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This article presents a study focused on the fire resistance of steel structures when solid wood cladding or OSB panels are used. The measured properties of wood at elevated temperatures are presented. Wood pyrolysis is studied with the use of available procedures for calculating the influence of pyrolysis on fire development. The development of the charred layer is studied as a desirable part that fills the insulating layer. When this effect is shown in the experiments, the charred layer slows down the heat transfer to the structure. The charred layer will last on the steel member throughout the investigation or will fall off and expose the steel member to more rapid heating. The paper presents insights identified in previous research. Our study proposes advanced procedures for predicting the charring layer. By calibrating the thermal characteristics of the wood, a method is established to address the analysis of a charred wood layer exposed to fire. The study presents the influence of input data on the accuracy of the charred layer calculation, the development of pyrolysis, i.e., the fire protection effect on the structure or on the progress of the fire.
Fire resistance is one of the most critical areas in the design of members and entire structures of buildings to ensure the resilience of the components or system when it is exposed to the effects of fire, and to ensure rapid and safe evacuation of people in the building. Protecting the immediate vicinity of the building is also an integral aspect of the fire resistance of a structure, i.e., preventing the fire spreading to surrounding buildings. When designing wood or steel members, the limits of the materials are often underestimated to the point that, without the application of fire protection or oversizing of individual parts, members or whole structures fail very early, e.g., within 7 mins. When designing wooden structures, it is necessary to consider wood flammability and the influence of flammability on the development of the fire, e.g., a faster increase in temperature, higher temperatures in a given compartment. The design of a structure should include the formation of a char layer and the development of a layer of pyrolysis. The standards indicate the basic procedures for the design according to empirical formulae or, for example, according to the non-linear methods presented in the Wood Handbook . Steel structures that have to withstand fire for more than 15 min have to be protected from high temperatures. A foam coating, an insulation layer spray, or gypsum board material is used for fire protection. In practical applications, gypsum board is the most widely used material. Another option is to use wood or OSB as a protective cladding for the steel member. The challenge is that the added material is flammable. The main question is therefore how best to protect a steel structure from the effects of fire by using wood or wood-based materials in such a way that the structure continues to be classified as non-combustible. An option for laying the groundwork for an amendment to the legislation is to look at modelling and experimenting with protective steel structures. A major challenge in modelling wood burning or any wood thermal stresses is how to correctly present the thermal degradation of the material and the influence of pyrolysis, which will influence the development of a fire in a compartment.
Thermal conductivity is affected by the diminishing humidity of the wood and by the formation of a layer of charring. There are two ways to accommodate the change in thermal conductivity. The first step is to change the thermal conductivity as the temperature rises. This procedure was established by Knudson and Schneiwind  as early as 1975, and because of its universality the procedure has subsequently been adopted by many other publications and standards. In 1999, König et al.  found that the thermal conductivity reached much more pronounced values when temperatures reached above 500 °C. The temperature conductivity pattern of Knudson et al. was therefore adjusted. Findings on the course of changes in thermal conductivity are always directly dependent on the type of wood and the location where it was harvested. The generalization of individual thermal conductivity development is therefore always approached with the assumption of thermal conductivity at an average temperature. Assuming the use of non-spruce wood with unknown thermal characteristics, where the detailed course is not known, a conversion employing a temperature-dependent reduction ratio is used. This is a simplified form, which approximates earlier experiments and can serve for a basic estimate of the properties of the material. The relationship between isotropic thermal conductivity and temperature is given in EN 1995-1-2:2005 , where wood unification is assumed. Temperature deformation may be considered in conjunction with the specific heat capacity, depending only on the loss of moisture in the wood or on a slight increase in the body due to the development of water vapor, which has a larger volume than the original water. These findings are transposed to the model as a temperature-dependent property of the members being studied. It is not dependent on force action and does not primarily trigger cracks in the body. The development of temperature deformation to 300 °C is no longer taken into consideration, due to the assumption that all the water vapor has already evaporated from the body. (Figure 1) clearly shows the increasing heat capacity, because heat is needed to start wood pyrolysis.
The most significant changes in volume density occur at around 220-320 °C, when, due to thermal degradation and burning of wood, the raw wood is converted into a charred layer. The wood burns in this temperature range. However, the burning process is not yet limited by a charred layer, which would act as insulation and protection for the lower layers of the wood. At higher temperatures, the density decreases more slowly. Another break occurs at 800 °C, when the charred layer has been wholly transformed into ash. For illustrative purposes, when comparing the density at room temperature and at 400 °C, the density loss due to the release of volatile substances and due to the development of charred layer is about 40 %. Thus, wood at 300 °C has 60 % of the density of the raw wood.
