Fire Resistance and Mechanical Properties of Wooden Dou-Gong Brackets in Chinese Traditional Architecture Exposed to Different Fire Load Levels

Abstract

Dou-Gong brackets, the distinctive structural element in ancient Chinese architecture, fulfill critical roles in load transfer, span reduction, and decoration, making its preservation vital for safeguarding wooden heritage buildings. This study investigates the combustion performance and residual load-bearing capacity of key Dou-Gong bracket components—Zuo-dou, Zheng-xin-gua-gong, and Qiao—exposed to varying fire conditions. The results reveal that an increasing fire load elevates heating rates and peak temperatures of wood substrates, resulting in a significant degradation of structural integrity. At a fire load of 55 kW, the peak temperatures at the bottom, joint edge, and top of the Dou-Gong brackets reach 755.3 °C, 489.9 °C, and 620.7 °C, respectively, representing increases of 2%, 65%, and 38%, respectively, compared to those observed at a fire load of 20 kW. Moreover, the charring rate of Dou-Gong bracket increases from 0.22–0.26 mm/min at a fire load of 20 kW to 0.50–0.56 mm/min at a fire load of 55 kW, accompanied by an increase in mass loss rate from 28.5% to 36.9%. These findings highlight the significant impact of fire conditions on the fire characteristic and structural integrity of Dou-Gong brackets, providing the first quantitative evidence of their degradation under fire exposure. By addressing this vulnerability, the study contributes to the scientific preservation of ancient wooden architecture under contemporary fire risk scenarios.

1. Introduction

As a distinctive and structurally critical feature of ancient Chinese architecture, Dou-Gong brackets exemplify the ingenuity and aesthetic sophistication of traditional craftsmanship. This composite structure, formed by interlocking and stacking individual wooden components, fulfills multiple architectural functions, such as supporting cantilevered eaves, redistributing structural loads, reducing span requirements, dissipating seismic energy, and integrating ornamental motifs. Given the critical role of Dou-Gong brackets in ensuring the structural safety of ancient wooden buildings, it is significant to analyze the combustion performance of its main load-bearing components and investigate the mechanisms of temperature rise, charring, deformation, and mechanical degradation during fire exposure.

In recent years, many scholars investigated the fire resistance of timber structures using theoretical analysis and numerical simulation. Schnabl et al. [1] proposed a mathematical model for timber column performance under fire conditions, revealing that increased moisture content and prolonged fire exposure diminish residual cross-sectional areas, leading to reduced ultimate load capacity. Firmanti et al. [2] conducted fire tests on timber beams of varying densities under different mechanical loads. Their results showed that the presence of knots did not affect the fire resistance, and timber density had no significant influence on residual load capacity. However, fire-resistant time is decreased with increasing applied mechanical load. Zhang et al. [3] employed both numerical simulations and experimental research to investigate the charring rate of timber beams exposed to fire on three sides, revealing that the vertical charring rate was higher than the horizontal rate and decreased progressively with prolonged fire exposure. Babrauskas et al. [4] conducted a full-scale fire test on timber structures, concluding that the charring rate was influenced by initial defects, placement, building characteristics, ventilation conditions, and the position of components. Gernay et al. [5] highlighted that the heating stage is the critical phase of fire in buildings, noting that two structural components with the same fire resistance rating under a standard fire can behave drastically differently under a natural fire. Yang et al. [6] determined the charring depth, charring rate, and heat release rate of glued laminated wood made from five types of cork using standard fire tests and the cone calorimeter method, finding that the average charring depth and charring rate on the bottom surface of the specimen were higher than those on the side surface at an identical fire exposure condition. In the study of Dou-Gong brackets, Wu et al. [7] constructed three full-scale bracket models and conducted eccentric load tests, finding that the failure mode of the bracket shifted to brittle failure under eccentric loading, and the elastic stiffness under eccentric loading was greater than that under axial loading. Zhang et al. [8] examined Song-style Dou-Gong brackets and analyzed the effect of the xia-ang on their mechanical performance through horizontal static tests. The results indicated that the horizontal stiffness of the Dou-Gong bracket with xia-ang component increased by 29.4% under a pressure load compared to one without mechanical load, while existing studies on timber beams and columns provide foundational insights into fire resistance of linear structural elements [9,10,11,12]. However, traditional timber beams and columns were differ fundamentally from Dou-Gong brackets in both geometry and load-transfer mechanisms. Unlike beams and columns, Dou-Gong brackets are composed of interconnected wooden blocks and mortise–tenon joints that redistribute loads through compression–friction interactions, which is vulnerable to localized stress concentrations under fire-induced material degradation. Prior mechanical analyses of Dou-Gong [13,14,15,16,17] neglect thermally driven joint loosening, charring-induced stiffness loss, and asymmetric collapse risks in heritage structures. This oversight undermines fire-safety assessments for culturally irreplaceable buildings, necessitating dedicated research on the post-fire mechanical behavior of Dou-Gong brackets.

