Bio-Inspired Materials E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Geisteswissenschaft
- Serie: Frontiers in Biomaterials
- Sprache: Englisch
- Veröffentlichungsjahr: 2019
Nature has provided opportunities for scientists to observe patterns in biomaterials which can be imitated when designing construction materials. Materials designed with natural elements can be robust and environment friendly at the same time. Advances in our understanding of biology and materials science coupled with the extensive observation of nature have stimulated the search for better accommodation/compression of materials and the higher organization/reduction of mechanical stress in man-made structures.
Bio-Inspired Materials is a collection of topics that explore frontiers in 3 sections of bio-inspired design: (i) bionics design, (ii) bio-inspired construction, and (iii) bio-materials. Chapters in each section address the most recent advances in our knowledge about the desired and expected relationship between humans and nature and its use in bio-inspired buildings. Readers will also be introduced to new concepts relevant to bionics, biomimicry, and biomimetics.
Section (i) presents research concepts based on information gained from the direct observation of nature and its applications for human living.
Section (ii) is devoted to ‘artificial construction’ of the Earth. This section addresses issues on geopolymers, materials that resemble the structure of soils and natural rocks; procedures that reduce damage caused by earthquakes in natural construction, the development of products from vegetable resins and construction principles using bamboo.
The last section takes a look into the future towards the improvement of human living conditions.
Bio-Inspired Materials offers readers - having a background in architecture, civil engineering and systems biology - a new perspective about sustainable building which is a key part of addressing the environmental concerns of current times.
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Ähnliche
Frontiers in Biomaterials
(Volume 6)
(Bio-Inspired Materials)
Edited by
Ulisses Targino Bezerra
Federal Institute of Paraíba - IFPB, Civil Construction, Av. Primeiro de Maio, 720, Jaguaribe, zipcode 58.015-435, João Pessoa-PB, Brazil
Heber Sivini Ferreira
Federal University of Paraíba - UFPB, Department of Engineering and Materials Science City University, S/N, Castelo Branco, zipcode 58.051-085, João Pessoa-PB, Brazil
&
Normando Perazzo Barbosa
Federal University of Paraíba - UFPB, Department of Civil Engineering City University, S/N, Castelo Branco, zipcode 58.051-085, João Pessoa-PB, Brazil
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PREFACE
One of the precursors of the current concepts of “bionics”, “biomimetics”, and “biomimicry” was the Italian, Leonardo da Vinci who, while observing the surrounding world, conceived extraordinary contemplating works and imitation of nature. Two examples are worth noting: the conception of a helicopter from a dragonfly and the design of an ornithopter from the birds and the bat.
The history of science has many examples of the use of materials and/or techniques aiming at certain finality and, after some optimizations and developments, the starting idea is deviated towards further ends, creating innovative products, consequently. This kind of change, all natural, seems to result in the consolidation of the three concepts, cited above. Created from the 50s of the 20th century, these three concepts have caused some misconceptions in the scientific community due to the similarity of their definitions. It can be said that biomimetics is the area of science that studies processes of nature and applies them in everyday problems; bionics is the area of science that studies processes of nature and applies them to technology by means of machines and/or equipment with no concern about the sustainability and/or ethics; while the most recent concept, biomimicry, follows the concept of bionics, ethics and sustainability. The bionics is more objective and focused on immediate results, without concerns with the man-ethics or the nature-sustainability relationships, while the biomimicry is more focused on the respect for nature and its imitation with sustainability (which is improbable to achieve, given the challenge to succeed at an entirely sustainable process). This means that the bionics has a broader scope, immediate and of easy reach, than better biomimicry.
This book covers some aspects of these frontiers of science and is divided into 3 sections:
Frontiers in bionics design;Frontiers in bio-inspired construction; andFrontiers in bio-materials.Each section has some manuscripts that aim at addressing the most recent advances of the desired and expected relationship between human and nature. This relationship also includes political connotations, once the following question is valid: which political regime, form of government or production system would be able to live in harmony with nature?
Section (i) presents the subjects that clarify the direct observation of nature and its use under human aspects, as the case of the construction of surfaces with controlled roughness for applications in high-friction conditions, or the case of textures similar to natural ones, or the observation of the mechanism of mosquito stings and the consequent design of a more efficient drill for medical purposes. Ultimately, it cannot be forgotten that the search for better accommodation/compression of materials and the higher organization/reduction of mechanical stress in structures was made by the humans by extensive observation of nature.
