Monotonic and Cyclic Behavior of CLT Diaphragms

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 ($\mathrm{4,570}\times \mathrm{4,570}\mathrm{mm}$) were used to quantify ductility through the diaphragm force reduction factor ($R_s$) and m-factors from ASCE standards, and validate common design methods. Tests demonstrated that CLT can function well as a diaphragm with a mean $R_s$ value of 1.19, comparable with precast concrete having an $R_s$ 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 $R_s$. 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, $6.86\mathrm{kN}/\mathrm{mm}$ ($39.8\mathrm{kip}/\mathrm{in.}$), was lower than the estimate of spline stiffness of $11.5\mathrm{kN}/\mathrm{mm}$ ($65.7\mathrm{kip}/\mathrm{in.}$) from individual fastener tests. Hysteretic loading resulted in a lower spline stiffness of $4.37\mathrm{kN}/\mathrm{mm}$ ($24.9\mathrm{kip}/\mathrm{in.}$), while monotonic testing showed a mean spline stiffness of $9.04\mathrm{kN}/\mathrm{mm}$ ($52.5\mathrm{kip}/\mathrm{in.}$). CLT diaphragm displacement based on National Design Specification methods was conservative compared to test results in determining diaphragm flexibility.