The skyline as a subject of protection of a historical settlement (on the example of St. Petersburg)

Introduction

At the present stage of development of historical cities, the urban environment is recognized as an object of urban heritage and a resource of socio-economic development ^[1]. In the structure of the historical urban landscape, the silhouette serves as a factor in the organization of development, diversity and originality of the architectural image, a source of visual information about the layout, buildings, relief and landscape, as well as about the urban composition as a whole. In the theory of architecture and urban planning, the development of the urban space silhouette category is associated with the study of such aspects of urban composition as the spatial-planning framework of vertical dominants, the regularities of the height ratios of vertical dominants and background buildings, the interaction of relief and landscape with buildings. In the works of G. Mertens, A.V. Bunin ^[2], N. N. Baranov ^[3], the main techniques of high-altitude interaction of background buildings and vertical dominants are investigated, the works of K. Zitte ^[4], A.V. Ikonnikova ^[5], M. G. Barkhina ^[6], I.A. Bondarenko ^[7] and other authors reveal aspects of spatial patterns of silhouette organization of buildings. The publications of E. L. Belyaeva ^[8], K. Lynch ^[9], A.N. Fedorov ^[10], A.V. Stepanov ^[11], Yu.A. Vedenin ^[12] are devoted to the problems of visual and cognitive perception of the silhouette.

With the development of the protection of architectural and urban heritage, the historical silhouette of urban space is considered as a quality of historical and cultural value. The analysis of the value characteristics of the historical urban landscape is devoted to the works of S.K. Regame, A. A. Nikiforov ^[13], E. A. Shevchenko^[14], A.V. Slabukhi ^[15], A. A. Emelyanova ^[16], S. A. Belskaya ^[17], S. V. Zelenova ^[18] and other authors. The issues of preserving the silhouette in the context of the reconstruction of the historical environment are given attention in the works of Y. L. Kosenkova, A.V. Makhrovskaya, B. V. Nikolashchenko, N. T. Vinokurova, S. A. Lamkina. 

The silhouette of the building of the historical center of St. Petersburg is one of the most striking features of its architectural appearance and genius loci. The history of the formation of silhouette expressiveness of St. Petersburg is reflected in the general works on the evolution of the layout and development of the city by V. G. Lisovsky ^[19], S. V. Sementsov ^[20], O. N. Zakharov ^[21]. Within the framework of the conducted research, the issues of the formation of vertical dominants of St. Petersburg were studied, patterns of their placement in space were revealed, a classification of architectural dominants and types of their visual perception was proposed. In the works of A. G. Weitens ^[22], O. M. Kormiltseva ^[23], the experience of urban planning regulation of St. Petersburg in the XIX-XX centuries is studied. 

At the present stage of protection of the historical and urban environment, the problem of preserving the historical silhouette of urban development is quite acute. In the conditions of active growth of cities, accompanied by the inclusion of new high-rise buildings in the existing fabric, the historical silhouette structure is transformed, the integrity and artistic expressiveness of the silhouette image are devalued.

The problem of preserving the silhouette of historical city centers has acquired national significance both in the Russian Federation and abroad. In the international practice of urban heritage protection, special strategies for the protection of urban landscape types are included in urban planning documentation, 3D modeling methods are used to determine the permissible height parameters of urban environment development. In 2012, the Russian Federal legislation introduced the requirement of protection within the borders of historical cities with the status of a historical settlement, valuable environmental characteristics (subject of protection), including, along with buildings and structures, the composition and silhouette of buildings [Federal Law No. 73-FZ of 25.06.2002]. The introduced innovations, however, are not supported by methodological recommendations for determining the composition of the component of the subject of protection, as well as methods of its preservation within the boundaries of a historical settlement.

