Construction Technologies and Conservation Strategies for the Bell Tower of Former Nanking University (Nanjing, China)—A Case Study of a Typical Architectural Heritage of the American Church School in the Late 19th Century

: The bell tower of Nanking University, built in 1888, is a milestone of modern educational architecture in Nanjing. It fully introduced Western architectural concepts and is an important historical witness to the cultural exchange between China and the West. At present, there are few studies on the conservation and reuse of the building, so the authors conducted the project. In this paper, we conducted research by combining qualitative and quantitative methods. The qualitative analysis includes analyses of historical maps, photos, text, surveys, mapping of buildings, and a comparative analysis. The quantitative analysis includes statistics on the distribution of the urban school architectural heritage, the number of buildings at Nanking University, and China’s school building design standards. Through the historical research and value analysis of the building, we explore its real information from the aspects of architectural style, space, structure, material, and construction technology, and propose a reasonable conservation method. According to the current urban planning policy and architectural heritage data, the functional upgrading and renovation of the building will be carried out. This paper explores authenticity and integrity-based restoration methods, as well as the adaptive-reuse strategy for China’s architectural heritage, which can provide a paradigm for regional development and historic campus renewal.


Introduction
Nanking University was the first church university founded by the American Christian Church in Nanjing, China, in the 1880s. At present, there is only one piece of architectural heritage on the campus, the bell tower, which is a major historical and cultural site that is protected at the national level. The authors completed historical research on the bell tower through archival document research, on-site surveying, mapping, and an exploration of the building in terms of its architectural style, space, structure, material, and construction technology. We conserve and renovate based on the principles of authenticity and integrity so that the bell tower not only adapts to modern life but also retains its original historical flavor.
Church school buildings generally refer to school buildings built by Western churches on the Chinese territory after the first Opium War in the 1840s to the founding of the People's Republic of China in 1949. After the first Opium War in the 1840s, the Chinese government lifted the more than a 100-year-old policy of prohibiting foreign religions and allowed Western churches to build churches and schools in China [1]. Church school buildings are was preserved [14], so that the historical information of each historical period could be truly presented. The author believes that the above-mentioned restoration of "return to the original state" can easily lead to the disappearance of historical information, and a large number of restorations have no authoritative basis for restoration, and many are conjectures. The author believes that adhering to the principle of authenticity is to truly protect the historical information of various historical periods. In regard to preserving the legitimate contributions of cultural relics in different historical periods, the purpose of restoration is not to pursue the unity of style (Venice Charter, 1964).
It is worth noting that the factors affecting the authenticity of the heritage include structures, materials, craftsmanship, functions, and the environment [15]. However, too much focus on architectural appearance in many current heritage conservation practices has led to the emergence of "facadeism" [16], forming a model of "external protection and internal reconstruction" [17], which is a response to authentically one-sided understanding.
As for integrity, the Chinese government delimits the "conservation scope" and "construction control area" to protect the surrounding environment of the cultural relics according to the cultural relics grade and status quo ("Law of the People's Republic of China on the Protection of Cultural Relics", Article 17). Therefore, although many scholars have conducted research on China's modern architectural heritage, summed up the conservation ideas and methods of different types of architectural heritage, and reached a consensus, in many respects, in architectural heritage conservation practice, the understanding of authenticity, integrity, and conservation methods are still controversial and often require a case-by-case strategy, which provides the opportunity for this study.
Adaptive reuse is an important conservation method, and its focus is on comprehensively considering the continuation of the original function or the upgrading and transformation of the function according to the value characteristics, conservation status, and environmental conditions of the heritage (Principles for the Conservation of Heritage Sites in China, Article 40). "The Barra Charter" promulgated in 1979 first proposed "compatible use"; that is, "use with little or no effect on cultural significance" [18]. Compatible utilization is an effective method of achieving a win-win situation for urban development and architectural heritage conservation that is related to the value assessment of heritage and social needs in reuse design. Academia has conducted extensive research on architectural heritage conservation and reuse, including adaptive reuse, sustainable reuse, and co-evolutionary approaches [19,20]. In recent years, the Chinese government has encouraged "compatible uses" of architectural heritage. In 2019, the National Cultural Heritage Administration issued Guidelines for the Opening of Architectural Heritage to ensure that heritage buildings are allowed to open to the public in the form of community services, cultural displays, tours, commercial services, and public welfare offices without compromising the safety of heritage buildings [21]. It provides guidelines for research on this topic.
To summarize, this article focuses on the core principle of "authenticity and integrity", analyzes the authenticity of heritage in various cultures, discusses the current status of "how to protect authenticity and to what extent to protect authenticity", and points out the deficiencies of current conservation practices (many heritages have removed historical traces, paid no attention to raw materials and original craftsmanship, paid attention to external protection but ignored internal functions, etc.). In view of this, the authors believe that under the current Chinese heritage protection system, the core issue of architectural heritage protection of modern Chinese church schools is to explore the strategy of heritage protection and adaptive reuse based on authenticity and integrity on the premise of value assessment, retain the traces of each period, protect the authenticity and integrity of the heritage itself and the surrounding environment, and upgrade functions according to current needs. This is the contribution of this paper. The following uses the protection of the bell tower in the former Nanking University campus in Nanjing City, Jiangsu Province, China, as an example (Figures 1-3). On the premise of retaining the traces of various periods, it focuses on the authentic and integral preservation of the architectural style, structure, materials, and surrounding environment, and analyzes the current requirements for functional upgrading. A reasonable protection strategy based on authenticity and integrity is proposed. assessment, retain the traces of each period, protect the authenticity and integrity of the heritage itself and the surrounding environment, and upgrade functions according to current needs. This is the contribution of this paper. The following uses the protection of the bell tower in the former Nanking University campus in Nanjing City, Jiangsu Province, China, as an example (Figures 1-3). On the premise of retaining the traces of various periods, it focuses on the authentic and integral preservation of the architectural style, structure, materials, and surrounding environment, and analyzes the current requirements for functional upgrading. A reasonable protection strategy based on authenticity and integrity is proposed.   assessment, retain the traces of each period, protect the authenticity and integrity of the heritage itself and the surrounding environment, and upgrade functions according to current needs. This is the contribution of this paper. The following uses the protection of the bell tower in the former Nanking University campus in Nanjing City, Jiangsu Province, China, as an example (Figures 1-3). On the premise of retaining the traces of various periods, it focuses on the authentic and integral preservation of the architectural style, structure, materials, and surrounding environment, and analyzes the current requirements for functional upgrading. A reasonable protection strategy based on authenticity and integrity is proposed.

Aims and Methodology
The bell tower has a history that spans more than 120 years. The use of the building has changed many times. The current architectural form is somewhat damaged, and the building structure and materials need to be repaired urgently. According to the Nanjing Historical and Cultural City Conservation Plan (2010-2020) [22], Zhongshan Road is located Buildings 2022, 12, 2251 6 of 28 on the axis of modern historical buildings and has high regional value. The research goal of this paper is to explore the building from the aspects of architectural style, space, structure, material, and construction technology on the basis of value analysis and to propose repair and adaptive restoration methods based on authenticity and integrity so that these strategies can be taken advantage of ( Figure 4).
Buildings 2022, 12, x FOR PEER REVIEW 6 of 29 Figure 3. Location of the bell tower on the campus (source: author).

