Introduction: Understanding and designing adaptability

Circular construction

Reflecting the harmful effects of the construction industry on the environment, there is high pressure to develop alternative concepts for a more sustainable practice of construction and operation of the built environment. In 2020, the construction and operation of buildings caused 37% of all global CO2 emissions (11.7 gigatons) and 36% of the global energy demand (149 EJ) [1]. The slight reduction compared to 2015 (38% of CO2 emissions) is attributed to the influence of the covid pandemic, and consumption is approaching the previous level again. At the same time, almost one in six dwellings in the EU was unoccupied with significant differences between the individual countries and regions, e.g., 14% unoccupied conventional dwellings in Belgium in 2011 [2]. The apparent demand to build more is often contrasted with the phenomenon of vacant buildings [3]. Of course, these aspects do not always meet locally, but in urban areas, real estate pressures tend to lead to demolition and new construction rather than conversion. The exceptionally high structural vacancy rate of office buildings (e.g., 13% of the office space in the Netherlands in 2009) also has a strong social impact as those buildings are rather located in urban areas where there is often high demand for housing. The long vacancy often leads to functional segregation or even abandonment and further degradation of neighbourhoods [4, 5]. Increasing vacancy rates of buildings or the decreasing lifespans are drastic signs of the necessity of reuse for a more sustainable building culture [6].

While many regulations still focus on energy efficiency or minimisation of harmful construction materials, a growing research focus is laid on the principles of a circular economy (European Green Deal, area 4) [7]. As many life-cycle assessments (LCAs) have shown, the most effective strategy for achieving a sustainable built environment is implementing circular construction practices on different levels. First, on the material level, energy-intensive materials are sought to be reduced [8] using hybrid material construction techniques or digital technologies for material optimisation, or they are replaced by less carbon-intensive substitute materials [9, 10]. At the component level, concepts for reuse study the impact and the organisation of reclaimed building parts, increasingly focusing on standardisation and designs for disassembly (DfD) [11,12,13,14]. The third level is the reuse and refurbishment of existing buildings structures where the general properties and qualities in general and the building layers and their capacity for longer life cycles are studied. Reuse at the building level can often be considered the most desirable as it leaves a large portion of embodied carbon untouched and stored. At the same time, it also minimises the energy effort of modifying, transporting and reprocessing the components. However, the reuse of buildings and their underlying mechanisms and boundaries are largely understudied [15,16,17], as convertibility and adaptability are currently not integrated neither into legal frameworks nor the architectural or technical design process in practice.

Adaptability as an architectural strategy

Current research on adaptability often sees the building as a generic entity and technology as the key to making changes. A large portion of recent discussions was driven by systemic top-down approaches developed by real-estate and construction specialists [18, 19] or focus on monofunctional buildings [20]. They seek to either evaluate a building as an element of the building stock on the market [21] or address technological aspects to assess “flexibility” or “modularity” broadly over the entire building [22,23,24]. Even if designers are recognised as agents through their design actions (“design-based enablers”), technology is identified as the key to flexibility [18]. Valuable surveys with designers have contributed to a better understanding of roadblocks and priorities, but they cannot, by nature, reflect individual or collective design experiences. Within the conversion of a building for its reuse, these design paths are very particular and usually challenging to present in a systematised way. Nevertheless, these cases offer reasonable indications of the limits of repurposing and reuse, be they technical, practical or financial [25].

Researchers are increasingly demanding “quantitative modelling” for a deeper understanding of how functional changes are connected to the building design and its construction [18]. Although it is well understood that buildings always must provide different functional zones with particular requirements, the theoretical modelling is still very generic. It describes buildings as homogenous systems without differentiated zones and their conditions. In this research, however, particular conditions and actual changes are the starting point. Instead of drawing conclusions from generalised assumptions, the question here is how and why buildings are changed. “All buildings are predictions. All predictions are wrong,” concluded Brand in his influential study of how buildings change over time [26]. Beyond technological developments, it is the role of architects in the first place to organise a building for needs that will most certainly change. As we can project tendencies of urban and social transformations, architecture would have to materialise these processes: “The Holy Grail is an architecture that is appropriately flexible, one that recognises modern tendencies for home, work and recreation to overlap.” [27] The processes of change reflect these needs and, at the same time, mirror the technical and functional conditions to which they are subject.

