FDS-Based Study on Fire Spread and Control in Modern Brick-Timber Architectural Heritage: A Case Study of Faculty House at a University in Changsha

Abstract

The modern Chinese architectural heritage combines sturdy Western materials with delicate Chinese styling, mainly adopting brick-timber structural systems that are highly vulnerable to fire damage. The study assesses the fire spread characteristics of the First Faculty House, a 20th-century architectural heritage located at a university in China. The assessment is carried out by analyzing building materials, structural configuration, and fire load. By using FDS (Fire Dynamics Simulator (PyroSim version 2022)) and SketchUp software (version 2023) for architectural reconstruction and fire spread simulation, explores preventive measures to reduce fire risks. The result show that the total fire load of the building amounts to 1,976,246 MJ. After ignition, flashover occurs at 700 s, accompanied by a sharp increase in the heat release rate (HRR). The peak ceiling temperature reaches 750 °C. The roof trusses have critical structural weaknesses when approaching flashover conditions, indicating a high potential for collapse. Three targeted fire protection strategies are proposed in line with the heritage conservation principle of minimal visual and functional intervention: fire sprinkler systems, fire retardant coating, and fire barrier. Simulations of different strategies demonstrate their effectiveness in mitigating fire spread in elongated architectural heritages with enclosed ceiling-level ignition points. The efficacy hierarchy follows: fire sprinkler system > fire retardant coating > fire barrier. Additionally, because of chimney effect, for fire sources located above the ceiling and other hidden locations need to be warned in a timely manner to prevent the thermal plume from invading other sides of the ceiling through the access hole. This research can serve as a reference framework for other Modern Chinese Architectural Heritage to develop appropriate fire mitigation strategies and to provide a methodology for sustainable development of the Chinese architectural heritage.

1. Introduction

1.1. Fire Protection of Architectural Heritage on University Campuses

Modern architectural heritage, an essential part of cultural heritage, bears witness to the significant societal changes in 20th-century China [1,2]. Nevertheless, fire poses a constant danger to these structures. Chinese university campuses remain numerous architectural heritage sites from the 20th century [3,4,5]. Examples include the Early Architecture of Hunan University, the Historic Buildings of Tsinghua University, and the Former National Central University Campus in Nanjing [4,6]. Most of these 20th-century Chinese architectural heritage are multi-story structures with brick-timber frameworks, extensive wooden interior adornments, and modern electrical equipment, resulting in poor fire resistance [2,7]. In the event of a fire, the timber elements facilitate rapid fire spread, presenting substantial difficulties for fire-extinguishing efforts [8].

The frequent occurrence of fires in campus buildings is related to the diverse functions of university architecture. These buildings must support teaching, laboratories, offices, and administrative activities [9]. Regular renovations and construction work are necessary to meet these needs and mitigate potential fire risks. Notably, some of China’s university heritage buildings are inadequately safeguarded and are partially accessible to the public. This practice brings in various risks such as fires, lightning strikes, insect damage, and water infiltration, which can severely damage these architectural heritages.

Fire incidents involving architectural heritages have occurred on campus frequently. For example, in 2011, a fire broke out in the Chemistry Building at Central South University. It was caused by improper handling of sodium metal in a top-floor laboratory, where contact with water led to explosive combustion. The flames quickly engulfed the wooden ceilings, destroying most of the timber roof structure. Firefighters could only bring the fire under control 20 min after it started. In 2023, the historic Great Hall of Henan University’s Preparatory School for Overseas Study in Kaifeng, Henan Province, was completely destroyed [10]. The accidental ignition of roof timber components during roof renovation work allowed the fire to spread into the interior. The contrasting realities before and after the fire in Henan University’s auditorium are shown in Figure 1. These cases illustrate different usage situations and layout arrangements of 20th-century architectural heritage in Chinese universities, contributing to their fire vulnerability.

1.2. Fire Protection of Modern Chinese Architectural Heritage

The collision and fusion of various architectural styles and the introduction of new materials, techniques, and construction methods have created Chinese modern architectural heritage. However, as of 2024, China has seen 38 National Key Cultural Relics Protection Units destroyed by fire over the past decade, including the Middle East Road Architectural Complex in Qiqihar, Heilongjiang, and the Yantai Mountain Modern Architectural Complex in Yantai, Shandong. Due to the unique structural forms and inherent fire risks of architectural heritage, physical fire experiments are still not feasible. Therefore, researchers increasingly use Fire Dynamics Simulator (FDS) software to model and analyze fire behavior in these structures. For example, Tongshuang Liu et al. [13] used FDS to study fire development in historical and newly built timber buildings in Xi’an and found that wood aging increases flammability and the probability of pine wood ignition. Fupeng Zhang et al. [14] used FDS simulation to study the fire spread law of traditional wind-rain bridges in Hunan Province and verified the effectiveness of fire retardant coatings, automatic fire extinguishing equipment, skylights, and other fire reduction measures. Biao Zhou et al. [15] analyzed the fire accident data of Chinese historical buildings in the past ten years, pointing out that electrical faults, improper use of fire, arson are the leading causes of fire, and reviewed the research progress in fire prevention and damage assessment. Huai, C. et al. [16] took a historical temple in Beijing as an example, carried out quantitative research on the fire risk of traditional timber structures, and verified the accuracy of FDS software through a large number of experimental tests.