Aims and Methodology
The bell tower has a history that spans more than 120 years. The use of the building has changed many times. The current architectural form is somewhat damaged, and the building structure and materials need to be repaired urgently. According to the Nanjing Historical and Cultural City Conservation Plan (2010-2020) [22], Zhongshan Road is located on the axis of modern historical buildings and has high regional value. The research goal of this paper is to explore the building from the aspects of architectural style, space, structure, material, and construction technology on the basis of value analysis and to propose repair and adaptive restoration methods based on authenticity and integrity so that these strategies can be taken advantage of ( Figure 4).

Aims and Methodology
The bell tower has a history that spans more than 120 years. The use of the building has changed many times. The current architectural form is somewhat damaged, and the building structure and materials need to be repaired urgently. According to the Nanjing Historical and Cultural City Conservation Plan (2010-2020) [22], Zhongshan Road is located on the axis of modern historical buildings and has high regional value. The research goal of this paper is to explore the building from the aspects of architectural style, space, structure, material, and construction technology on the basis of value analysis and to propose repair and adaptive restoration methods based on authenticity and integrity so that these strategies can be taken advantage of ( Figure 4).  This paper employs a range of qualitative research methods, including historical research, field surveys, and mapping. Its academic contributions are as follows: (1) Through systematic and complete historical research, field surveys, and mapping, the historical background, cultural significance, and current situation of Nanking University and its bell tower are explored. According to the value types based on architectural heritage in the "Principles for the Conservation of Heritage Sites in China" (2015), this paper qualitatively studies their practical significance and practical solutions. Based on the principles of authenticity and integrity, conservation principles and repair strategies are formulated.
(2) Quantitative analysis includes statistics on the distribution of urban school architectural heritage, the number of buildings in Nanking University, and China's school building design standards. Starting from the development needs of the campus, combined with the "Nanjing Historical and Cultural City Conservation Plan" and regional conservation needs, we discuss the feasibility and suitability of the bell tower's reuse. Based on the functional scheme of the current architectural spatial characteristics, the scheme has a minimal effect on the original structure. Additionally, we develop a restoration strategy compatible with conservation and reuse. Nanjing, as the capital of the Republic of China, was the political, economic, and cultural center of China in the first half of the 20th century, and was, therefore, an important base for the missions of Western churches.

Case Presentation
The establishment of schools is an important part of the missionary causes of Western churches in China [23]. The earliest kindergartens, primary schools, and middle schools in Nanjing all started as mission schools. Initially, churches were created inside rented private houses, then schools and churches were separated, and, eventually, land was purchased to build mission schools. The educational model and campus construction model provided a reference for Chinese schools in the same period.
The location of Nanking University along the Gan River (now no. 169 Zhongshan Road) in the middle of Nanjing City was influenced by missionary activities. At first, missionaries preached in the area of the Gan River and Drum Tower [24], where they built churches, hospitals, and schools, which was conducive to the formation of a religious cultural circle. The bell tower is a representative church building.

The History of the Bell Tower
In 1888, American Christian missionary C.H. Fowler founded Nanking University near the Gan River in Nanjing and hired Canadian missionary J.C. Ferguson (1866-1945) as its first president. In 1910, after Nanking University, Christ College and Yizhi College merged to form "The University of Nanking", the university department was set up in Huiwen College, the middle school was set up in Hongyu College, the primary school was set up in Yizhi College, and the middle school was changed. It was called Jinling High School [25]. In 1921, after the University of Nanking moved to a new campus near the Drum Tower, the former campus was handed over to Jinling High School for use, and it continues to be used by this school to this day.
Nanking University was the largest church school in Nanjing in the late 19th century, with its bell tower (built in 1888, rebuilt after a fire in 1917) ( Figure 5), chapel (built in 1888), west teaching building (built in 1893), east teaching building (built in 1893), Collins dormitory (built in 1893), YMCA library (built in 1902), gymnasium (built in 1934), and seven other representative buildings, all of which were built in a Western style with the outer walls being built from gray brick masonry. The buildings were designed by an American architect and built by Chen Mingji Construction Factory (the first Chinesefounded construction factory in Nanjing. The owner of the factory, Chen Lieming, was a Christian, and he built many church buildings in Nanjing) [26]. Until the end of the 1980s, the school maintained the campus space form of the late nineteenth century. After the 1990s, due to a shortage of school land, all other historical buildings except the bell tower were removed. In 2006, the bell tower was rated as a major historical and cultural site protected at the national level as the earliest existing school building built by Christians in Nanjing.
founded construction factory in Nanjing. The owner of the factory, Chen Lieming, was a Christian, and he built many church buildings in Nanjing) [26]. Until the end of the 1980s, the school maintained the campus space form of the late nineteenth century. After the 1990s, due to a shortage of school land, all other historical buildings except the bell tower were removed. In 2006, the bell tower was rated as a major historical and cultural site protected at the national level as the earliest existing school building built by Christians in Nanjing. The bell tower was the first three-story western-style building built in Nanjing, and it is a symbol of education in Nanjing. The building survived the war and still stands tall. It not only records the historical development of Chinese society but is also an important reference for studying the history of modern Chinese education and architecture. It has important historical and cultural value.

Location and Current Function of the Building
The bell tower is now located on the campus of Jinling High School, no. 169, Zhongshan Road, Gulou District, Nanjing. It served as a classroom, dormitory, and office, and was an important meeting place for missionaries. It is currently an office building.

General Architectural Form
When the bell tower was first built, the main structure was three floors. After the fire in September 1917, it changed to two floors. The original third floor was changed to an attic with dormer windows. The original flat roof was changed to the sloping roof. The tower was originally five floors but was later changed to four floors. At present, The architectural form of the bell tower is slightly damaged, and its architectural style is the American colonial style (Figures 5 and 6). The bell tower was the first three-story western-style building built in Nanjing, and it is a symbol of education in Nanjing. The building survived the war and still stands tall. It not only records the historical development of Chinese society but is also an important reference for studying the history of modern Chinese education and architecture. It has important historical and cultural value.

Location and Current Function of the Building
The bell tower is now located on the campus of Jinling High School, no. 169, Zhongshan Road, Gulou District, Nanjing. It served as a classroom, dormitory, and office, and was an important meeting place for missionaries. It is currently an office building.

General Architectural Form
When the bell tower was first built, the main structure was three floors. After the fire in September 1917, it changed to two floors. The original third floor was changed to an attic with dormer windows. The original flat roof was changed to the sloping roof. The tower was originally five floors but was later changed to four floors. At present, The architectural form of the bell tower is slightly damaged, and its architectural style is the American colonial style (Figures 5 and 6).  The floor is an attic, the highest point is 3.2 m, and the dormer windows are opened using a sloping roof [28].
The building is a brick and wood structure. The exterior walls are built with clay bricks, the interior walls of the first and second floors are brick walls, and the interior walls of the third floor are wooden. Roof trusses, stairs, floors, and joists are all made of wood (Figure 7). At present, the building has some damage, for example, some walls are damaged or cracked, some roof tiles are falling off, many windows are missing, many window sills are damaged and a large number of drainage pipes are seriously damaged, which are in urgent need of repair ( Figure 8). According to the "Safety Appraisal Report of the bell tower of the Nanking University" [29], the structural system needs to be strengthened. There are four rooms on the first and second floors, with floor heights of 3.9 m and 3.3 m, respectively. The floor is an attic, the highest point is 3.2 m, and the dormer windows are opened using a sloping roof [28].
The building is a brick and wood structure. The exterior walls are built with clay bricks, the interior walls of the first and second floors are brick walls, and the interior walls of the third floor are wooden. Roof trusses, stairs, floors, and joists are all made of wood (Figure 7). At present, the building has some damage, for example, some walls are damaged or cracked, some roof tiles are falling off, many windows are missing, many window sills are damaged and a large number of drainage pipes are seriously damaged, which are in urgent need of repair ( Figure 8). According to the "Safety Appraisal Report of the bell tower of the Nanking University" [29], the structural system needs to be strengthened.  We excavate the real information from the architectural style, space, structure, material, and construction process, and propose reasonable protection methods. There are close logical relationships between these five points. We used raw materials and original

