Tectonics has been seen as a fundamental third pole in architectural technology in, as the visual communication of creating buildings by linking the construction processes with structural form [19]. It becomes an often unconscious yet integral component of a designer’s vocabulary, a creative approach that can transcend mechanical and rational conditions of the technology involved [14]. Beyond the designer, tectonics can also show an “organisation of physical things which allows the user … to have some sense that this building has been ordered in a way that has some meaning” (David Chipperfield. Along these dimensions, this paper attempts to highlight and balance some associated underlying concepts in the field of conservation: firstly, how such an important component in the vocabulary of designers informs their role in conservation; secondly the value of this communication for the users, i.e. how they can appreciate historical fabric by connecting with more familiar forms from contemporary architecture.

Beyond the creative stage, a stronger engagement of designers with the tectonics of their project can enhance the educational dimension of architecture, by bringing the user a sense of confidence with the designer, but also curiosity and engagement with the technology. This dimension mirrors and opens a window to the education the designers have gone through themselves. Architecture training is often compared to learning a language, which according to theorists like Deplazes [7] has vocabulary, grammar (elements) and syntax (structure).

When working with existing fabric, a designer will have to align its architectural language into that of the concept of a design project through the step of construction, from assembly to tectonics. And in the case of ruined historic fabric, these original languages are fragmented or belong to many fragments, remnants of their own different architectural language(s), that designers have to agglomerate their observed technical and structural disruption in a conservation project.

These connections depend on the recording and reading of the figurative and material characteristics of the fabric, the only legitimate witnesses of the creative processes that happened [10]. Nowadays, the increasing variety of the types and levels of significance of heritage (to include public housing, office or industrial buildings), have challenged the rigour of such building analysis approaches, since the priorities are often to keep the buildings in use and safe from aggressive treatments. The extent but also nature of this range when seen in the context of contemporary architectural design theories has promoted the concept of change management, recognising conservation as part of the continuous transformations the building undergoes in its constant interaction with its environment [10].

This concept and new ranges of heritage make even the precise knowledge of the original architecture less imperative. Eventually, they are all reasons of placing less importance to the original artistic unity and making any attempts in recreating it to become meaningless at an intervention, pursuing a physical instead of aesthetic wholeness [4, 17]. Creation of a new wholeness, which can be highlighted by tectonics, is inevitable in cases of high fragmentation like ruins. Their reconstitution as architecture would rather spring from a critical act that lets them (as a work of art) indicate their intervention criteria [2]. Tectonics can offer an essential visual communication of this continuum of change and critical evaluations by developing an intervention project around the processes that took place in the selection of the remnants, the consolidation techniques, essential new parts added to make an arch work as such again for example etc. and linking them with the final structural form. This deeper engagement with the essence of the transformations that have taken place may also reduce the risk of future arbitrary interpretations by the client or the user, especially those driven by narrow acts, like controversial political narratives or development priorities [18].

In a further step in the potential of tectonic communication, when the creative intent of the original designer has been lost altogether, the focus of interpretation may shift from the "artist" to the "respondent", i.e. the user [17]. This may facilitate the reconciliation of the meaning of conservation to wider groups and cultures, an important contemporary debate, and this includes even emerging cultures that show a distinctly less or challenging degree of care than the traditional groups and professions [9]. Such alternative and innovative designs will need to contextualise the cultural values of the site and communicate their appraisal in ways that engage the users’ interpretation, so tectonics of the design can shift the emphasis.

Dealing with the tectonic, consciously or not, is fundamental in design, explored as Kenneth Frampton stated [11] “in the revealed ligaments of the construction and in the way in which the syntactical form of the structure explicitly resists the action of gravity”. This paper starts exploring conservation tectonics firstly in how they affect form: essential completions that restore the design essence (and often confer structural stability) but also interventions that enter a dialogue with the original fabric, like load-bearing walls (interweaving in the layering of the cross section), frames (complementing or enhancing their fundamental hierarchy) or vaults (interpreting the network of ribs in their continuity with the pier shafts, the ashlar blocks of the webs or the thrust resisting strategies) and their tectonics. Moreover, an emphasis on the structure brings a sense of comfort when delicate fabric is safely underpinned or additions have to find an equilibrium with the existing.