Changsha, Hunan Province, has a long history and culture. Taking Yuelu Mountain as the center, it preserves a large number of modern university architectural heritage. The Early Architectural Complex of Hunan University and the Early Architectural Complex of Xiangya, built in the 20th century, represent China’s modern architectural history. However, the dense forest coverage in Yuelu District and the climate characteristics of hot summer, cold winter and dry winter increase the fire risk [17,18]. In January 2024 alone, the district recorded 247 fire incidents, the highest among all districts in Changsha. Scholar Long Yan et al. [17] improved the existing fire risk assessment model for Yuelu Mountain cultural relics by introducing forest fire dynamics, climatic factors, and regional fire-fighting capacity, improving risk prediction accuracy. At present, the heritage research mainly focuses on ancient architecture and traditional dwellings, and there is a lack of FDS-based research on the fire of independent Chinese modern architectural heritages or buildings [15,19,20]. Given the concentrated distribution of vulnerable modern architectural heritages in Yuelu District, it is urgent to conduct special fire protection research on such buildings.

PyroSim (version 2022), a software platform based on FDS, is widely used in the fire risk analysis of timber structures [16,21,22,23,24,25,26]. By directly importing SketchUp (version 2023) models into PyroSim, researchers can conduct detailed numerical simulations and visual analyses of fire dynamics [14]. In this study, The First Faculty Building of Central South University in Changsha, Hunan Province, was modeled using SketchUp, and its fire spread characteristics were simulated in PyroSim. This method not only combines the needs of historical protection with modern safety standards but also provides a new technical framework for the protection of Chinese modern architectural heritage.

2. Methodology

This study (Figure 2) of brick-timber architectural heritage used a dual approach combining field Survey and FDS software simulation. The framework consists of three key components: the field survey on the fire risk assessment of the First Faculty Schoolhouse, the comparative summary of different fire control strategies for fire mitigation based on FDS software simulation, the fire risk assessment of original situation and fire control study of the house based on FDS software simulation.

In the study framework (Figure 2), the first part involved the field survey selecting the representative The First Faculty Schoolhouse on campus for measurement and mapping. The survey mainly investigated the building structure, building construction, moisture content of materials, and fire loads of the First Faculty Schoolhouse, these elements embody the typical characteristics of 20th-century Chinese architectural heritage. The second part utilized FDS software simulation to analyze the fire spread of the First Faculty Schoolhouse in its original situation. The final part proposed three fire prevention strategies based on the FDS-simulated fire spread results of the First Faculty Schoolhouse: Fire Sprinkler Systems, Fire Retardant Coatings, and Fire Barriers. FDS software was then used again to verify and compare the effectiveness of these strategies through visual assessment of results, flashover time, and other combustion parameters.

The study integrates actual field data with virtual computational modeling, providing practical solutions to mitigate fire risks while adhering to heritage conservation principles [27,28].

2.1. Field Survey

Changsha, a renowned cultural city in China, is home to numerous historic universities and educational buildings that embody its rich heritage. During the Second World War, institutions like Tsinghua University (originally in Beijing) relocated temporarily to Changsha, constructing campus facilities and academic buildings to sustain operations amid wartime upheaval [29]. These brick-timber structures, later integrated into the early campus of the Central South Institute of Mining and Metallurgy (now Central South University) (Figure 3), feature a unique fusion of Soviet and traditional Chinese architectural elements [30,31]. This hybrid design not only imparts exceptional aesthetic and historical value but also provides tangible case studies for cultural and architectural research. Due to their significance, the Changsha Government has set these buildings as protected cultural heritages, underscoring their role as vital witnesses to modern Chinese educational and architectural history.

2.2. Case Study—The First Faculty House

The First Faculty House (Figure 4) was built in 1955. In 2016, it was listed as a third-batch municipal-level protected building by the Changsha Government. It is located at the foot of Yuelu Mountain in Changsha City, Hunan Province, within the Yuelu Mountain Campus of Central South University. The form of the house is of the traditional Soviet-style architecture. Its exterior facade incorporates decorative elements of traditional Chinese architecture, including a five-cornered pyramidal roof, dougong (brackets under the eaves), and so on. The roof is covered with small curved blue-grey ceramic tiles, which is characteristic of classic Chinese architecture. The interior is partitioned by brick walls, and the roof is supported by a full-wood truss.

The First Faculty House is composed of a lightweight wooden roof, a red brick or concrete enclosure, and a load-bearing structure (Figure 5). Due to the limitations of structural construction technology at that time, this type of building exhibits a structural form characterized by a highly flammable timber roof and a non-combustible masonry or concrete load-bearing enclosure, with a wooden ceiling arranged between the two components. Because the most severely damaged part of the heritage with the brick-timber structure is the roof [16,32,33]. In this study, FDS (Fire Dynamics Simulator) simulations were conducted, focusing on the wood eave and roof sections of the building connected to the top floor, which have a higher fire risk and omitted the numerical simulation of the indoor movable fire loads (sofa, closet, etc.).