Present-Existing State of the Building
We excavate the real information from the architectural style, space, structure, material, and construction process, and propose reasonable protection methods. There are close logical relationships between these five points. We used raw materials and original craftsmanship for repairs to maximize the preservation of the original historical appearance and architectural style. Since authenticity is the core principle of architectural heritage conservation, the preservation of architectural style is the most important and difficult to implement in restoration practice. The use of historical buildings is a process in which authenticity is constantly disappearing. We used raw materials and original techniques to repair them as they are, and only preserved the authenticity of the architectural heritage to the greatest extent, including the true and complete preservation of the information of each historical period. In addition, safety is an important principle of conservation practice, and structural strengthening can eliminate potential safety hazards. At the same time, the building functions are updated according to the current regional development needs. Based on the above analysis, this paper focuses on the preservation of architectural style, structural strengthening, and functional renewal.

Artistic Characteristics
From 1840 to 1949, the dissemination of Western modern architectural culture in Nanjing mission schools went through two stages; according to the architectural form, it is roughly divided into the early "transplantation" and the later "combination of Chinese and Western". "Transplantation" is a direct adaptation of Western architectural styles.
The bell tower is a transplant of the colonial style of the United States, which originated from the rural and urban dwellings in Europe in the Middle Ages. After Europeans arrived in the Americas, this form became popular. The roof of the bell tower is steep, the outer wall is built with brick walls, not painted, and the round-arched windows are made of fine brick lines in the middle of the wall, the lower part of the cornice, the corners, and the entrance to form a more complex decoration [30].
The facade of the bell tower is symmetrical, the towering tower is located in the middle of the building facade, and the building is divided into three vertical sections; the platform base, the wall body, and the roof form the three horizontal sections. The facade is carefully composed according to geometric proportions involving equilateral triangles, golden sections, similar rectangles, etc. (Figure 9). The building as a whole creates a solemn and elegant architectural form through the symmetrical plan and elevation pattern of the central axis and the rigorous geometric proportions and scales. To summarize, as a representative of American architecture in early modern Nanjing, the bell tower shows the superb skills of American architects, reflects the rigorous geometric composition, and distinctive artistic features of Western architecture; it is of high historical and artistic value.

Structural System
The building is a brick and wood structural system. Wood roof trusses, wood purlins, wood-square columns, wood beams, wood joists, brick walls, and brick foundations constitute the load-bearing structural system of the building. In order to increase the stability of the structural system, wood beams are pre-embedded in the surrounding exterior walls to connect with the wood roof trusses. On the third floor, the pre-embedded wood beams are connected to the bottom of the wood-square columns ( Figure 10).
The structural system bears the vertical load and horizontal load. The vertical load is transmitted as follows: the vertical load of the roof is transferred to the purlins, square columns, joists, and walls; the live load and dead load on the third floor are transferred to the wood joists, the wood beams are embedded in the wood joists and the wall on the second floor, and so on; the vertical load is transferred from the brick wall to the brick foundation and foundation soils.
carefully composed according to geometric proportions involving equilateral triangles, golden sections, similar rectangles, etc. (Figure 9). The building as a whole creates a solemn and elegant architectural form through the symmetrical plan and elevation pattern of the central axis and the rigorous geometric proportions and scales. To summarize, as a representative of American architecture in early modern Nanjing, the bell tower shows the superb skills of American architects, reflects the rigorous geometric composition, and distinctive artistic features of Western architecture; it is of high historical and artistic value.  The structural elements of the building are described as follows: the brick foundation is in the form of spread footing, the thickness of the exterior brick wall is 460 mm, and the thickness of the interior brick wall is 270 mm (Figure 10a,b). The walls are built with grey clay bricks and white lime mortar. The first to third floors are all wood floors with wood joists. The thickness of the wood floor is 50 mm. The height of the wood joists is 300 mm, the width is 50 mm, and the spacing is 500 mm. At present, a small number of wood joists are rotted and cracked, and we plan to add I-shaped beam strengthening at the bottom of the wood joists. After the mechanical calculation, the reasonable I-shaped beam size is selected according to the space scale. For example, when the span is 6.8 m, the height of the I-shaped beam is 300 mm, the width is 200 mm, and the thickness is 8 mm; when the span is reduced, the size of the I-shaped beam is also reduced. The third floor is the attic. In order to reduce its own weight, wood roof trusses, wood purlins, and wooden pillars are used. The bottom of the pillars is connected with the pre-embedded wood beams in the wood joists, and the wood roof trusses are connected with the surrounding external walls, which together form a structural system (Figure 10c,d). The thickness of the interior walls of the attic is 150 mm; they are wooden walls and are used to separate rooms (Figure 10b), not a structural system [31].

Structural System
The building is a brick and wood structural system. Wood roof trusses, wood purlins, wood-square columns, wood beams, wood joists, brick walls, and brick foundations constitute the load-bearing structural system of the building. In order to increase the stability of the structural system, wood beams are pre-embedded in the surrounding exterior walls to connect with the wood roof trusses. On the third floor, the pre-embedded wood beams are connected to the bottom of the wood-square columns ( Figure 10).

Roof Truss System
The roof truss system is special. The authors learned through detailed historical research that after the bell tower caught fire in 1917, the American church funded the conversion of the third floor into an attic. Since it was only a partial renovation, there was no clear record of whether architects were involved. Therefore, the authors speculate that the attic was built by Chinese craftsmen based on experience and did not fully meet the requirements of the building structure. It was a common phenomenon at the time. In the early 20th century, Western construction technology had not been fully introduced into China [32]. Some missionaries would draw architectural sketches, and Chinese craftsmen would build according to the wishes of the missionaries and their own experiences. Ginling College, built at the same time in Nanjing, was built by Chinese craftsmen, who often discussed construction techniques with professional architects [33].
Because Chinese craftsmen participated in the construction, the roof truss is similar to the "cross type wood structure". It is a typical structural system of traditional Chinese architecture. Its characteristic is that the columns are erected along the depth direction of the house, and the spacing between the columns is relatively dense. Along the direction of the purlins, the columns in series with fangs (small beams) are connected. This forms an overall structural system [34] (Figure 11a).
Buildings 2022, 12, x FOR PEER REVIEW 14 of 29 Figure 11. Comparison of the Chinese traditional roof-truss system and bell tower roof-truss system (source: author). (a) Chinese traditional roof-truss system "cross type wood structure" (b) Bell tower roof-truss system.
Similar to the bell tower, the columns are connected in series with fangs, and the roof trusses, purlins, and square columns are closely connected with the pre-embedded wood beams in the wood joists to form an integral structural system ( Figure 11b). The connection of roof trusses, purlins, and square columns includes iron nails and "mortise and tenon" (a combination of concave and convex parts on two components). It is a connection method of traditional Chinese wooden components, which can make two wooden components tightly connected.

