Structural Reciprocity: Critical Overview and Promising Research/Design Issues
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Reciprocity is a structural principle that has fascinated designers and builders throughout the world since ancient times. Despite the topic’s having been studied by various academics, designers and researchers, a critical overview of the references is still missing, as is an outline and discussion of the current and future promising research/design issues. Further, no single text provides an exhaustive definition of the principle of structural reciprocity and it must be critically reconstructed from several different sources. This paper aims to fill in these gaps, providing a complete and annotated list of references, in which historical examples, as well as patents, research articles and terminological issues are discussed. A consistent definition of structural reciprocity is also proposed, and the promising developments of such a principle are outlined in order to guide designers and researchers in the future.
KeywordsStructural reciprocity Reciprocal frames (RFs) Nexorades Mutually supported elements (MSE) Leonardo da Vinci Leonardo grids Spatial structures Timber constructions Form-finding Morphogenesis History of construction
The principle of reciprocity is based on the use of load-bearing elements which, supporting one another along their spans and never at the extremities, compose a spatial configuration with no clear structural hierarchy.
Such a system has been used throughout the world since ancient times. However, a comprehensive, annotated list of references regarding them has not been developed yet. Furthermore, since designers and researchers works frequently in independent ways, the current studies still need to be grouped and compared systematically in order to outline the future promising design/research activities. It is an explicit aim of this essay to respond to these needs by proposing a critical overview of the topic supported by: (1) an annotated list of references in both Western and Eastern cultures; (2) a complete lists of patents on reciprocity; (3) a complete list of publications grouped by topic; (4) a complete and annotated list of scientific terms coined and used by different authors; (5) a consistent definition of structural reciprocity; (6) several future research and design directions based on the current state of the art.
Differences and Similarities of a Fragmented History
The first applications of structural reciprocity date back to Neolithic pit dwellings, Eskimo tents and Indian tepees, as documented by Popovic Larsen (2008). However, it was also present during the Roman Empire, when Julius Caesar used it for the construction of a bridge over the Rhine: the structure was made of interlocked timber elements, with the main purpose of simplifying the joints. The project was described in Caesar’s “Commentaries on Gallic and Civil Wars” and was later reconstructed by Palladio (Gros and Beltramini 2003). Apart from these ancient applications, independent from one another, reciprocity has been independently studied and used in both Western and Eastern cultures for centuries. The needs and purposes have been different, but the structural outcomes have been similar. During the last few decades, reciprocity has also become a research topic for academics, who have started to study the mechanical, geometrical and construction aspects of reciprocal structures.
Reciprocity in Western Culture: Timber and Short-Beams
In Europe, structural reciprocity has mainly been used, at least until the twentieth century, to span distances longer than the length of the available timber beams.1
After Leonardo, Sebastiano Serlio discussed the construction of planar floors with short-beams in his first book on architecture, but unfortunately proposed an unbuildable structure (Serlio 1566; Yeomans 1997). John Wallis wrote about different types of floors made of reciprocal elements in his “Opera Mathematica” (Wallis 1695), and this was the first scientific text to be supported with structural calculations, as it has been described in detail by Houlsby (2014).
Amongst the treatises on carpentry, Émy, who was professor of Fortification at the Royal Military School in Saint-Cyr, included some reciprocal examples in his “Traité de l’art de la charpenterie” (Émy 1837). Reciprocity was also present in the so-called ‘flat-vaults’ described by Frézier (1737), but which were invented by Joseph Abeille and Sébastien Truchet. It was finally described by Rondelet (1810) and Tredgold (1837).
Reciprocity in the East: Interwoven Structures and Symbolism
In Eastern culture, interest in reciprocal structures derives from two separate concepts. On the one hand, especially in China, the use of interwoven strips of bamboo for the realization of baskets is an old tradition that has been transferred to objects of larger scale; the so-called “Rainbow Bridge” in Shandong is the main example (Baverel 2000; Rizzuto et al. 2002; Di Carlo 2008). On the other hand, the religious concept of Mandala, with its symbolic ‘magic circle’ shape (Gombrich 1979), has inspired the construction of circular reciprocal roofs in many Buddhist temples all over Asia. Unfortunately, written documents concerning this practice are only found after the year 1275, i.e., when Marco Polo arrived in China at the end of the Song Dynasty.