2.3. Building Layout and Form

The building consists of three stories with a central corridor flanked by dormitory rooms on both sides, forming an elongated rectangular floor plan (Figure 6). The roof features a timber-framed pitched structure topped with blue-grey ceramic tiles, a hallmark of traditional Chinese roofing. Outdoor observation halls and rooms on both lateral and central axes are integrated into the building’s main volume. With a total floor area of 2730 m² and a footprint of 1179 m², the structure exemplifies 20th-century Chinese architecture, blending utilitarian spatial planning with architectural ornamentation that reflects its Soviet-Chinese hybrid heritage [34].

2.4. Electrical Equipment and Fire Use in the House

The interior of the building is currently utilized as residential accommodation. In the corridors, some residents employ open-flame gas stoves for cooking, while modern electrical distribution cabinets and network adapters are installed along both walls. Cable trays for high-voltage wiring have been added beneath the wooden ceiling above. The coexistence of these electrical installations and corridor obstructions such as stacked woven bags, books, and fabrics, are substantially exacerbates the fire risk of the structure. Concurrently, the existing fire safety infrastructure is insufficient to handle emergencies, necessitating urgent modernization to address these vulnerabilities. Figure 7 shows the scene on the top floor of the house. This scenario underscores the dilemma between preserving historical authenticity and adhering to contemporary safety standards in heritage buildings.

2.5. Wooden Roof of the House

The building is constructed with a masonry load-bearing structure, and its wall-bearing roof system is supported by triangular roof trusses (Figure 4). The base of each truss directly rests on the walls, while wooden ceiling panels are used to separate the roof framework from the top-floor masonry (Figure 7). This configuration of roof assembly and material composition is exemplified by the representative structural typologies in 20th-century modern architectural heritage across China, which are characterized by hybridized construction techniques blending traditional craftsmanship with early modern engineering practices [35]. The design is reflected to adapt functionally to local climatic conditions and is constrained by the technological limitations of its era (Figure 8).

2.6. Software Simulation

2.6.1. Simulation Model

Given architectural heritage’s inherent uniqueness and irreproducibility, physical experiments for studying fire spread characteristics are impractical. Computational Fluid Dynamics (CFD)-based computer simulation methods to accurately simulate fire propagation in modern architectural structures were employed in this research. PyroSim is an FDS software developed by the National Institute of Standards and Technology (NIST) of the United States based on CFD, which is specifically used to simulate fluid flow, heat transfer, combustion and smoke diffusion in fire scenarios. It belongs to the specific application of CFD technology in the field of fire safety [36,37]. The methodology comprises three key steps: (1) Field Documentation: The entire building and its roof components are comprehensively surveyed and cataloged, including material properties and structural details; (2) Digital Reconstruction: A simplified 3D model of the faculty dormitory is created using SketchUp, with curved surfaces streamlined and non-combustible stone ornaments that do not affect the simulation removed; (3) Simulation Integration: The model is imported into PyroSim and coupled with the Fire Dynamics Simulator (FDS) to establish a fire spread simulation and evaluation framework. This approach preserves the integrity of the heritage structure while enabling precise analysis of fire behavior through advanced computational modeling.

FDS employs a Fast Fourier Transform (FFT)-based Poisson solver for fire simulations. The scale of the fire plume is determined by the dimensionless parameter D*/δ_x (recommended range: 4–16), where (characteristic fire diameter) is calculated as [38]:

$$D*={(\frac{Q}{{\rho }_{\infty }{C}_P{T}_{\infty }\sqrt{\mathrm{g}}})}^{{\displaystyle \frac{2}{5}}}$$

(1)

Variables in Equation (1):

D*: Characteristic fire diameter (m)

Q: Heat release rate of the fire source (8000 kW)

ρ_∞: Ambient air density (1.29 kg/m³)

T_∞: Internal ambient temperature (290.2 K/17.2 °C, annual average for Changsha)

C_p: Specific heat at constant pressure (1.004 kJ/(kg·K))

g: Gravitational acceleration (9.81 m/s²)

To standardize variables, the ambient temperature was set to Changsha’s annual average of T_∞ = 290.2 K (17.2 °C) [39], with a natural ventilation environment of 0 m/s wind speed (all windows open). Based on field measurements of The First Faculty House, Equation (1) the computational domain for the simulation was defined as 74.0 m × 22.2 m × 14.2 m, yielding a total volume of 23,321.8 m³. According to Equation (1), the grid size δ_x is obtained as 0.25m× 0.25m× 0.25m. This value conforms to the recommended value in the user manual of PyroSim software and a total of 1,501,608 grids are generated [40].

2.6.2. Fire Setting

Figure 5 shows the roof structure, primarily composed of fir and camphor timber, classifies the fire as a fast-growing type (α = 0.0469 kW/s²) based on

Table 1. Fire Growth Coefficient α.

Fire Classification	α (kW/s2)	Typical Combustible Materials
Ultra-fast	0.1876	Oil pool fires, flammable furniture, lightweight curtains
Fast	0.0469	Filled mail bags, plastic foam, stacked wooden racks
Medium	0.0117	Cotton polyester spring mattresses, wooden office furniture
Slow	0.0029	Heavy timber products

[16]. According to

Table 2. Maximum heat release rate values for typical fire locations [41].