Materials and Technology Characteristics
The space layout of the bell tower is very good. The interior of the building is centered with corridors and stairs for the transportation system. The building plan is divided into four rooms with roughly equal areas, which not only reflects the Christian spirit of "equality of all living beings" but also makes the room-scale very suitable for modern teaching and office spaces. From the perspective of architectural design, the room is 5 m × 7.6 m, roughly rectangular, and suitable for desks and chairs; there are windows on both sides of the room, the light is bright, and the window-to-floor ratio design can ensure the brightness of the room so as to meet the teaching needs of modern teaching (e.g., blackboard writing and drills). For China in the late 19th century, this was a brand-new space design concept, while traditional Chinese academies of the same period were dimly lit Figure 11. Comparison of the Chinese traditional roof-truss system and bell tower roof-truss system (source: author). (a) Chinese traditional roof-truss system "cross type wood structure" (b) Bell tower roof-truss system. Similar to the bell tower, the columns are connected in series with fangs, and the roof trusses, purlins, and square columns are closely connected with the pre-embedded wood beams in the wood joists to form an integral structural system (Figure 11b). The connection of roof trusses, purlins, and square columns includes iron nails and "mortise and tenon" (a combination of concave and convex parts on two components). It is a connection method of traditional Chinese wooden components, which can make two wooden components tightly connected.

Materials and Technology Characteristics
The space layout of the bell tower is very good. The interior of the building is centered with corridors and stairs for the transportation system. The building plan is divided into four rooms with roughly equal areas, which not only reflects the Christian spirit of "equality of all living beings" but also makes the room-scale very suitable for modern teaching and office spaces. From the perspective of architectural design, the room is 5 m × 7.6 m, roughly rectangular, and suitable for desks and chairs; there are windows on both sides of the room, the light is bright, and the window-to-floor ratio design can ensure the brightness of the room so as to meet the teaching needs of modern teaching (e.g., blackboard writing and drills). For China in the late 19th century, this was a brand-new space design concept, while traditional Chinese academies of the same period were dimly lit and accommodated few people.
In terms of building materials, the building's foundation and walls were built with local clay-ash bricks. It embodied the wisdom of construction according to local conditions. The masonry method of the brick wall was Flemish bond. They used white lime mortar for jointing. When the building was first built, the exterior walls were all made of clay-ash bricks. In daily use, the outer wall from the outdoor ground to the first window sill was often wetted by rainwater. Therefore, when repairing, we brushed a layer of cement mortar on the surface of the clay brick as a waterproof layer [35]. To accentuate the main entrance, the surface of the brick porch was covered with a hammered granolithic finish. It is a kind of artificial stone ( Figure 12). During production, cement mortar mixed with stone chips and stone powder was applied to the surface of the building. After hardening, the mixture was made into a textured stone surface style by cutting and chiseling. They are widely used in modern China in important public buildings.

Equipment and Facilities
Nanjing is located in a climate zone with hot summers, cold winters, and abu rainfall. Therefore, the comprehensive designs of drainage, ventilation, and equip systems are particularly important.
The design of the drainage system is scientific and reasonable, and it is still i after more than one hundred years. The main body of the building and the roof tower are steep four-pitched roofs. Rainwater is collected by the cornice gutter usin height difference and is opened into multiple catchment areas. Drainage pipes a stalled at the four corners of the house and the porch. It flows into the outdoor gr open ditch and then flows into the school drainage pipe to the municipal drainage ( Figure 13).

Equipment and Facilities
Nanjing is located in a climate zone with hot summers, cold winters, and abundant rainfall. Therefore, the comprehensive designs of drainage, ventilation, and equipment systems are particularly important.
The design of the drainage system is scientific and reasonable, and it is still in use after more than one hundred years. The main body of the building and the roof of the tower are steep four-pitched roofs. Rainwater is collected by the cornice gutter using the height difference and is opened into multiple catchment areas. Drainage pipes are installed at the four corners of the house and the porch. It flows into the outdoor ground open ditch and then flows into the school drainage pipe to the municipal drainage pipe ( Figure 13).
The building focuses on natural ventilation. Since the dominant wind direction in Nanjing is the southeast wind in the summer and the northwest wind in the winter, the building is arranged facing south, with porches on the south and north sides, windows all around, dormer windows on all sides of the roof, and ventilation holes in the basement to form a natural ventilation system ( Figure 14). Under well-ventilated conditions, the wood roof trusses, wooden pillars, and wooden stairs can be air-dried in time even if they are temporarily damp, and there is no major decay after more than one hundred years.
The design of the drainage system is scientific and reasonable, and it is still in use after more than one hundred years. The main body of the building and the roof of the tower are steep four-pitched roofs. Rainwater is collected by the cornice gutter using the height difference and is opened into multiple catchment areas. Drainage pipes are installed at the four corners of the house and the porch. It flows into the outdoor ground open ditch and then flows into the school drainage pipe to the municipal drainage pipe ( Figure 13). The building focuses on natural ventilation. Since the dominant wind direction in Nanjing is the southeast wind in the summer and the northwest wind in the winter, the building is arranged facing south, with porches on the south and north sides, windows all around, dormer windows on all sides of the roof, and ventilation holes in the basement to form a natural ventilation system ( Figure 14). Under well-ventilated conditions, the wood roof trusses, wooden pillars, and wooden stairs can be air-dried in time even if they are temporarily damp, and there is no major decay after more than one hundred years. In the building's early days, a fireplace was installed indoors as a heating system, with a chimney protruding from the roof. At present, the fireplace is broken, and the building's cooling and heating systems need to be redesigned.

Damage to the Building and Assessment of the Technical Condition
In China, current building laws require historic buildings to be structurally identified before they can be repaired. For example, it is mentioned in the "Technical Standards for Building Structure Testing [36]" (GB/T50344-2019, Article 3.1.4) that we need to detect structural damage, structural component bearing capacity, structural deformation, and other items involving structural performance. It includes structural safety assessment, structural anti-seismic assessment, reliability assessment before building repairs, etc. In order to understand the safety condition of the bell tower and provide a basis for strengthening, we commissioned Nanjing Southeast Construction Engineering Safety Appraisal Co., Ltd. (Nanjing, China) to conduct a technical condition assessment.
The first is to examine the general condition of the structure. The building has no obvious uneven land subsidence, no cracks in the foundation, and no cracks in the superstructure caused by uneven land subsidence of the foundation. Wall materials are clay bricks and lime mortar. At present, the quality of the bricks is still good, but some exterior walls are damaged due to dampness and weathering (Figure 15a), and a small number of windows have oblique cracks at the corners (Figure 15b). Therefore, the load-bearing capacity of the walls is basically acceptable based on a small number of structural cracks and material degradation assessments. Currently, small numbers of wood joists and In the building's early days, a fireplace was installed indoors as a heating system, with a chimney protruding from the roof. At present, the fireplace is broken, and the building's cooling and heating systems need to be redesigned.

