Exploratory study of high-strength, low-cost through-bolt with cross-pipe and nut connections for square and roundwood timber frame construction

Through-bolt with dowel-nut connectors are commonly used in furniture where high strength mechanical connections are required. During early periods of use, dowel-nuts were often called barrel nuts (Eckelman 1977), but the term “dowel-nut” is more descriptive, and the author has referred to them as such in the above and all subsequent publications (Eckelman 1989, 1999; Eckelman and Erdil 2000). The high pull-out strength noted for dowel-nuts inserted cross ways through the ends of table legs led to their investigation as potential timber frame connectors by Eckelman and Senft (1995). The principal objective of that study, carried out with 6to 7-inch-diameter yellow-poplar peeler cores and 1-1/2-inchdiameter steel dowel-nuts with a 6-inch end spacing, was to obtain initial estimates of the ultimate tensile strengths that could be obtained with this connection (Fig. 1). Results of the tests indicated that maximum strength values are obtained when the longitudinal axis of the dowel-nut is located perpendicular to the plane of any major drying split. Strength values for specimens with the dowel-nut located in this manner averaged 22,820 pounds. Specimens with dowel-nuts placed in the plane of the split averaged only 61 percent as strong as those placed perpendicular to the split with minimum values only 31 percent as strong. Further, it was found that strength could be increased significantly by banding the ends of the members with a 3/4-inch steel strap. Banded specimens with the dowel-nut located in the plane of the split averaged 92 percent as strong as those with the nut located perpendicular to the plane of the split. Subsequently, the high strength of these solid steel dowel-nut connections was noted by both Wolfe (2000) and Stern (2001). Research was carried out by Wolfe et al. (2000) to determine the withdrawal strength of dowel-nuts in Douglas-fir peeler cores and small-diameter round ponderosa pine. They found that 1.75-inch-diameter dowelnut connectors were economically feasible at a design capacity of 10,000 pounds for 5-inch-diameter Douglas-fir peeler cores (Wolfe et al. 2000). The potential value of dowel-nuts was illustrated in construction of a space-frame roof system (Anon. 2000), a demonstration structure (Anon. 2000), and an information kiosk (Green 2001).