Typical Fire Locations	Maximum Heat Release Rate/MW
Offices and guest rooms with sprinklers	1.50
Offices and rooms without sprinklers	6.00
Shopping malls with sprinklers	5.00
Public places with sprinklers	2.50
Public places without sprinklers	8.00
Supermarkets and warehouses with sprinklers	4.00
Supermarkets and warehouses without sprinklers	20.00

[41], the first Faculty House in its original situation belongs to the public places without sprinklers, and the heat release rate (HRR) at its typical fire location is set to 8 MW.

As combustible materials in the building are predominantly wooden furniture and structural components, the simulation employs a t² growth curve to model fire development, with calculations based on the peak HRR during combustion [42].

The t² fire growth model categorizes fire progression into four tiers (Equation (2)):

Q = αt²

(2)

Variables in Equation (2):

Q: Heat release rate (kW)

α: Fire growth coefficient (kW/s²), categorized by fire types as shown in Table 1 (Fire Growth Coefficient Classification)

t: Simulation time (s)

The fire growth coefficient α for different fire types is listed in Table 1. According to Equation (2), with Q set to 8 MW, the fire development time t₂ can be calculated as 413 s, and the simulation time is configured to 900 s.

The combustion of wood is divided into five major stages: heating, thermal decomposition, ignition, combustion, and combustion spread. Heating marks the beginning of the wood combustion process, during which heat is supplied from external sources such as open flames, overheating, or electric sparks. When the heat source has sufficient energy, the wood temperature gradually rises until thermal decomposition, ignition, and combustion occur; otherwise, a fire will not ensue. Thermal decomposition occurs when the wood is heated to a certain temperature, causing its chemical structure to be destroyed, releasing numerous low-molecular volatile substances and heat, and altering its original physical and chemical properties. Ignition happens when the wood temperature, concentration of flammable gases in the environment, and oxygen content simultaneously meet the required conditions. Thereafter, the wood burns with the flammable gas mixture in the environment to form flames (i.e., flaming combustion), releasing heat that heats adjacent areas through radiation, convection, and thermal conduction. When the heat is sufficient to cause thermal decomposition and combustion in adjacent areas, the flames will spread in all directions, leading to fire propagation and large-scale combustion spread [43].

To streamline data collection, the following model configurations and assumptions were adopted [16,44]:

• Complete Combustion Assumption: All timber components in the building are assumed to participate in combustion and achieve complete burnout.
• Heat Release Rate (HRR) Measurement: The moisture content of the timber was measured to calculate HRR, but internal heat loss caused by moisture evaporation during combustion was not considered.
• Ignition source: Considering the most disadvantageous fire scenario, the simulation locates the ignition source above the ceiling on one side of the corridor (Figure 9), 9.20 m from the building’s central axis. This concealed position mimics real-world conditions where electrical short circuits could trigger fires undetected due to ceiling obstructions in time.
• Ceiling Destruction Simulation: The ceiling connected to the roof was divided into 21 ceiling panels. Two thermocouples were installed above and below each panel (totaling 42 thermocouples), When either thermocouple in a panel reached 600 °C, that panel was removed from calculations to simulate ceiling collapse due to fire. Each set of thermocouples is located at the geometric center of each ceiling panel, as shown in Figure 9.

2.6.3. Simulation Parameters

• Slice and text Point Parameters

To better monitor fire spread and hazardous gas generation, Z, Y-axis slices were set in the model for gas temperature and visibility analysis. (Figure 10) Y-axis slices were placed across the central walkway, above the ceiling, and in stairwells to track how different factors affect the indoor environment and building structure during a fire. Z-axis slices were positioned at 1.6 m relative to the top floor’s ground elevation (matching human face height) and above the ceiling to monitor hazardous gas impacts on people and the building. Five text points (A, B, C, D, E) monitoring of CO, CO₂ and thermocouples were installed at the top floor (1.6 m from the ground) and three text points (A, C, E) at the first and second floor staircase exits to simulate the effects of hazardous and thermal gases on occupants during a fire (Figure 10).

2.

Conditions for Flashover

When a flashover occurs in the building, the temperature within the interior space rises rapidly and remains persistently high. Flames damage windows and doors, while the inflow of fresh air through these openings supplies additional oxidizing agents, sustaining and intensifying combustion. This process significantly reduces the load-bearing capacity of building components, eventually causing damage and collapse of the main structural framework.

Currently, based on the fire studies conducted by Francis [45], Zhang [46] and Cheng et al. [47], four primary criteria are used to determine the occurrence of flashover, with confirmation achieved if any one of the following conditions is met: (1) The temperature of thermal smoke gas near the interior ceiling reaches 600 °C; (2) The heat radiation intensity on the floor surface reaches 20 kW/m²; (3) Flames are ejected from ventilation openings or windows; (4) A sudden change occurs in the heat release rate (HRR) from the combustion of indoor combustibles.

2.7. Mitigation Strategies

2.7.1. Fire Barriers

The roof structure of The First Faculty House is primarily constructed of wood and is slender overall, forming a relatively enclosed and narrow attic space [48]. To preserve the original functional use of the building without altering its floor plan, architectural appearance, or structural framework, lightweight vertical fire barriers obscured by ceiling were installed between each roof truss above the ceiling (Figure 11). These barriers are designed to partition smoke and prevent the rapid fire spread within the long, narrow space. Constructed from fire-resistant glass [49], the lightweight vertical fire barriers can be maintained intact for no less than 30 min under high-temperature conditions of (620 ± 20) °C.

