Experimental Characterization of Cantilever Cross-Laminated Timber Diaphragms under In-Plane Shear Load

Abstract

Diaphragms transfer the forces exerted by lateral loads to vertical elements through an in-plane shear action. Cross-laminated timber (CLT) has garnered considerable interest as a diaphragm material, particularly for midrise nonresidential construction. This article discusses the findings of tests performed on two full-scale 6.1 m (20 ft)×6.1 m (20 ft) cantilever CLT diaphragms composed of three-ply CLT panels. The diaphragms were subjected to in-plane shear loads with a cyclic loading protocol. Two different layouts of CLT panels were considered for this study (simple span and two-span continuous panels). The displacement, force, or strain responses of various components of the diaphragms were measured and analyzed. The peak capacity of the diaphragms was governed by the fasteners used and exceeded the LRFD capacity by a factor close to two. Test results showed the primary energy dissipation mechanism for the diaphragms was via the slip of diaphragm components parallel and perpendicular to the direction of load and rotation of CLT panels, nearly all of which could be attributed to fastener deformations. Observation of strain data revealed deep-beam-like behavior of the diaphragms where the resistance to coupled tensile and compressive forces was primarily contributed by the outer portion of the edge panels, with little to no contribution from the inside panels. In the compression zone, a near-uniform compressive strain pattern was observed at the top and bottom ply, aligned along the major axis of the CLT panels. In the tension zone, the tensile strain was observed only in the top ply aligned along the major axis (where tension straps were installed), and compressive strain was observed in the bottom ply. The results of the strain data were used to suggest a design procedure for the tension and compression chords.