Noting the significance of the results of previously performed studies, it should be said that the problems of introducing the concept of building silhouette into the system of urban heritage protection have not been sufficiently reflected. At the present stage of studying the historical environment and searching for new approaches to the protection of urban heritage, the development of the concept of a building silhouette as an object of protection is of strategic importance. Accumulation of existing knowledge on the development of the spatial composition of the building of a historical city, taking into account the traditions of urban regulation and practical experience can become the basis of an integrated approach to the protection of the silhouette originality of the building, taking into account the value features of the silhouette and the factors influencing its perception.

Methodology and methods of research

The study was based on a systematic analysis of data from previous studies, a historical and urban planning analysis of the spatial composition of the historical center of St. Petersburg, which allows us to identify patterns of changes in spatial and height relationships between background buildings and vertical dominants, a comparative analysis of legislation in the field of cultural heritage protection, a graph-analytical method for assessing the silhouette structure, as well as a value analysis of urban planning objects based on the criteria for assessing historical urban landscapes adopted in domestic and international practice for the protection of urban heritage.

The main part

The formation of ideas about the value of the silhouette correlates with the development of issues of protection of the historical environment as an object of urban heritage at the end of the XIX century. The appeal to the value of the historical environment in the works of K. Zitte served as an impulse to realize the need to protect not only individual monuments, but all elements of the environment ^[4]. At the present stage of development of the protection of urban heritage, the value approach is implemented in the system of protection of historical cities. In international practice, an integrated approach to the preservation and sustainable development of the historical environment of cities is being actively developed – "historical urban landscape" ^[24]. Within the framework of this approach, the objects of urban planning protection are not individual monuments, but a set of elements of the historical urban landscape in their interaction.

Modern researchers include the composition and silhouette of the building, along with the layout and spatial structure, as part of the urban value recognized as a key criterion for assessing the urban environment, however, the main value indicators of the silhouette are the characteristics of views and panoramas, the ratio of lost and preserved dominants and accents, the significance of panoramas of historical buildings in the appearance of the settlement, inclusion in the structure of panoramas historical and cultural monuments of federal significance ^[13]. No special silhouette assessment systems have been developed.

The silhouette of the building in the system of protection of the historical environment

The practical experience of regulating urban planning activities in the aspect of protecting the silhouette organization of the development of European and Russian cities testifies to the established tradition of protecting the silhouette through high-altitude regulation in the perception zones of protected urban views and panoramas. In a number of foreign countries, in order to protect the silhouette, special documents on the protection of species are being developed, including criteria for the assessment and selection of species, measures for monitoring protected species ^[25-26]. The altitude restrictions established in the visual perception zones of the species adapt to the topography of the terrain and differentiate for the territories of the near-plan (in front of the object of observation) and the far-plan (behind the object of observation).

The system of heritage protection includes atlases containing, along with urban planning restrictions, standard solutions for the placement of vertical dominants, the organization of specific corridors and sites aimed at forming general ideas about permissible interventions in the prevailing historical environment ^[27].

An innovative method of protecting the visual integrity of the historical environment is the construction of a spatial 3D model of urban development and the calculation of the maximum permissible height that does not adversely affect the perception of protected species – the "visual surface protection method" (VPS) ^[28]. Modeling of the restriction surface allows us to illustrate the influence of the parameters of new inclusions located outside the center on the spatial composition of protected species. 3D modeling methods can serve as a tool for assessing the structure of visual perception of both the existing silhouette and the projected one, taking into account the inclusion of new buildings and structures.

In the national legislation, the protection of the historical silhouette of urban development is not provided with a special regulatory framework. The silhouette of individual objects of cultural heritage is protected as part of the subject of protection of these objects, however, this approach does not provide protection of the silhouette line of integral fragments of the historical environment.

A tool for the protection of the historical and natural environment of cultural heritage objects is security zoning. The zones of protection of cultural heritage objects can cover significant territories in the case of combining the zones of protection of several monuments (single protection zones). The system of restrictions on urban development activities in protected areas includes special land use regimes and requirements, including the preservation of compositional and visual connections of cultural heritage objects in their historical and natural environment ^[29]. The listed requirements aimed at the protection of cultural heritage objects cannot fully ensure the preservation of the silhouette originality of the building in the interaction of various architectural forms and scales of vertical dominants with background buildings.