In the last 50 years, change-based research conceptualised design for adaptability (DfA) in architecture very broadly [28,29,30]. In 1976, N. John Habraken detailed why and how structures need to be capable to be repeatedly appropriated by the users [31]. In the 1970s, Bob van Reeth developed the concept of the intelligent ruin, a building that is designed to grow with and adapt to changes over time. Stewart Brand focused on the changing capacity of individual buildings and introduced functional ‘shearing’ layers of different life spans immanent in each building. Bernard Leupen developed the concept of a frame and generic space in 2006. He used a visual technique of exploded axonometric views of the essential building layers ‘structure’, ‘skin’, ‘circulation’, ‘service’ and ‘scenery’ to understand their impact [32]. In 2016 Robert Schmidt and Simon Austin extended Brand’s layers concept with social and urban layers [33]. All these change-oriented models aim for a generic design for flexibility, still mainly without understanding the mechanism and circumstances of change and differences within a building.

Change based design in practice

There is momentum in the architecture and construction practice incorporating circularity (e.g. Rotor in Belgium or Baubüro insitu in Switzerland). In recent issues of architecture magazines, e.g., the Belgian Architecture magazine A + “Structure & Reuse” (286) or “Practices of Change” (287), architects extensively debated the culture of adaptations and the urgent need to understand the process behind it. Next to the already well-established French office Lacaton & Vassal, a few young French offices have been featured in the influential German magazine Arch + (240, 2020) with their work on adaptable architecture. Alex Lifschutz (Lifschutz Davidson Sandilands) has converted many buildings in the UK and promoted loose-fit design for an adaptable architecture [27] together with many influential projects by Squire and Partners. In their debate, architects often refer to the sociologist Richard Sennett’s Building and dwelling, ethics for the city and his plea for “porous” and adaptable buildings as the very fabric of an ever-changing city [34]. However, methods are limited, and designers broadly refer to a lack of systematic understanding. Although international research into circularity has established a sound basis, particularly for the sustainable reuse of components, there is a lack of research into adaptability in building design on an architectural level, differentiated according to parts of a building, its functions and the materials used.

This paper seeks to understand better how buildings have been changed if adapted and what we can learn from them. Following the methodology of looking at functional layers, i.e., their characteristics and flexibility, it will be focused on the load-bearing structure of the building and its construction materials, the functional areas and the circulation around and accessibilities of them. Based on three case studies in Belgium and the Netherlands, converted for new functions, the actual change processes will be traced, modelled and analysed. Changeability will be mapped using planning documents of both the original and the converted building and additional information from individual designers involved in the projects. For this purpose, a new methodology is proposed as well as the central concept of structural porosity. Finally, building on the empirical study of the three cases and their adaptations, crucial factors will be identified that suggest the paths in which a sustainable building design is most likely to produce long-living buildings.

Method: mapping zones of change

Layering permanences

The methodology of this study builds on the work of Stewart Brand [26] and Bernard Leupen [32]. Both have argued for a differentiated view in the study of the adaptability of buildings. They have taken a similar approach towards the mobility and autonomy of functional building layers. A building is typically expected to have a lifespan of several decades, depending on its function and importance. However, this longevity does not apply equally to all parts of the building. Therefore, it has been argued that the lifespan of functional building layers should be studied separately and, consequently, be considered adequately in the overall design process. In the course of the service life, adjustments are made for various reasons, both technically for repairs or retrofitting of technical equipment and functionally for changes in the requirements of new users. The load-bearing structure of the building, on the other hand, is very rarely affected.