The Basis of a Scientific Approach
Even though structural reciprocity has not had a linear history, and the evidence of its use seems to be fragmented, a main point in common in both Western and Eastern cultures is the use of timber as a construction material. In Europe, this occurred for functional reasons, with the aim of developing flat configurations. In Asia, designers were mainly guided by spiritual considerations in the construction of 3D structures, which recalled symbolic shapes.
In this historical framework, Leonardo da Vinci was the only one in the West to study the potential of elaborating reciprocal structures as complex 3D geometries (Leonardo da Vinci 2000). It was John Wallis who first approached structural reciprocity scientifically, like a modern research topic (Wallis 1695; Houlsby 2014).
The Years of Patents and Scientific Research
The German engineers Erwin Walle and Sigurd Prinz patented a reinforced concrete ceiling made of prefabricated units hinged together, which curiously recalls the reciprocal slabs designed by Kahn for the Mill Creek public housing project in Philadelphia, dated 1952–1953. The patent was filed in 1971 and then published in three different parts. The building element for the construction of interlocking grids patented by Gat is also worth mentioning. This was actually the first work to be also published as an article in a scientific journal (Gat 1978).3 However, it was the 3D closed circuit of sticks patented by Graham Brown that definitively inspired and kick-started academic research on structural reciprocity. Brown established contact with the University of Nottingham, with the intention of increasing his understanding of the mechanical behaviour of such a system, which he called “Reciprocal Frame”. This led John Chilton’s group to make several publications during the 1990s (Chilton and Choo 1992, 1994; Choo et al. 1994; Chilton et al. Chilton 1995a, b), including the Ph.D. thesis by (Popovic 1996), which has recently been revised and printed as a book (Popovic Larsen 2008). A complete list of patents involving structural reciprocity is given in “Appendix 1”.
Even though John Chilton’s group was the first to use the term ‘Reciprocal Frame’ in a scientific paper (it is here that a definition of the concept of closed circuits of elements is found), other names by other academics and research groups also exist (see “Appendix 2” for further details). Such terminological heterogeneity creates a certain level of confusion that has been avoided, in this text, by adopting the most logical and generic term ‘structural reciprocity’, or simply ‘reciprocity’.
Structural reciprocity is a very extensive research topic, and has been approached from very different points of view. Looking at the current publications, at least fourteen different sub-topics can be defined: (1) morphology and geometry of reciprocal spatial structures, including (2) polyhedrical configurations; (3) form-finding and morphogenesis of such structures with computational tools, recently also combined with (4) the rapid prototyping of timber elements; (5) the design of joints and connections; (6) analysis of the structural behaviour; (7) the study of the kinematic potential; (8) investigations on materials and sections for construction; (9) the use of planar panels; (10) discussion/criticisms of projects and prototypes; (11) teaching structural reciprocity; (12) history of reciprocal structures; (13) art and sculpture, (14) reciprocal structures based on Leonardo’s grids. “Appendix 3” lists the main research works published in each category.
Reciprocity in Contemporary Architecture, Art and Industrial Design
Some experimental pavilions, with the Arup Group’s Advanced Geometry Unit group as the engineering consultant, have also been constructed: the Forest Park by Shigeru Ban (McQuaid 2003), the H-edge pavilion by Cecil Balmond, realized with the students at Penn University (Balmond 2007), and the Serpentine Gallery 2005 by Álvaro Siza (Sakamoto et al. 2008). These examples clearly illustrate that the architectural potential of structural reciprocity still needs to be explored—the use of different materials, as well as the definition of element sections and joints, are some of the possible research/design issues of the next few years.
Characteristics and Interesting Aspects
The term ‘reciprocity’ comes from the Latin reciprocus, a composite of recus, backwards, and procus, forwards. The etymological significance of reciprocity is therefore that of ‘back and forth’, evoking an exchange for mutual benefit. In the world of construction, situations in which structural systems imply an exchange of actions is frequent—the voussoirs of an arch, for instance, achieve equilibrium through mutual action/reaction. However, reciprocity differs from mutuality because a transitive relation between at least two elements occurs only if such a relation is perfectly symmetric.
The concept of structural reciprocity therefore refers to a specific subset of structures, which are characterized by two main properties.