The experience of security zoning on the territory of St. Petersburg reflects the search for methods of protecting the silhouette of buildings within the boundaries of integral sections of the historical environment. Since the 1960s, the practice of establishing uniform height restrictions for groups of blocks has been implemented on the territory of the united protection zones ^[23]. The massive reconstruction of the urban environment of the historical center of Leningrad in the 1970s and 1980s, accompanied by the superstructure of background building objects, was carried out taking into account the features of silhouette organization and visual connections of objects, which contributed to the maintenance and continuous development of silhouette characteristics of historical buildings. An important stage in the development of security measures for the silhouette organization of buildings was the study of the visibility zones of architectural dominants under the leadership of N. T. Vinokurova in the 1980s. The distances of potential influence on the perception of various types of dominants were determined on the basis of instrumental calculations and were 10-25 km for vertical dominants with a height of 70 to 120 m, 10-15 km for a height of 40 to 70 m, and 6 km for dominants with a height of less than 40 m ^[30]. The proposed method can be qualified as a tool for determining the height parameters of buildings that do not have a negative impact on the perception of vertical dominants.

The current high-rise regulation of development within the boundaries of the united zones of protection of cultural heritage objects on the territory of St. Petersburg, fixed by the regional law [St. Petersburg Law No. 820-7 of 19.01.2009] provides for the differentiation of height restrictions for historically established central districts and peripheral (suburban) districts of St. Petersburg, but does not take into account the depth of visibility zones as separate dominants, and integral silhouette compositions. The "belts" of protection zones with stricter altitude restrictions on the right-bank and left-bank parts of the city cover the territory to a depth of up to 3 km relative to the historical core, as they move away to the peripheral areas up to 4-4.5 km, the restrictions on altitude parameters are weakened. Along with the height restrictions within the boundaries of the protection zones, general requirements for the preservation of the silhouette of the building and the main visual directions for architectural dominants are established, however, these norms are general in nature and are not detailed either in the system of height regulation or in the content of special requirements for subzone sections. At the same time, land use regimes within the boundaries of protection zones allow changing the elevation and roof configuration of historical buildings. The listed set of measures, therefore, does not fully ensure the preservation of the existing variety of vertical dominant and background building height ratios, as well as the protection of visibility zones not only of individual vertical dominants, but also of integral silhouette compositions from inclusions of high-rise construction at a depth of more than 5 km from the historical center of St. Petersburg.

Along with the security zoning, a new tool for the integrated protection of the historical and urban environment is being developed in the domestic legislation - the protected status "historical settlement". The composition of the subject of protection of a historical settlement includes historically valuable city–forming objects - buildings and structures, planning structure, including its elements, spatial structure, composition and silhouette of buildings, the ratio between different urban spaces (free, built-up, landscaped), compositional and species connections (panoramas), the ratio of natural and man-made environment ^{[31, v.59]}. Thus, a historical settlement can be correlated with a territorial object of cultural heritage, while the subject of protection of a historical settlement is the historical environment in the totality of valuable elements and parameters, including the silhouette and composition of the building. This method of protecting the integrity of the historical environment seems to be the most effective tool for preserving the historical silhouette of the building, ensuring the protection of individual valuable features and silhouette parameters of different sections of the urban landscape.

In regulatory documents, the silhouette of a building relative to the subject of protection of a historical settlement is revealed through the ratio of vertical and horizontal dominants and accents, compositional axes and connections with other parts of the city, the relationship between historical buildings and urban planning dominants, the surrounding natural and anthropogenic landscape, large-scale, high-rise and silhouette ratios of various types of buildings ^[31-32].

