Influence of densification on structural performance and failure mode of cross-laminated timber under bending load

Abstract

The effect of densification of the laminas was studied relative to the shear performance of cross-laminated timber (CLT) specimens submitted to the bending test. The three-layered CLT panels were fabricated using loblolly pine (Pinus taeda L.). A compression ratio (CR) of 50% was used to densify the lumber using the thermomechanical densification technique. The process included plasticizing the lumbers by soaking them in boiling water for 10 minutes and then hot-pressing to the target thickness at 140 °C. Four groups were made, i.e., a control sample with all three layers non-densified, only mid-layer densified, all layers densified, and all layers planed to the same thickness of densified layers. Specimens were tested in short-span bending with a span-to-depth ratio of eight. For specimens having densified mid-layer, the failure mode changed from rolling shear to tensile failure of the outer layer, and the maximum shear stress was increased by 34%. Densification of the mid-layer at CR of 16% was sufficient to change the failure mode from rolling shear in mid-layer to tensile in outer layer. In the case of all-layer-densified specimens, the maximum rolling shear strength was increased by 129% compared to the control.

Optimizing Lumber Densification for Mitigating Rolling Shear Failure in Cross-Laminated Timber (CLT)

“…Given that the majority of densified specimens experienced tensile failure at the bottom layer, comprehensive analysis led to the determination that the developed bending stress (σ max ) corresponds to the bending strength (MOR) of the sample. This observed bending strength value averaged 47.45 MPa, closely aligning with the initial value reported by Pradhan et al [36]. This observation underscores the fact that the bending strength of CLT is predominantly governed by the bending strength of the bottom layer.…”

“…The findings clearly demonstrated that densification at 16.667% notably enhanced rolling shear strength, leading to a shift in the failure mechanism from rolling shear to tension. Similarly, in the study conducted by Pradhan et al [36], densification at 50% also induced a tension failure mode, restricting overall strength, primarily governed by the bending strength of the outer layer. However, densification at higher compression ratios would decrease the overall moment of inertia as higher densification is associated with a reduction in thickness, thereby impacting the bending stiffness of CLT.…”

“…Utilizing Equation ( 4), with the known value of MOR for specific species and grades of lumber, the minimum required shear strength to resist the maximum shear stress developed in CLT made from the same species and grade can be calculated for all the span-to-depth ratios. For loblolly pine lumber, #1 grade, with an average MOR of 42.28 MPa reported by Pradhan et al [36], a graphical representation illustrating the relationship between the minimum MRSS and the span-to-depth ratio (L/h) was generated from Equation (4), as shown in Figure 2. This graph serves as a valuable tool to determine the precise MRSS requirement based on the specific span-to-depth ratio of their CLT beam configurations.…”

“…Through the densification process, a higher density can be achieved, resulting in improved mechanical strength [29][30][31][32][33][34][35][36]. Pradhan et al [10] used thermomechanical densification at varying compression ratios and reported a linear relationship between the percentage increase in rolling shear strength and the compression ratio.…”

“…The study also reported that at a 50% compression ratio, the rolling shear strength of the lumber increased by double with respect to non-densified lumber. Similarly, Pradhan et al [36] evaluated the bending performance of densified lumber and reported a change in failure mode from rolling shear failure to bending failure when the transverse mid-layer was densified at a 50% compression ratio. Furthermore, in the case of all three-layer densified samples, CLT experienced failure in rolling shear, with the rolling shear strength elevated by 115% compared to that of normal, non-densified CLT samples.…”