Withdrawal Strength of Nail-Timber Joints for Kenyan Grown Cypress and Pines

Abstract

Nailed joint specimens of general structural and special structural grade cypress and pine timber were constructed and tested to determine the withdrawal resistance in order to generate a database for basic withdrawal loads for nailed Kenyan grown cypress and pine timber joints for use in the development of design codes. Timber specimens were obtained from graded 150mm by 50mm timber while nails were purchased from local hardware dealers. Seventy two pairs of matched 50mm by 25mm by 300mm joint members were prepared from the timber then conditioned to an equilibrium moisture content of about 16 to 18 percent. The members were laid on flat surface and nails driven into them using a claw hammer in a direction perpendicular to grain. Each specimen was subjected to a withdrawal load and strength determined according to British Standards (BS 6948: 1989). The maximum load and mode of failure were recorded. Data analysis was done using General Linear Model tools in SPSS. Results show that wood species, grade, nail size and penetration have a significant effect on the nail withdrawal strength. The differences were probably due to variation in specific gravity of the wood species and increased fastener contact area with penetration. Selected structural grade wood and 15 cm nails yielded significantly higher strength for both pine and cypress. Nailed pine and cypress joint strengths were comparable to those of species with similar specific gravity. Withdrawal strength design values should therefore put into account the combined effect of these factors.