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.
This study aimed to investigate the effect of layer arrangement on bending properties of CLT panels made from poplar (Populus deltoides L.). A total of 20 three-layer CLT panels with the same dimensions of 1300 × 360 × 48 mm3 (Length, Width, Thickness) were fabricated in five configurations: 0/30/0, 0/45/0, 0/90/0, 45/0/45, and 45/45/45. The apparent modulus of elasticity (MOEapp), modulus of rupture (MOR) and apparent bending stiffness (EIapp) values in major and minor axes of CLT panels were calculated using experimental bending testing. In the major axis, the highest values of MOR, MOEapp, and EIapp were obtained from the 0/30/0 arrangement, while the least values resulted from the arrangements of 90/60/90 and 90/45/90 in the minor axis. Besides, in all arrangements, the average of the experimental apparent bending stiffness values (EIapp,exp) of specimens was higher than that of the shear analogy apparent bending stiffness values (EIapp,shear). The bending and shear stress distribution values over the cross section of samples were also estimated using the finite element method. Moreover, the numerical apparent bending stiffness (EIapp,fem) values of samples were compared to experimental apparent bending stiffness (EIapp,exp) values. Based on experimental and finite element method results, in all groups of layer arrangements, the EIapp,fem values concurred well with the EIapp,exp values.
Project contact is Henry Quesada at Virginia Polytechnic Institute and State University
This project is a multistate industry-university collaboration between SmartLam, the Northeastern Lumber Manufacturers Association (NELMA), the American Plywood Association (APA), IKD Architectures, Virginia Tech, and Purdue University to advance the utilization of hardwood lumber for the fabrication of Cross- Laminated Timber (CLT). This new proposal builds upon a previous Wood Innovation project. The collaboration among the organizations proposes to: 1) apply the shear analogy method to hardwood species listed in the National Design Standards (NDS) supplement to assure these species are feasible for the construction of structural CLT panels, 2) create a custom grade CLT layup made of yellow poplar (Liriodendron tulipifera) lumber and get its approval by the Engineered Wood Association (APA), 3) train the hardwood industry in the Midwest and in the Southeast on the application of hardwood structural lumber grading rules, and 4) perform mechanical testing on the hardwood CLT panels used in the Conversation Plinth project by IKD Architectures in Columbus, IN. In 2012 Virginia Tech conducted the first experimental tests on hardwood CLT panels. Results indicated that bonding, strength, and stiffness of yellow poplar CLT panels matched or were superior to some of the softwood CLT layups in the APA standard. Similar results were also obtained by independent testing conducted by the American Hardwood Export Council (AHEC) in 2018. However, further investigation by Virginia Tech found that the main limitations for the use of yellow poplar and other low value hardwood species in CTL panels are 1) lack of experimental data on other hardwood species used in CLT panels, 2) lack of supply of structurally graded hardwood lumber, and 3) acceptance and validation of hardwood CLT panels by the APA standard. Overcoming these limitations is critical for the hardwood lumber industry in order to gain access to the CLT market. Currently, the annual production of CLT panels in the US is about 35,000 m3 but it is expected that in 10 years production will be close to 2 million m3 per year. The outcomes of this project are to increase the utilization of low-value hardwood species from national and private forests and to increase economic development in rural areas in the hardwood regions of the US.
Cross laminated timber (CLT), as a structural plate-like timber product, has been established as a load bearing product for walls, floor and roof elements. In a bending situation due to the transverse shear flexibility of the crossing layers, the warping of the cross section follows a zigzag pattern which should be considered in the calculation model. The Refined Zigzag Theory (RZT) can fulfill this requirement in a very simple and efficient way. The RZT, founded in 2007 by A. Tessler (NASA Langley Research Center), M. Di Sciuva and M. Gherlone (Politecnico Torino) is a very robust and accurate analysis tool, which can handle the typical zigag warping of the cross section by introducing only one additional kinematic degree of freedom in case of plane beams and two more in case of biaxial bending of plates. Thus, the RZT-kinematics is able to reflect the specific and local stress behaviour near concentrated loads in combination with a warping constraint, while most other theories do not. A comparison is made with different methods of calculation, as the modified Gamma-method, the Shear Analogy method (SA) and the First Order Shear Deformation Theory (FSDT). For a test example of a two-span continuous beam, an error estimation concerning the maximum bending stress is presented depending on the slenderness L/h and the width of contact area at the intermediate support. A stability investigation shows that FSDT provides sufficiently accurate results if the ratio of bending and shear stiffness is in a range as stated in the test example. It is shown that by a simple modification in the determination of the zigzag function, the scope can be extended to beams with arbitrary non-rectangular cross section. This generalization step considerably improves the possibilities for the application of RZT. Furthermore, beam structures with interlayer slip can easily be treated. So the RZT is very well suited to analyze all kinds, of shear-elastic structural element like CLT-plate, timber-concrete composite structure or doweled beam in an accurate and unified way.
This paper deals with the influence of the rolling shear deformation on the flexural behavior of CLT (Cross-Laminated Timber) panels. The morphological configuration of the panels, which consist of orthogonal overlapped layers of boards, led to a particular shear behavior when subjected to out-of-plane loadings: the low value of the shear modulus in orthogonal to grain direction (i.e., rolling shear modulus) gives rise to significant shear deformations in the transverse layers of boards, whose grains direction is perpendicular with respect to the tangential stresses direction. This produces increases of deflections and vibrations under service loads, creating discomfort for the users. Different analytical methods accounting for this phenomenon have been already developed and presented in literature. Comparative analyses among the results provided by some of these methods have been carried out in the present paper and the influence of the rolling shear deformations, with reference to different span-to-depth (L/H) ratios investigated. Moreover, the analytical results have also been compared with those obtained by more accurate 2D finite element models. The results show that, at the service limit states, the influence of the rolling shear can be significant when the aspect ratios became less than L/H = 30, and the phenomenon must be accurately considered in both deflection and stress analysis of CLT floors. Contrariwise, in the case of higher aspect ratios (slender panels), the deflections and stresses can be evaluated neglecting the rolling shear influence, assuming the layers of boards as fully-connected.
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.