Damage to the Building and Assessment of the Technical Condition
In China, current building laws require historic buildings to be structurally identified before they can be repaired. For example, it is mentioned in the "Technical Standards for Building Structure Testing [36]" (GB/T50344-2019, Article 3.1.4) that we need to detect structural damage, structural component bearing capacity, structural deformation, and other items involving structural performance. It includes structural safety assessment, structural anti-seismic assessment, reliability assessment before building repairs, etc. In order to understand the safety condition of the bell tower and provide a basis for strengthening, we commissioned Nanjing Southeast Construction Engineering Safety Appraisal Co., Ltd. (Nanjing, China) to conduct a technical condition assessment.
The first is to examine the general condition of the structure. The building has no obvious uneven land subsidence, no cracks in the foundation, and no cracks in the superstructure caused by uneven land subsidence of the foundation. Wall materials are clay bricks and lime mortar. At present, the quality of the bricks is still good, but some exterior walls are damaged due to dampness and weathering (Figure 15a), and a small number  (Figure 15b). Therefore, the load-bearing capacity of the walls is basically acceptable based on a small number of structural cracks and material degradation assessments. Currently, small numbers of wood joists and trusses are rotted or cracked (Figure 15c,d). The second is to analyze the bearing capacity of the main structure and evaluate the safety level of the house. We checked and calculated according to the structural layout and the geometric dimensions of the components based on the site investigation and mapping, combined with the Chinese structural standard. In Nanjing, this includes snow pressure (q = 0.65 kN/m 2 ), floor live load (qk = 3.5 kN/m 2 ), and wind load (w = 0.40 kN/m 2 ). The seismic grade is 7. According to the current structural standard in China (Technical Standards for Maintenance and Strengthening of Wooden Structures of Ancient Buildings (GB/T50165-2020) [37]", "Building Structure Load Code (GB50009-2012)" [38], "Code for Seismic Design of Buildings (GB50011-2010)" [39], we concluded that the bearing capacity of the wood roof trusses can basically meet the national standards; the compressive capacity and seismic bearing capacity of some brick walls on the first and second floors do not meet the standards; the bearing capacities of wood joists do not meet the current national standards. In order to confirm the safety of the building and to make the analysis credible, we conducted a structural safety status assessment according to the appraisal procedures and methods of the "Dangerous House Appraisal Standard" (JGJ125-2016) [40], combined with on-site inspections. We evaluated the safety level of the building foundation and upper load-bearing structure (including dominant members, such as brick walls, wood joists, and wood roof trusses). According to the "Dangerous House Appraisal Standard" (JGJ125-2016) (Article 6.1.4), Class B basically meets the requirements for safe use, and Class C means that some load-bearing structures cannot meet the requirements for safe use, and the house is partially dangerous.
The details are as follows: According to on-site inspections, the building site is flat and has no slopes. The building does not have any obvious settlement cracks, deformations, displacements, or other uneven settlement phenomena, indicating that the foundation is stable. The safety level assessment of the foundation is Bu level .
The safety appraisal grade of the upper load-bearing structure is comprehensively assessed according to the safety grade of the dominant member, the integrity grade of the The second is to analyze the bearing capacity of the main structure and evaluate the safety level of the house. We checked and calculated according to the structural layout and the geometric dimensions of the components based on the site investigation and mapping, combined with the Chinese structural standard. In Nanjing, this includes snow pressure (q = 0.65 kN/m 2 ), floor live load (qk = 3.5 kN/m 2 ), and wind load (w = 0.40 kN/m 2 ). The seismic grade is 7. According to the current structural standard in China (Technical Standards for Maintenance and Strengthening of Wooden Structures of Ancient Buildings (GB/T50165-2020) [37]", "Building Structure Load Code (GB50009-2012)" [38], "Code for Seismic Design of Buildings (GB50011-2010)" [39], we concluded that the bearing capacity of the wood roof trusses can basically meet the national standards; the compressive capacity and seismic bearing capacity of some brick walls on the first and second floors do not meet the standards; the bearing capacities of wood joists do not meet the current national standards. In order to confirm the safety of the building and to make the analysis credible, we conducted a structural safety status assessment according to the appraisal procedures and methods of the "Dangerous House Appraisal Standard" (JGJ125-2016) [40], combined with on-site inspections. We evaluated the safety level of the building foundation and upper load-bearing structure (including dominant members, such as brick walls, wood joists, and wood roof trusses). According to the "Dangerous House Appraisal Standard" (JGJ125-2016) (Article 6.1.4), Class B basically meets the requirements for safe use, and Class C means that some load-bearing structures cannot meet the requirements for safe use, and the house is partially dangerous.
The details are as follows: According to on-site inspections, the building site is flat and has no slopes. The building does not have any obvious settlement cracks, deformations, displacements, or other uneven settlement phenomena, indicating that the foundation is stable. The safety level assessment of the foundation is B u level.
The safety appraisal grade of the upper load-bearing structure is comprehensively assessed according to the safety grade of the dominant member, the integrity grade of the structure, and the lateral displacement grade of the structure. The safety grade of the dominant member is assessed as the C u level (an evaluation of the dominant members, such as brick walls, wood joists, and wood roof trusses. The overall seismic resistance of the walls is poor. According to on-site inspections, some load-bearing walls are damaged, and there are cracks at the corners of the windows. The two ends of the wood joists were placed in the wall for a long time, and some wood joists were rotted and broken due to humidity. Due to the deflection and deformation of some wood joists, the support length of the ends of the wood joists in the wall is less than 100 mm, and the ends are loose, which affects the overall stability of the floor. Some wood trusses are rotten or cracked, affecting the safety of the house). The integrity of the structure is evaluated as the C u level (the building is not equipped with structural columns and ring beams, the roof truss is not equipped with vertical and horizontal supports, and the integrity and earthquake resistance of the house are poor). The lateral displacement of the structure is evaluated as the B u level (according to the on-site inspection, the maximum inclination rate (to the east-west direction) of the building is 0.16%, which is less than the limit value (2%). In general, the safety level of the upper load-bearing structure is evaluated according to the lower level. Therefore, the safety level of the comprehensive assessment of the upper load-bearing structure is C u level.
To summarize, according to the assessment results of the foundation and the upper load-bearing structure, the safety level of the building is assessed as the C su level [41]. Some dominant members, such as walls, wood joists, and wood roof trusses must be strengthened due to material degradation. In addition, the building did not consider earthquake resistance in its previous design, and its structural integrity and earthquake resistance were poor. It should be combined with repairs and seismic strengthening to basically meet China's current codes.