The implementation of the concept of silhouette protection within the boundaries of a historical settlement requires the definition of the subject of protection, that is, the identification of representative silhouette compositions from the general silhouette structure, as well as the development of requirements for land use regimes and urban planning regulations in the form of special restrictions aimed at preserving silhouette compositions assigned to the subject of protection.

Determination of the composition of the subject of protection of the historical settlement "composition and silhouette of the building"

Terms and concepts related to the vertical composition of the city – silhouette, sky line, architectural and urban dominant - are included in the national standard in the field of preservation of cultural heritage [GOST R 59124-2020. Item 3]. In the theory of urban composition, the "silhouette of the city" is understood as the projection of a space organized by means of architecture onto a vertical plane ^[2]. In relation to a historical city, the concept of "historical silhouette" is used, denoting a silhouette of a building that has developed to a certain historical period and has historical value. For the purposes of this study, it seems necessary to refer to the concept of "silhouette composition", the content of which, in relation to urban development, is limited to more general categories of urban planning and spatial composition.Thus, "urban composition" means architectural-the order of spatial coherence of parts of the city, which determines its artistic integrity ^[33]. The content of the concept of "three-dimensional composition" is much narrower, since it does not include the artistic aspect ^[34]. Based on a comparative analysis of the conceptual apparatus, the author's definition of the concept of silhouette composition is proposed – a visually perceived outline of combined architectural volumes and surfaces against the sky, which has a special urban planning and architectural and artistic significance.

The identification of silhouette compositions should be based on the analysis of the silhouette structure from the point of view of the laws of the silhouette organization of the building, the formation of the typology of silhouette compositions, the determination of their properties and means of achieving artistic expressiveness. The category of silhouette composition is considered on the examples of the development of the central urban space of St. Petersburg that had developed by the 1950s.

The types of silhouette compositions of the historical city can be determined based on the analysis of the formation of the silhouette structure and the conditions of perception. By the beginning of the XX century . within the central urban space of St. Petersburg, there are two main spatial types of silhouette compositions of buildings: frontal and deep-spatial axial. Despite the instability of the frame of the vertical dominants due to the construction of new and massive losses in the Soviet period, the transformation of the background building by increasing its height and density, the main types of silhouette compositions of the historical core have not undergone significant changes during the more than 200-year period of urban development. The factors of positive influence on the ordering of the silhouette structure were the strict regulation of placement in the areas of background development and its height parameters.

The formation of a frontal front building with the placement of vertical dominants along the main planning axes, the width of the Neva riverbed and the low relief of coastal areas predetermined the formation of a tradition of panoramic perception with a system of platforms and perception paths along the embankments. The prevailing conditions for the perception of the silhouette organization of the Nevsky panoramas provide an optimal depth of visibility (400-700 m) and the width of the view openings. The placement of bridges across the Neva River at a distance of 1200-1300 m within the boundaries of the main urban space contributes to the allocation of spatial links in the structure of the Neva panoramas with a horizontal angle of perception in the range from 800 to 120 degrees, as well as points of perception of an elevated level. 

The silhouette structure of the building of St. Petersburg, formed on the territory of the historical core by the middle of the XX century, provides extensive material for the study of the basic properties of silhouette compositions of various types - the architectural form and scale of vertical dominants, height and spatial relationships of elements, patterns of rhythmic organization. The silhouette of the building of the historical city unites vertical dominants of various stylistics and scale of architectural forms, from simple geometric to complex curved, from large-scale citywide dominants to local accents. Despite the wide variety of architectural forms in the silhouette of St. Petersburg, which developed by the early 1910s, the silhouette expressiveness of the main urban space is determined by a laconic combination of multi-scale domes and spires opposed to the extended horizontal ridges and cornices of the background building, while the range of sound of such combinations is extremely wide - from dynamic contrasts to subtle nuance. The traditions of silhouette organization were successively developed in the period of the 1930s-1950s.

Spatial and height ratios of building elements are of key importance for the perception of the silhouette structure. High–rise dominants of citywide significance, being the focal points of frontal silhouette compositions, are concentrated along the main planning axes - the embankments of the Neva and its branches, highways of citywide significance.

