High-performance grout-reinforced shear connectors for hybrid steel-cross-laminated timber building systems: Experimental study

Abstract

Wood-based construction advancements have been observed recently with the adoption of hybrid assembly and offsite construction methods, relying on prefabricated components, such as volumetric cross-laminated timber (CLT) modules or composite panel-type wall and floor elements. Within the high-rise timber building framework, mass timber structural assembly requires connections designed to accommodate innovative technologies to limit damage under seismic loads and mitigate lateral vibration of the building under serviceability loads. As part of the performance-based capacity and constructability enhancements, current research efforts have been on the development of high-performance and reliable mass timber connections. This paper presents a new connection design method for hybrid steel-CLT composites and assemblies. Shear connectors are fabricated by encasing steel rods into CLT panels using an epoxy-based grout, with geometry specified to provide stiffness and shear capacity target levels. An insight into their behaviour and performance is given based on an experimental dataset comprising 240 full-scale push-out shear tests and an array of rod diameter, grout thickness, and mechanical properties of steel and CLT. Maximum shear strength of 289.6 kN and elastic stiffness of 243.2 kN/mm were attained for the tested specimens, showing that strong and stiff behaviour were attainable. Elastic mechanical properties of connectors were shown to be governed by the rod diameter, whereas strength resistance and failure mode from the grout thickness, allowing for decoupling of design parameters and fine-tuned design. The yield point was shown clearly identifiable, supporting optimum capacity design strategies. Observed failure modes were mostly ductile and with a maximum slip ductility of 6.4, and on average comparable with common wood bolted connections. A resistance factor of 0.8 was shown adequate to design the shear connectors using the calculated 5th percentile values of the yield shear resistance and maximum shear resistance, relatively, for elastic and plastic design strategies. Large-scale building applications were shown attainable by intrinsic individual mechanical properties of the connector, supporting deconstruction and reuse of the hybrid steel-timber structural system components when the connector is capacity-protected and behaves elastically; thus, designed using its 5th percentile yield shear strength.