Specific thermal capacity
In particular, the humidity contained in the wood is a fundamental factor in influencing the change in the specific heat capacity. At initial temperatures, i.e., up to 200 °C, the course of the specific heat capacity is related to the evaporation of moisture and its conversion into water vapor, which in turn makes its way through the wood to the surface. When the free moisture in the material evaporates, the density of the wood decreases due to the thermal degradation of the wood, i.e., due to the course of pyrolysis. An essential representation of the course of the specific heat capacity is given in EN 1995-1-2, where a large increase in thermal capacity is shown at 100 °C. At this point, much energy is consumed to convert moisture into water vapor (Figure 2). Similar behaviour is observed for concrete exposed to fire, where the water bound in the concrete is also converted into water vapor.
Pyrolysis is the thermal decomposition of organic matter in an inert atmosphere. In the first stage of pyrolysis an endothermic reaction occurs, in which energy is consumed up to the initial temperature of the wood . Constant chemical reactions are assumed, and a specific amount of flammable gases released into the environment and released by the heat source can also be assumed. The chemistry of pyrolysis is influenced by the composition of the wood, with the proportion of volatile substances contained in the wood reaching up to 77 % of the weight of dry wood (Table 1).
Table 1: Representation of chemicals in wood.
Ratio of chemical elements in wood
Type of wood
For simplification, the overall mechanism of pyrolysis is broken down into primary and secondary responses. The primary reaction relates to the basic wood building units, namely, lignin, cellulose, and hemicellulose, where pyrolysis of the individual substances occurs simultaneously. The primary reaction is affected only by the local temperature. The secondary reaction involves the decomposition of the resulting substances from the primary reaction, thus affecting the further development of the charred layer due to the burning of volatile substances .
The individual reaction components are described using a first-order differential equation, where the mass fraction at the beginning is equal to 1.0. The calculation is addressed using the Runge-Kutty method to determine the residual mass fraction for each wood component [6-7].
By adjusting the equations obtained from the Runge-Kutty method, the effect of temperature changes due to pyrolysis is calculated, depending on the mass speed of the ignition dependent on the initial temperature and the surface temperature of the element under study.
From the calculated values, the rate of heat release to the surroundings by pyrolysis is calculated using the effective calorific value of the wood or wood-based material, as follows:
When determining the mass rate of burning, the wood needs to be divided into basic building blocks, namely, cellulose, hemicellulose, and lignin. This step concerns wood composition, e.g., 70% cellulose and hemicellulose, 20% lignin, and 10% water. For each type of wood, the distribution is noticeably different. Each wood has different reaction factors, which must always be taken into consideration. The, and response rates are obtained by burning wood samples in a conical calorimeter. It is necessary to consider the error of the difference in wood construction and samples from other parts of the tree. Experimental tests in a calorimeter are carried out in order to obtain the kinetic coefficients. Pyrolysis is measured as a complex problem, i.e., without differentiating the temperatures for the onset of pyrolysis for lignin, cellulose, or hemicellulose. This step produces a higher calculation error rate if the heat consumption for developing the pyrolysis of each component of an exothermic event is not described (Figure 3). Kinetic coefficients of pyrolysis have been published, e.g., by Wang et al. in 2016 . A more detailed approach addresses the pyrolysis of the three major parts: lignin, hemicellulose, and cellulose. This step provides a more sensitive record of the pyrolysis process and thus prevents the possibility that the temperature will increase more slowly due to pyrolysis (Figure 4).
The detailed reaction of the cellulose at each step is described as follows:
The easiest way to include the influence of pyrolysis in small-scale models is to combine the burner power with the released energy of wood pyrolysis. The power added to the fire site is calculated on the basis of the mass rate of decanting obtained by the cost coefficients. A cyclic calculation is still needed in order to include the effect of the increase in the furnace temperature and thus the faster course of the pyrolysis. Let us suppose that computational programs are used that allow pyrolysis to be calculated concurrently with the simulation (FDS). In that case, it is appropriate to use these procedures to verify the contribution of the influence of pyrolysis to the secondary calculation. This predicted process is possible with small wooden members. Ideally, this solution is offered when there is a need for an initial prediction of the behaviour of the oven temperature or to ascertain the contribution of pyrolysis to the evolution of the temperature in the furnace (Figure 5).