Silhouette compositions of the frontal type are characterized by a sufficiently large variability of spatial and height ratios of elements, at the same time, it is possible to distinguish stable types of height ratios, indicating the maintenance of silhouette schemes of development of coastal territories within the boundaries of the central urban space of St. Petersburg for more than two centuries.

In the structure of deep-spatial axial silhouette structures, spatial-altitude patterns have a narrower range. The main spatial idea – the placement of the main dominant along the central axis is supported by the order of the building along the path of perception, as well as various techniques for identifying the dominant vertical.

Based on the analysis of the main properties of the silhouette organization of the building, the criteria for the value of silhouette compositions are determined. The system of evaluation criteria includes urban planning value, historical, architectural and artistic and intangible. These criteria are correlated with the main methods of assessing the historical environment of a number of scientists ^[13-18]. The approbation of the assessment based on the analysis of the key silhouette compositions of St. Petersburg showed the possibility of identifying integral compositions from the silhouette structure of the city that have special significance for the space of the historical city and are subject to protection. The study of silhouette compositions using the grapho-analytical method allowed us to identify factors of negative impact on the value of silhouette compositions, the most significant of which are changes in the architectural form of high-rise dominants, the composition of the frame of vertical dominants, deformation of spatial and height ratios of dominants and background buildings (Fig.1).

As a result of the analysis of the silhouette of buildings along the embankments of the central core of St. Petersburg, silhouette compositions of citywide significance were revealed. Embankment development complexes are limited by elements of the planning framework – streets, squares, river channels, bridges. The town-planning value of silhouette compositions is determined by the location and significance in the structure of the planning framework of the historical core of the city. The development along the embankments of the Neva River – the main planning and compositional axis of the central urban space – has a special urban planning significance. The architectural design of the "ceremonial" embankments" has been given special attention since the initial periods of its construction. Mirror The Neva River contributes to the unification into a single urban-planning ensemble of the development complexes of the right and left banks, as well as the ensemble of the arrow of Vasilievsky Island and the preservation of the conditions for the perception of panoramas. Architectural dominants on coastal territories included in the silhouette compositions of embankment buildings are planning nodes that tighten the directions of streets and accentuate the intersections of planning axes.

The architectural and artistic value of the building silhouette is determined by the quality of architectural solutions of buildings and structures included in the boundaries of silhouette compositions. Almost all buildings in the structure of the embankment front have a special architectural and artistic value, have the status of historical and cultural monuments. The construction of the Neva embankments representatively reflects the features of architectural solutions and techniques of silhouette organization of buildings and structures of different stylistic trends of the XVIII–early XX centuries, illustrates the large-scale subordination and stylistic consistency of the objects of background development, the expressiveness of the outlines of vertical dominants and accents. In the structure of the existing silhouette compositions, historical vertical dominants and accents, engineering accent structures, green spaces, elements dissonant to historical buildings and negatively affecting the perception of silhouette compositions are highlighted.

The historical value of the silhouette composition of the Nevsky panoramas development within the historical core of St. Petersburg is determined by an extremely high degree of authenticity of the historical outline of the weddings of buildings and structures of various typologies and stylistics. Inclusions in the street fronts of embankment buildings of late periods (the second half of the XX-XXI centuries) are isolated (Fig. 1).

Fig. 1. Grapho-analytical method of silhouette structure analysis (on the example of Petrovskaya and Pirogovskaya embankment development in St. Petersburg). A – silhouette organization of the building, B - architectural and artistic value, C – historical value.

