Timber has been used for building construction for centuries, until the industrial revolution, when it was often replaced by steel and concrete or confined to low-rise housings. In the last thirty years however, thanks to the development of mass timber products and new global interest in sustainability, timber has begun to make a resurgence in the building industry. As building codes and public perception continues to change, the demand for taller and higher-performance timber buildings will only grow. Thus, a need exists for new construction technology appropriate for taller mass timber construction, as well as for fabrication and deconstruction practices that respect wood’s inherent sustainable nature. With this in mind, this research program aims to develop a new hybrid shear connection for mass timber buildings that allows for easy construction, deconstruction, and reuse of the structural elements.
This report includes results of Phase 1, which focused on connections consisting of partially threaded 20M and 24M steel rods bonded into pockets formed in CLT and surrounded by thick crowns of high-strength three-component epoxy-based grout. A total of 168 specimens were designed and fabricated, and push-out shear tests carried out with a displacement-controlled monotonic loading protocol. Strength and stiffness values were assessed and effective failure modes in specimens identified. These latter, along with the recorded load-deformation curves, indicate that it is possible to develop mechanics-based design models and design formulas akin to those already used for typical dowel-type fastener timber connections. Additionally, the specimens were easily fabricated in the lab and quickly fastened to the test jig by means of nuts and washers, suggested such connections have a strong potential for prefabrication, disassembly, and reuse.
The usage of holes in glulam and LVL beams is a common practice in timber constructions and requires in many cases the application of reinforcement. At present, Eurocode 5 does not contain design rules for holes, nor for their reinforcement, which are, however, regulated in the German National Annex to EC5. Although it has been proven that internal rod-like reinforcements improve the shear force capacity of a beam with holes, several problems still remain, particularly the inability to successfully reduce peak stresses at the periphery of the hole, especially shear stresses. Inclined internal steel rod reinforcements were studied and compared with vertically oriented rods, which is currently the only regulated application. The analysis revealed a reduction of both perpendicular to grain tensile stresses and shear stresses, which for the case of vertical rods are not reduced at all. A first attempt at the design of such inclined reinforcements was made by deriving an equation based on the results from FEM simulations. The design approach was then applied to an example case. Experimental verification of the theoretical observations is still necessary and ongoing, though a very promising approach for an improved internal reinforcement and its respective design can already be observed.