Building Function Update
According to the "Nanjing Famous Historical and Cultural City Conservation Plan (2010-2020)", the former Nanking University area is defined as a cultural and educational area, forming a representative historical campus area with the nearby University of Nanking and Ginling College in the "Drum Tower-Qingliang mountain Historic District". The plan calls for maintaining the spatial pattern of the historic campus and the environmental features of high green coverage. At the same time, the former Nanking University is located on the historical axis of the Republic of China of "Zhongshan North Road-Zhongshan Road-Zhongshan East Road" [42], and the planning requires maintaining the public building functions along the line. According to the quantitative analysis of the quantity and distribution characteristics of the Nanjing school's architectural heritage, it can be concluded that the school's architectural heritage is concentrated in the Gulou District and distributed along the historical axis of the Republic of China (Figure 16).
To summarize, the bell tower is intended to continue to be used as a school building, which is a prerequisite for the renewal of the building, but its specific function requires further research and analysis. According to an analysis of the number of teachers and students, the types of buildings, and the number of buildings, the school has 3128 students [43]. The area of the teaching space and teaching auxiliary space is 25,283 m 2 (16,742 + 8541 = 25,283) [44] (Figure 17), and the average area of middle schools in China is 9.0 m 2 per student (dormitory not included) [45], so the total construction area is 28,152 m 2 (9.0 × 3128 = 28,152). Thus, it is necessary to provide a construction area of 2869 m 2 (28,152 -25,283 = 2869).
At present, it has the most teaching space, followed by logistics and sports spaces. There is a lack of teaching auxiliary space, such as office and clubhouse spaces. Combined with the current architectural functional requirements and in view of the important historical status of the bell tower, the National Cultural Heritage Administration, Jinling High School (the current user), and the Architectural Design Institute of Southeast University (the designer) have jointly decided that upgrading the bell tower and transforming it into an exhibition, clubhouse, and office space is more reasonable. To summarize, the bell tower is intended to continue to be used as a school building, which is a prerequisite for the renewal of the building, but its specific function requires further research and analysis. According to an analysis of the number of teachers and students, the types of buildings, and the number of buildings, the school has 3128 students [43]. The area of the teaching space and teaching auxiliary space is 25,283 m 2 (16,742 + 8541 = 25,283) [44] (Figure 17), and the average area of middle schools in China is 9.0 m 2 per student (dormitory not included) [45], so the total construction area is 28,152 m 2 (9.0 × 3128 = 28,152). Thus, it is necessary to provide a construction area of 2869 m 2 (28,152 -25,283 = 2869). The space and scale of the new functions should conform to the spatial characteristics of the historic building. According to the building's bearing capacity and functional requirements, there are plans for the exhibition space with a large load to be on the first floor. Combined with the outdoor exhibition hall and the environmental landscape, it will display the difficult process of the Western Church missionary's mission in China, the brilliant achievements of running a school, and the construction history of Nanking University. It is proposed that a historical building be set up on the second floor to create a small-scale exchange club for domestic and foreign architectural heritage protection enthusiasts and professionals in order to achieve the purpose of "meeting friends through architecture" and promoting the culture of historical buildings. The office space will be placed in the attic to reproduce the office scene of the principal (Table 1). To summarize, the utilization of architectural heritage is a balance between historical value and use value after the rational analysis to achieve a win-win situation of protection and development. At present, it has the most teaching space, followed by logistics and sports spaces. There is a lack of teaching auxiliary space, such as office and clubhouse spaces. Combined with the current architectural functional requirements and in view of the important historical status of the bell tower, the National Cultural Heritage Administration,Jinling High School (the current user), and the Architectural Design Institute of Southeast University (the designer) have jointly decided that upgrading the bell tower and transforming it into an exhibition, clubhouse, and office space is more reasonable.
The space and scale of the new functions should conform to the spatial characteristics of the historic building. According to the building's bearing capacity and functional requirements, there are plans for the exhibition space with a large load to be on the first floor. Combined with the outdoor exhibition hall and the environmental landscape, it will display the difficult process of the Western Church missionary's mission in China, the brilliant achievements of running a school, and the construction history of Nanking University. It is proposed that a historical building be set up on the second floor to create a small-scale exchange club for domestic and foreign architectural heritage protection enthusiasts and professionals in order to achieve the purpose of "meeting friends through architecture" and promoting the culture of historical buildings. The office space will be placed in the attic to reproduce the office scene of the principal (Table 1). To summarize, the utilization of architectural heritage is a balance between historical value and use value after the rational analysis to achieve a win-win situation of protection and development.

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural Heritage Administration, the current architectural design specifications of the People's Republic of China, the "Venice Charter", "The Cultural Relics of the People's Republic of China" Conservation Law, other international and domestic cultural relic conservation rules, and the above analysis, the following principles of building repair are formed:

2.
Integrity principle It is necessary not only to conserve the individual historic buildings but also to maintain the overall spatial form and inseparable relationship with the surrounding environmental elements, especially the relationship between the historical buildings and the district and the overall landscape of the campus (according to the Venice Charter, 1964,  Set up a conservation scope and construction control zone ( Figure 18) to completely protect the cultural relic building's body and demonstrate the authenticity and integrity of the cultural relic building [46].

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural

Repair Target and Principles
As a symbol of modern educational buildings in Nanjing, the bell tower is a national key cultural relic conservation unit. Based on the above analysis of heritage conservation and utilization, we plan to restore the building's external form, internal characteristics, and surrounding historical features and integrate new functions and supporting facilities. It has become an important "historical and cultural" landmark in Nanjing.
According to the "Renovation Design Plan for the Bell Tower" and "Structural Safety Appraisal Report of the Bell Tower", which were approved by the National Cultural First of all, conduct a comprehensive and in-depth investigation into cultural relic buildings and strive to fully grasp the complete history and information of cultural relic buildings according to survey drawings, historical photos, and related text materials so that the repair process can be truly authentic and grasp the historical building's authenticity.
protect the cultural relic building's body and demonstrate the authenticity and integrity of the cultural relic building [46]. First of all, conduct a comprehensive and in-depth investigation into cultural relic buildings and strive to fully grasp the complete history and information of cultural relic buildings according to survey drawings, historical photos, and related text materials so that the repair process can be truly authentic and grasp the historical building's authenticity. Figure 18. Set up a conservation scope and construction control zone (source: author). Figure 18. Set up a conservation scope and construction control zone (source: author).

Completely Preserve the Appearance of the Building
The original wooden doors and windows are rotten and damaged, and the wooden doors were replaced with similar copies. According to the style and color of the original wood window, it was replaced with a plastic-steel window, the surface of which imitates wood grain; double-layer insulating glass is used to improve the building's sound insulation and heat insulation performance. The diamond-shaped cement roof tiles are special features of this project. The two acute angles of the diamond-shaped tile are 65 degrees. They are laid obliquely and have the advantages of no dust accumulation, smooth drainages, and beautiful appearances. They were brought to China and were once used in church and mission school buildings. During this renovation, a batch of new tiles was ordered to replace the old tiles according to the style, size, and color of the original tiles.
We repaired the damaged brick wall by using brick powder with a similar color to the original wall, mixing it with water, and adding an appropriate amount of adhesive to repair the severely damaged part of the wall. We removed the decorations added to the interior.
We maintained the original appearance of the bell tower with original materials and original technology and strove to highlight the historical, cultural, and artistic values of the architectural heritage ( Figure 19).

Structural System Strengthening
According to the safety assessment of the building (in Section 3.5 of this paper), the materials of the main structural components were degraded and needed to be strengthened in combination with repairs. Durability maintenance is required for the main structural components (to improve the durability of the structure) and the integral and seismic strengthening of the main structure.
We repaired the damaged brick wall by using brick powder with a similar color to the original wall, mixing it with water, and adding an appropriate amount of adhesive to repair the severely damaged part of the wall. We removed the decorations added to the interior.
We maintained the original appearance of the bell tower with original materials and original technology and strove to highlight the historical, cultural, and artistic values of the architectural heritage ( Figure 19). Figure 19. Photos of the bell tower before, during, and after repairs (source: author). (a) Before conservation. (b) Renovation in progress (c) After conservation.