The role of the silhouette in maintaining the intangible value of the architectural landscape of the historical city is extremely high. The silhouette line, revealing the outlines of the city against the background of the firmament, is realized by the observer as the boundary of the space of material objects and the sphere of poetic images. In works of art, silhouette compositions are an integral quality of the image of the city, a background for the life of literary heroes, an illustration of the processes of urban development. The perception of the silhouette of St. Petersburg is carried out through the prism of the works of A.S. Pushkin, F.M. Dostoevsky, M.V. Dobuzhinsky, A.P. Ostroumova-Lebedeva, associated with the poetic tradition of the XIX century and the literary works of A. Akhmatova, I. Brodsky, "the magic of white nights".

Silhouette compositions with special urban planning, architectural and artistic, historical and intangible value can be included in the subject of protection of the historical settlement of St. Petersburg. In order to identify and localize this subject of protection, it is proposed to establish the boundaries of the subject of protection "composition and silhouette of the building", indicating the territory of especially valuable silhouette compositions of the historical settlement (Fig. 2).

Figure 2. Outline of the boundaries of the territory of silhouette compositions within the central urban space of St. Petersburg

Recommendations for the protection of the silhouette within the boundaries of the historical settlement

The urban environment is a continuously evolving and changing object in time and space. The development of the environment presupposes the preservation of some stable, relatively unchangeable elements that ensure continuity in the development of the whole. The stability of the silhouette organization of development within the central urban space was ensured by strict regulation and the continuous development of historical principles of silhouette formation for more than two hundred years.

The value analysis of the silhouette structure of the central urban space of St. Petersburg allows us to draw conclusions about the significant influence on the perception of the silhouette organization of the historical development of late buildings and structures, dissonant to the established silhouette by the nature of the architectural forms of the crowning parts, height parameters, spatial solution. A negative impact on the perception of the silhouette of buildings in the structure of Nevsky panoramas is caused by buildings that significantly exceed the height parameters of the background building and stand out with a significant length (plate buildings), structures with a chaotic configuration of roof lines and cornices / or the absence of crowning cornices, the presence of vertical accents of low architectural and artistic quality. Such inclusions into the structure of both medium and long-range plans contribute to the distortion of the historical configuration of the silhouette line of the embankment development, a decrease in the value of the visual image of the historical urban landscape, and the loss of the cultural identity of the space. 

Integral silhouette compositions of river panoramas are sensitive to high-rise and large-scale construction on coastal territories outside the boundaries of the historical center. Complexes of multi-storey buildings erected at considerable distances from the historical center intrude into the structure of perception of panoramas of the Neva River water area due to bends of the riverbed (bend), violating the established artistic and spatial relationships of the silhouette line.

Taking into account the results of studies of the depth of visual perception of silhouette compositions, as well as the visibility zones of architectural dominants, the boundaries of the silhouette preservation zones (ZSS) are determined, including the following subzones:

– the silhouette perception zone (zone A) is the territory of the near, middle and far plans of silhouette compositions, viewed when perceived to a depth of 1500-1700 m and characterized by clear visibility of the general outline of buildings and structures, facade divisions;

– zone of compositional and species influence (zone B) - the territory of the far–plan of the existing silhouette compositions of the development of the central urban space, when perceived from the main viewpoints and perception paths to a depth of up to 6000 m. The depth of zone B is due to the potential impact of high-rise building parameters of more than 30 m on the perception of valuable silhouette compositions;

– the zone of extreme visibility (zone B) – the territory of the far plan of the protected silhouette compositions of the development of the central urban space, viewed when perceived from the main viewpoints and ways of perception of high-rise elements (height more than 100 m) to a depth of 10,000 m (Table 1. Fig.3-4).

Table 1.