Based on Francis Duffy's [35, 36] analysis of the city and office spaces from 1989 and 1992, Brand considered building layers according to their permanence and their location within the building: site, structure, skin, services, space plan, stuff (Fig. 1a). He called these organisational units shearing layers: "Because of the different rates of change of its components, a building is always tearing itself apart." [26] Too often, Brand argues, can buildings not be changed as higher maintenance layers are not accessible or too closely interconnected with other layers and thus cause structural vacancy. Consequently, in an adaptable and thus durable building, the components of these layers should be constructionally and spatially separated as much as possible so that they can change independently of each other. Although Brand also suggests the connection of the building's specific geometrical and technical configuration to its program, he does not consider them in his layer model.

Fig. 1
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Brand’s shear layer diagram (left) [26] and Leupen’s spatial layer diagram (right) [32]

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Leupen takes up the concept of parts featuring different speeds of change. He departs from Brand's technical notion of functional and spatial layers and defines his concept of layers as follows: loadbearing structure, skin, scenery (internal walls and finishes), servant elements (services) and access (circulation) (Fig. 1b). Unlike Brand's onion-like understanding of a system, Leupens' layering is driven by the idea that each layer is potentially a frame that constitutes conditions for the generic space it creates, which is allowed to change. In this way, the layers are much more spatially interlinked and dependent. Both Brand and Leupen have established a differentiated spatial understanding of building layers that refer to independent speeds of change or, in other words, layers of various permanences. However, although these models connect spatial components of a building, they do not reflect the fact that there are differences and hierarchies within these layers or that these do not feature the same characteristics throughout the building. For example, there are various types of stairs and circulation areas in many large buildings that purposefully establish redundant paths. Facades may vary along with their orientation and function, and even structural parts are more permanent than others, e.g., core, walls or columns.

Based on the models of Brand and Leupen, a novel method of mapping permanence and adaptability will be presented in this paper. This mapping will focus on distinct layers of the models mentioned above, namely the load-bearing structure and the circulation. Both layers are not only essential parts of a building, but they also profoundly overlap with the architectural functioning of the space around them and are thus often critically linked with the use thereof. Next to the physically most present layer, the load-bearing structure, the circulation system is a functional space and virtually intangible but rarely subject to changes due to its critical necessity. Both layers are thus dominantly determining the building’s functional capacity for change and are considered the most permanent parts. A new layer to those two is the function spaces that have been established in the initial and the converted building, respectively, i.e. housing, office or production, and retail. Since not only convertibility is essential for adaptation but also the possibility of mixed-use, the extreme case is tested here, i.e., whether and to what extent uses can be combined not only within the building but also within floors. However, the skin is only partly and the services not considered at all in the adaptability study because they do not play a significant role in the modern layout of the buildings studied in terms of changeability.

Porosity and membranes

To discuss how a building can be approached, appropriated and adapted — ideally, from a variety of different users and parties — the building shall also be understood as a permeable structure. This plea follows the concept of Richard Sennett’s discussion of the porosity of a city which connects the permeability of buildings and its role in separating private and public spaces within the city [34]. Sennett reflects the qualities of the Nolli map, describing building blocks as closed or permeable solids, making visible the open boundaries of the built environment. Consequently, the building then also represents a mass dissolved through perforations that, depending on the configuration, allows more or less access, various inner spaces and their connections. The interplay between private and public space becomes a central theme in the building, understanding the corridors as streets and thus as the most permanent zones that mediate access and separation [37].

Like building blocks, street facades or the building volume, walls and slabs of a building structure are not fully closed but also have numerous openings for various reasons, e.g., for supply shafts or doors. But even more than that, these structural surfaces (especially from the first half of the twentieth century) often have an internal substructure as they consist of a skeleton filled with surface elements. As a porous structure, the skeleton, which was preferred mainly for the material economy and the faster and thus cheaper construction process, allowed much better flexibility in later conversions by making openings more quickly and easily. The structurally hierarchical ribbed slabs thus became architectural membranes that allowed for grades of permeability where needed. These internal capacities of change through potential penetrations without compromising the overall integrity of the structure shall be integrated into the discussion of the porous structure and be reflected in the mapping method. The fundamental property here is the spacing of beams within slabs, just as the columns are the larger pattern of the storeys and the building.

