The use of engineered timber products such as cross-laminated timber (CLT) is of increasing interest to architects and designers due to their desirable aesthetic, environmental, and structural properties. A key factor preventing widespread uptake of these materials is the uncertainty regarding their performance in fire. Currently, the predominant approach to quantifying the structural fire resistance of timber elements is the charring rate, which allows estimation of residual cross-section and hence strength. The charring rate is usually determined by testing timber specimens in a furnace by exposure to a ‘standard fire’. However, it is recognized that the resulting charring rates are not necessarily appropriate for non-standard fire exposures or for characterizing the structural response in a real timber building. The effect of heating rate on the charring rate of CLT samples is investigated. The charring rate resulting from three heating scenarios (constant, simulated ‘standard fire’ and quadratically increasing) was calculated using interpolation of in-depth temperature measurements during exposure to heating from a mobile array of radiant panels, or in a Fire Propagation Apparatus (FPA). Charring rate is shown to vary both spatially and temporally, and as a function of heating rate within the range 0.36–0.79 mm/min. The charring rate for tests carried out under simulated ‘standard fire’ exposures were shown to agree with the available literature, thus partially verifying the new testing approach; however under other heating scenarios the Eurocode charring rate guidance was found to be unconservative for some of the heat flux exposures in this study. A novel charring rate model is presented based on the experimental results. The potential implications of this study for structural fire resistance analysis and design of timber structures are discussed. The analysis demonstrates that heating rate, sample size and orientation, and test setup have significant effects on charring rate and the overall pyrolysis, and thus need to be further evaluated to further facilitate the use of structural timber in design.
Architectural Institute of Japan Structural System
Timber elements, which are different from other structural elements, have a characteristic problem in that the load bearing capacity decreases due to self-burning in the case of a fire, and this self-burning may continue after other fuel in the room has been exhausted. Therefore, the structural fire performance of timber elements should be clarified during not only the heating phase, but also the cooling phase. However, in examining the load bearing capacity of timber elements in a fire, few studies have considered the cooling phase. In the present paper, the fire performance of glued, laminated timber beams is discussed based on load-bearing fire tests that take the cooling phase into consideration.
Inspection, Testing, and Monitoring of Buildings and Bridges
Depending on the severity, fire damage can compromise the structural integrity of wood structures such as buildings or residences. Fire damage of wood structures can incorporate several models that address (1) the type, cause, and spread of the fire, (2) the thermal gradients and fire-resistance ratings, and (3) the residual load capacity.
The investigator should employ engineering judgment to identify those in-service members that are to be replaced, repaired, or can remain in-service as they are. Suchjudgment will likely be based on the visual inspection of damaged members, connections, and any protective membranes.
The charring behavior of timber structural elements, such as the charring rate of timber elements and delamination of glue-laminated timber, affects the structural stability of timber buildings. The charring rate of timber elements varies depending on the severity of fire exposure. However, charring rates have been ordinarily investigated in fire tests under the standard fire exposure defined by ISO 834. It is important to accumulate and analyze data on the charring behavior of timber elements under actual fire exposure. The aim of this study was to clarify the charring behavior of glue-laminated timber structural elements exposed to actual fire in full-scale fire tests of three-story timber school buildings. Charred and uncharred areas of the timber structural elements were carefully observed and investigated after the fire tests. The charring rates of timber elements in full-scale fire tests ranged from 0.6 mm/min to 1.3mm/min. The charring rates were greater than the nominal charring rates reported in past studies because of preheating and severe fire exposure.
This thesis describes a series of 5 tests that were conducted at Carleton University Fire Research Laboratory to assess the contribution of Cross Laminated Timber (CLT) panels to the development, duration and intensity of room fires. The tests were conducted in rooms constructed from 105 mm thick 3-Ply CLT panels and measured 3.5m wide by 4.5 m long by 2.5 m high. Propane and furniture fires were used with the CLT panels in protected and unprotected configurations. Data was collected on Heat Release Rate (HRR), room temperatures and charring rates. In protected configurations, no noticeable contribution was observed from the CLT panels, however in unprotected configurations, the CLT panels contributed to the fire load and increased fire growth rates and energy release rates. When charring advanced to the interface between the CLT layers, the polyurethane based adhesive failed resulting in delamination. Delaminated members contributed to the fire load and exposed uncharred timber which increased the intensity and duration of the fire. When delamination occurred, the fire in unprotected rooms continued to burn at high intensity well after the combustible contents in the room were consumed by the fire. These fires were extinguished as they could have resulted in structural failure of the test rooms.
The performance of timber in fire is often assessed by measuring the temperature at different positions in the specimen. As timber is a low conductive material, it can be difficult to measure the correct temperature.Therefore, this paper shows how to correctly measure the temperature in timber members and how to describe temperature measurements of fire tests and experiments non-ambiguously.Typical temperature measurement setups used in tests and experiments were experimentally assessed under ISO/EN fire exposure and a constant incident radiant heat flux. By comparing the charring depth and the thermocouple readings(charring temperature 300°C) it was found that only the wire thermocouples inlaid parallel to the isotherms deliver correct temperature readings. For other temperature measurement setups, the underestimation was between 5 and 20 minutes.Due to the numerous factors influencing the measurement error, no correction factor could be defined.
International Conference on Performance-based and Life-cycle Structural Engineering
December 9-11, 2015, Brisbane, Australia
Tall timber building designs have utilized cross-laminated timber (CLT) significantly over the past decade due the sustainable nature of timber and the many advantages of using an engineered mass timber product. Several design methods have been established to account for the composite action between the orthogonally adhered timber plies. These methods assume perfect bonding of the adjacent plies by the adhesive. CLT designs methods for timber in fire have also been formulated. These methods rely on the relatively constant charring rate of timber to calculate a sacrificial layer to be added onto the cross-sectional area. While these methods focus on the timber failure mode of reduced cross section by charring, the failure mode of ply delamination is often overlooked and understudied. Due to the reduction of shear and normal strength in the adhesive, the perfect bond assumption can be questioned and a deeper look into the mechanics of CLT composite action and interfacial stress needs be conducted. This paper seeks to highlight the various design methods for CLT design and identify the failure mode of delamination not present in the current design codes.
The objective of this work is to generate fire performance data for NLT assemblies to address gaps in technical knowledge. This project aims to study how the size of gaps between NLT boards might affect charring of an assembly and its overall fire performance. This research will support designers and builders in the use of mass timber assemblies in larger and taller buildings, by ensuring fire safe designs.
Cross-laminated timber is a relatively new engineered timber material that can be used in the design and construction of modern timber buildings. A key factor that raises concerns in the wide application of cross-laminated timber is the uncertainty of its fire performance. This article describes experimental and numerical investigations on the fire behaviour of loaded cross-laminated timber panels manufactured with Canadian hemlock. A total of 10 cross-laminated timber panels with different number and thickness of layers were tested under ambient and standard fire conditions to investigate the flexural capacity at ambient temperature, and temperature distribution, charring rate, fire resistance, mid-span deflection under fire exposure. Three-dimensional finite element model was developed using the Hashin criterion and cohesive elements to predict the failure of wood and adhesive, respectively. The thermal model implicitly considers the rapidly increased temperature of inner fresh timber after the protective charred layers have fallen off. The numerical model was validated with the results obtained from experimental tests and was found to have the ability to simulate the fire behaviour of loaded cross-laminated timber panels in reasonable accuracy.