Structural System Strengthening
According to the safety assessment of the building (in Section 3.5 of this paper), the materials of the main structural components were degraded and needed to be strengthened in combination with repairs. Durability maintenance is required for the main structural components (to improve the durability of the structure) and the integral and seismic strengthening of the main structure.
The bell tower was built more than one hundred years ago, and some materials have degraded. According to on-site inspections, some exterior walls were damaged ( Figure  20a), and oblique cracks appeared in some window corners. Since the earthquake resistance was not considered in the design at that time, the overall earthquake resistance of the structure was poor. Therefore, we attach great importance to the maintenance of the wall to ensure its integrity, and strengthen the bearing capacity and integrity of the wall to improve its load-bearing capacity and ductility to deal with earthquake risks (Figure 20b). The bell tower was built more than one hundred years ago, and some materials have degraded. According to on-site inspections, some exterior walls were damaged (Figure 20a), and oblique cracks appeared in some window corners. Since the earthquake resistance was not considered in the design at that time, the overall earthquake resistance of the structure was poor. Therefore, we attach great importance to the maintenance of the wall to ensure its integrity, and strengthen the bearing capacity and integrity of the wall to improve its load-bearing capacity and ductility to deal with earthquake risks (Figure 20b). A pressure pump was used to inject a cement slurry to seal the cracks in the wall. The broken brick walls were repaired to ensure wall integrity and improve the load-bearing capacity. Clay bricks adopt different repair methods according to the damage depth; when the damage depth of the brick is less than 10 mm, it will not be repaired, but only cleaned and protected. When the damage depth of the brick is 10-20 mm, it should be repaired with brick powder. The brick powder material has good compatibility with the original wall, and the color is similar to the original wall. After adding water and mixing well, an appropriate amount of adhesive was added, the damaged part of the wall was repaired, and then the mortar joints were filled. When the damage depth of the brick is greater than 20 mm, the damaged surface will be cleaned and repaired with brick bat.
The strengthened wall adopts the steel mesh and polymer mortar surface layer, which not only improves the bearing capacity and earthquake resistance but also minimizes the impact on the use of indoor space. A total of 40 mm of thick steel mesh and polymer mortar were added to the inside of the exterior wall for strengthening; 40 mm of thick steel mesh and polymer mortar were added to both sides of the interior wall for strengthening. The strengthening method is shown in Figure 20c.

Wood joists and wooden stair strengthening.
A small number of wood joists were cracked (Figure 21a) and needed strengthening. We first applied water-resistant glue to the two cracked surfaces, then used more than two steel hoops to hoop the cracked wood joists, and finally wrapped them with CFRP to secure them. Considering that the two ends of the wood joists were embedded in the wall for a long time and are easy to rot or loosen after deformation, we strengthened the ends of the wood joists with L-section steel. The height of the L-section steel was 120 mm, the width was 90 mm, and the thickness was 8 mm. Not only does this increase the lengths of support at the ends of the wood grating, but it also forms a steel ring beam on the wall [47]. A pressure pump was used to inject a cement slurry to seal the cracks in the wall. The broken brick walls were repaired to ensure wall integrity and improve the load-bearing capacity. Clay bricks adopt different repair methods according to the damage depth; when the damage depth of the brick is less than 10 mm, it will not be repaired, but only cleaned and protected. When the damage depth of the brick is 10-20 mm, it should be repaired with brick powder. The brick powder material has good compatibility with the original wall, and the color is similar to the original wall. After adding water and mixing well, an appropriate amount of adhesive was added, the damaged part of the wall was repaired, and then the mortar joints were filled. When the damage depth of the brick is greater than 20 mm, the damaged surface will be cleaned and repaired with brick bat.
The strengthened wall adopts the steel mesh and polymer mortar surface layer, which not only improves the bearing capacity and earthquake resistance but also minimizes the impact on the use of indoor space. A total of 40 mm of thick steel mesh and polymer mortar were added to the inside of the exterior wall for strengthening; 40 mm of thick steel mesh and polymer mortar were added to both sides of the interior wall for strengthening. The strengthening method is shown in Figure 20c.
A small number of wood joists were cracked ( Figure 21a) and needed strengthening. We first applied water-resistant glue to the two cracked surfaces, then used more than two steel hoops to hoop the cracked wood joists, and finally wrapped them with CFRP to secure them. Considering that the two ends of the wood joists were embedded in the wall for a long time and are easy to rot or loosen after deformation, we strengthened the ends of the wood joists with L-section steel. The height of the L-section steel was 120 mm, the width was 90 mm, and the thickness was 8 mm. Not only does this increase the lengths of support at the ends of the wood grating, but it also forms a steel ring beam on the wall [47].
strengthening. The strengthening method is shown in Figure 20c. 2. Wood joists and wooden stair strengthening.
A small number of wood joists were cracked (Figure 21a) and needed strengthening. We first applied water-resistant glue to the two cracked surfaces, then used more than two steel hoops to hoop the cracked wood joists, and finally wrapped them with CFRP to secure them. Considering that the two ends of the wood joists were embedded in the wall for a long time and are easy to rot or loosen after deformation, we strengthened the ends of the wood joists with L-section steel. The height of the L-section steel was 120 mm, the width was 90 mm, and the thickness was 8 mm. Not only does this increase the lengths of support at the ends of the wood grating, but it also forms a steel ring beam on the wall [47]. After the renovation, the building will serve as a school history museum and club and will be visited by a large number of people. We set the standard value of the live load of the wood floor as 3.5 kN/m 2 (according to the Chinese standard "Building Structure Load Code (GB50009-2012)"), and according to the structural calculation, the wood joists need to be strengthened as a whole. We added steel beams along the vertical direction of After the renovation, the building will serve as a school history museum and club and will be visited by a large number of people. We set the standard value of the live load of the wood floor as 3.5 kN/m 2 (according to the Chinese standard "Building Structure Load Code (GB50009-2012)"), and according to the structural calculation, the wood joists need to be strengthened as a whole. We added steel beams along the vertical direction of the wood joists in the middle of the room span for strengthening. The I-shaped beam height was 300 mm, the width was 200 mm, and thickness was 8 mm (Figure 21b).
The deformation of the wooden staircase was large, and it needed to be strengthened according to the structural inspection. We used steel plates to make U-shaped steel beams to strengthen the stairs. The height of the steel beams was 200 mm, the width was 80 mm, and the thickness was 10 mm. Both sides of the beam were fastened with M20 bolts (Figure 21c) 3.
Wood roof truss strengthening.
Before the strengthening of the wood roof truss, the president ofJinling High School (the current user) organized the design, supervision, and construction personnel to jointly inspect and determine the wood roof truss strengthening repair plan. Because the ends of some wood roof trusses have rotted, most of the roof truss components have cracked, and the connection nodes of most components have become loose and disengaged. Therefore, it is necessary to strengthen and repair the roof members and nodes to improve their integrity and durability.
(1) The rotten parts of the wood roof truss were replaced. Some wood roof trusses had rotted ends (Figure 22a). We sawed off the rotten parts and used the original wood to redo this place. First, we used water-resistant glue on the joint surface of the old and new materials. Then we used two pieces of anti-corrosion wood to clamp the old and new wooden components; we then used water-resistant glue to fasten and screw the bolts. Finally, we used CFRP to strengthen the wooden components and wrap the old and new materials tightly.
Buildings 2022, 12, x FOR PEER REVIEW 25 of 29 the wood joists in the middle of the room span for strengthening. The I-shaped beam height was 300 mm, the width was 200 mm, and thickness was 8 mm (Figure 21b). The deformation of the wooden staircase was large, and it needed to be strengthened according to the structural inspection. We used steel plates to make U-shaped steel beams to strengthen the stairs. The height of the steel beams was 200 mm, the width was 80 mm, and the thickness was 10 mm. Both sides of the beam were fastened with M20 bolts ( Figure  21c) 3. Wood roof truss strengthening.
Before the strengthening of the wood roof truss, the president ofJinling High School (the current user) organized the design, supervision, and construction personnel to jointly inspect and determine the wood roof truss strengthening repair plan. Because the ends of some wood roof trusses have rotted, most of the roof truss components have cracked, and the connection nodes of most components have become loose and disengaged . Therefore, it is necessary to strengthen and repair the roof members and nodes to improve their integrity and durability.
(1) The rotten parts of the wood roof truss were replaced. Some wood roof trusses had rotted ends (Figure 22a). We sawed off the rotten parts and used the original wood to redo this place. First, we used water-resistant glue on the joint surface of the old and new materials. Then we used two pieces of anti-corrosion wood to clamp the old and new wooden components; we then used water-resistant glue to fasten and screw the bolts. Finally, we used CFRP to strengthen the wooden components and wrap the old and new materials tightly. (2) For cracked roof trusses, CFRP was used to enhance durability in a circular direction.
The thickness of CFRP was 0.167 mm, the paving width was 100 mm, and the interval was 400 mm (Figure 22b). (3) CFRP paste strengthened the joints of the beams, purlins, and columns to enhance the integrity of the wood roof truss (Figure 22c). When the cracks of the other wood components were less than 3 mm, we used putty to apply it tightly; cracks of 3-30 mm were filled with wooden strips and water-resistant glue, and finally strengthened with CFRP.
The wood columns under the roof of the tower were strengthened with CFRP. The (2) For cracked roof trusses, CFRP was used to enhance durability in a circular direction.
The thickness of CFRP was 0.167 mm, the paving width was 100 mm, and the interval was 400 mm (Figure 22b). (3) CFRP paste strengthened the joints of the beams, purlins, and columns to enhance the integrity of the wood roof truss (Figure 22c).
When the cracks of the other wood components were less than 3 mm, we used putty to apply it tightly; cracks of 3-30 mm were filled with wooden strips and water-resistant glue, and finally strengthened with CFRP.
The wood columns under the roof of the tower were strengthened with CFRP. The tower was much taller than the main building, which was prone to the whiplash effect during an earthquake and required seismic strengthening. We added L-shaped steel on the inner side of the tower wall to form structural columns and ring beams, which improved the integrity and earthquake resistance of the building and weakened the whiplash effect without affecting the appearance of the building [48] (Figure 23). It is worth mentioning that all of the wood components of the building were brushed with preservatives and insect repellant before strengthening; after strengthening, the surface of the wood was painted with high-quality tung oil to prevent moisture, corrosion, and insects.