Mapping and quantifying use and structural porosity

The mapping aims to determine how the layers of the load carrying structure and circulation are entangled with each other and the functional spaces and how those aspects have changed during the conversion. In general, the diagram should hint at certain immanent constraining conditions of a building which could contribute to a better understanding of the capacity to change. The layers and functions will be related in space in the true scale of the floors, with only the room heights shown as 1.5 times excessive for visibility reasons in the diagrams. The geometrical dimensions of all components are suppressed so that only system lines remain to indicate the structural layout and internal separation walls.

To determine the usability of the spaces in the building structure, next to the horizontal circulation area (C), the contiguous units are determined, here called single Pockets of Use (PoU). To determine a spectrum, the largest (LPoU) and smallest pocket of use (SPoU) are specified along with their share of the total floor area. This variety of continuous, usable areas, which indicates the mixing ratio of the available spaces, is referred to here as the diversity of pockets of use (DPoU). It is given as the size ratio of all pockets per floor in relation to the smallest pocket of the entire building. In this way, a spectrum of available pockets of use becomes apparent within the entire building, and the size grades thus indicate their diversity, and, beyond that, a mixability of uses in the building.

Regarding the structural porosity, on the one hand, the spatial characteristics of the structural grid are considered, such as the floor height and the spans between the columns, with the spatial structural porosity (SSP) here being the ratio of span and floor height, and its higher level of porosity. On the other hand, the characteristics of the porosity of the structural surfaces are understood as the capacity to open walls or slabs. This planar structural porosity (PSP) is the lower level of porosity and looks at both the use of walls within the floor plan that may complicate circulation or changes of use, and the articulation of slabs. Here, the hierarchy of the slab construction is considered between a non-hierarchical flat slab (0) and articulated beam slabs with one (1) or two beam levels (2). The arrangement of these slabs with their beam spacing and the resulting slab thickness makes it rather easy or difficult to open them up to accommodate the building.

Cases: conversions as traces of adaptation modes

Three case studies have been chosen to discuss the adaptation process of converted buildings and demonstrate a novel mapping method. They have been chosen for their different scale, function and qualities as architectural spaces. Moreover, they date from different periods of the twentieth century. Therefore, even though they are all skeleton buildings, they show subtle differences in the scale of the components and the complexity of the construction. It is no coincidence that most of today's large-scale conversions are originally industrial buildings, which in their generosity and robustness allow for many potential reuses, but even in their possibly modernist expression have qualities all their own. Unfortunately, many such buildings are nevertheless demolished, and the following discussion is intended to reveal decisive characteristics of good adaptability.

Belgrade building, Brussels, 1972/2022

The first case already has a dense history of reuse within its 50 years of existence. Built in 1972 by Brussels architecture firm AUSIA, the building was conceived as a compact Cinzano factory. It was then turned into a warehouse with offices in 1995. Embedded in a residential area, the building stands out from its surroundings, mainly because of its brutalist façade. It has a very regular concrete structure consisting of a wide span skeleton and superimposed slabs spanning in both directions. The spans are consistently 8 m with cantilevers varying between 0.5 m and 1.5 m with clear room heights between 3.61 and 4.50 m. Next to the main loadbearing skeleton structure stands a structural core providing stability to the building and allowing vertical circulation of people and services (stairs, elevators, technical shafts). Thanks to the core, the building featured vast open space for the warehouse and offices (Figs. 2 and 3a).

Fig. 2
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Original construction by AUSIA (left) (source: hertiage.brussels) and rehabilitation (project) by Bogdan & Van Broeck (right) (source: Bogdan & Van Broeck)

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Fig. 3
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Diagram showing the loadbearing structure, circulation and functional layout of the original (left) and the converted building (right)

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The municipality decided to develop the empty building as social housing in 2020 (https://www.bogdanvanbroeck.com/projects/belgrade). The architects Bogdan & van Broeck won the design competition, and construction works started in early 2021. They left most of the existing structure but opened the slab at two locations, a larger part of roughly an entire span to create an open garden in the back and a smaller part to create a patio and introduce daylight in the heart of the building. All storeys are used for apartments. Each of them is split up into three or four living units. They are organized around the existing circulation shaft and a corridor and distribution area varying from floor to floor (Fig. 3b).

