Bending Characteristics of Custom CLT Layups Laminated with Ponderosa Pine Harvested from Restoration Programs

Abstract

National forest restoration programs aimed at effective mitigation of catastrophic wildfires and pest outbreaks in the Western region of the United States yield a substantial amount of small-diameter ponderosa pine logs. Lumber produced from these logs is considered low-value due to lower mechanical properties compared with commercially harvested lumber. The US Forest Service is seeking a value-added market for this material to offset the high costs of forest restoration operations. Cross-laminated timber (CLT) is considered one potential market for the material. This study was part of a larger project to determine the feasibility of utilizing CLT fabricated from restoration harvested ponderosa pine in low-rise modular mass-timber construction. A CLT layup was designed based on the design requirements of a modular construction system determined in a parallel study. The bending characteristics of that layup were empirically verified through mechanical tests on prototype panels, three fabricated at a pilot-plant line at Oregon State University and two at an industrial manufacturing line. In all prototypes, grades No. 1, 2, 3, and ungraded laminations were assigned to all layers randomly. Standard ASTM D198 methods for long-and short-span flatwise bending tests were conducted to derive effective moment capacity, effective stiffness, and shear capacity of the layups. Optical measurement based on digital image correlation was used to derive effective shear rigidity of the specimens. There were no significant differences between results obtained from prototypes fabricated in the pilot plant compared with those fabricated in an industrial setting. Mechanical tests indicated that prototype CLT panels exceeded the structural requirements. Effective moment capacity, stiffness, and shear rigidity were higher than values estimated by the shear analogy method. Shear capacity was lower than predicted. Restoration program ponderosa pine CLT layups can be custom-designed to meet mechanical requirements for structural elements in certain classes of modular buildings.