain bolts at random, the model may be used in simulation procedures or in probabilistic models.     

Modelling the long-term strength of timber columns

Summary This paper studies the influence of the used creep function, and the duration of the experimental sustained loading from which the creep function was derived, in modelling the long-term buckling strength of timber columns. The sensitivity of this buckling strength to creep and initial deflection is numerically studied using an energy approach that takes into account the initial deflection. Various creep functions — power laws as well as exponential laws — derived from bending experiments of different durations are used for the numerical simulation. The crucial need for experimental results issued from very long load period experiments is identified. Such experiments may lead to a more credible creep analysis of timber structures.Symbols i radius of giration - s stress level - E_L effective MOE - E₀ instantaneous MOE - I inertia moment - K,K geometrical matrices - L length of beam or column - L_f effective length - N_cr axial force, buckling load - U elastic strain energy - W external forces work - t time variable - u moisture content (m.c.) - v total deflection - v_c, v_e, v₀ creep, instantaneous elastic and initial deflections respectively Greek Letters (t) time function - , _e, ₀ total, instantaneous and initial strains respectively - twist rotation - (t) creep coefficient or fractional creep or relative creep - (x) shape function - load factor - longitudinal shortening - total potential energy - creep factor - relaxation time     

Estimation of the internal shear strength distribution of the element for laminated veneer lumber by nonlinear least-squares method

Until now we developed an estimation method for strength distributions of laminated veneer lumber (LVL) element by nonlinear least-squares method (NLM). Estimated strengths by this method were modulus of elasticity (MOE) and modulus of rupture (MOR) in the horizontal use direction and the vertical use direction, tensile strength and compression strength. But to use LVL for structural members, shear strength was also needed. Therefore, we tried to estimate the shear strength distribution of LVL element by NLM same as MOE and MOR in the horizontal use direction and the vertical use direction, the tensile strength of LVL and the compression strength of LVL in the previous reports. We conducted shear strength test for LVL and estimated element shear strength distribution by LVL strength data in the horizontal and vertical use direction. Next, we simulated LVL shear strength distribution using element shear strength distribution and compared with experimental ones in each use direction. They were overlapped in both use direction. Therefore, we could validate NLM for estimating element shear strength distribution.     

Study on the estimation of the strength properties of structural glued laminated timber I: determination of optimum MOE as input variable

There have been many attempts to predict the performance of glulam beams. Several approaches have been taken, from early empirical techniques to more sophisticated stochastic methods. In recent years, more emphasis has been placed on the modeling of material properties. Generally, the modulus of elasticity (MOE) has been used as a criterion of laminar strength for the prediction of glulam performance in the traditional models. Most of the current models are based on MOE that was measured using the long span test; that is, they account only for variability between pieces of lumber. Therefore, these models do not account for the variation of material properties within a given piece of lumber. Five methods were considered to choose the appropriate one that could effectively predict the performance of glulam in this study. Prediction of glulam performance was done by the transformed section method. MOEs measured with the five methods were applied to a strength prediction program to compare the actual test results and the predicted results. MOEs used as input variables are as follows: long span MOE of the static bending test, localized MOE of the static bending test, long span MOE of the stress wave test, localized MOE of the stress wave test, and MOE of the machine stress rating (MSR) test. Results of the localized test showed excellent signification compared to those of the long span test. The MSR method, when used as input variable, obtained the most approximate result, so it is considered adequate for predicting the strength of glulam.An outline of this paper was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998     

木材单板层积材现状及趋势探讨

叙述了木材单板层积材的发展现状及趋势。认为利用小径材研究开发单板积材正是解决天然林保护及木材短缺矛盾的有效方法。根据目前LVL的发展现状及趋势，综述LVL的研究方向及其生产设备要求。     

Estimation of shear strength of dowel-type timber connections with multiple slotted-in steel plates by European yield theory

The shear strength of dowel-type timber connections with multiple slotted-in steel plates was estimated based on European yield theory. The values calculated based on the yield theory were compared with experimentally obtained results. An experiment was performed on dowel-type timber connections with two and three slottedin steel plates under lateral loads parallel to the grain. The yield mode of the dowel-type connection assumed in this study corresponds approximately to the failure mode of the connection obtained from the experiment. The shear strength of the dowel-type connections calculated based on the yield theory showed good agreement with the results for shear strength obtained in the experiment. The yield theory was useful for estimating the shear strength of the doweltype connection with multiple slotted-in steel plates. The shear strength of the dowel-type connection was greatly affected by the spacing of the steel plates, the number of steel plates, and the timber thickness. The values of these parameters that showed the proper shear strength of the dowel-type connection could be estimated based on the yield theory. Part of this paper was presented at the 9th Timber Engineering Forum of the Japan Timber Engineering Society, December 2005     

