Monotonic and Cyclic Behavior of CLT Diaphragms

Abstract

Mass timber is emerging in new markets for wood buildings, requiring more design knowledge to compete with other structural materials. Large, in-plane tests on cross-laminated timber (CLT) diaphragms (4,570 ×4,570 mm) were used to quantify ductility through the diaphragm force reduction factor (Rs) and m-factors from ASCE standards, and validate common design methods. Tests demonstrated that CLT can function well as a diaphragm with a mean Rs value of 1.19, comparable with precast concrete having an Rs from 0.7 to 1.4. Like precast concrete systems, CLT diaphragms depend on the interpanel connections for ductility. ASTM methods validated the assumption that a CLT diaphragm is shear-controlled for the purposes of determining Rs. ASCE m-factors are indirect measures of the nonlinear deformation capacity of a component and are multipliers on the expected strength. The m-factors observed (0.46 to 1.9 for immediate occupancy to collapse protection performance levels, respectively) were lower than those in previous studies on similar panel-to-panel connections. The initial stiffness of the large diaphragm panel-to-panel connections, 6.86 kN/mm (39.8 kip/in.), was lower than the estimate of spline stiffness of 11.5 kN/mm (65.7 kip/in.) from individual fastener tests. Hysteretic loading resulted in a lower spline stiffness of 4.37 kN/mm (24.9 kip/in.), while monotonic testing showed a mean spline stiffness of 9.04 kN/mm (52.5 kip/in.). CLT diaphragm displacement based on National Design Specification methods was conservative compared to test results in determining diaphragm flexibility.