This paper focuses on the main load-bearing components of the Dou-Gong brackets and aims to reveal the influence of fire conditions on the fire evolution characteristics of Dou-Gong brackets. By varying the fire load intensity, the morphology, temperature, and charring behavior of the Dou-gong brackets under different fire scenarios were investigated carefully. Additionally, a hydraulic universal testing machine was used to examine the deformation characteristics, residual load-bearing capacity, and other mechanical properties of the Dou-gong brackets before and after the fire test.

2. Experimental

2.1. Materials

The main load-bearing components of the Dou-Gong brackets include the Zuo-dou, Zheng-xin-gua-gong, and Qiao. The Zuo-dou component serves as the primary compression component, the Qiao component functions as the main bending component, and the Zheng-xin-gua-gong component acts as the connecting component between the Zuo-dou component and Qiao component. A schematic diagram and dimensional details of the main load-bearing components are shown in Figure 1 and Figure 2. The assembled structure measures approximately 280 × 250 × 160 mm³ (length × width × height). The dimensions of the components are as follows: the Zuo-dou component measures 120 × 120 × 96 mm³, the Zheng-xin-gua-gong component measures 250 × 48 × 96 mm³, and the Qiao component measures 280 × 48 × 96 mm³.

2.2. Fire Source Setting

In actual wooden heritage buildings, ranging from large temples and palaces to ethnic minority wood-framed villages, activities such as incense burning and oil rituals are common. Therefore, studying the burning characteristics of wooden components using oil pool fires is of significant importance [9]. Based on this, the study employs oil pool fires as the fire source, with anhydrous ethanol (C₂H₅OH, ≥99.8 wt% purity, calorific value 29.7 kJ/g) as fuel. Oil pool sizes with diameters of 20, 25, and 30 mm were selected, referencing fire resistance tests of wooden components from the literature. The corresponding fire load of the oil pools was calculated using Equation (1).

$$D^{\ast }={\left({\displaystyle \frac{\dot{Q}}{{\rho }_0{c}_{\mathrm{p}}{T}_0\sqrt{g}}}\right)}^{{\displaystyle \frac{2}{5}}}$$

(1)

where D* is the fire source diameter (m); $\dot{Q}$ is the heat release rate (kW); T₀ is the ambient temperature (293 K), ρ₀ is the density of the air in the environment, the value of 1.205 kg/m³ at 293 K; c_p is the specific heat of the air in the environment, the value of 1.004 kJ/(kg·K) at 293 K; and g is 9.8 m/s². Substituting oil pool diameters of 20, 25, and 30 mm, the corresponding heat release rates were approximately 20, 35, and 55 kW, respectively. The fire load calculation (Equation (1)) assumes uniform burning behavior across the ethanol pool surface and steady-state heat flux consistency, validated by <5% fluctuation in flame height and radiative heat flux. The thermocouples (Type K, 0.5 mm diameter) were calibrated at 293 K, 473 K, and 673 K prior to the experiments, with a maximum error margin of ±1.5 K.

Additionally, the mass loss rate of C₂H₅OH during combustion was calculated using Equation (2),

$$\dot{Q}=\dot{m}\times h_{\mathrm{c}}$$

(2)

where $\dot{m}$ is the rate of mass loss of C₂H₅OH, g/s; h_c is the calorific value of C₂H₅OH, 29.7 kJ/g. The corresponding mass loss rates were found to be 0.67, 1.18, and 1.85 g/s, respectively. Taking the continuous burning time of t = 10 min, the required amounts of C₂H₅OH were calculated as 402, 708, and 1110 g, respectively. Ignition was achieved via an electric spark ignitor that was removed within 2 s post-ignition.