Deplazes [7] identifies modern attributes of tectonics, and the ways they can interpret historical character and enhance its restoration are used in this work to explore tectonic concepts in conservation. Regarding structural form, these can be the expression of a tectonic, well-subdivided, hierarchy (often in frames); the monolithic vs layered construction processes in mass load-bearing types; or the choice between a geometrical vs organic form. Assembly becomes an important over-arching attribute, reminding of the original definition of Tectonics as the art of connections (or “joining rigid, linear parts”) by Carl Bötticher in early nineteenth century.

It is more obvious though for the material and how it informs its construction system to take central stage. Modern tectonic attributes that can be explored [7] are: masonry is characterised either by massiveness or lightness; timber frames express clearly the performance objectives of its constituent elements (frame, sheathing layers, water-proofing, insulation, external skin); steel is the ultimately logical material; concrete is the regulator of stiffness across the entire building. Modern, non historical materials like the increasingly popular mass timber systems combine many of these attributes and is interesting to see how they are changing the focus and expression in design, supporting more the sustainability agenda that can highlight better the relevant messages in conservation. Finally, other modern techniques like 3D laser scan and printing bring a potential of faithful reproduction of essential missing parts, and can re-frame the debate about authenticity in the material or the structural form.

Structural form

Completions of missing fabric in partial or large ruins usually have an educational aim or attempt to restore the design essence of important phases, while in some cases they are deemed as fundamental (and compatible) solutions to guarantee structural stability of the fabric (see [21] for a wider range of this discussion). Where any remaining tectonic values are detected, their reproduction is a reasonable initial reaction. An example is the modern restoration of classical Greek temples, the buildings that inspired Bötticher to coin the term Tectonic. The completion of many of the unstable structures in the monuments of the Acropolis in Athens (Fig. 1) aimed among others to improve their seismic safety. While the original structural hierarchy was restored, the interfaces with the necessary new pieces of Pentelic marble can be found to distract the appreciation of the historical beam or column connections and the precision in the ashlar joints, the key aspects of the original tectonics [19]. This aspect may tone down with the long term darkening of the marbles.

Fig. 1
figure 1

The hierarchical beam frame of the coffered roof in the Propylaia of the Acropolis in Athens (photo taken in 2008)

Different options occur in other hierarchical frames, where essentially the tectonics of the original joints can be kept intact: in early 20C buildings the steel frames are meant to be concealed for durability reasons and they remain so even after their encasement is repaired (De la Warr Pavilion, Reliance Building). Timber frames in Chinese and Japanese sacred architecture though are exposed and they are part of the spiritual experience of the sites – they are constantly and ritually renovated, guaranteeing their assembly is “always ancient, always new”.

The sizing of any new members in modernist frames may also have a Tectonic dimension. The original analysis can often be reliable, well documented and complemented today by a sizing process of similar principles. A technical issue then is how far the new distribution of the loads due to the introduction of portions or beams with higher stiffness can be controlled and the critical area may be the joints, more than the joining beams. The stiffness of the original joints and its effect on the analysis of the performance can be difficult to assess, as a study of timber roof trusses has shown [13], so this uncertainty should be acknowledged or even bypassed by the choice of new joints and members. This goes beyond necessary prosthetics in decayed joints of roof trusses, like for example the new timber roof over the vaults of the San Agustin church in Valencia [15].

The dialogue with the original tectonics gets more complex to resolve in load-bearing walls [21]. It is not usually difficult to reproduce the bond of the original cross section when substantial repairs in specific areas are needed (wallheads, window surrounds). This may extend to partial reconstructions to restore the original perception of space and its scale. The good practice is to key additions in the original fabric, which aligns with Gottfried Semper’s observation of masonry as a product of an “original” weaving technique. The need for distinction (and authenticity) in the phases though can be served by keeping the massiveness of masonry and challenging its weaving and stereotomy tectonics, as was explored in the completions of the bastions in the Castle of Baena, where an open-jointed, almost English bond was adopted for the new limestone stonework.

