Date of Award
Program or Major
Master of Science
Timber Framing is one of the earliest forms of construction, utilizing large cross-section timbers that connect to one another with interlocking joinery. These joinery styles were developed over millennium through trial and error, so there is limited research on the failure modes, structural capacities, and factors of safety associated with current design practices. This research project fills a knowledge gap, identified by the Timber Frame Engineering Council (TFEC) of Alstead, NH, regarding the joinery design of traditional birdsmouth (angled bearing) connections and how the connections act under compressive loads.The three most popular truss peak joints (two angled compression members, single tension member) and truss heel joints (single angled compression member, single tension member) were determined through a survey designed by the author and distributed through the TFEC. A king post truss was chosen to simultaneously load the truss peak and truss heel in compression by applying a load to the truss king post, loading it in tension and forcing compression into the truss top chords. Thirteen truss specimens (nine unique and four replicates) were loaded to failure in The University of New Hampshire reaction frame. The specimens were designed with different joint types but with consistent geometry and materials to limit variability between the specimens. Each specimen was fabricated by the same team at Vermont Timber Works of North Springfield, VT. The performances of the different joinery styles were compared using multiple factors: ease of fabrication (determined from survey), measured and calculated responses (determined from vertical applied load data and vertical deflection data), and failure modes with corresponding loads (determined using Digital Image Correlation analysis of each joint). All thirteen trials carried maximum imposed loads that exceeded their calculated design load by a factor of 2.0 or greater. All four predicted failure modes from the current code practices were observed (block shear, peg yielding, shoulder bearing failure, and mortise and tenon bearing failure), as well as additional serviceability failure modes that did not affect the specimen’s structural integrity. Block shear was the only observed failure that prevented a specimen from carrying additional load, which occurred in nine of thirteen specimens. The optimum joints performed the best regarding the measured and calculated responses (e.g., maximum sustained load, maximum stiffness) and had the highest load carrying capacities, with corresponding factors of safety, for their respective joint types (truss peak or truss heel). The best performing peak option was found to have a full-width, continuously sloping shoulder, a centered tenon with bearing face perpendicular to the king post (vertical member), and was joined with one 1” peg per side. The best performing heel option was found to have a full-width shoulder with bearing face perpendicular to the top chord (angled member), no tenon, and was joined for constructive purposes with one ¼” timber screw centered on the truss width.
McGinnis, Matthew, "Compression Testing of Traditional Timber Framing Truss Joinery" (2022). Master's Theses and Capstones. 1652.