2.3. Fire Source Location Determination

To determine the vertical clearance between the main load-bearing components of the Dou-Gong brackets and the fire source, preliminary combustion tests were conducted using C₂H₅OH as fuel. The comparison of temperatures at a height of 30 cm for oil pool fires with different power outputs is shown in Figure 3. As expected, the temperature at 30 cm is increased with the oil pool diameter, clearly reflecting the difference of fire load level. In summary, the height of the Dou-Gong brackets used in this test was set at 30 cm above the fire source.

2.4. Fire Resistance Test

To maintain experimental integrity by mitigating environmental interference (e.g., wind speed), the primary load-bearing Dou-Gong bracket components were positioned within a sealed chamber during fire exposure testing, as shown in Figure 4. The combustion chamber included a total net height of 1.75 m and a volume of 2.50 m³. During the fire test, the burning behavior of the Dou-Gong brackets was recorded in real time using a camera. After the C₂H₅OH combustion ceased, a high-pressure fine mist sprayer was used to extinguish the fire until no visible flames remained.

The placement of temperature measurement points is shown in Figure 5. The mp1 point was located at the center of the bottle of the Zuo-dou component, while mp2 and mp4 points were positioned at the contact points of the three components, and mp5 was located at the center of the top of the Qiao component. These four measurement points were used to measure the temperature differences at different heights along the vertical axis of the central region of the Dou-Gong brackets. The mp3 point was placed at the connection edge between the Zuo-dou and the Zheng-xin-gua-gong components and aligned horizontally with the mp2 point to compare temperature variations between the edge and the central regions.

2.5. Residual Bearing Capacity Test

The MC-10 electronic universal testing machine was used in this experiment to test the residual bearing capacity of the Dou-gong samples before and after the fire test at a running speed of 5 mm/min.

3. Results and Discussion

3.1. Combustion Process Analysis

The combustion process of the main bearing components of Dou-Gong brackets is shown in Figure 6. In the fire load condition, the Dou-Gong brackets enter the combustion phase at different times. When the fire load is 20 kW, the fire spread is more slowly, entering the steady combustion phase later and reaching the top of the component at approximately 270 s. As the fire load increases to 55 kW, the fire spread much faster and reaches the top of the component in approximately 85 s. These results clearly demonstrate that an increase of fire load substantially intensifies the fire hazard.

3.2. Combustion Temperature Analysis

The temperature changes in the Dou-gong components at different measurement points under different fire loads are shown in Figure 7, Figure 8 and Figure 9. It is evident that the temperature variations at the measurement points correspond closely with the burning process shown in Figure 6. The temperature changes in the wooden Dou-Gong brackets components under a fire load of 20 kW are shown in Figure 7. The temperature at the bottom (mp1) increases steadily during the initial combustion stage, reaching a peak temperature of approximately 760 °C at around 150 s, followed by a gradual decline. The temperature at the top measurement point (mp5) rises slowly for the first 140 s. As the flame gradually reaches the top of the component, the temperature at mp5 point increases steadily and reaches the peak value of 450.6 °C. The temperature variations at mp2 and mp4 points, located at the connection center of the components, sustained minimal change throughout the fire exposure. The peak temperature at mp2 point is only increased to 99.3 °C, while mp4 point peaks at just 51.8 °C, indicating little heat is transferred to the interior structure of Dou-gong component. Additionally, the mp3 point, located at the connection edge in the same horizontal plane of mp2 point, shows significant temperature changes, following a steady upward trend throughout the fire exposure.

The temperature changes in the wooden Dou-Gong brackets under a fire load of 35 kW are shown in Figure 8. The temperature at mp1 point exhibits a linear increase during 75 s, reaching a peak temperature of 752.6 °C. When the flame reaches the top of the component, the temperature at mp5 point rises sharply to 556.7 °C at around 150 s. Meanwhile, the temperature variations at mp2 and mp4 points are relatively small, with relatively low peak temperatures of 98.9 °C and 76.7 °C, respectively. The temperature at mp3 point shows a continuous upward trend during the fire, ultimately reaching a peak of 426.1 °C.