Due to the personal character of the authors and their dedication to research in civil construction, Section (ii) is devoted to all who are dedicated to what can be defined as “artificial construction” of the Earth, in other words, the construction performed by the humans without prioritizing the future. This section addresses the issues on geopolymers (inorganic polymers), materials that resemble the structure of soils and natural rocks; procedures that reduce the damage caused by earthquakes in natural construction; development of superplasticizers from vegetable resins and construction principles using bamboo, as a construction material.
The last section embodies all other subjects not yet covered and takes a look into the future towards the improvement of human living conditions. The goal is to make the human species learn more about nature to live harmonically with it. The life cycle of biomaterials is analyzed, using a new concept of maintenance (here “maintenance” is no longer characterized by periodic interventions performed by humans, but becomes something natural, where maintenance mechanisms are capable of self-regulating, since they are conceived through the biomimetic way).
The structure of this book covers areas of knowledge that have received more contributions from new concepts of bionics, biomimicry, and biomimetics. It is expected to achieve the release of this new world view and further: it is expected that humanity awakes from its lethargy regarding aggressions against nature and starts to think more in the biomimetic way.
Finally, this book is devoted to all the professionals and students who still believe, or have come to believe, that the world can become much better.
Let the burden of the future be light!
List of Contributors
Bio-Pulse Oscillations Driven Design of Kinetic Structures
Marios C. Phocas*,Odysseas Kontovourkis,Niki I. GeorgiouAbstract
Natural organisms consist of an integral, multi-layered, finely tuned and differentiated combination of basic components and may act as role models in providing kinematic principles to structures subjected to time-varying external conditions. The term biomimetic design is introduced in the present chapter to refer to the synthetic integration of nature mechanisms considered in the development and composition of kinetic structures. Respective kinematic principles transferred in the design of structures follow performance-based open-loop design processes, made possible through interdisciplinary modes of operation in research and design. Following initial visions of kinetic architecture in achieving structural flexibility and adaptability, respective prototype developments, achieved until recently, follow a hard-mechanical approach that enables the development of deployable and transformable actuated bar structures. Confronted with implicated geometrical limitations, the mechanical complexity and high energy consumption of such systems, a design approach derived from soft mechanics is hereby proposed. So far, few realizations of bending-active elements and hybrid systems with enhanced reversible elastic deformability are based on plant movement principles. Following an overview of related form-finding simulation and analysis methods, further bio-pulse oscillation processes, present in nature, are proposed to extend the biomimetic design spectrum. In a case example, motion principles of jellyfish pulse oscillations have been transferred, scaled-up, and integrated into the design of a high-rise structure.
1. INTRODUCTION
The basis for a physically dynamic architecture that is supported by improved understanding of biological systems, and scale in particular, has been increasingly investigated in recent years. In this respect, three main development directions may be identified [1]: Biomimetics functional morphology (form and function),
biocybernetics, and sensor technology, and robotics and nanobiomimetics. The term ‘biomimetics’ implies adaptation or derivation from biology, embracing the practical use of mechanisms and functions of biological science in architecture and engineering. Research on biomimetics seeks to contribute to the development of operational, effective and energy efficient systems as to varying functional needs and external conditions. In particular, biomimetic design seeks to integrate aspects of nature’s role model mechanisms into the development and design of structures, through performance-based approaches [2]. An important aspect is the effective exchange of research between the disciplines on a methodological level. In this respect, knowledge gained from biological paradigms and processes in biomechanics and functional morphology, through analysis and evaluation, needs to be understood and abstracted in its principles. The derived technology is then implemented through prototyping and testing. Respective nonlinear design developments enable integration supported by technology transfer of related technical aspects to serve, on one side, the holistic design approach and open-loop research-based interdisciplinary design processes, experimentation and advances on the other. Results obtained aim at an architecture able to homogenize the discipline areas concerned, in technical terms for example, those of materiality, structure and form [3].