Mapping the structural porosity diagram, we can see the very regular structure of the building. The façade is not part of the load-bearing structure and can thus be adjusted independently. The skeleton has a generous spacing and allows for a flexible interior space free of any inner columns. The regular beam pattern allowed the partial removal of the slabs. However, as the spans between them are relatively long (8 m), the slabs feature a much greater thickness than the traditional Hennebique system, resulting in beam depths between 45 and 76 cm and the slab between 27 and 47 cm. Opening the ceiling creates large openings because of the beam spacing; therefore, the ceiling has a large porosity that is technically enabled through its thickness and less through the beams.

The generous room height from 3.61 to 4.50 m allows acceptable light conditions for all possible use scenarios of the building’s depth of 17.50 m. As the old use was almost monofunctional, the circulation concept is highly pragmatic by providing access through a core connected to the side of the building leaving all storeys intact as large, uniform and undisturbed spaces. For the new functional scenario of monofunctional housing, the existing circulation system determines the configuration of possible units. There had to be corridors in front of the core from which access to the units is provided. However, the length of the units is then given and cannot be easily varied. Also, technical services, which might be added later, can connect to the units (arriving through the core). However, the primarily small corridors, which remain on the side of the core, allow one large PoU for one tenant.

Although the room height and the floor areas allow for various functions, a mix of them within the same storey is complicated as users of different functions would have to take the same access route. In addition, the circulation area would have to increase (along the building axis) to allow for smaller units of use. However, this relatively long-term arrangement would be in conflict with the flexible space plan and the possible rearrangement of uses. Therefore, a second vertical circulation route would be very valuable, ideally placed on the opposite side of the building, to allow for a more extensive diversity of circulation.

Figure 4 gives an overview of the key figures of the building structure regarding its porosity and usability. While the small corridor on the two upper floors creates a continuous useable area, the largest PoU in the entire building, the access areas on the two lower floors separate the useable area into small PoU. The Belgrade building has a rough porosity of its structure, providing wide open spaces and equally wide-open areas within the slab. However, the low hierarchy of the slab, together with its massive thickness, renders it relatively rigid for changes.

Fig. 4
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Porosity chart for the converted Belgrade building in Brussels

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Building Anton, Eindhoven, 1929/2014

Designed as a factory building for Philips, Building Anton was constructed in 1927 by de Broekert (architect) and Bouten (engineer). The seven-story building has a regular reinforced concrete skeleton structure with a grid of 7,5 by 7,5 m, featuring two internal rows of columns and creating long wide and open construction halls (Fig. 5a). At the backside of the building, three cores organize the vertical circulation of people and services and provide stability to the building. Next to the most outer cores, technical areas were set up on each floor (Fig. 6a). However, in 1990 the industrial activities in the factory stopped, and the building became vacant.

Fig. 5
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Original production hall (left) and conversion to lofts by Diederendirrix (right) (source: Diederendirrix)

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Fig. 6
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Diagram showing the loadbearing structure, circulation and functional layout of the original (top) and the converted building (bottom)

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In 2014 Dutch architecture firm Diederendirrix converted the building with a broad mix of functions creating an internal metropolitan area. Since the building is protected as a volume with its outside appearance, changes were limited to the inner part. Five large elliptical voids were introduced to connect the new program vertically and add some daylight, penetrating the original floors but leaving the beams intact. These voids form the new main vertical circulation and are connected by a central corridor on each floor (Fig. 6b). The new apartment areas only consist of loft units inviting the users to use the large room height with built-in mezzanines (Fig. 5b). The entire structural skeleton remained untouched, and future changes could make use of it again. Diederendirrix strengthened the structure of the building to allow for a later addition of two extra top floors in the future (https://www.diederendirrix.nl/nl/projecten/gebouw-anton).