2.7.2. Fire Retardant Coatings

Fire retardant coatings are crucial for preventing wood from burning. This simulation used an efficient phosphorus-nitrogen-silicon fire retardant coating [50,51]. Numerous long-term studies have shown that fire retardant coatings applied to building timber structures can maintain long-term fire resistance performance if subjected to proper daily maintenance [52]. For the architectural heritage cases discussed in this paper, if the airtightness of the building can be maintained, especially by ensuring the relative closure of the roof space. fire retardant coatings will serve as a cost-effective and long-lasting mitigation strategy. It enhances fire resistance and maintains transparency. When applied to wood surfaces, it creates thermal properties different from other materials, does not change the building’s original interior appearance. The phosphorus and silicon in the coating work together to resist fire during combustion. They can form a stable char layer to protect the wood matrix. The retardant coating improves the material’s surface thermal stability, achieving fire protection for the wood and will have virtually no impact on the appearance of the interior, which is located on the top floor below the ceiling.

Table 3. Thermal properties of major building materials.

Material	Density kg/m3	Specific Heat Capacity kJ/(kg·K)	Thermal Conductivity W/(m·K)
Fir wood	500	2.52	0.108
Camphor wood	700	4.02	0.115
Small green tile	2800	0.92	0.760
Fire barrier	1800	0.84	0.750
Concrete	2400	0.90	1.080
Red bricks	2000	0.50	1.500
Fire retardant coating	634	0.90	0.167

shows thermal properties of major building materials including fire retardant coatings and fire barriers in the study.

2.7.3. Fire Sprinkler Systems

The fire sprinkler system is a critical means of preventing fires in modern buildings, effectively controlling and extinguishing incipient fires, though it may have certain impacts on the load-bearing structure of the building [53]. A total of 2473 research papers have shown that fire sprinkler systems can effectively prevent the further spread of fires when they occur [54]. The dry fire sprinkler system can operate normally in extreme temperatures (below 4 °C or above 70 °C), and there is no accumulated water in the dry pipe, so there is no possibility of leakage corroding the wooden components of the building [55,56]. This is extremely important for the protection of wooden buildings in areas with hot summers and cold winters. In Changsha, recorded winter temperatures have dropped below 0 °C [57,58]; to prevent water in the pipelines from freezing at low temperatures, the experiment adopted a dry pipe fire-extinguishing method. Each sprinkler is installed in the middle of two roof trusses within the roof space directly below the ridge of the sloped roof, with a total of 15 nozzles. The sprinklers are spaced approximately 3.15 m apart, each with a flow rate of 50 L per min (5000 particles per s) [59] (Figure 12).

2.8. Survey Result

2.8.1. Features and Structure of the First Faculty House

The overall structural form of The First Faculty House is characterized by a support system comprising inner and outer brick walls for the three floors, with each floor having a height of 3.6 m. The upper part is composed of 14 vertical roof trusses arranged at a spacing of 3150 mm, with 3 additional roof trusses positioned at the main corners of the structure. The roof trusses above are supported by the brick walls, which are triangular roof trusses made of camphor wood (Figure 13). The trusses are braced by cross wooden connecting rods between each pair, and purlins, roof panels, and blue-grey ceramic tiles are installed above the trusses. The structural connections between the trusses are made using a combination of mortise and tenon joints and wooden pegs.

2.8.2. Moisture Content of the Construction Materials Used for the First Faculty House

Measurements of the main wooden structural materials in the First Faculty House show that the average moisture content of the wood used for the roof trusses’ main beams is 9% (range: 7–11%), which falls within the “oven-dry wood” moisture content range. Influenced by Changsha’s environmental conditions, the roof materials have become dry. This creates a high fire load in the entire roof frame and significantly lowers their ignition points, making fire accidents more likely to occur.

Table 4. Fire load statistics for roof sections.

Roof Components	Volume (m3)	Quality (kg)	Heat of Combustion (MJ)
Roofing panels	24.08	12,040.00	237,790.00
Purlin	39.72	19,860.00	392,235.00
Roof truss	64.80	45,360.00	991,116.00
Ceiling keel	10.77	5385.00	106,353.75
Ceiling	19.04	9520.00	188,020.00
Pavilion frame	1.02	510.00	10,072.50
Eaves	3.30	1650.00	32,587.50
Bracket set	1.83	915.00	18,071.25
Total	164.56	95,240.00	1,976,246.00

shows the fire loads of the main materials of the building in that environment.

2.9. Simulation Results

2.9.1. Spread of Fire

When the fire starts, flames gradually intensify, mainly gathering beneath the roof panels. Dense smoke continuously rises from the flames. At 150 s, thick smoke fills half of the roof space. By 450 s, smoke largely obscures the entire roof. At 550 s, a small amount of smoke leaks from the ceiling access hole. At this moment, the fire area in the roof space suddenly expands, indicating that the fire has entered the development stage. Some flames reach the farthest end of the roof space from the ignition point shortly after. By 650 s, the nearest ceiling panel below the flames is burned through, causing flames and smoke to spread instantaneously to the space beneath the ceiling. During the 100 s from 650 to 750, a total of 7 ceiling panels are damaged, leading to a large inflow of flames and smoke into the third-floor interior and central corridor. At this point, the fire enters the flashover stage. When the simulation ends at 850 s, both the entire roof truss system and the third-floor space have entered the flashover stage. High heat from smoke and flames fills the third-floor corridor and most rooms in the building. Figure 14 shows the spread of flame and smoke over a period of 850 s.

