A timber renaissance has occurred in the built environment in an effort for architectural freedom and increased sustainable construction. Current building codes place limits on the extent to which timber can be exposed. Overall building heights and floor areas are capped due the perception that timber burns continuously. Original fire resistance requirements were derived on the concept of complete burnout of the fuel within a compartment. Whether non-combustible or combustible materials are used, materials must be capable of withstanding a certain fire load until burnout at which point flaming combustion of the material must cease. If not, then the fire resistance framework loses its meaning and the concepts used in the design process need to be re-evaluated. This study focuses on the underpinning behavior of timber exposed to a fire in an attempt to extract meaning processes that enable an adequate assessment of the structural performance of timber as a construction material. The focus is on cross laminated timber (CLT) as an example of relevant engineered wood products but the concepts presented in this work could be applied to any other timber product.
The first aspect researched in this study was the self-extinction characteristics of CLT. Self-extinction is an unavoidable starting point since it is the critical process that defines the relevance of fire resistance as an assessment methodology for structural performance. CLT samples were exposed to a range of heat fluxes (6-100 kW/m2). Samples were heated by an external heat flux until either flaming or smoldering combustion occurred. Exposure continued throughout the transient stage of burning until steady-state was achieved. After a pre-established heating period the heat flux was completely removed. In every case tested, flaming combustion ceased. A series of tests were then conducted to determine the critical heat flux and mass loss rate for extinction. The tests were repeated except, instead of removing the heat flux completely, the heat flux was gradually decreased until flaming combustion ceased. The experiments showed that for all conditions studied an almost constant critical mass loss rate was measured at extinction. However, in some cases, flaming combustion continued due to debonding of the timber plies. Where cracks and gaps opened at the bond line, re-ignition of flaming combustion would occur. The test allowed the threshold for self-extinction to be quantified and showed that careful detailing in the design and construction process is likely to be necessary in order for self-extinction to be predictable.
Once self-extinction of flaming combustion of timber was confirmed, the relevant mechanics that influence debonding were studied. To quantify the effects of temperature between timber plies, series of tests were conducted to investigate thermal penetration and thermal profiles within the CLT. The samples were exposed to a range of heat fluxes (6-100kW/m2) and the results showed that above 30kW/m2 the thermal profiles were approximately the same. The only difference between the heat fluxes was the speed at which the pyrolysis front moves through the material.
Once the thermal profiles had been measured, a single-lap shear test was performed using a range of temperatures (20-150°C). The samples were heated to the target temperature and then a tensile force was applied to the joint until failure. Digital image correlation was used to capture the displacement and strains of the adhesive bond. The results showed that the strain experienced by the bond was greater as the sample increased in load and temperature. The interfacial stresses (normal and shear) were compared and the normal stresses in the bonded joint at areas of discontinuity were shown to be orders of magnitude greater than the shearing stresses. The study found that the mechanics of the debonding were primarily governed by the normal forces creating a separation at the local discontinuities and then unzipping the bond until failure.
Following the small scale tests, a larger scale testing program was executed to identify whether the behaviors observed in the single-lap shear test could be observed in industrial CLT. CLT beams both at ambient and elevated temperature were loaded in a three-point bend test until failure. Differences in the bond performance between the two tests were not observed as the primary failure mode was rolling shear. In each case, debonding occurred but was initiated at cracks formed by the rolling shear. Once the local discontinuities appeared on the bond line, the increased normal forces proceeded to unzip the bond until failure. Failure occurred due to insufficient effective bond length.
This work develops methodologies and provides quantification for many of the fundamental parameters required for analyzing the performance of CLT structures in fire. Designing timber structures for fire conditions was concluded to be possible but the need for further research is evident. Self-extinction of flaming combustion is required for any fire safety strategy to be valid but self-extinction is not only a combustion problem, but it is also a problem that is intimately related to the mechanical behavior of the bonds in engineered timber. Thus, a detailed understanding of the combined thermos-mechanical behavior is necessary before timber (and timber structural systems) can be engineered to deliver adequate fire performance.