Mapping the structural porosity diagram for the converted building, the repetitive structure of the building becomes clear (Fig. 6). Here, the façade is part of the load-bearing structure and protected, so it is a given outer boundary. The columns have a generous spacing of 7.5 m allowing for a flexible interior space. In the Anton building, the skeletal structure is even more hierarchical. There are large main beams in the longitudinal direction of the building with 1 m in depth and above them, transversely, smaller beams of 40 cm depth, contributing to the ribbed slab. Therefore, the slab itself is only 8 cm. This, together with the regular beam pattern, allowed the partial removal of the slabs. These openings are limited to the vertical circulation areas. The architects left all existing beams intact so that those changes could be turned back or adapted differently.

In relation to the depth of the building, but in particular to the column grid that indicates a use layer, the clear room height is very generous and allows good lighting for all uses. Like the Belgrade building, the maximum continuous, usable space is made possible by placing the circulation cores to the side. No special corridors were provided in the old factory use as people went from the access zone directly into the production rooms. Now, the cores are preceded by distribution areas, and a narrow corridor connects them longitudinally through the entire building, slightly off-centre so that two usage zones of different widths are created. This cutting of the large usable area is necessary to make the entire building accessible, as an exterior corridor is not possible due to the protected façade. This division creates two smaller PoU between the distribution zones in front of the cores and a very large one on the opposite side. By linking all vertical accesses through the longitudinal corridor, there are always several access options for all parts of the building. The newly inserted internal vertical shafts further strengthen the high DoC. In this way, it is possible to allow a large mix of uses, even within one floor. Service installations can also be inserted and replaced at any time along with the cores and corridors.

Figure 7 shows the key figures of Anton building’s structure regarding its porosity. On each floor, two small and a large PoU make a moderate DPoU, but the redundant access to the areas allows excellent usability in differently sized units. In addition, the room heights, different use depths and sizes, and a high DoC allow for excellent mixability of uses in the building. With the generous column grid and the room height, the SSP is very good, and due to the lack of load-bearing interior walls and the fine gradations of the beams and the thin slab, the PSP is also very good.

Fig. 7
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Porosity chart for the converted Anton building in Eindhoven

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Lamot brewery, Mechelen,1922/2005

Founded in 1855, the Lamot Brewery has a long history of adaptations. The brewery is located in the historic centre of Mechelen, next to the river Dijle. In 1922 two main brew towers and a warehouse were added. All three structures have a reinforced concrete skeleton, with a load-bearing firebrick façade which also stabilises the building. The structural skeleton of the brew towers features small spans and a layout particular to their function. The warehouse has larger spans, creating wide open areas (Figs. 8 and 9a). The brewery was extensively adapted during the following seventy years, with three significant interventions, each adding and subtracting new structures.

Fig. 8
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The vacant ensemble of structures in early 2000 (left) (source: 51N4E) and rehabilitation by 51N4E icw Architektenkoöperatief (right) (source: Mirte Wouters)

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Fig. 9
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Diagram showing the loadbearing structure, circulation and functional layout of the original use (left) and the re-use (right)

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After being vacant for ten years, the building was converted in 2005 by 51N4E architects collaborating with Architektenkoöperatief. A new program was introduced with a combination of cultural and commercial activities. Some smaller structures were demolished and gave way to a new central circulation area, connecting the different volumes and spaces. The first floor of this new part functions as a large foyer, separating the commercial and cultural functions. To visually open up this floor of the building towards the city, the firebrick façade was demolished and replaced by large glass panels exposing the building's interior. The brew towers host smaller functional spaces, such as a café, a restaurant and offices, while the warehouse contains larger auditoria and exhibition spaces. To accommodate these larger functions, the three top floors were demolished and replaced by two new levels, introducing column-free spaces and greater room heights (Fig. 9b).

As this research mainly focuses on adapting the long-lasting existing structure, the new part with the auditoria is left out in the following analysis and the diagrams (indicated with a dashed volume in Fig. 9b at the front right). Figure 9 shows the change in structure, circulation and use. The skeleton in each part of the building corresponds to its load-intensive use and space requirements. The spans of the dense grid in the towers are only 5 m, while it is between 6 and 18 m in the warehouse. In the dense grid, there are deep beams (43 cm) above each column and regular smaller beams (23 cm) above those as part of a ribbed slab. Thanks to the massive beams and the dense column-beam arrangement, the slab is only 14 cm deep. The slabs have not been removed in these parts, but they could, similar to the Anton building. In the upper storeys of the towers, the structure features wider spans with columns standing on the structure below and holding slabs above without beams whatsoever.

