With the introduction of Cross Laminated Timber (CLT) into North America and gaining popularity it is of interest to the design and code development community to have codified provisions to facilitate the design of CLT structures. This paper addresses the design aspect of CLT panels subjected to combined bending and compressive axial loads. Fifteen specimens covering different combinations of grades and layups of commercially available Canadian CLT panels were tested at different eccentricities to validate the proposed interaction equation. Testing program was reported in this paper and the test data was compared with the model prediction. It was concluded that the current nonlinear interaction equation given in the Canadian timber design standard (CSA O86) for timber and glulam tends to overestimate the capacity of CLT wall panel. A linear interaction equation was found to be appropriate
Cross laminated timber (CLT) is a wood panelling building system that is used in construction, e.g. for floors, walls and beams. Because of the increased use of CLT, it is important to have accurate simulation models. CLT systems are simulated with one-dimensional and two-dimensional (2D) methods because they are fast and deliver practical results. However, because non-edge-glued panels cannot be modelled under 2D, these results may differ from more accurate calculations in three dimensions (3D). In this investigation, CLT panels with different width-to-thickness ratios for the boards have been simulated using the finite element method. The size of the CLT-panels was 3.0 m × 3.9 m and they had three and five laminate layers oriented 0°–90°–0° and 0°–90°–0°–90°–0°. The thicknesses of the boards were 33.33, 40.0, and 46.5 mm. The CLT panel deformation was compared by using a distributed out-of-plane load. Results showed that panels with narrow boards were less stiff than wide boards for the four-sided support setup. The results also showed that 2D models underestimate the displacement when compared to 3D models. By adjusting the stiffness factor k88, the 2D model displacement became more comparable to the 3D model.
Cross-laminated timber (CLT) has been used extensively in timber construction. CLT panels are typically used in roofs and floors that carry a continuous load, and it is important to examine the long-term loading capacity of CLT. However, studies that focus on the long-term loading capacity of CLT are limited. To this end, we conducted long-term out-of-plane bending tests on seven-layer CLT made from Japanese larch (Larix kaempferi) under constant environmental conditions, investigated creep performance and duration of load, and experimentally analyzed creep rupture behavior. The mean estimated relative creep after 50 years was 1.49. The sample showed a satisfactory resistance to creep as a building material. The duration of load of most of the specimens in this study was shorter than the conventional value of small clear wood specimens. Specimens had a lower duration of load capacity than solid lumber. According to the results of survival analysis, a loading level of 70% or more caused the initial failure of specimens. Creep rupture of most of the specimens occurred at less deflection than displacement at failure in the short-term loading test. Additional studies focusing on the effects of finger joints, transverse layers, and width of a specimen on creep rupture behavior are suggested.
The outcome of an experimental campaign on the long-term behaviour of timber floors retrofitted with timber-to-timber composite methods is presented. Four diaphragm specimens, 5.2 m long (5 m span) were tested out-ofplane. Each specimen consisted of a solid wood-spruce joist strengthened with a crosslam panel. A layer of timber boards was placed in between the joist and the panel to simulate the existing flooring. The specimens, were subjected to uniformly distributed loading in a climatic controlled chamber. A patented procedure that enables to apply a pre-stressed state and a pre-camber to the composite floor joists by just using screw fasteners, was adopted. Different typologies and arrangements of screws were tested in order to maximize the performance (cost/effectiveness) that can be achieved by employing the above mentioned procedure. Uplift values of approximately 1/300th of the diaphragm span were registered at the end of the cambering procedure. After an initial testing phase (duration approximately equal to 3h) where the loading was consistent with the characteristic combination, the specimens were set for long-term testing under an imposed load equal to that specified by the quasi-permanent combination.
A systematic investigation is still lacking for tension out-of-plane in cross laminated timber (CLT), as a planar timber construction product. The objectives of the present study are the determination of the tensile properties of CLT made of Norway spruce, the identification of essential product-specific influencing parameters and a comparative analysis with glulam. For this purpose, seven test series were defined, which allowed the determination of the tensile properties on board segments and thereof produced glulam and CLT specimens by varying the number of layers, layer orientation and number of elements within a layer. The orthogonal laminated structure of CLT led to between 50% and 70% higher tensile properties out-of-plane, which is explained by the different stress distribution compared to glulam; the regulation of 30% higher properties than for glulam is suggested. In addition, the lognormal distribution turned out to be a more representative distribution model for characterizing the tensile strength out-of-plane than the Weibull distribution. This was also confirmed with regard to the investigated serial and parallel system effects, in which a clearly more homogeneous behavior was found in CLT compared to glulam, which in turn can be attributed again to the different stress distributions.
Project contact is Karl Englund at Washington State University
Cross laminated timber (CLT) has energized the wood industry, not only throughout the US but also across the globe. Potential for lower construction costs and a sustainable building material has provided proponents of CLTs the fuel for their growth. However, to obtain lower feedstock costs and provide a truly sustainable building product the use of small diameter timber (SDT) and other lower quality woods is imperative, but not yet realized. The out-of-plane (OOP) defects such as twist, cup and bow commonly found in SDTs, make processing CLTs prohibitive due to the press load requirements that are needed to “flatten” these defects out and create intimate contact at the glue line. Due to this issue, many CLT manufacturers utilize high grade lumber, while SDT and other low value woods are culled out and not used. Our proposal will characterize the OOP defects commonly found in SDT Douglas-fir (DF) and ponderosa pine (PP) from the Inland Northwest, will develop a tool to calculate anticipated forces to compress out the OOP defects and evaluate the durability performance of a full-scale CLT panel that includes commonly rejected lumber from SDT due to presence of OOP defects. The tool developed in this project will provide the CLT industry with the know-how to determine the press loads required to make a panel from SDT feedstocks and how to lower these accumulated loads through reducing or changing the laminate cross-sectional dimensions. Results of this study will promote increased utilization of SDT lumber, currently rejected, for CLT production and will contribute to healthy forests and rural economic development.