Effect of butt joints on flexural properties of nail laminated timber

Abstract

Nail laminated timber (NLT) is manufactured using dimension lumber laminations, stacked on edges, and fastened with nails, to create large flat structural components that are widely used for constructing timber bridge decks in North America. Butt joints usually exist due to the length limits of lumber, leading to concerns about the decrease of structural performance of NLT. This project was undergone to understand the impact of butt joints on flexural properties of NLT and investigate the methods for reinforcing the NLT containing butt joints. To reach this goal, four-point bending tests were performed on 7 types of 3-layer NLT specimens, 1 without butt joints as reference, 3 with butt joints and 3 with butt joints reinforced with nails. It was found that the effective stiffness (𝐸𝐼𝑒𝑓𝑓) and load capacity (𝑃𝑝𝑒𝑎𝑘) of 3-layer NLT specimens with butt joints, in comparison to those without butt joints, could be reduced up to 64% and 71%, respectively. The reduction in 𝐸𝐼𝑒𝑓𝑓 of NLT specimens with a certain 1-in-3 frequency pattern of butt joints was found to be 34%, which proved the applicability of the stiffness reduction factor in current CSA S6 standards. It was found that the nailing reinforcement was an effective approach to recover the 𝐸𝐼𝑒𝑓𝑓 and 𝑃𝑝𝑒𝑎𝑘 of NLT specimens with a certain 1-in-3 frequency pattern of butt joints by 21% and 38%, respectively. This approach could be used to make NLT with butt joints, but more research is required. Keywords: Effective stiffness, Load capacity, Nail laminated timber, Nailing reinforcement