In this procedure, the effect of a wooden element slowing the passage of the released water vapour is not taken into consideration in the calculation, for the following reasons:
The element is so small/thin that the influence of water vapour does not manifest
The simplified model fails to consider vapour evaporation options
Due to the simplicity of the calculation, the effect of the added released energy takes time to manifest, and the assumption is accepted that the deceleration due to the calculation will include the effect of the released steam.
The division of pyrolysis into three parts was addressed in 2000 by Sinha et al. , and completely new pyrolysis coefficients for spruce wood were set in 2021 by Rinta-Paavola et al. . The pyrolysis input data of Wang , Sinha  and Rinta-Paavola  were compared using the same example, the same computing network and the same computational step. In the Room corner test, the burner was set at 50 kW, and Rinta-Paavola used propane. In a corner near the room’s burner, a 20 mm thick poultice of spruce wood was simulated. (Figure 6) presents the apparent gradual evolution of the kinetic coefficients of pyrolysis, with a gradual refinement of the data that has been obtained, i.e., with the avoidance of thermal declines, which are caused by the significant influence of the endothermic part of the individual component or by the possible development of pyrolysis of one of the wood components, and the consequent reduction in pyrolysis, which results in a significant drop in temperatures.
Thermal degradation and burning transforms wood into a charred layer with physical-mechanical properties different from those of the raw wood. The main significant change is the insulation property, the change in thermal conductivity, in (Figure 7), which increases significantly and prevents further degradation of the wood member. The charred layer has little or no resistance to force action, with only crude non-degraded wood transferring the load. It is therefore necessary to define as precisely as possible the depth of the wood charring, which reduces the original cross-section to effective dimensions. The conversion to an efficient cross-section that is considered for the transmission of stresses ensures safe design of the structural members. The formation of a charring layer is taken into consideration for all wood surfaces which are exposed to fire and are not protected . First of all, it is important to note that wood carbon monoxide occurs in the protected member and the charring rate is reduced.
The temperature distribution is taken into consideration. The temperature under the charred layer of 288-300 °C is determined by the carbonation depth, using the isotherm 300 °C method. Due to its distinctive insulating properties, the temperature of the charred layer is reduced to 180 °C by a layer 6-7 mm in depth. Numerical simulation or an advanced analytical calculation may be used to analyses the position of the charring line in detail. The speed of wood burning depends on many factors, in particular the intensity of the temperature load, the density and the humidity of the wood, the emissivity of the material surface, the direction of fire spread over the members (horizontal, vertical), etc. For reliable results, however, appropriate input data must be applied. Examples include the heat needed to evaporate dry wood, the precise coefficient of emissivity of the material surface, the surface temperature of the member as a function of temperature, load variation, the average temperature of pyrolysis, etc. The calculation is very complex, in particular due to the specific input data that are required. In the case of wood-burning models, when even the calculation involves the comprehensive removal of a part of the sample, the multiphysical problem needs to be addressed. This issue was discussed by Ira et al.  in their description of the basic approaches to model building using OOFEM. Part 5 of this article compares the linear charring rate according to the results of experiments with the FEM model reflecting the results of the experiments.
For the purposes of determining the temperatures in the small-scale furnace and the early stages of the influence of the flammable gases released by pyrolysis of the samples, temperature patterns have been determined using the FDS program, after the exact geometry of the small-scale test furnace has been modelled (for the dimensions, see section 6). The simulation verifies the burner settings for maintaining temperatures up to 800 °C, which is the temperature that is important for maintaining wood density of at least 20% of the original value. Two simulations, MF.1 and MF2, were performed. In the MF.1 simulation, the burner power was set according to ISO 834, reaching a temperature of 900 °C. In the MF.2 simulation, the effect of spruce wood pyrolysis was considered with the Rinta-Paavola kinetic coefficients of pyrolysis . The kinetic coefficients of pyrolysis are shown in (Table 2). For simulation MF.3 with OSB cladding, values of the kinetic coefficients of pyrolysis from Ira et al. were used  and are shown in (Table 3). (Figure 8) When the influence of pyrolysis (MF.2) was included, 150 °C higher temperatures were reached, on average, than in the first model, in which pyrolysis is not taken into consideration (MF.2). The MF.1 and MF.2 simulation runs are shown in (Figure 9). On the basis of this finding, a course of experiments in a small-scale furnace with burner settings up to 800 °C was established to take into consideration the charred layer forming the protective layer in at least 20 % of the raw wood density.
Table 2: Spruce-wood kinetic coefficient values by Rinta-Paavola .
42,39 × 1012
19,51 × 104
54,26 × 1012
16,81 × 104
24,60 × 1011
15,75 × 104
Table 3: OSB Board kinetic coefficient values according to Ira .