Form-Fitting Mass Timber Connections: Experimental Investigation of the Effect of Tenon Flange Angle on Structural Performance

Abstract

Mass timber construction has expanded rapidly as a lower-carbon alternative to conventional structural systems. However, connection design remains a critical factor affecting constructability, cost, and structural performance. Contemporary mass timber connections rely on steel hardware, proprietary connectors, and mechanical fasteners. While these systems are reliable and code-supported, advances in digital fabrication create opportunities to reconsider form-fitting connections. Despite their historical precedent in heavy timber structures, the structural behaviour of form-fitting glulam connections remains underexplored, particularly how geometry influences load transfer, deformation capacity, and failure mode.

This thesis investigates form-fitting glulam connections for purlin-to-girder applications, with an emphasis on tenon flange angle in mortise-and-tenon geometries. The research program included a literature review, two stages of rapid prototyping, material characterization, full-scale experimental testing, and development of a preliminary mechanical model. The literature review established that wood anisotropy, especially its low tensile strength perpendicular-to-grain, and geometry govern load transfer and failure mode. It also identified inclined bearing surfaces as a potential means of redistributing load into compression and promoting embedment mechanisms.

The rapid prototyping phase first evaluated traditional single-tenon, multiple flat-tenon, and multiple triangular-tenon configurations. The traditional tenon exhibited low capacity and brittle mortise splitting, while multiple tenons improved strength and deformation capacity through progressive engagement. The triangular-tenon configuration further improved performance, demonstrating that geometry can improve both capacity and failure response. A second prototyping phase then extended this concept to open-top wedge geometries compatible with vertical installation, supporting their selection for full-scale testing.

Twelve full-scale glulam wedge-tenon specimens were tested under static loading. Three full-depth series varied the tenon flange angle, while one partial-depth series examined the influence of reduced tenon height. The results showed that flange angle significantly affected yield load, peak load, stiffness, post-yield behaviour, and failure mode. Larger angles produced higher yield and peak loads, whereas smaller angles reduced stiffness and capacity but increased the role of wedging, horizontal thrust, and progressive deformation. The partial-depth series behaved differently from the full-depth specimens, transitioning from side-face wedging to a hybrid wedge/notch mechanism once bottom bearing developed.

A mechanical model was developed to estimate the yield load from inclined-face bearing, friction, and compression perpendicular-to-grain embedment. The model more closely reproduced the experimentally observed yield load trends than the CSA O86 fracture-shear provisions and a rounded dovetail-based prediction method. The proposed method applies to the onset of connection softening; ultimate failure mechanisms require separate consideration. Overall, this thesis showcases the promise of form-fitting connections