The original buildings provided a linear circulation system. Between the various sub-buildings, all adjacent floors were reached via the vertical circulation. From there, one passed through all areas of the storeys. This corresponded to the mono-functional use of each building and floor. In the converted building, additional corridors were inserted between the old sub-buildings and the new auditorium building, which run along its entire depth. In this way, the usable areas of the respective floors can be divided and made accessible individually. Similarly, the technical supply ducts can also run through the corridors and can be accessed at any time. This circulation method is therefore very good for mediating between sub-buildings and making them accessible effectively. The SPoU and LPoU then always correspond to the entire floor areas of the respective sub-building. Because of the central vertical circulation, the route to and from the areas of use is always predetermined, i.e., the DoC is low. Thus, within the floors of a partial building, uses cannot be highly mixed, as they are always connected via the same corridor. An additional vertical circulation point would be beneficial to increase the DoC.

Figure 10 summarizes the essential porosity figures of the converted Lamot brewery. With the various sub-buildings, a moderate DPoU is provided. However, the variety of clear room heights is extraordinary, but because the structural configuration varies significantly in the sub-buildings, so does the SSP. It is very good in the old warehouse and the new storeys, as they allow flexible use and changes, e.g., partly additional mezzanines can be included. The ceilings of the new floors have a poor PSP as flat slabs, while in the old floors below, it is not good because of the massive and densely arranged beams in the slab.

Fig. 10
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Porosity chart for the converted Lamot factory in Mechelen

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Conclusion

In this paper, a new methodology is presented based on the models of Stewart Brand and Bernhard Leupen to describe the adaptability of buildings as a function of their structure. Apart from the structure, the modelling approach also includes the circulation system and the specific areas of use. In this discussion of structurally determined usability, the concept of diversity of pockets of use is introduced regarding the size ratios of the continuous, usable areas. In further evaluating the mixability of uses, the concept of diversity of circulation is introduced, reflecting the necessary redundancy of circulation for different neighbouring uses. Finally, for the discussion of structurally enabled adaptability, the concept of structural porosity is introduced. On a higher level, it describes the proportions of the spatial grid and, on a lower level, the dissolvability of the slabs or walls.

The method was applied to three specific case studies, the Belgrade building in Brussels, the Anton building in Eindhoven and the converted Lamot Factory in Mechelen. All three buildings have proven to be successfully convertible buildings. Not only are they structurally robust and still usable after up to 90 years later, but they are also convertible within their structural substance. The vertical circulation is placed adjacent to the building volume in all three cases, thus enabling very good and flexible utilisation options. In the buildings, different variants of horizontal circulation are set up through differently positioned corridors, each of which creates very distinct qualities and flexibilities of use. Central to this is the realisation that the diversity of the circulation requires at least two different vertical circulation systems, which must also be regularly interconnected. This principle is most effective in the Anton building. There, but also in the Belgrade building, the Anton building shows that a dissolved slab structure with beams and a superimposed slab makes it easier to provide subsequent slab openings, which can be essential for later vertical combinations of uses. The delicate beam system of the Anton building, therefore, seems to be the more flexible structure. Together with the spatiality (room height), which allows good lighting and possibly additional denser utilisation through mezzanines, it proves to be a helpful porosity criterion for good appropriability and convertibility.

The method has proven helpful in understanding the dependence of the change of use on a given structural configuration. It helps to name the essential aspects, objectify them and make substantially different buildings comparable. In the following research step, these key aspects will be reasonably quantified so that their contribution to adaptability can be understood more refined and tendencies would become better visible. In another step, the actual professional planners behind the conversions will be involved using interviews to better understand the given limits and possibilities of the structure. Furthermore, the actual used building processes and material systems of both the existing and the converted building will also be included to verify and broaden the insights developed. In the long term, these principles can be incorporated into a design framework for new or further converted buildings.