Advances in embedded computation, material design and kinetics on the technological side, and increasing concerns about sustainability, social and urban changes on the cultural side, provide a background for emerging kinetic architectural solutions, based on initial visions proposed in 1970 by Zuk and Clark for a ‘Kinetic Architecture’ [4]. The exploration of kinematics and responsiveness in architecture relies on changing the properties of structures and materials, in order to create reconfigurations or real transformations of surfaces and systems. Kronenburg argues that for a building to be ‘flexible’, it must be capable of adapting, as a way to better respond to various functions, uses and requirements, transformation, defined as alterations of the shape, volume, form, or appearance, movability and interaction, which applies to both the inside and outside of a building [5]. Engineering precedents, for the development of kinetic structures, include commonly known systems, such as deployable tensegrity and scissor-like systems [6]. These precedents form an important part of the practical knowledge currently available for the development of shape control within our architecture. Nevertheless, in the majority of examples developed and implemented so far, the resultant system reconfigurations are limited between a ‘closed’ and an ‘open’ state, or realized through integration of ‘locking’ techniques and rigid body mechanisms. In cases where real transformability is aimed at, the systems rely on embedded computation and mechanical actuators [7]. Such mechanisms often lead to an energy inefficient and complex kinetic behavior [8].
Flexible motion principles have, thus far, been transferred to architecture following plant movement principles that have been scaled-up and integrated into lighter and less complex bio-inspired kinetic elements. In particular, global flexibility is often achieved through the versatile behavior of locally differentiated regions with special morphological features that act as living hinges and allow for large elastic deformations [9]. In this frame, elastically deformable members may act as primary components in hybrid structures with enhanced capabilities in their kinematics. Planar bending-active members of relatively thin section for instance, prompt controllable elastic deformations, while they preserve geometrical reversibility, thus achieving different configuration transitions [10]. The embedded energy, stored inside the material’s molecular structure, allows bending-active members to compose autonomous kinetic mechanisms, able to undergo geometrical deformation without the empowerment of additional energy. The deformation process can increase the member’s stiffness and establish sufficient global stabilisation of the system. This behavior offers potential new forms of flexibility, adaptability and deformation using the memory effect in structural members [11].
Kinetic structures in general, and the complexity of material-inherent behavior more specifically, necessitate a coordinated series of design, exploration and analysis. In this way, early design investigations are further informed and supplemented with full mechanical descriptions [12]. Complexity is innate; a repercussion of the inextricable relations between material, structural capacity and form. Nevertheless, within each mode of design, the complexity needs to be decoded into manageable components, determining the variable and invariable properties. To do so, each mode of design can be addressed via the distinct definitions of topology, force and materiality. Such definitions establish the character of the system, where integration generates behavior. In this frame, simulation approaches of force-density, dynamic relaxation methods and finite-element analysis (FEA) will be explained in providing a holistic methodology of preliminary topological investigation and detail load-deformation analysis, respectively.
Following this state-of-the-art analysis, the soft-mechanical approach may include aspects of elastic and fluid dynamics, as suggested with respective examples of biological processes of bio-pulse oscillation mechanisms existent in nature. Such mechanisms extend from the human heartbeat for blood circulation, over fluid drops in motion, to the jellyfish motion behavior, and are briefly presented in the chapter. A biomimetic design example, developed at a conceptual level, has been derived from the jellyfish motion principles and is presented in the last section of the chapter. The example refers to the design, simulation and analysis of a high-rise hybrid structure of 250 m height and 25 m diameter [13]. The structure consists of an innovative lightweight load-bearing system and incorporates a kinetic core mechanism that provides through vertical airflow, improvement of the environmental conditions of the building spaces and the surrounding urban areas of high density. The kinetic structure is envisioned to operate as an urban ventilation chimney for air polluted cities and contribute to microclimate improvements.
2. BIOMIMETIC DESIGN
Biomimetics, also synonymous with ‘biomimesis’, ‘biomimicry’, ‘bionics’, ‘biognosis’, ‘biologically inspired design’ etc. implies copying or adapting or deriving from biology [14]. Recently, a definition of biomimetics has been developed [15]: ‘interdisciplinary cooperation of biology and technology or other fields of innovation with the goal of solving practical problems through the abstraction, transfer, and application of knowledge gained from biological systems’. Although this definition primarily highlights the fundamental steps in knowledge transfer, at the same time, the investigation of natural structures provides clarification of their respective function, which may be a result of related forms, processes and interactions [16].The term biomimetic design is then, used to characterize the transfer of structural mechanisms and construction processes in architecture based on principles of biological systems behavior and their technological interpretation. This definition should also include the application of mechanisms from nature in the design of structures in architecture. In this respect, the process of biomimetic design consists of three stages: research, abstraction and application.