The temperature changes in the wooden Dou-Gong brackets under a fire load of 55 kW are shown in Figure 9. The temperature rise rate at mp1 is faster than other two fire load conditions, reaching a peak of 755.3 °C at around 80 s and stabilizing at that level. Additionally, the temperature rise rate at mp5, located at the top of the component, is significantly higher than that of low fire load, reaching a peak of 620.7 °C. This accelerated heating process correlates with a high fire source of 55 kW, which produces a larger flame dimension. Furthermore, the size of the oil pan (30 cm diameter) exceeds the base dimensions of the Dou-Gong brackets (25 cm × 28 cm), enabling flames to the component apex prior to combustion initiation.

The temperature comparisons at mp3 and mp5 points under three different fire loads for the main load-bearing components of the Dou-Gong brackets are shown in Figure 10. As the fire load increased, the temperature rise rates at the connection edge between the Zuo-dou and Zheng-xin-gua-gong components increase significantly. Their peak temperatures also rose substantially, indicating a significant increase in fire hazard. Specifically, when the fire loads are 20 kW, 35 kW, and 55 kW, the corresponding peak temperatures at mp3 point are 296.8 °C, 426.1 °C, and 489.9 °C, respectively, while the peak temperatures at mp5 point are 450.6 °C, 556.7 °C, and 620.7 °C, respectively. Notably, the peak temperatures at mp2 and mp4 points are significantly increased when the fire load is 55 kW, indicating that the high fire scale accelerates the heat transfer rate within the interior of the Dou-Gong brackets, exacerbating the damage of its internal structure.

3.3. Charring Behavior Analysis

The overall charring condition of the main load-bearing components of the Dou-Gong brackets under different fire loads is shown in Figure 11. The 3D diagram clearly shows that the number of surface cracks on the component is relatively low at a fire load of 20 kW. As the fire load increases to 35 kW and 55 kW, the number of surface cracks grows significantly. Particularly, when the fire source reaches 55 kW, an upward view reveals deeper and more extensive crack networks on the surface of the Zuo-dou component caused by the fire. The expansion of these cracks not only increases in depth but also shows a clear upward trend in quantity. Moreover, the top-view pictures further confirm the density and prominence of cracks in the top area of the Qiao component, which aligns with the previously discussed temperature distribution characteristics. The formation and propagation of these cracks may be related to the degradation of the thermodynamic and mechanical properties of wood substrates at high temperatures.

The thermal contraction experienced by the Dou-gong components during the fire process leads to significant expansion of gaps at the joints. This gap expansion is altering the interaction between components and adversely affecting the overall mechanical performance of the wooden structure. By comparing the top and frontal views, it is evident that the contact surfaces between the Zuo-dou, the Zheng-xin-gua-gong, and the Qiao components are tightly fitted before the fire test, demonstrating good structural integrity of Dou-gong brackets. However, after the fire test, noticeable gaps appeared in the contact areas between the three components of Dou-gong brackets. The formation of these gaps is attributed to the thermal expansion of the wooden materials under high temperatures and subsequent uneven contraction during the cooling process. The enlargement of these gaps at the joints likely weakens the interaction between each component of Dou-gong brackets, concomitant with the reduction in the overall stiffness and load-bearing capacity of wooden Dou-gong brackets. In terms of mechanical performance, the presence of gaps leads to stress concentration, increasing the risk of localized failure and affecting the stability and durability of the structure. Additionally, the widening of these gaps may be providing a pathway for flame and heat transfer, further exacerbating thermal damage to the components.

Table 1. The average charring depth and speed.

Fire Load (kW)	Component Name	D (mm)	t (min)	v (mm/min)
20	Zuo-dou	3.0	11.5	0.26
	Zheng-xin-gua-gong	2.8		0.24
	Qiao	2.5		0.22
35	Zuo-dou	5.0	11.3	0.44
	Zheng-xin-gua-gong	4.5		0.40
	Qiao	5.0		0.44
55	Zuo-dou	6.5	11.7	0.56
	Zheng-xin-gua-gong	5.8		0.50
	Qiao	6.0		0.51

