Building Type

The building type refers to the industrial heritage in the form of a building, which generally includes the heritage building and the movable cultural relics, such as industrial equipment within it. This type of industrial heritage generally covers an area of several hundred square metres to several thousand square metres and is not necessarily less valuable than the plant-type industrial heritage, but is less likely to be reused, as it occupies less physical space.

The distribution of the kernel density of building-type industrial heritage in Tianjin is shown in Figure 11a, which shows that this type of industrial heritage is mainly concentrated in the six districts of the downtown area; there is very little in other areas. Statistics on the number of building-type industrial heritage site and the number of renovation and reuse sites of the same kind in all districts of Tianjin are shown in Figure 11b. Heping and Hebei Districts have the highest total numbers of building-type industrial heritage sites and the highest number of reuse sites; the other districts have lower total numbers of heritage sites and fewer reuses.

In summary, the current state of renovation of architectural industrial heritage in the Heping and Hebei districts is good, and there is still a certain amount of unimproved industrial heritage, so there is also greater potential for future adaptive reuse.

#### Structure Type

The structure type refers to the presence of the industrial heritage site in the form of a structure. This type of industrial heritage includes linear railways, bridges and smaller structures, such as wharves, water towers and chimneys. The former is more limited in the way they can be reused due to their overly narrow footprint and functional limitations (e.g., industrial tourist train lines), and the latter also become less likely to be reused than the plant type and building type due to their size and structure.

The kernel density distribution of structure-type industrial heritage in Tianjin is shown in Figure 12a, which shows that this type of industrial heritage is mainly concentrated in Binhai District; there is less of it in other areas. Statistics on the number of structure-type sites in each district of Tianjin are shown in Figure 12b. There are 13 items in Binhai District, accounting for 62% of the total. Moreover, there are no cases of reuse of the structure-type industrial heritage in Tianjin, and the current status of reuse is poor. Maintaining its existing

function as a "living landscape" of industrial heritage along the Haihe River is a good way to keep it active.

#### **4. Discussion**

In general, there are three distinct clusters of industrial heritage in Tianjin, but there is clear heterogeneity in the spatial distribution of the sites from different periods. From 1860 to 1978, the industrial development of Tianjin was divided into two periods, with the "Third Line Construction" in the 1960s as the nodal point. Between 1860 and 1957, the construction of early modern military industries by the "westernization group", the construction of private capital industries, the planning and construction of the "nine country concession" and the industrial construction of the Hebei New District all objectively contributed to the development of early modern industry in Tianjin [28] and thus established the spatial pattern of three clusters of modern industrial heritage in Tianjin along the important transport routes. After the founding of the People's Republic of China in 1949, Tianjin experienced a short recovery period (1950–1957) and lost its status as the industrial centre of the north in the context of Soviet aid [50,51], after the start of the "Third Line Construction" in 1964, in response to the demand for decentralisation and concealment of the country's defence [52,53], Tianjin's industries began to expand outward from the three clusters. The spatial distribution of Tianjin's industries showed a change of "gathering first, then scattering". On the whole, the centre of gravity of the distribution of Tianjin's industrial heritage shifted dramatically. The overall distribution range was relatively stable between 1914 and 1957, concentrated along the Haihe River and gradually expanded after 1958.

According to the buffer-zone analysis, Tianjin's industrial heritage is clearly distributed along the main transport routes including the Haihe River, the Jinpu Railway and the Jingfeng Railway. There are 86 items of industrial heritage within 1 km of the main trunk, accounting for 77.5% of the total. Recently, the concepts of "heritage corridors" and "cultural routes" proposed by the US and Europe have become new directions in the field of cultural heritage conservation, and large-scale, cross-regional linear cultural heritage is receiving more and more attention [2,54–57]. The spatial and temporal distribution characteristics of Tianjin's industrial heritage are compatible with the concept of linear industrial heritage, and the construction of the "Tianjin Industrial Heritage Route" will provide new ideas for the overall conservation of Tianjin's industrial heritage on a regional scale.

Like other regions in China, Tianjin has only a few industrial properties located in the heritage conservation system. While more attention has been paid to Tianjin's industrial heritage in recent years, adaptive reuse of the heritage is perhaps a more practical approach than its alarming rate of loss [58]. The resource types to which industrial heritage belongs and its distribution are important conditions for its successful reuse. Based on the above analysis, the three types of industrial heritage are ranked in descending order of reuse potential: plant type, building type and structure type. In terms of their spatial distribution, the plant type and building type are mainly concentrated in the six inner districts of the downtown, which is in the leading position in terms of economic development and population in Tianjin, and therefore, has a greater potential for reuse. The structure type is mainly located in the Binhai District, and given its own resource type characteristics, this type of industrial heritage has a low use value in reuse and can be explored as a way to transform the industrial "living landscape". A holistic conservation strategy for the heritage corridor is also important for the reuse of this type of industrial heritage.

Tianjin's industrial heritage, as a witness to the city's development, should be given due protection in the context of urban development. In the operation of cities, where governments and investors prioritise urban prosperity, while heritage conservation units and research institutes prioritise the preservation of architectural monuments, finding the link between the two is a key part of heritage conservation [59]. Industrial heritage sites are an important part of the city, and their conservation cannot be separated from the urban context in which they are located but should pay more attention to their interaction with the city [60]. From 2008 to 2021, 17 cities in China have joined the UN Creative Cities Network [61], and the reuse of industrial heritage in Tianjin to support the construction of Tianjin's creative city may become the connecting point between the conservation of industrial heritage and urban development in Tianjin.

The lack of knowledge of industrial heritage by the author's team at the time led to the absence of information on industrial equipment heritage in that particular census of Tianjin's industrial heritage. The value of industrial equipment heritage as a material carrier of the history of industrial-related technological development and social development has now been widely recognized [32,62,63]. Therefore, no specific discussion was presented in this study for the time being, and the team will make up for it in future studies. In addition, this study does not provide an in-depth discussion and scientific classification of the conservation status of industrial heritage, which is a direction for future work.
