Withdrawal Strength of Nail-Timber Joints for Kenyan Grown Cypress and Pines
George Muthike, Fred Muisu, Joseph Githiomi

Abstract
Nailed joint specimens of general structural and special structural grade cypress and pine timber were constructed and tested to determine the withdrawal resistance. The main objective of this study was to generate a database for basic withdrawal loads for nailed Kenyan grown cypress and pine timber joints for use in the development of timber structural design codes. Timber specimens were sampled from the sawmills in all plantation growing areas. 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 a hard 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 in a USTM and strength determined according to British Standards (BS 6948: 1989). The maximum load and mode of failure were recorded. Statistical analysis was done using General Linear Model and ANOVA tools in Statistical Package for Social Sciences. Results showed that wood species, grade, nail size and penetration had a significant effect on the nail withdrawal strength. The differences between species and grade are due to variation in specific gravity of the wood species and increased fastener contact area with increased penetration. Special structural graded wood (SS) and six inch nails yielded significantly higher strength for both pine and cypress than general structural (GS) and other nail lengths. Nailed pine and cypress joint strengths are comparable to those of species with similar specific gravity. These findings imply that establishment of withdrawal strength design values should put into account the combined effect of these factors.

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