Fire safety remains a major challenge for engineered timber buildings. Their combustible nature challenges the design principles of compartmentation and structural integrity beyond burnout, which are inherent to the fire resistance framework. Therefore, self-extinction is critical for the fire-safe design of timber buildings.
This paper is the first of a three-part series that seeks to establish the fundamental principles underpinning a design framework for self-extinction of engineered timber. The paper comprises: a literature review introducing the body of work developed at material and compartment scales; and the design of a large-scale testing methodology which isolates the fundamental phenomena to enable the development and validation of the required design framework.
Research at the material scale has consolidated engineering principles to quantify self-extinction using external heat flux as a surrogate of the critical mass loss rate, and mass transfer or Damköhler numbers. At the compartment scale, further interdependent, complex phenomena influencing self-extinction occurrence have been demonstrated. Time-dependent phenomena include encapsulation failure, fall-off of charred lamellae and the burning of the movable fuel load, while thermal feedback is time-independent. The design of the testing methodology is described in reference to these fundamental phenomena.
This paper describes selected observations, measurements, and analysis from a series of large-scale experiments on cross-laminated timber (CLT) slabs that were exposed to fire from below, using four different heating scenarios, with a sustained mechanical loading of 6.3 kN m per metre width of slab. The deflection response and in-depth timber temperatures are used to compare the experimental response against a relatively simple structural fire model to assess the load bearing capacity of CLT elements in fire, including during the decay phase of natural fires. It is demonstrated that the ventilation conditions in experiments with a fixed fuel load are important in achieving burnout of the contents before structural collapse occurs. A mechanics-based structural fire model is shown to provide reasonably accurate predictions of structural failure (or lack thereof) for the experiments presented herein. The results confirm the importance of the ventilation conditions on the fire dynamics, burning duration, and the achievement of functional fire safety objectives (i.e. maintaining stability and compartmentation), in compartments with exposed CLT.
An experimental study of the influence of an exposed combustible ceiling on compartment fire dynamics has been performed. The fire dynamics in compartments with combustible cross-laminated timber ceilings vs non-combustible reinforced concrete ceilings in otherwise identical compartments with three different ventilation factors were investigated. The experimental results are compared against predictions from two theoretical models for compartment fire dynamics: (a) the parametric fire model given in EN 1991-1-2, and (b) a model developed at Technische Universität Braunschweig, which are the parametric fire models currently used in Germany. It is confirmed that the introduction of a combustible timber ceiling leads to higher temperatures within the enclosure, both under fuel-controlled and ventilation-controlled scenarios. It is also demonstrated that the theoretical models considered in this article require refinement in order to adequately represent all relevant scenarios when combustible ceilings are present. A refinement of the German model, by adding the fuel from the combustible ceiling to the occupancy fuel load, was shown to not adequately capture the response for the ventilation-controlled fires.
A novel dynamic testing method has been used to study changes in flexural elastic modulus of Cross-Laminated Timber (CLT) with elevated temperature. The elastic modulus is an important parameter for estimation of the structural response of CLT structures to fire and, using this dynamic method, it can be measured with a small, unobtrusive sensor.
A sectional analysis model was developed to estimate the reduction in stiffness of CLT beams in bending at elevated temperatures, and experiments were conducted using modal analysis in an attempt to validate the predictions made. The model comprised two parts: a two-dimensional heat transfer model that estimates the temperature profile within the CLT elements, and a stiffness reduction model which is based on the decrease in the elastic modulus of CLT at elevated temperatures along with a sectional analysis.
The model was validated by conducting experiments on four different CLT beams of dimensions 3 x 0.1 x 0.3 m. The beams were heated from ambient conditions within a specialised heating chamber capable of maintaining a gas temperature of 140 °C. Thermocouples were used to record the thermal gradients within the beam with time, and to compare these values against those predicted by the model. The reduction in dynamic flexural stiffness was measured periodically by exciting the beam and recording the response of an attached high-temperature accelerometer.