First, in each and every element the functions of supporting and being supported by other elements must be separated instead of being overlapped. For instance, in simply supported beams constraints are placed at the extremities and loads act at the midpoints—the functions correspond to different position on the element and no inversion is possible. In contrast, constraints and loads both occur at the ends of truss bars—so the functions overlap here and the definition of supporting and supported ends becomes merely conventional. This first property also implies that only beams and two-dimensional elements can form a reciprocal structure. The forces are transferred through bending and shear with beams, both in-plane or out-of-plane for 2D components.
Second, each and every element must be supported by the other one it supports. As stated above, a perfectly symmetric relationship is needed to distinguish a reciprocal structure from a simply mutual one.
Reciprocity vs. Hierarchy
According to the second property mentioned above, each and every element of a reciprocal structure must be simultaneously supported by and supporting other components. It must generate, with the repetition of such a scheme, a pattern in which all the members of the system play the same role, without differences in terms of structural behaviour. This situation is clearly in contrast to that of other structures in which the elements have distinct functions depending on their respective positions and relationships. The difference between reciprocity and hierarchy is exemplified by the schemes shown in Fig. 13.
In reciprocal structures, an intuitive understanding of the structural behaviour is intrinsically lost. This was already clear in the seventeenth century, when, for the first time, John Wallis attempted to calculate reciprocal grillages by following the load paths within the structure in order to evaluate the internal forces acting in each element (Wallis 1695; Houlsby 2014). However, the choice of a specific path (i.e., the sequence of elements to use to compute the internal reactions) was totally arbitrary. Recent research by (Kohlhammer and Kotnik 2011) also seems to confirm this.
Reciprocal structures have an intrinsic generative property, which can be considered more as a reproduction of elementary/minimum patterns, or fans, rather than instantiations of higher level abstract schemes. The absence of hierarchy affects the architectural as well as the structural design of such systems. In the Western culture in particular, the presence of a hierarchical logic in composition has always been an integral part of the building experience. Furthermore, many aspects of the mechanical behaviour of a structure, such as redundancy and robustness, are also related to hierarchy.
Form-Finding/Morphogenesis Techniques and Design Approaches
The form-finding of 2D and 3D configurations of reciprocal structures, as well as of those based on regular polyhedra, can be performed through different computational techniques. Illustrative case studies have been provided by Baverel (2004), Popovic Larsen (2008) and Stacchetti (2005).
At present, three main families of such techniques can be identified. The first is known as ‘iterative additions’. Starting from the basic configurations of three or four elements, these techniques work by adding sets of new bars to the elementary frame (Proll et al. 2010). The second is related to the use of optimization strategies, such as genetic algorithms (Baverel et al. 2004) or relaxation (Douthe and Baverel 2009). Such techniques find the frame geometry (the solution) by iteratively reducing a measured value of the geometrical errors (the fitness function) from a set of non-compatible configurations (the tentative solutions). The third models the behaviour of kinematically undetermined configurations in order to define their final shapes.
However, in architecture, the use of physical models is probably still the most diffused way of exploring reciprocal configurations, as designers can intrinsically take advantage of the characteristics of reciprocity. With the aid of computation, instead, a design process can only start from an over-arching reference geometry/shape, a forced ‘top-down’ approach in which reciprocity does not inspire the project but is just adapted to it.
In addition to these problems of structural and geometrical form-finding, a new set of questions and fields of experimentation are emerging from the development of digital fabrication techniques, mostly because the new way in which the components are produced automatically requires a different approach to their design (Proll et al. 2010).
The Use of Timber: Members and Joints
In reciprocal systems made of elongated members, all the elements are beams generally joined by simple overlapping. This naturally relates them to timber construction, in which high flexural strength members are usually available, while the realization of joints is a complex problem. Furthermore, working with raw timbers without the need of iron keys or complex connections made reciprocal construction fast and reliable, as shown by the application to military bridges, such as the one described by Caesar (Gros and Beltramini 2003).
The idea of joining members by overlapping has also a conceptual relevance: the overlaps do not require any specific device, such as bolts and pins which are made of timber or steel, but they influence the shape and the organization of the whole structure. The curvature of a reciprocal bridge made of raw timbers is ruled by the size of the timbers themselves and by the position of overlaps. Hence, there is an intrinsic relationship between members and joints (joints are in fact parts of the members) and the design and detailing is an integral part of the whole conception. In terms of mechanical behaviour, such as for the number of degrees of freedom, the effect of friction and of local deformability, and displacement capability, joints directly contribute to define the global behaviour of the structure.