Composition of silhouette preservation zones (ZSS)

Figure 3. Silhouette preservation Zones (ZSS). A is the zone of perception of the silhouette, A-1 is the territory of the established silhouette compositions of the development of the central urban space, B is the zone of compositional and specific influence of the established ones, C is the zone of extreme visibility

Figure 4. The zone of preservation of the silhouette composition of the right bank of the Neva River from the Exchange Bridge to the Trinity Bridge. A is a sub–zone of perception of silhouette composition, B is a sub–zone of compositional-specific influence of vertical dominants, C is a sub–zone of extreme visibility of vertical dominants

Restrictions on urban development activities in order to preserve and maintain the value of the building silhouette within the boundaries of the listed subzones include requirements for the reconstruction of existing buildings and the construction of new facilities, the placement of high-rise dominants, high-rise regulation of development. The introduction of such restrictions, differentiated depending on the purpose of the zone, will avoid a negative impact on the perception of valuable silhouette compositions of new construction and reconstruction of existing buildings, will contribute to the continuous development of silhouette expressiveness of the building.

Height regulation

Within the framework of this study, the principles of establishing the height restrictions of the building, ensuring the preservation of valuable characteristics of the silhouette of the building, are proposed. Based on the need to preserve the historical outline of the silhouette, it is advisable to establish a ban on increasing the height within the zone of clear visibility (zone A), limiting the maximum height of new construction and reconstruction parameters in the zone of compositional and specific influence of the main urban dominants (zone B), and the zone of maximum visibility of silhouette compositions of the main urban space (zone B). Within the area of potential formation/reorganization of the silhouette, it is recommended to establish height restrictions in accordance with existing objects in the structure of the street front, while observing the height ratios of the background building to architectural dominants of no more than 1:3 (Table 2).

Table 2.

Height regulation of buildings within the boundaries of silhouette preservation zones

Requirements for the silhouette organization of development

The chaotic lines of the crowning cornices and roof ridges, acquired due to the inclusion of new buildings and structures in silhouette compositions, or the superstructure of existing buildings with a change in the configuration of roofs, have a negative impact on the valuable characteristics and parameters of the silhouette. 

The artistic quality of the contour line of the building, perceived against the sky, is one of the key criteria for the value of the silhouette of the building. The tasks of maintaining the artistic integrity of the outline of the building can be solved by establishing the requirements for preserving the historical architectural solution of the crowning parts of the building, including the configuration of the crowning cornice, the configuration of the roof and decorative completions from the front facades located in the area of the established silhouette (A-1) and establishing the requirements for reproducing traditional types of silhouette organization of the building in the area of formation / reorganization of the silhouette (A-2).

In the Zone of compositional and specific influence of the main urban dominants (Zone B), it is advisable to adopt the requirements for historical buildings in terms of silhouette preservation by analogy with the restrictions in the silhouette preservation zone, the requirements for the architectural solution of the crowning parts of new capital construction projects should include the predominance of horizontals and the correspondence of the appearance of decorative elements/volumes protruding above the roofs to historical types of vertical accents, characteristic of the historical development of the central districts of St. Petersburg (Table 3).

Table 3.

Requirements for the silhouette organization of buildings in silhouette preservation zones

Conclusions

The silhouette of historical buildings is of particular importance in the perception of the urban landscape, being a carrier of information about the historical features of the natural landscape, planning, buildings and cultural traditions. In the modern system of protection of architectural and urban heritage, the silhouette and composition of the building is recognized as a valuable element of the historical and urban environment, subject to state protection along with other valuable elements and components of the urban landscape as part of the subject of protection of the historical settlement.

As a result of the analysis of the formation of the silhouette structure of the central core of St. Petersburg from the 1703-1950s, the regularities of the development of the silhouette of the building of the central urban space were determined, a system of criteria for evaluating the silhouette was proposed, the building sites characterized by a high degree of preservation of the historical silhouette organization – silhouette compositions subject to special protection were identified. Silhouette compositions of buildings that have a special value in terms of a set of parameters are recommended for inclusion in the subject of protection of the historical settlement "Composition and silhouette of buildings".

Based on the data obtained on the value parameters and features of the silhouette of the building of St. Petersburg, a silhouette protection mechanism has been developed, including the definition of the boundaries of the silhouette preservation zone, the introduction of restrictions on urban development within the boundaries of these zones aimed at preserving, maintaining the value and continuity of the silhouette of the building of the historical city.