Innovative mass timber panels, known as composite laminated panels (CLP), have been developed using lumber and laminated strand lumber (LSL) laminates. In this study, strain distributions of various 5-layer CLP and cross-laminated timber (CLT) were investigated by experimental and two modelling methods. Seven (7) different panel types were tested in third-point bending and short-span shear tests. During the tests, the digital imaging correlation (DIC) technique was used to measure the normal and shear strain in areas of interest. Evaluated component properties were used to determine strain distributions based on the shear analogy method and finite element (FE) modelling. The calculated theoretical strain distributions were compared with the DIC test results to evaluate the validity of strain distributions predicted by the analytical model (shear analogy) and numerical model (FE analysis). In addition, the influence of the test setup on the shear strain distribution was investigated. Results showed that the DIC strain distributions agreed well with the ones calculated by the shear analogy method and FE analysis. Both theoretical methods agree well with the test results in terms of strain distribution shape and magnitude. While the shear analogy method shows limitations when it comes to local strain close to the supports or gaps, the FE analysis reflects these strain shifts well. The findings support that the shear analogy is generally applicable for the stress and strain determination of CLP and CLT for structural design, while an FE analysis can be beneficial when it comes to the evaluation of localized stresses and strains. Due to the influence of compression at a support, the shear strain distribution near the support location is not symmetric. This is confirmed by the FE method.
An innovative multi-layer (3 and 5) composite laminated panel (CLP) with various layups were developed using sawn lumber and structural composite lumber (SCL) to address the rolling shear and gap issues of cross laminated timber (CLT). The bending properties including apparent and effective bending stiffness, shear stiffness, moment capacities and failure modes of CLPs were evaluated by a combination of modal tests and third-point bending tests of beam specimens cut from the panels. The static bending test results showed that the apparent bending stiffness values of 3-layer and 5-layer CLPs were up to 20% and 43% higher than the corresponding values of 3-layer and 5-layer generic CLT, respectively. The bending moment capacity values of 3-layer and 5-layer CLPs were up to 37% and 87% higher than the corresponding values of 3-layer and 5-layer generic CLT, respectively. The use of SCL in transverse layers eliminated the potential rolling shear failure in CLT and increased the stiffness properties. The apparent and effective bending stiffness predicted by shear analogy method had a good agreement with corresponding values measured by bending tests and/or modal tests. The prediction of bending moment capacity using shear analogy method cannot be validated due to the rolling shear failure and tension failure modes observed in certain groups.
As an emerging building solution, cross-laminated timber (CLT) floors have been increasingly used in mass timber construction. The current vibration design of CLT floors is conservative due to the assumption of simple support conditions in the floor-to-wall connections. It is noted that end fixity occurs as a result of clamping action at the ends, arising from the gravity load applied by the structure above the floor and by the mechanical fasteners. In this paper, the semi-rigid floor-to-wall connections are treated as elastically restrained edges against rotations to account for the effect of partial constraint. A rotational end-fixity factor was first defined to reflect the relative bending stiffness between CLT floors and elastic restraints at the edges. Then, for the design of vibration serviceability of CLT floors as per the Canadian Standard for Engineering Design in Wood (CSA O86), restraint coefficients were defined and their analytical expressions were derived for natural frequencies and the mid-span deflection under a concentrated load, respectively. In particular, a simplified formula of the restraint coefficient for the fundamental frequency was developed to assist engineers in practical design. At last, by comparing with reported experimental data, the proposed design formula showed excellent agreement with test results. In the end, the proposed end fixity factor with their corresponding restraint coefficients is recommended as an effective mechanics-based approach to account for the effect of end support conditions of CLT floors.
It has been shown that measurement of elastic constants of orthotropic wood-based panel products can be more efficiently measured by modal testing technique. Identification of vibration modes and corresponding natural frequencies is key to the application of modal testing technique. This process is generally tedious and requires a number of measurement locations for mode shape identification. In this study, a simplified method for frequency identification was developed which will facilitate the adoption of the vibration-based testing technique for laboratory and industrial application. In the method, the relationship between frequency order and mode order is first studied considering the boundary condition, elastic properties of the orthotropic panel. An algorithm is proposed to predict the frequency values and mode indices based on corresponding normalized sensitivity to elastic constants, initial estimates of orthotropic ratios and measured fundamental natural frequency. The output from the algorithm can be used for identification of sensitive natural frequencies from up to three frequency spectra. Then the algorithm is integrated with the elastic calculation algorithm to extract the elastic constants from the sensitive frequencies. The elastic constants of cross laminated timber panels were measured by the proposed method. The moduli of elasticity agree well with static testing results. The calculated in-plane shear modulus was found to be within the expected range.