The distinction is sharp and discontinuous if lightweight, independent structures are added in such completions. These are often reversible efforts to reproduce the scale of the original space, like the timber frame over the Roman republican ruins in Domus Fregellae in central Italy, or shelters to fragile fabric [21]. Scale and interfaces are very crucial here, with many unsuccessful examples: the shed over the mosaics in the Roman villa in Veranes, Spain bypasses any relationships with the ruined fabric by locating an elegant yet massive steel frame on its outer face (Fig. 2). This is an awkward solution of an interface that has always intrigued architects. Franco Minissi’s steel frames over another Roman villa, in Piazza Armerina, Sicily (main phase 1963–67) reproduced the original volumes in a lightweight manner, but treated the wallheads as part of the visitors itinerary and the frames were located on the wall, close to their external edges [22]. The lightweight brick walls of the upward extension that created the Columba Museum in Cologne shows the limits of a tectonic solution to this interface—the two fabrics are structurally compatible and align volumetrically, but the new and extensive brickwork challenges all the tectonic traits of the original fabric. Interpretating the original architectural experience without committing a fake is the common point in these projects, but the busy and distinct steelwork of the first two detract the visitor from any sort of a coherent spatial experience.

Fig. 2
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The massive shelter over the ruins of the Roman villa in Veranes in N. Spain

Koldinghus Castle offers an interesting resolution of this relationship across the cross section of the wall. The aim of the project (1972–94) was to protect the ruins with a simple enclosing structure and leave them untouched [12]. The originally proposed extent of the reconstruction stirred a debate and eventually a compromise was reached, to give the exterior a general form similar to the condition before the catastrophic fire of 1808 and exhibit the ruins at the interior. The new wall and roof structure was clad in timber shingles and simply met the remains underneath in a sympathetic texture but without any structural continuity (Fig. 3). This fundamental interface for the project eventually conditioned other parts of the building by requiring a new structure that had to be free-standing. The glulam columns of this structure reinterpret the motif of engaged Gothic pillars, making a notion to earlier phases of the castle, and generate a brand-new set of tectonics that indeed stems from a clear disassociation at the interface with the fabric. This fine-tuning of the interface problem requires skills as it is structurally demanding – a bolder but maybe less demanding treatment is often lightweight steel glazing behind ruined walls. This is the example of the restoration in the W wing of Moritzburg Castle (2008) to create spaces for exhibitions in an area of rich historic windows, formed at the end of a folded plane that characterised the new roof unifying the original fabric and the extension planned by Nieto & Sobejano.

Fig. 3
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Koldinghus Castle, the interior showing the relationship of the new timber shingles envelope with the original fabric, and how is held by the very expressive independent glulam structure (photo by Szilas 2018,,_interior_2018-07-26-3.jpg)

Moving to another historical form, the interpretation of the network of ribs in their continuity with the pier shafts is an expression of tectonics in solid vaults, as also the elaborate often ashlar masonry for the webs and the thrust resisting strategies and their interfaces. Especially in Gothic, these build upon construction techniques of various complexity, like generation processes from geometric design, the resulting stereotomy of the ribs (and often the shells) and force-channelling strategies. Tectonics drove for example the original design of the spectacular net vaults of late medieval times in England and Germany, or elaborate intersections of corridors or staircases with vaults and domes in early baroque architecture in Spain, Paris or Rome. The original elaborate skills in stereotomy and stone carving are still necessary nowadays for their conservation but are in growing shortage or poorly adapted from modern stone technology. Moreover, very different tectonics characterise current shell technology, like communicating continuity and fluidity, rather than assembly, the outcome of exploring concrete and its fibre-reinforced derivatives in sprayed mixes on an innovative range of formworks (e.g. Darwin Centre at the Natural History Museum in London, Centro Commerciale di Chiasso).

The digital design and fabrication environment can provide a deeper understanding and even prototyping of areas of vaults where stone blocks need replacement, supporting techniques accessible to stone-masons. An example from scientific research is the groin vault fabricated for the SEBESMOVA3D project, which had to be tested in seismic action and be rapidly reconstructed between tests (Fig. 4). The vault was assembled by hollow voussoirs made in a 3D printer by one of the partners and were then filled with mortar to simulate the inertia of the block [20]. More complex physical explorations on the geometric generation process have been made through the extensive work of the group on cross vaults led by JC Palacios in ETSA Madrid [3].

