Evaluation of bending performance of nail laminated and dowel laminated timber

Abstract

Dowel-type fastener-laminated timber, such as nail laminated timber (NLT) and dowel laminated timber (DLT), is gaining back popularity in timber construction in the world, which can be used for floors, roofs and walls of a building, and bridge decks. One of its advantages over other glue laminated mass timber products, such as glue laminated timber (GLT) and cross laminated timber (CLT), is its environmentally friendly nature. In recent years, proportion of mid-rise structures built with laminated timber products has increased, especially in North America. This research was mainly aimed at examining the effects of fastener types (nail and wooden dowel), fastening patterns (straight and zigzag) and spacing on the mechanical performance of dowel-type fastener-laminated timber. To this end, three phases of research were conducted. The first phase examined the withdrawal capacity (WC) of wooden dowels in DLT in terms of pilot hole diameter and dowel swelling. Spruce-Pine-Fir wood substrate and 6-mm-diameter maple wooden dowels were used. Results showed that the group made of 5-mm pilot holes and 1% moisture content (MC) dowels had the largest WC of 507N, which was 87% higher than that group of 6-mm pilot holes and 15% MC dowels. The second phase studied the flexural properties of downscaled 3-layer laminated wood specimens connected by nails and wooden dowels and bonded by adhesive. The fastening spacing and fastener type were also examined. Results from the third-point bending tests showed that the 60-mm-spacing NLT specimens had an average MOEapp of 10,145 MPa, which was only 2% and 1% higher than 85-mm-spacing ones, and an average MORapp of 75 MPa, which was 9% and 4% higher than 110-mm-spacing ones. While for the fastener type, DLT had an average MOEapp of 10,575 MPa and MORapp of 78 MPa, which were 6% and 4%, respectively, higher than NLT one. As for the DLT, the zigzag pattern performed better than the straight one. Findings also revealed that NLT had a lower MOEapp variability than DLT. The final phase investigated the mechanical performance of full-scale 7-layer NLT, DLT and GLT beam-type specimens based on the findings from the second phase. The influencing factors considered were fastener spacing (250 mm and 450 mm), fastener types (nails and wooden dowels), and fastening pattern (parallel and inclined nailing). Results from the third-point load bending tests showed that group NS2 made with 89-mm-long nails and 250-mm nailing spacing had average MOEapp and MORapp of 11,724 MPa and 34 MPa, which were 4 % and 9 %, respectively, greater than group NM4 made with 102-mm-long nails and 450-mm nailing spacing that are recommended in the Canadian Standard CSA O86. The NLT beams had average MOEapp of 11,534 MPa and MORapp of 32 MPa that were 2 % and 16 % higher than DLT ones, respectively. No significant difference existed between parallel and inclined nailing patterns. This work made some contributions to understanding of the structural performance of dowel-type fastener-laminated timber products for the structural applications. Keywords: Nail laminated timber, dowel laminated timber, fastener type, fastening pattern, fastening spacing, apparent modulus of elasticity, apparent modulus of rupture, withdrawal capacity