Cross laminated timber (CLT) is leading the evolution of wood construction throughout the world. As atwo-dimensional plate-like construction product, the in-plane elastic constants of CLT panels are the fundamental parameters for serviceability design. The elastic constants including moduli of elasticity (MOE) in major and minor strength direction ( and y) and in-plane shear modulus ( xy) of full-size CLT panels with different dimensions and layups from three CLT producers were measured by a non-destructive test (NDT) method developed by the first author. In total, 51 CLT panels were tested with most of the testing conducted at CLT mills. The measured values were used to examine the existing effective stiffness prediction models of CLT. Results show that k-method can be used for predicting and y values of industrial size CLT with a large length/ width to thickness ratio. xy cannot be well predicted by k-method and is greatly affected by edge bonding and gaps. Gamma method and shear analogy method can include the effect of transverse shear to different extents into account in predicting apparent or y. Shear analogy method appears to predict closer apparent to the measured values than gamma method for CLT with small length to thickness ratio. However, the effect of transverse shear on apparent y is not as much as predicted by shear analogy method for CLT panels with width from 1 to 3 meters. NDT by modal testing was proven to be an efficient mechanical property evaluation method for full-size CLT panels.
Eucalyptus is one of the most important plantation species in south China. The need of alternate applications of plantation grown hardwood species including eucalyptus is concerned because of the sharp decrease of the demands from pulp and paper industry. The feasibility of manufacturing cross-laminated timber (CLT) using fast-grown small diameter eucalyptus wood (Eucalyptus urophylla × Eucalyptus grandis) was evaluated here. The effects of adhesives, priming treatment, adhesive spread rate, pressure, and pressing time duration on block shear strength (BSS), wood failure percentage (WFP), and rate of delamination (RD) of CLT via block shear tests and cyclic delamination tests. The results indicated that eucalyptus CLTs made of small diameter lumbers with four types of EP, EPI, PRF, and PUR adhesives could qualified bonding and mechanical requirements according to ANSI/APA PRG 320-2012. The maximum wood failure percentage (WFP) and block shear strength (BSS) values at dry stage condition were 85.2% and 3.51 MPa obtained from specimens bonded with PUR adhesive meanwhile those values at wet stage condition were 58.2% and 1.62 MPa obtained from specimens bonded with EPI and PUR adhesives, respectively. The minimum rate of delamination (RD) value was 7.6%, which was obtained from specimens bonded with PUR adhesive. The optimal pressing parameters were adhesive spread rate 160 g/m2, pressure 0.8 MPa, and pressing time duration 200 minutes when one-component polyurethane adhesive was used to manufacture eucalyptus CLT. The values of MOEs and MORs in the major and minor direction were 11,466 MPa, 24.5 MPa, 681 MPa, and 8.6 MPa, respectively. The values of transverse shear moduli and interlaminar shear strength in the major and minor strength were 91.8 MPa, 1.3 MPa, 241.6 MPa, and 0.5 MPa, respectively. The mechanical properties of eucalyptus CLT were equivalent to those of commercial CLT made of traditional softwoods available in market. Generally, HMR priming treatment was effective to enhance bonding performance and mechanical properties of eucalyptus CLTs. It is safe to conclude that fast-grown small diameter eucalyptus lumber was feasible to manufacture CLT for structural applications.
Cross laminated timber (CLT) has the potential to play a major role in timber construction as floor and wall systems. In order to meet specific design needs and to make the use of CLT more effective, property evaluation of individual CLT panels is desirable. Static tests are time-consuming and therefore costly, and for massive products such as CLT practically impossible to implement. Modal testing offers a fast and more practical tool for the property evaluation of CLT and timber panels in general. This paper presents a comparison of different boundary conditions in modal testing in terms of accuracy, calculation effort and practicality. Single-layer timber panels as well as scaled CLT panels were fabricated. Three elastic properties of the panels were evaluated using modal testing methods with different boundary conditions (BCs). The results were compared with results from static test.