It was found that the thermal properties suggested in Eurocode 5 resulted in reasonable estimates of the temperature profiles recorded during the tests. Results also showed that most used modulus reduction models over-predicted the measured reduction of dynamic elastic modulus at elevated temperatures. The results suggest that the reduction of modulus of elasticity in timber is not only a complex function of temperature, but is also influenced by other parameters dependent on temperature such as moisture transport, cracking, and creep.
This paper presents a review of the pyrolysis, ignition, and combustion processes associated with wood, for application in tall timber construction. The burning behaviour of wood is complex. However the processes behind pyrolysis, ignition, combustion, and extinction are generally well understood, with good agreement in the fire science literature over a wide range of experimental conditions for key parameters such as critical heat flux for ignition (12 kW/m2 ± 2 kW/m2) and heat of combustion (17.5 MJ/kg ± 2.5 MJ/kg). These parameters are key for evaluating the risks posed by using timber as a construction material. Conversely, extinction conditions are less well defined and understood, with critical mass loss rates for extinction varying from 2.5 g/m2s to 5 g/m2s. A detailed meta-analysis of the fire resistance literature has shown that the rate of burning as characterised by charring rate averaged over the full test duration is observed to vary with material properties, in particular density and moisture content which induce a maximum 18% variability over the ranges expected in design. System properties are also shown to be important, with stochastic phenomena such as delamination and encapsulation failure resulting in changes to the charring rate that cannot be easily predicted. Finally, the fire exposure as defined by incident heat flux has by far the largest effect on charring rates over typical heat fluxes experienced in compartment fires. Current fire design guidance for engineered timber products is largely prescriptive, relying on fixed ‘‘charring rates’’ and ‘‘zero-strength layers’’ for structural analyses, and typically prescribing gypsum encapsulation to prevent or delay the involvement of timber in a fire. However, it is clear that the large body of scientific knowledge that exists can be used to explicitly address the fire safety issues that the use of timber introduces. However the application of this science in real buildings is identified as a key knowledge gap which if explored, will enable improved efficiencies and innovations in design.
Modern building construction is increasingly applying laminated timber products as structural members for larger and more ambitious projects, both commercial and residential. As a consequence, designers require reliable knowledge and design tools to assess the structural capacity of laminated mass timber elements in fire. This paper reviews and assesses available data and methods to design for fire resistance of laminated mass timber compression elements. Historical data from fire resistance tests is presented and compared against the available design calculation methods. The underlying assumptions of the thermal and structural analyses applied within the presented calculation methodologies are discussed. The resulting meta-analysis suggests that the available methods are all able to make reasonable predictions (with an average mean absolute error (MAPE) of 22% across methods) of the fire resistance of glued-laminated columns exposed to standard fires; however, the available methods for CLT walls give inconsistent (MAPE of 46% across all methods and 30% excluding extreme outliers) and potentially non-conservative results (up to 88% of investigated cases are statistically non-conservative). Additional research on loaded compression elements is therefore needed.
With increasing regularity, compartments with exposed timber boundaries are being proposed in high-rise buildings. However, due to the combustible nature of timber, the fire-specific risks associated with these decisions must be thoroughly explored. In particular the requirement that the timber stops burning after the imposed fuel load has been consumed must be fulfilled. By means of reduced scale experiments it was determined that sustained burning was dependent on both the configuration of exposed faces and, to a lesser extent, the imposed fuel load. The principal factor for auto-extinction or otherwise was found to be in the configuration of exposed surfaces, with two exposed walls (in this case back and side wall) consistently resulting in sustained burning. When a wall and the ceiling were left exposed (wall opposite the compartment opening and ceiling), auto-extinction occurred for all but the highest fuel load considered. The occurrence of char fall-off (delamination) was significant in promoting sustained burning and was observed to cause a transition from apparent extinction back to flaming in one experiment.