presents the key charring parameters of the Dou-Gong components under different fire loads. The charring depth is noted as D, the actual fire time is noted as t, and the average charring speed is noted as v. It is evident that the Zuo-dou component exhibits significant charring during the fire. When the fire load is 20 kW, the charring depth of the Zuo-dou component is approximately 3.0 mm, with an average charring rate of about 0.26 mm/min. At a fire load of 55 kW, the charring depth increases to around 6.5 mm, and the average charring rate rises to approximately 0.56 mm/min, consistent with the charring patterns shown in Figure 4, Figure 5 and Figure 6. Furthermore, as the fire load increased, the average charring rate of each individual load-bearing component of the Dou-Gong brackets structure is increasing significantly. Specifically, at a fire load of 20 kW, the average charring rate of the Dou-Gong brackets components ranges from approximately 0.22–0.26 mm/min. The average charring rates of Dou-Gong brackets are increased to 0.40–0.44 mm/min at a fire load of 35 kW and 0.50–0.56 mm/min at a fire load of 55 kW, respectively.

Table 2. Mass loss characteristics of the Dou-Gong bracket components.

Fire Load (kW)	Component Name	m1 (g)	m2 (g)	w (%)	wall (%)
20	Zuo-dou	413.7	287.1	30.6	28.5
	Zheng-xin-gua-gong	301.1	219.9	27.0
	Qiao	398.5	288.5	27.6
35	Zuo-dou	412.3	262.2	36.4	33.9
	Zheng-xin-gua-gong	295.2	201.7	31.7
	Qiao	387.1	259.4	33.0
55	Zuo-dou	411.9	255.0	38.1	36.9
	Zheng-xin-gua-gong	321.6	211.3	34.3
	Qiao	403.6	250.7	37.9

presents the mass loss characteristics of the Dou-Gong bracket components under different fire loads. The mass of the Dou-Gong bracket components before the fire is noted as m₁, the mass of the Dou-Gong bracket components after fire is noted as m₂, the mass loss rate of the single component is noted as w, and the mass loss rate of the whole Dou-Gong bracket components is noted as w_all. The data clearly indicate that the Zuo-dou component at the bottom of the structure exhibits particularly significant mass loss during the fire process, which is closely related to the charring depth and rate of the components. When the fire load is 20 kW, the mass loss rate of the Zuo-dou component reaches 30.6%, while this rate increases further to 38.1% at a fire load of 55 kW. This trend aligns with the charring rate data in Table 1, further confirming the direct influence of fire load on the charring behavior of the components. Additionally, as the fire load increased, the mass loss rate of each individual load-bearing component of the Dou-Gong brackets exhibits a clear upward trend. At a fire load of 20 kW, the total mass loss rate of the Dou-Gong bracket components is approximately 28.5%. When the fire load is increased to 35 kW and 55 kW, the total mass loss rate rises to 33.9% and 36.9%, respectively. These phenomena reveal a positive correlation between the fire load and mass loss rate of the Dou-gong components, indicating that a higher fire load results in more severe pyrolysis and mass loss. The mass loss and charring rate of the Dou-Gong brackets components are key factors affecting their mechanical properties. During the charring process, the chemical composition and microstructure of the wood material are obviously changed, resulting in a significant decline in its physical and mechanical properties. The increase in mass loss rate reflects the extent of pyrolysis in the material, which in turn impacts the load-bearing capacity and overall structural stability of the wooden components. Therefore, the mass loss and charring behavior of Dou-Gong bracket components in fire conditions is of critical importance for evaluating and improving the fire resistance of ancient buildings.

3.4. Residual Bearing Capacity Analysis

The damage forms of the main load-bearing components of the Dou-Gong brackets under different fire loads are shown in Figure 12. Before the fire test, the primary failure characteristics of the main load-bearing components of the Dou-Gong brackets include multiple transverse cracks appearing on the Zheng-xin-gua-gong and Qiao components, which are related to tensile stresses experienced during the loading process. Additionally, significant compressive deformation is being observed in the Zuo-dou component, indicating that localized stress concentrations are occurring in the ear regions under compressive loads. In contrast, after exposure to fire, the typical failure features of the main load-bearing components of the Dou-Gong brackets primarily involve compressive deformation and the breaking of the Zuo-Dou component, with minimal damage to the Zheng-xin-gua-gong and Qiao components. These phenomena indicate that the Zuo-Dou component, located at the bottom, is sustaining the most significant damage during fire exposure, as it serves as the main compressive component of the entire structure. Consequently, during the loading process, the Zuo-Dou component is more prone to compressive deformation after fire exposure. Moreover, as the primary compressive component of the structure, the damage to the Zuo-Dou component after fire exposure significantly compromises the load-bearing capacity of the entire Dou-Gong brackets due to the thermal degradation of wood caused by fire, compressive deformation and fracture during the loading process could lead to partial or even total structural failure.