2.9.2. Variations in the Parameters at Each Slices and Text Points

• Flue Gas Temperature

When the fire starts, thermal smoke accumulates beneath the roof panels due to the localized high temperature from the ignition source, spreading horizontally along the space. By 550 s, part of the ceiling above the fire source is destroyed, and flames rapidly spread to both sides along the narrow attic space of the roof trusses. At 650 s, the ceiling directly below the fire source is burned through. Between 650 and 750 s, the fire’s heat release rate (HRR) surges rapidly, and six ceiling panels are burned through within 100 s. High-temperature gases quickly spread downward from the roof truss space through the ceiling to adjacent areas, engulfing most of the third-floor space. Temperatures exceeding 600 °C are detected at multiple points by thermocouples 1.6 m above the ground. By 750 s, the temperature across a large area of the roof reaches over 270 °C, and the entire attic space of the building is in a flashover condition, with the roof trusses forming a massive burning body. Due to the destruction of the ceiling and continuous air inflow, the thermal plume spreads rapidly along the trusses to the farthest ceiling panels at the roof’s extremities, causing their destruction. At 800 s, the third-floor space is completely filled with high-temperature gases, which jet out from both sides of the windows. At this point, the roof temperature exceeds 400 °C, with the maximum temperature reaching 570 °C. When the simulation ends at 850 s, large sections of the roof panels and ceiling are destroyed, flames spread below the ceiling, and the third-floor space becomes a massive fire scene. Most of the roof trusses are also engulfed in flames. Figure 15 shows the temperature slice variations above the ceiling as well as in the center walkway.

2.

CO Concentration

During the 850-s simulation, CO and CO₂ variations at 1.6-m elevation observation points on the third floor followed a consistent pattern: they began to rise rapidly at 650 s (after the first ceiling panel was destroyed) and reached their peak at 800 s. In contrast, CO concentrations monitored at observation points near the stair entrances on the first and second floors remained low, with a maximum of only 2.90 × 10⁻¹¹ mol/mol and no significant increase. Neither the top five text points CO variation (Figure 16) nor the first and second floors three text points CO₂ variation (Figure 17) plots exceeded the human tolerance limit values (0.1 mol). This indicates that CO and CO₂ concentrations did not pose a fatal threat to human life during this fire.

3.

Visibility

The visibility slices at 1.6 m height on the third-floor show that at 350 s, visibility changes occurred around the ceiling access hole farthest from the fire source (Figure 18). This indicates that due to the chimney effect and hot-cold air exchange [60], thermal plumes first emerged from the farthest enclosed space relative to the fire source. By 550 s, significant visibility changes appeared near all ceiling access holes. At 650 s, when the first ceiling panel was destroyed, visibility in all rooms and corridors dropped rapidly. From 750 s to the simulation’s end at 850 s, visibility across the entire third floor dropped to 0 m, indicating complete obstruction by dense smoke. The overall visibility changes show that smoke first spreads to other clear-space areas through hot-cold air exchange.

3. Simulation Results of Mitigation Strategies

3.1. Different Mitigation Strategies

3.1.1. Fire Barrier

When vertical fire barriers are installed (Figure 19), the spread of high-temperature smoke is restricted, causing smoke to accumulate in the relatively enclosed area between two roof trusses and leading to localized high temperatures. These high temperatures begin to transfer to both sides of the building through pathways such as roof panels and ceilings, though the spread rate of dense smoke and thermal plumes is slowed, preventing the thermal plumes from rapidly igniting the farthest ceiling panels via air pressure differences. Flashover occurs after 650 s as high-temperature gases quickly rush into the top-floor space through the breach created by a burned-through ceiling panel. After 750 s, a total of 7 ceiling panels are burned through; at 846 s, the eighth ceiling panel is destroyed (Figure 20). When the simulation ends at 850 s, most of the third-floor space is under high-temperature conditions.

At a height of 1.6 m on the third floor, observation points recorded a maximum temperature of 820 °C at 789 s (Figure 21). The CO concentration did not exceed the human tolerance limit, with the highest measurement being only 0.0006 mol/mol. Visibility dropped sharply after 500 s, and all five measurement points approached 0 m visibility after 700 s. Among the five measurement points above the ceiling, the central point C first recorded a high temperature of 600 °C at 732 s, indicating the house roof had entered the flashover condition.

3.1.2. Fire Retardant Coating

The fire-retardant coating contains a phosphorus-containing structure (Figure 22). When pre-decomposed by high temperatures, this structure accelerates the dehydration process of the substrate, promotes carbonization, and helps form a phosphorus-rich, dense char layer. The char layer inhibits the transfer of flammable volatiles and heat, thus preventing further combustion of the underlying wooden structure. Both the ignition point and heat release rate (HRR) of the wood applied fire retardant coating are lower than those of original wood. The first ceiling panel was not destroyed until 750 s. By the end of the 850-s simulation, a total of 4 ceiling panels had been burned through, and high-temperature smoke had invaded part of the third-floor space (Figure 23).

