The cross-laminated timber (CLT) technology is also perceived as a potential for utilization of lumber oflower grades and underused species, because the core layers perpendicular to the principle loading direction transferloads through rolling shear, which is not correlated to the grade of lumber. Current the product standard however specifies the minimum grade requirements for all lumber to be used as CLT laminations. In this study the effect of the presence of knots in the transverse core layer of CLT billets was examined in matched CLT samples where the heavy presence of knots in the transverse core layer was the only variable compared to knot free reference. All samples were tested as short-beams in three point bending and all failed in rolling shear in the transverse core layer. The presence of knots had no measurable effect on the shear capacity expressed as nominal MOR of the tested CLT beam samples
The goal of this study was to analyze behavior of the cross-laminated timber (CLT) panel subjected to torsion and develop an efficient procedure for quick verification of numerical model of CLT that subsequently may be used for virtual prototyping of non-standard CLT products. Study used both experiments based on optical measurement using digital image correlation (DIC) and numerical modeling by means of finite element method (FEM). A physical torsion test of the CLT panel was first analyzed in terms of a displacement field that was computed on its surface. The FE simulation of the torsion test followed real boundary conditions and was carried out with use of 2 geometrically different FE models of the CLT. The first FE model did not take into account edgebonding of the lamellas, the second one demonstrated alternative manufacturing option by considering the lamellas’ edge-bonding. The experiment and FE simulations were mutually compared based on displacement paths created on the panel surface. Results showed that the presented procedure offers relatively easy way of verification of FE analyses of CLT. FE model with edge-bonding of lamellas exhibited higher stiffness and higher relative error to DIC measurement than FE model without edge-bonding. Edge-bonding of lamellas introduces influential factor in FE modeling of CLT and should be omitted for accurate and realistic FE analyses of their behavior. Study also showed that lack of orthotropic properties of Oregon hybrid poplar can be in FEA sufficiently substituted by using cottonwood properties. Combining the DIC measurement and FEM in the analysis of the CLT is favorable since it offers an full-field validation of numerical models, which can be subsequently used for virtual prototyping.
Fire concerns are one reason for building code restrictions against tall wood buildings in US. Despite growing body of empirical data on fire performance of cross-laminated timber (CLT) generated in Europe and Canada, lack of full-scale tests performed on structural CLT “made in the USA” is often quoted as a barrier for approval of massive timber in tall buildings in the US. The goal of this project was to contribute towards removing this barrier by testing the fire resistance of three full-scale unprotected CLT floor assemblies fabricated by two US-based manufacturers. The assemblies represented two species groups (SPF and DF-L), and two adhesive systems (PUR and MF).
All assemblies met ASTM E119 standard qualifying criteria for 2-h fire rating in loaded condition. While char fall-off was observed in assemblies bonded with PUR, the differences in mean char rates in the first two layers between assemblies bonded with PUR and MF were smaller than the variability of char rates within individual assemblies. Statistical significant difference was observed only for cumulative char rate for the entire time of exposure. The effect of softening of the PUR bonds was most apparent as an accelerated rate of assembly deflection in the late stage of the tests. The unprotected half-lap joints provided adequate barrier against transmission of hot gases and flames through the assemblies.
Cross-laminated timber (CLT) is a prefabricated solid engineered wood product made of at least three orthogonally bonded layers of solid-sawn lumber that are laminated by gluing longitudinal and transverse layers with structural adhesives to form a solid panel. Previous studies have shown that the CLT buildings can perform well in seismic loading and are recognized as the essential role of connector performance in structural design, modelling, and analysis of CLT buildings. When CLT is composed of high-grade/high-density layers for the outer lamellas and low-grade/lowdensity for the core of the panels, the CLT panels are herein designated as hybrid CLT panels as opposed to conventional CLT panels that are built using one lumber type for both outer and core lamellas. This paper presents results of a testing program developed to estimate the cyclic performance of CLT connectors applied on hybrid CLT layups. Two connectors are selected, which can be used in wall-to-floor connections. These are readily available in the North American market. Characterization of the performance of connectors is done in two perpendicular directions under a modified CUREE cyclic loading protocol. Depending on the mode of failure, in some cases, testing results indicate that when the nails or screws penetrate the low-grade/low-density core lumber, a statistically significant difference is obtained between hybrid and conventional layups. However, in other cases, due to damage in the face layer or in the connection, force-displacement results for conventional and hybrid CLT layups were not statistically significant.