Richard Emberley

Fire Behavior of Cross-Laminated Timber (CLT) Slabs: Two-Way Action

Two-way action is often stated to be a major benefit of cross-laminated timber; however, it is seldom considered when designing tall timber buildings. A research project has been carried out to investigate this inconsistency with the objective of understanding the significance of cross-laminated timber slabs acting as two-way members. This objective was met through semi-parametric analytical modeling carried out in conjunction with bending tests on small-scale, three-layer cross-laminated timber samples. The fire variable was reduced to the process of charring which was simulated by varying the number and thickness of layers. Two-way and one-way tests were run in parallel to allow direct comparison between strain, stiffness, failure modes, and load-carrying capacity. Results of the modeling and experiments showed that, for standard panel lay-ups, two-way load transfer is enhanced by the charring process. The load-carrying capacity of charred two-way slabs was improved by as much as 50 % over the one-way equivalent. In instances, the elastic stiffness was more than doubled. It was concluded that utilizing the two-way ability of cross-laminated timber could lead to more optimal use of the material, and assuming one-way behavior in all instances may be overconservative.

Components and Consequences of Cross-Laminated Timber Delamination

Mass timber products such as cross-laminated timber are increasingly being used in tall timber building designs. The high-density wood used in mass timber products has a natural self-extinguishment capacity which chars only under exposure from an external heat flux. This makes timber a suitable building material giving timber buildings the potential to withstand full fire burnout of the building furnishings. However, ply delamination poses a significant challenge as the loss of the char layer exposes the unburnt wood and prevents the insulating char from regulating the heat flux to the unburnt wood. Even further, ply delamination means the structurally designed composite action of the mass timber product has been lost, and the ply could fall off into the compartment fire increasing the room fuel load. Ply delamination is extremely complex involving the resolution of heat transfer and structural equations as well as variables which are functions of temperature, moisture content, and many other factors. To date, the structural and thermal effects of cross-laminated timber delamination have not been comprehensively studied. This paper seeks to review the current research into delamination and highlight the research gaps that still exist. The current market focuses primarily on charring and eliminates the thermal penetration depth at the expense of losing understanding on a major failure mode of engineered timber. More detailed research into delamination is necessary to confidently ensure tall timber buildings meet the stakeholder’s goals of providing a safe building which can withstand complete fire burnout.