Observation points at a height of 1.6 m on the third floor recorded a maximum temperature of 325 °C at 833 s. The CO concentration remained below the human tolerance limit, with the highest measurement at 0.0004 mol/mol. Visibility dropped sharply after 600 s, and all five measurement points had visibility approaching 0 m by 800 s. Among the five measurement points above the ceiling, central point C first recorded a high temperature of 600 °C at 836 s, indicating the house roof had entered the flashover condition (Figure 24).

3.1.3. Fire Sprinkler System

When the house uses a fire sprinkler system, the dry pipe starts spraying water 30 s after detecting a temperature of 70 °C (with a 30-s water storage delay). At activation, the main roof truss structure remains unignited, and the fire is still in the ignition stage. The conical water curtain formed by water droplets significantly disturbs the fire plume inside the roof, effectively curbing the escalation of smoke spread. By 500 s, only part of the smoke disperses through doors and windows. At 650 s, as the roof enters the initial fire growth stage, the wooden structure is ignited; however, due to the control of the sprinkler water curtain, high-temperature smoke fails to spread widely. From 750 s to the simulation’s end at 850 s, only a small area around the ceiling ventilation openings continues to burn, with no flashover occurring in the overall roof structure. By the end of the simulation, none of the 21 ceiling panels are burned through (Figure 25 and Figure 26).

Observation points at 1.6 m height on the third floor recorded a maximum temperature of 22.6 °C at 823 s. The overall temperature in the third-floor space remained within human-tolerable ranges throughout the simulation, and the CO concentration did not exceed the human tolerance limit. Among the five visibility measurement points, points A, D, and E (farthest from the fire source) showed a sharp decline in visibility. Among the five measurement points above the ceiling, the highest temperature recorded was 300 °C, which did not meet the flashover conditions (Figure 27).

3.2. Mitigation Strategies Assessment

Figure 28 shows the heat release rate (HRR)-time variations of house fires for four operating conditions. Parameter comparisons under the four operating conditions are as follows

Table 5. Monitoring values at five measurement points on the third floor of the building.

	Original	Fire Barrier	Fire Retardant Coating	Fire Sprinkler System
TTI	511 s	509 s	589 s	577 s
PHHR	112,000 kW	89,000 kW	52,000 kW	3000 kW
T-HHR	845 s	836 s	804 s	667 s

: time to ignition (TTI), peak heat release rate (PHRR), and time to reach PHRR (T-PHRR).

Relative to the original case, For the fire barrier, fire retardant coating, and fire sprinkler system ignition times (TTI) were respectively: 2 s faster, 78 s slower, 66 s slower. However, Peak heat release rates (PHRR): 21% lower, 54% lower and 97% lower; Time to peak heat release rate (T-PHRR): 9 s faster, 41 s faster and 178 s faster.

In summary, among the three measures, although vertical fire barriers can block the rapid spread of thermal plumes and form local fire compartments, high-temperature gases are concentrated in specific spaces by them, leading to localized overheating and subsequent spread to other directions, resulting in the poorest fire protection effect. The combustion of the wooden roof truss structure can be effectively controlled by fire retardant coating and the fire sprinkler system, thereby slowing the spread of fire. The fire sprinkler system is the most effective in fire control: no ceiling panels were burned through by the end of the simulation, the fire was contained within the roof structure, high-temperature gases did not spread to the third-floor space overall, and no flashover occurred. The spread of thermal plumes is effectively controlled by the sprinkler system’s conical water curtain, thus managing the entire fire progression. A stable phosphorus-silicon-rich char layer is formed on the wooden surface by fire retardant coating under high temperatures, providing better protection for the wood substrate during combustion—its effectiveness is second only to the fire sprinkler system. However, the impact of heat release from the instantaneous ignition of large-area painted wood retardant coatings on the building must be considered.

4. Discussion

4.1. Application Scenarios of the Fire Risk and Control Assessment Model

This paper is applicable to Chinese 20th-century architectural heritage with brick-timber structural systems. Such buildings mostly adopt timber truss roofs, wooden interiors, and masonry walls, presenting high fire risks. According to numerous fire cases of similar buildings, ignition points often start from the roof of the building and spread horizontally or vertically downward. Therefore, this paper selects 20th-century architectural heritage with typical long-narrow floor plans and enclosed ceilings in Chinese universities to simulate the fire spread from the ignition to flashover growth stage in the upper part of the building. For potential fire sources such as electrical short circuits in concealed spaces above the ceiling, it simulates the imperceptible spread inside the roof during the early and growth stages of the fire.

Meanwhile, the house is located at the foot of a mountain and in front of a giant air-raid shelter, with complex surrounding terrain. The low-temperature air in the air-raid shelter easily generates thermal convection with the air around the building. Field measurements show that the wind speeds on the four facades of the architectural heritage vary significantly (0.6–1.9 m/s) and are unstable. Setting it to 0 m/s can eliminate the influence of surrounding unstable factors on the experiment, reflecting the building in a calm environment and more realistically reproducing the scenario. Secondly, this setting is commonly used to construct a “windless control condition” for comparison with future simulation studies under other wind speed conditions (such as 1 m/s, 5 m/s). Compared with scenarios where some windows are open or damaged, the fully open state can avoid the flow field asymmetry caused by differences in opening positions and sizes, facilitating the extraction of the linear relationship between ventilation volume and fire parameters. Moreover, the current condition of the building shows severe damage to external enclosure structures such as doors and windows. During an actual fire, it is difficult to achieve the ideal state of full closure, and scenarios of doors and windows deforming or breaking due to high temperatures will also exist in the early stage of the fire. The above variable settings are suitable for mimicking the real situation when a fire occurs in this architectural heritage.