Fig. 4
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The groin vault tested by the SEBESMOVA3D project made of hollow, 3D printed voussoirs

Digital documentation has the potential to integrate the above and combine them with 3D laser scanning, which can capture an extremely objective and rich view of the geometry of a monument. This enabled the virtual exploration of the original form of the vaults from the Vila Vella church in Dénia, Spain by identifying and recomposing the scattered remains of the ribs on the park where the church was [16]. It would be an intriguing exercise to use such methods to re-compose poor reconstructions, like the Trinity Aisle as was relocated off the Royal Mile in Edinburgh in late nineteenth century.

Such an environment can lead to the exact fabrication in stone of missing components or re-composition of existing ones, and can be communicated to the public as a quite spectacular operation, beyond technical efficiency, equally becoming a teaching tool for operational knowledge of history among Architecture students. Such techniques nowadays can be combined with CNC machinery to fabricate complex or precise ashlar blocks and their composition in vaults or facing of walls can be technically accurate and compatible. This on the other hand can challenge some authenticity reservations about the degree modern interventions can disturb too much the order of the original construction.

Material systems/ fabric

Moving the focus on the tectonics of the original material systems, their conservation can also explore the modern attributes discussed earlier [7]. Structural timber, to start with, can express the assembly process and performance of its constituent elements to a variety of forces, ideally axial tension or compression in lightweight frames and trusses. Preserving the integrity of the connections is paramount as the key witness of the tectonic attitudes of the past. A challenging case was the reconstruction of the major roof truss in Reims Cathedral by H. Deneux (1919–38) driven by the need for alternatives to timber, which was diverted to housing construction in N France after the destruction of the Great War. The genious solution was to adapt contemporary prefabricated concrete trusses techniques by connecting the bars in patented variations of mortice-and-tenon joints. This eventually resulted in celebrating the tectonics of the original system by using a completely different one – actually not far from the use concrete was originally conceived for in the 1820s.

The curious case of masonry construction is that of a lack of tectonics [7]: the external face is often decorative, independent from the massive load-bearing fabric behind. Massiveness and lightness can equally characterise such walls, traits that are often juxtaposed for example in the consolidation of Roman ruins that have lost their cladding or render – the remaining concrete or brickwork core is often repaired with bricks of a uniform quality that is more suitable as facing. This was done in a more autonomous design mode at the Neuesmuseum in Berlin where the load-bearing brickwork of the 1850s building exposed by the wartime destruction became the visual bond across the reconstruction project and was echoed in the brick replacement of the front elevations (Fig. 5).

Fig. 5
figure 5

Brickwork enabling architectural continuity across the reintegration of the ruined Neuesmuseum in Berlin (photo taken in 2009)

Tectonics in concrete, another material with historic resonance, refer to an ability to distribute conveniently the important stiffness for the stability and efficiency of the structure, but more often are associated with exhibiting the process that made a fluid material get a hardened and textured consistency (like the seminal work of Luis Kahn in Salk Institute in 1965). These are usually the explorations in exposed, brutalist concrete that characterise the 1950-70 s modernist architecture, in various degrees of durability. The refurbishment and restoration of Denis Lasdun’s National Theatre complex in London in 2015 called for cleaning and repairs of the carefully specified and very durable concrete structure of 1977. Boardmarks and a uniform texture based on medium sized aggregates were characteristics to preserve, while decay from spalling was very localised due to the excellent original construction. Oddly enough, this lack of extensive damage limited the structural effectiveness of a compatible concrete mix for the repairs and a polymer-modified mortar was used instead [8], which meant that no research was necessary to reproduce the texture and strength of the original mix.

A few words can be said into the possibility to expand the tectonics canon in conservation with systems like mass timber which in a way summarise all what we have learnt from the mainstream materials: the natural character of timber and the expression of the performance of its constituents, the rationality of steel, the geometric adaptability and stiffness tuning of concrete. These are valuable attributes that only recently have been used to produce flexible and robust structural solutions, like the bracing to stabilise the slender flying buttresses in Notre Dame, Paris (Fig. 6) since they laid vulnerable due to the lack of balance by the damaged vaults after the 2019 fire.