Update and add Construction Equipment
First, repair the existing drainage and power supply system, including the roof waterproofing and gutters, and replace drains, electrical wiring, and lighting.
Then, according to the needs of the new users, add fire hydrants, lightning conservation belts, and air conditioning systems. Install fire hydrants in corridors, roof lightning conservation belts at the eaves and ridges of the buildings, 20 m × 14 m lightning conservation nets on the roof, strengthen the steel columns as lightning conservation down conductors, and use the building foundation steel bars as grounding devices. Adopt a VRV small central air-conditioning system, and hide the air-conditioning unit in the green belt around the building (Figure 24). It is worth mentioning that all of the wood components of the building were brushed with preservatives and insect repellant before strengthening; after strengthening, the surface of the wood was painted with high-quality tung oil to prevent moisture, corrosion, and insects.

Update and add Construction Equipment
First, repair the existing drainage and power supply system, including the roof waterproofing and gutters, and replace drains, electrical wiring, and lighting.
Then, according to the needs of the new users, add fire hydrants, lightning conservation belts, and air conditioning systems. Install fire hydrants in corridors, roof lightning conservation belts at the eaves and ridges of the buildings, 20 m × 14 m lightning conservation nets on the roof, strengthen the steel columns as lightning conservation down conductors, and use the building foundation steel bars as grounding devices. Adopt a VRV small central air-conditioning system, and hide the air-conditioning unit in the green belt around the building (Figure 24). It is worth mentioning that all of the wood components of the building were brushed with preservatives and insect repellant before strengthening; after strengthening, the surface of the wood was painted with high-quality tung oil to prevent moisture, corrosion, and insects.

Update and add Construction Equipment
First, repair the existing drainage and power supply system, including the roof waterproofing and gutters, and replace drains, electrical wiring, and lighting.
Then, according to the needs of the new users, add fire hydrants, lightning conservation belts, and air conditioning systems. Install fire hydrants in corridors, roof lightning conservation belts at the eaves and ridges of the buildings, 20 m × 14 m lightning conservation nets on the roof, strengthen the steel columns as lightning conservation down conductors, and use the building foundation steel bars as grounding devices. Adopt a VRV small central air-conditioning system, and hide the air-conditioning unit in the green belt around the building (Figure 24).

Conclusions
Church school buildings are important historical achievements of cultural exchanges between China and the West and, globally, are common cultural heritages of mankind. At present, China is facing the difficult problem of rapid urban development and achieving win-win scenarios when it comes to heritage conservation. This paper uses the bell tower, Figure 24. Renewal strategy of construction equipment (source: author).

Conclusions
Church school buildings are important historical achievements of cultural exchanges between China and the West and, globally, are common cultural heritages of mankind. At present, China is facing the difficult problem of rapid urban development and achieving win-win scenarios when it comes to heritage conservation. This paper uses the bell tower, the symbol of modern educational architecture in Nanjing, China, as the research object, and explores the path from an "investigation-value analysis-reconstruction strategy" to construct an architectural heritage conservation strategy that maintains the building's authenticity and integrity. This can serve as a reference for similar renewal and heritage conservation projects.
Authenticity and integrity are the core principles of architectural heritage conservation and are in line with the leading concept of "keeping cultural relics in their original state" advocated by China. The "Guidelines for the Conservation of Cultural Relics and Monuments in China" (2015) pointed out that "the original state of cultural relics" includes not only the state before restoration but also the state after historical restoration and reconstruction. However, in terms of the restoration of architectural heritage in China, sites have been restored to their original states on many occasions over a long period of time, which can easily lead to the disappearance of historical information.
The author believes that the "original state" is the superposition of real information in various historical periods. It requires a reasonable assessment of valuable information. The bell tower was originally a Western-style building with a flat roof (1888), but it was later converted into a building with a sloping roof due to a fire (1917). Its architectural form has been preserved to this day and has high artistic-historical value. Therefore, during this renovation, we do not advocate restoration to the original state, the emphasis is on sorting and restoring the existing information of the building. As the earliest Western-style building in Nanjing, the bell tower is an important historical witness of the cultural exchange between China and the West. The architectural style must be completely preserved, and the exterior walls, roofs, doors, and windows that affect the architectural style must be repaired as they are. The bell tower is an American colonial-style tower that combines ancient Chinese architectural techniques. It is an important architectural example in Nanjing and even in China. It reflects the architectural characteristics under the influences of Chinese and Western cultural exchanges, and its architectural technical characteristics have also been well preserved. Therefore, only through rigorous historical research and analysis of authenticity information can we avoid over-repairing and covering up of the historical information of the building, to preserve the authenticity of the information in different historical stages.
"External protection and internal reconstruction" is an inappropriate repair method, and this case avoids these defects. We keep the exterior of the building complete and minimize the internal changes. Therefore, while the functions are upgraded, the old and new functions are compatible and the original office functions are retained [49]. Adaptive reuse revitalizes architectural heritage, revitalizing it through functional enhancement and equipment renewal. Under the analysis of regional development and campus renewal needs, this paper proposes a reasonable heritage space renewal strategy from the aspects of culture and publicity. Based on the principle of minimal intervention, air conditioning, water supply, drainage, network, fire protection, lightning protection, and other facilities are implanted according to the needs of new functions, and structural strengthening is carried out according to structural inspection. After the renovation of the clock tower, it can better adapt to contemporary life while retaining its original historical appearance, achieving a win-win situation of protection and restoration. The strategy presented in this paper is not the only one. In the future, it will be possible to propose renewal strategies according to new needs to make the architectural heritage sustainable.