Embedment Properties of Self-Tapping Screws Loaded at Different Angles to the Grain

The embedment properties including embedment strength and foundation modulus govern the load-carrying capacity and stiffness of timber joints with dowel-type fasteners. As the common fasteners, self-tapping screws (STSs) have been widely used for timber connections. However, the empirical formulas for the embedment strength of STSs are still not exclusively mentioned in the current design standards for timber structures. Those empirical formulas for screws in the design standards are given by referring to the same status as nails or bolts with similar dimensions. A large number of experimental campaigns can contribute to the develop the reliable formulas for the embedment properties. In this study, the embedment tests for the fully threaded STSs with 7, 9, and 11 mm diameters in spruce-pine-fir (SPF) were performed for five load-to-grain angles of 0°, 30°, 45°, 60°, and 90°, with and without predrilled holes, using full-hole and half-hole test configurations, under two ambient environments with a temperature of 20°C and two relative humidities (RHs) of 65% and 85%, to determine the influences of load-to-grain angles, predrilling, test configurations and moisture contents of wood on the embedment properties of STSs. The experimental results show that the embedment properties of the specimens with and without predrilled holes had no significant differences, and the test configurations had no statistically significant effects on the embedment strengths. The empirical formulas for the embedment strength and foundation modulus were derived.