Comparison of three processing methods for laminated bamboo timber production

Three processing techniques, split-squaring(SS),V-grooving(VG), and split-edging(SE), were evaluated and compared in terms of their processing time and recovery. Semantan bamboo(Gigantochloa scortechinii Gamble) was used as a raw material and the results showed that the VG-method required the longest processing time(32.1 min), followed by the SS-method(18.2 min), and the SE-method(17.9 min). However, the VG-method showed the highest recovery(82.0%) compared to the SS-method and SE-method, 31.0 and 49.4%, respectively. When both processing time and recovery factors were combined with the same weighing factor, the highest value was recorded with the VG-method(49.1), while the SE-method had values of 31.1 and the SS-method 12.8. The results suggest that the VG-method is the best option. However, the V-grooving machine is still a prototype and therefore the SE-method could be an alternative option until the improved V-grooving machine is available.     

Tensile strength of thermally modified laminated strand lumber and laminated veneer lumber

Laminated strand lumber (LSL) and laminated veneer lumber (LVL) were thermally modified as a post-treatment at 140°C, 150°C, 160°C, 170°C, and 180°C. The tension modulus of elasticity (MOE) of LSL was not significantly impacted by the treatments, with the 180°C treatment group exhibiting the highest tension MOE (11.8?GPa). The LVL also experienced minimal impacts, with the 150°C treatment group having the highest tension MOE (19.4?GPa) and the 160°C treatment group exhibiting the lowest (17.1?GPa). The maximum tensile strength (MTS) of the LSL and LVL significantly decreased with increasing temperatures, with the control and 180°C treatment groups experiencing the highest and lowest MTS, respectively. The lowest MTS for LSL was 10.8?MPa (180°C treatment), which was 70% lower than the controls. The lowest MTS of the LVL was 24.4?MPa (also at the 180°C treatment), which was a 49% decrease compared to the controls. These results suggest that thermal-modification post-treatments minimally impact tension MOE, but can significantly reduce MTS at higher treatment temperatures. Combined with previous work improving the moisture properties and equilibrium moisture content of thermally modified LSL and LVL, it may be possible to optimize the treatment technique(s) to yield products with desirable properties.     

Determination of shear modulus on construction size timber

Shear modulus determination of construction size timber was examined. Three different Techniques were used: torsional vibration, simultan E&G, and variation of span technique. We found that the simultan E&G technique is quite sensitive to the error of natural frequency determination and provides only an estimate for G. Better shear modulus determinations are the static and torsional techniques, but the torsional technique is most precise. The torsional vibration technique can be a good solution for shear modulus determination in an industrial environment because it provides quick and precise etermination, but standardization of the method would first be necessary. We found a high correlation coefficient (0.80) between the screw withdrawal resistance and shear modulus of 2 by 4 timber. Based on this result it is possible to estimate the shear modulus of timber even if the timber is installed in a structure.This study was conducted with financial support of the Science and Technology Agancy Japan via an STA fellowship     

Effect of grain angle on shear strength of glued end grain to flat grain joints of defect-free softwood timber

To evaluate the effect of grain orientation on the adhesive bond strength, three-layered Norway spruce wood specimens were tested in shear. The two axial-oriented outer layers were jointed with the middle layer using three typical glues for load bearing constructions, i.e. one-component polyurethane (PUR), melamine–urea–formaldehyde (MUF) and phenol–resorcinol–formaldehyde (PRF). The grain orientation of the middle layers was varied from 0° (parallel to grain of the surface layer) to 90° (perpendicular to grain of the surface layer) in incremental steps of 10°. Samples with middle layers oriented parallel to the outer layers showed shear values in the range of solid spruce wood. Decreased shear strength values were expected for increased grain angles of the middle layer. However, no explicit tendency was observed for the shear strength in dependence of the grain direction. In general, MUF-bonded samples showed slightly higher shear strength values compared with PUR- and PRF-bonded specimens.     

Strength properties of glued laminated timber made from edge-glued laminae I: strength properties of edge-glued karamatsu (Larix kaempferi) laminae

The object of this study was to investigate the strength properties of edge-glued laminae and to propose a suitable grading method based on the lamina modulus of elasticity (MOE). Edge-glued laminae composed of lumber with similar MOEs (uniform laminae) and edge-glued laminae produced by randomly gluing lumber independent of MOE (random laminae) were made from karamatsu (Larix kaempferi) lumber having the same thickness and length, but various widths. For both the uniform and random laminae, there was a strong correlation between MOE values measured using the longitudinal vibration technique, the static bending test, and a grading machine. The average values of bending, tensile, and compressive strengths of the uniform laminae were similar to those of the random laminae. On the other hand, the average strength of laminae without end joints was significantly higher than that of finger-jointed laminae for both uniform and random laminae. Finger-joints and knots played a significant role in the failure of specimens, but the edge-gluing and the difference in MOE within an edge-glued lamina did not appear to affect the strength properties. The bending, tensile, and compressive strengths of edge-glued laminae were strongly correlated to the lamina MOE.