Fig. 6
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Glulam arches to temporarily stabilise the flying buttresses in Notre Dame (as in December 2019)

This flexibility was explored in spatial forms for reconstructions like the slender engaged pillars in brickwork textures used to suspend the independent new enclosure in Koldinghus (Fig. 3), re-introducing the emphasis of the rib network in the Gothic concordance [19]. This system further enables permanent additions that can be minimal and flexible, like the exposed roof designed to shelter the unearthed ruins of the Oratory of the 40 Martyrs in the Roman Forum (Fig. 7), aiming in particular to protect the frescoes and frame them at the right visual scale [6]. All the carpentry techniques and joints are designed to be visible, and the minimum form of the roof distinguishes its load path in the context of the original brickwork.

Fig. 7
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The interface of the new glulam roof with existing masonry at the Oratorio dei XL Martiri, Rome

Mass timber can eventually generate quite different drivers, expanding on the structural concept from Koldinghus or simply providing more flexible, long-spanning neutral shelters like the glulam arches that protect the remains of the Royal Garrison Church of St George, London [5]. CLT in particular, as a system of stiff panels, currently serves mainly for subdivisions in large historical spaces creating independent, lightweight and reversible units (Gare Maritime in Brussels). It is a logical step to explore CLT for completing missing load-bearing walls in a reversible and distinguishable way where stiffness is sought in distinction to the (authentic) massiveness, but the interface of the cross-sections build-up, layering and performance have to be reconciled. Stone or brick masonry are still preferred when the original tectonics have to be completed or enhanced, and choices like open-jointed sandstone bonds as used in the restoration of the Baena Castle in Spain (2005–15) actually balance the lightness and massiveness that characterise historic masonry [7], and even set the current vocabulary in such interventions. In line with the current concept of framing conservation within the continuous changes in the life of a monument, such a balance may be more effective in reconciling the contradictory characters of obsolescence and memory in ruins, or the temporary and unstable with the permanent and stable [1].

Communicating with the communities

A clearer emphasis on the tectonic in conservation can address another significant, yet less technical dimension that is growing in importance due to heritage funding mechanisms, that of community engagement and associated public consultations, especially in the UK. The concepts and material systems discussed earlier have a dimension of innovation and autonomous, creative thinking that may not always be shared between a designer (architects or engineers who see the global reach of a monument) and communities (with a primarily local focus). The disposition of such communities to challenge very innovative designs was put in test in the proposal for an independent structure in steel as a viewing platform over for the ruins of the Clachtoll broch in NW Scotland. The form and material may have been too urban for an essentially rural community and the proposal of a notional completion of the fragmented original form was not enclosed enough for a comfortable and traditionally acceptable assumption of how the space was used by the prehistoric inhabitants (Fig. 8).

Fig. 8
figure 8

A proposed steel frame observatory for the ruins of Clachtoll, Sutherland

The author’s experience in that project showed how intangible aspects of heritage (lifestyle, interior arrangement) are implicitly (over)valued by lay local communities (see for example the successful construction of a new Iron Age roundhouse in Whithorn) but may be too narrow for monuments rich in character. Projects with innovative yet not very contextualised techniques have to set up educational strategies to guide the design processes over set outcomes, giving opportunities and time for the communities to engage with the tectonics of the project that reveal the creation of form, space and architectural experience. Then modern material systems can start a meaningful engagement with local materials and techniques, as also cultural preferences.

Concluding remarks

A review of a range of tectonic approaches has shown ways where the composition of historical fabric with new architecture (and its fabric) has consolidated or expanded its values by connecting with the creative process that generate forms and use matter. Most of the project reviewed showed that a realisation of the potential of these tectonic traits is a starting point and subsequently can focus on the crucial resolution of the interfaces and then the cross-sections leading to much wider implications for the layout of the structure. New tectonics can be generated eventually from this process and modern materials like mass timber need to mature with design applications towards this direction.

Furthermore, modern attitudes to tectonics can consolidate interventions even more into the contemporary architectural debate – the traditional technical aspects in such projects (stiffness, geometric adaptation, load transfer) can be balanced with current cultural drivers of technology, like climate change, carbon content reduction, digital fabrication etc. Digital fabrication and reconstructions appear to have a strong potential to resolve complex issues of the restoration of spatial and very fragmented vaulted forms, communicating the role and potential of architectural technology. Beyond the designer, tectonics can be a valuable vehicle for stronger engagement of users if used with some degree of humility when communicating contemporary values of interventions to local communities.