4.2. Limitations and Future Research

Given the unpredictability, developmental uncertainty of fire, and its stage characteristics in enclosed spaces, this paper focuses on simulating the wooden structure roof part of modern brick-timber architectural heritage. This part is basically composed of wooden roof trusses and corresponding components as non-movable fire loads, forming a relatively enclosed independent space. The uncertainty and probability of external influence during a fire are relatively low. Movable fire loads such as furniture and sofas are not included in the indoor part of the simulation experimental environment, allowing for further fire simulation in future research on the full combustion stage in the late fire period and the stage of fire continuing to invade downward from the roof.

In addition, the simulation environment in this paper is set only for the typical climate of Changsha. In the future, simulation conditions can be adjusted for extreme climates in Changsha, such as high temperature, dry climate, and the subtropical monsoon climate with hot summers and cold winters, to analyze the impact of climate on fire development and dry pipe fire sprinkler systems. Moreover, adding movable fire loads in the top-floor space connected to the roof space and modeling the growth of trees and branches around the building can realistically simulate the situation where thermal plumes invade the interior during the late fire period of this architectural heritage, ignite the top-floor interior, and embers ignite other objects around the building.

Regarding the selection of ignition sources, after analyzing numerous fire cases of similar buildings [33,61,62], this paper mainly explores fire spread in relatively enclosed spaces within the roof. Following the principle of the most unfavorable conditions during a fire, the area above the ceiling, which is difficult to detect, is set as the ignition point. However, other potential ignition positions (such as indoor furniture, stairwells, and areas with concentrated electrical equipment) are not considered. In actual buildings, fires may occur in various positions due to different causes such as cooking, smoking, and equipment overload, and the setting of a single ignition point is difficult to cover all-scenario risks. In future research, multi-fire source coupling scenarios can be introduced to simulate the chain reaction of simultaneous combustion of multiple ignition points in the same building, such as the mutual promotion between electrical fires and furniture fires.

5. Conclusions

Three fire prevention strategies were proposed and validated by simulating the roof fire of The First Faculty House and addressing fire risks in 20th-century Chinese modern architectural heritage. For fire accidents in China’s modern brick-wood architectural heritage, measures such as fire-retardant coatings, fire sprinkler systems, and fire barriers can reduce fire risks to varying degrees. The conclusions are summarized as follows:

• The roof structure of The First Faculty House primarily uses wood, mostly covered by tiles and exposed to high-temperature, dry environments. The wood is highly prone to fire with an average moisture content of 9%. The third-floor stores large quantities of flammable materials (e.g., paper, fabric), and high-voltage cable trays with exposed wires are installed under the wooden ceiling. Inadequate existing fire protection equipment further increases risks. The typical fire load is 1,976,246 MJ. Located at the foot of Yuelu Mountain and surrounded by forests, a fire in the flashover stage could easily spread to surrounding trees, leading to severe disasters.
• Simulation results indicate that the ceiling access holes on the third floor are key pathways for fire spread. When high-temperature gases fill the roof space, a pressure difference with the cooler air below creates a channel for flames to spread downward. Because of chimney effect, the holes draw in air to sustain combustion, and thermal plumes can surge through them into the third floor, igniting objects and escalating the fire. Strengthening ceiling fire resistance and prioritizing holes protection that stops the thermal plume from invading the sides of the ceiling. can reduce damage from roof fires. For long, narrow heritage buildings, installing fire barriers inside roof trusses and exhausting accumulated smoke can prevent rapid high-temperature spread in enclosed areas.
• Under untreated conditions, the first ceiling panel is burned through at 650 s, allowing thermal plumes to flood the third-floor corridor. Flashover occurs in the roof at 700 s, with a sharp rise in HRR and flames jetting from windows. The maximum ceiling temperature reaches 750 °C, risking structural collapse. Harmful CO gas concentrates on the third floor, though concentrations at stair entrances remain low. We recommend regular fire monitoring of top floors and roof spaces, especially for hidden ignition sources above ceilings. Reducing storage of dangerous goods in the top floor and clearing flammable debris are critical for preventing the indirect fire. Maintaining the surrounding environment (e.g., cleaning trees, minimizing open flames) can reduce external fire risks.
• We proposed three strategies based on the building’s characteristics: fire sprinkler systems, fire retardant coatings, and fire barriers. Their effectiveness was validated via simulation in the order: fire sprinkler systems > fire retardant coatings > fire barriers. If structural conditions permitted, we recommend the sprinkler system as the priority to enhance fire resistance. Both sprinklers and fire barriers can slow fire spread in narrow buildings. Combining all three measures provides comprehensive protection. Hidden fires above ceilings may show minimal smoke at 500 s and enter flashover by 650 s (less than 3 min), necessitating smoke and fire warning devices in large roof spaces.