The load–displacement curves of the main load-bearing components of the Dou-Gong brackets under different fire loads are shown in Figure 13. The slope of the load–displacement curve for the Dou-Gong bracket components is decreased as the fire load increases. As for the Dou-gong brackets without fire exposure, the load–displacement curve exhibits a distinct plastic deformation yield plateau, a typical characteristic of material yielding, indicating that the component can withstand further deformation without failure once a certain stress level is reached. As for the Dou-gong components with fire exposure, the yield plateau is not appearing, suggesting that the yielding behavior of the material is affected by thermal damage, accompanied by a reduction in ductility. Overall, the stiffness of the main load-bearing components of the Dou-Gong brackets decreased with an increasing fire load, which is consistent with the trend of wood pyrolysis and wooden structure damage at high temperatures. The ultimate load capacity of Dou-Gong bracket without fire exposure is 77.6 kN. At a fire load of 55 kW, the ultimate load capacity of the Dou-Gong bracket drops from 77.6 kN to 34.3 kN, demonstrating the profound influence of fire intensity on structural performance.

The temperature thresholds observed in Dou-Gong brackets align with three critical wood decomposition stages: (1) initial decomposition (100–200 °C): at ~100 °C, free water evaporates with minimal strength loss; between 150 and 200 °C, hemicellulose and lignin degrade, releasing combustible gases. (2) Charring and strength decline (200–300 °C): surface charring initiates, forming an insulating layer, while compressive strength drops by ≥50%—a common failure threshold for structural wood. (3) Combustion and collapse (>300 °C): self-ignition occurs at 300–400 °C, accompanied by rapid mass loss and structural failure, as seen in mp5 peaks (450–620 °C) exceeding traditional glue-line stability limits (>450 °C). These phases contextualize the 55 kW fire load (mp5: 620 °C) as catastrophic, with char depths exceeding 6.5 mm.

4. Conclusions

This investigation systematically evaluates the combustion and mechanical properties of key load-bearing components of Dou-Gong brackets —Zuo-Dou, Zheng-xin-gua-gong, and Qiao under varying fire conditions. The morphology, temperature profiles, charring behavior, and residual load-bearing capacity exposed to different fire conditions were carefully characterized. The results show that a high fire load significantly accelerates the temperature rise and charring rate, thus greatly reducing the residual load-bearing capacity of the Dou-Gong brackets. At a fire load of 55 kW, the Zuo-Dou component reaches a peak temperature of 755.3 °C, the Zheng-xin-gua-gong component reaches a peak temperature of 489.9 °C at the junction and 620.7 °C at the top of the Dou-Gong brackets. Moreover, the charring rate of Dou-Gong brackets increases from 0.22–0.26 mm/min at 20 kW fire load to 0.50–0.56 mm/min at 55 kW fire load, while the total mass loss rate increases from 28.5% at 20 kW fire load to 36.9% at 55 kW fire load. The residual load-bearing capacity gradually decreases with increasing fire load, and the residual load-bearing capacity of Dou-Gong brackets decreases from 77.6 kN to 34.3 kN when exposed to a fire load of 55 kW. The highest temperature of Zuo-Dou reflects its exposed loading face and coupled radiation–convection. The rapid mass loss of Zheng-xin-gua-gong and Qiao components reflects their slender profiles and elevated surface-area-to-volume ratios, accelerating volatile release during combustion. The steep decline in residual load-bearing capacity suggests that Dou-Gong brackets exposed to fires exceeding 35 kW may lose compliance with the Historic Timber Structure Safety Code of China minimum capacity thresholds. These findings underscore the vulnerability of Dou-Gong brackets to high-intensity fires, highlighting the need for fire protection designs. Developing advanced fire protection strategies tailored to Dou-Gong components, such as the targeted application of multifunctional transparent fireproof coatings and early warning sensors, is essential for preserving the stability and safety of these heritage structures under fire conditions.