Compressive Strength of Nigerian Gmelina Arborea Cross-laminated Timber: A combined experimental and probabilistic approach for polyurethane and epoxy resin adhesive bonding

Cross-laminated timber (CLT) is gaining recognition as a sustainable alternative to traditional building materials like steel and concrete, offering enhanced structural rigidity and dimensional stability. However, its performance depends heavily on the wood species used. This study explores the potential of Gmelina arborea , a fast-growing hardwood widely cultivated in Nigeria but underexplored for CLT applications. The research evaluates the compressive strength of CLT panels made from Nigerian-grown Gmelina arborea under in-plane and out-of-plane loading conditions. Panels were fabricated using visually graded planks bonded with one-component polyurethane (PUR) and two-component epoxy resin (ER) adhesives. Compressive strength tests were conducted following the BS EN 408:2010 + A1:2012 standard, and reliability analysis was performed using Python and MATLAB to compute reliability indices (β) and probabilities of failure (P _f ) via the first-order reliability method (FORM). Results revealed that compressive strength perpendicular to the grain ranged from 11.37 to 13.38 N/mm², while parallel strength ranged from 14.30 to 30.26 N/mm², with PUR-bonded panels exhibiting slightly higher values. PUR-bonded panels achieved a mean perpendicular strength of 12.47 N/mm² compared to 12.24 N/mm² for ER-bonded panels. Parallel strength in the minor direction was higher than perpendicular strength, with PUR and ER panels averaging 18.19 N/mm² and 15.80 N/mm², respectively. The major strength direction showed the highest values, with PUR at 27.86 N/mm² and ER at 26.79 N/mm². Reliability analysis indicated that PUR adhesives outperformed ER, with higher reliability indices (β = 3.6637, P _f = 0.0243%) compared to ER (β = 3.1646, P _f = 0.077641%). These findings highlight the superior structural reliability of PUR-bonded CLT panels.