Static and Kinematical Determination
According to the first property of reciprocity mentioned above, the supported elements of reciprocal systems cannot come into contact with their supports at the vertices. This allows the constraint between the supporting and supported members to be defined in an extensive manner: sliding hinges and prismatic joints become possible substitutes of rotation hinges, and this offers unique kinematic properties to the obtained configurations, which are difficult to predict intuitively.
Preliminary research, carried out through the analysis of the kinematic matrix, has shown how different constraint patterns can lead to unexpected kinematic behaviour (Parigi et al. 2009; Parigi 2011; Parigi and Sassone 2011a). Recent developments have proposed a kinetic system based on the concept of reciprocity, which is called kinetic reciprocal system (KRS). Through a morphogenetic procedure, suitable Kinetic Reciprocal Frames are generated with assigned overall behaviour; the process starts from sets of kinematic parameters that create complex geometries of intersecting curves and lines, and which cannot be predicted directly from the input data (Parigi 2011; Parigi and Sassone 2011b). The latest results of such a research activity led to the development of a form-finding tool which is called the ‘Reciprocalizer’ (Parigi and Kirkegaard 2014; Parigi et al. 2012, 2014). Other conceptually similar research is found in Goto et al. (2011) and Kidokoro and Goto (2011).
Use of Planar Elements
When a reciprocal system is designed with elongated elements (i.e., with a set of components that behave like beams) design efforts are mainly focused on three aspects: (1) the definition of the elementary fans that have to be assembled, (2) the study of their composition possibilities and (3) the selection of the jointing systems. However, as suggested by the rare natural example of the cocolith, planar components of different shapes can also be used to transfer forces in a reciprocal way. The cocolith features circular tiles, but also squares, triangles and more articulated, or irregular, geometries could also be considered.
In order to guide future morphological research activities, we have distinguished five main categories of reciprocal configurations based on planar components. The first type uses planar elements as ‘thick’ elongated elements. This category includes all those reciprocal structures in which the planar elements are approached in the same way as elongated ones. For instance, Werner Blaser designed tables and chairs made of fans, in which planar timber panels were reciprocally interlocked. A few buildings have also been designed with thick linear elements; the Serpentine Gallery 2005 by Álvaro Siza and Cecil Balmond is probably the most relevant of all. A second category considers planar elements as groups of elongated elements. This includes all those reciprocal structures in which the planar elements can be substituted by fans made of sticks. A simple example is that of triangular tiles that replace fans of three linear elements each. The tiles can have three different engagement lengths, whereas only two are possible for elongated elements. A third family includes all those configurations in which the planar elements are part of a truss. A forth category groups the reciprocal systems in which the members can transmit bending moment via the notches, but are assembled differently than in the previous three categories. The fifth and last category collects all the possible remaining configurations (Baverel and Pugnale 2012, 2013).
The research and experiments on reciprocal structures currently underway are the heirs of an ancient tradition in both the East and West. We hope that the attempt at synthesis of the state of the art (definition, terminology, methodologies of form-finding and analysis) will allow those engaged in such research to view and present their efforts in terms of a broad context, overcoming the fragmentation that has prevented reciprocal structures from receiving the attention they deserve, and fostering a fruitful exchange of information, resources and results.
This problem was also solved using an alternative construction technique, based on the overlapping of layers of short load-bearing elements. This technique can be found, for instance, in the roof structure of the Square Hall building in the Old Nisa, Mesopotamia, as described by Pizzetti and Zorgno (1980). It was also proposed by Guarini for his Chapel of the Holy Shroud in Turin, Italy.
A more detailed description of historical works on reciprocity can be found in Popovic Larsen (2008). Instead, a complete list of historical treatises, dealing with reciprocal structure, has been provided by Vito Bertin on his website: http://www.vitobertin.hk/lw/10-reference/02-studies/index.html.
Even though Gat’s system was the first patent also published in a scientific journal, it was not the first article ever to discuss reciprocity. A paper by Donald Dean about a ‘new’ structural system called ‘Lamella Grid’ had in fact already been published in 1964 (Dean 1964).
Vito Bertin proposes an alternative description of how structural reciprocity works mechanically. He states that a reciprocal configuration should present three characteristics: (1) a set of levers that form a static system, (2) a set of elements that mutually support each other and (3) self-connected components, i.e. which behave at the same time as elements, nodes and connectors. Visit Vito Bertin’s website for further details: www.vitobertin.hk.
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