Biomimetic design offers a promising perspective within the field of architecture, requiring an instrumental and rigorous approach to the supporting and driving modes of the design process. In this sense, biomimetic design also requires an overarching theoretical framework to ground any relevant observation, experimentation, transfer and application and to give it a direction in relation to the production of kinetic structures, in terms of a performative architecture. Relevant research constitutes a specific field of growing complexity and can be successfully pursued only if it is based on certain discipline interrelations [17]. In this frame, interdisciplinarity, i.e. a group of related disciplines having a set of common purposes, coordinated from a higher purposive level, gains significance throughout the development process. In parallel, digital platforms of operation may support interdisciplinary design processes on the basis of the latest advancements through the introduction of computing facilities, simulations and numerical methods of analysis [18]. Digital design enables designers to collaborate with other disciplines from outside fields, visualize, research and modify structure and system performance with relatively high accuracy. At the same time, this mode of operation requires designers to rethink alternative strategies, in order to establish a robust connective link between disciplines and specializations.
In particular, biomimetics, and by extension biomimetic design, may be developed on a vertical scale. The approach followed by a biology push consists of a first step on carrying out basic biological research in biomechanics and functional morphology. As a second step, new insights are prepared for, and made available to, technology for further processing, i.e. bottom-up approach. Additionally, an alternative strategy may be followed by a technology pull, through the investigation of possible biological model solutions for specific technical problems, i.e. top-down approach. The latter approach enables the development of bionically inspired products in shorter time, whereas the former approach has the potential to yield higher innovation [19]. Furthermore, a stochastic investigation utilizes large databases of phenomena from nature, to systematically access information delivered by life sciences for questions arising in technological disciplines. The European Space Agency for example, uses a biomimetics database to look for suitable role models from nature. The ordering system for the database is the ‘technology tree’, ordering natural role models in the fields of structures and materials, mechanisms and processes, behavior and control, sensors and communication and generational biomimicry. Still, such collection of data does not directly aim at a technology transfer in architecture. Instead, the interdisciplinary design team is responsible for the evaluation of alternatives and the application of principles in the design process. In respect to the architectural design, a clear linear development of a biomimetic design is often difficult to maintain, since the design and development of solutions in architecture follow a rather nonlinear combination of rational and subjective aspects, which underlie, among others, various social, morphological, aesthetic and technical criteria. Nature role models may connect at different stages of the design process, whereas the latter often contains unidirectional orientation. In this way, biomimetic design may even constitute a creative transformative process.
3. KINEMATICS
The necessity for an architecture that is not static but instead, has the ability to adapt to time changes through systems with embedded kinetic mechanisms was initially proposed [4], while active control concepts envisioned, were directly influenced by respective advances in aerospace and mechanical engineering [20]. In particular, concepts for structural deformation control have been proposed, such as the ‘variable controlled deformation’ method, through the application of stressing tendons within the structure. The control members should be capable of being variably and automatically tensioned to counteract excessive deformations. Such a control mechanism was conceptually applied in five classes: axial, flexural, torsional, instability and vibration and seismic control. Along these lines, the human body may be considered as the most representative example of a dynamically interactive living organism. The engineer Guy Nordenson describes the phenomenon in active kinetic systems as creating a building like a body: a system of bones and muscles and tendons and a brain that knows how to respond [21]. Thus, the structural mechanism, responsible for different geometrical configurations of the lightweight components through folding, sliding, expanding and transforming in size and shape, among others, and the control system, which directs the structure towards specified transformations, are significant for the kinetic operability of the system. In the so-called hard-mechanical approach, the geometry of the members, the connections’ typology and the boundary conditions comprise critical parameters. They define the kinematics and therefore the transformational possibilities of the system [22].
Kinetic systems have mainly been studied in terms of deployable structures [23]. Τensegrity structures, i.e. self-stressed systems composed of tension and compression members [24], may be transformed from a closed configuration to a predetermined expanded form, in which they are stable and can carry loads [25] by altering the compression or tension of members’ length [26, 27]. Planar and spatial scissor-hinge elements comprise further common types of deployable structures, able to expand horizontally or in both, horizontal and vertical direction [28-30]. While such structural elements need additional stabilizing members, like cables or other locking devices, self-stable structures can be achieved by applying special geometrical configurations through additional inner scissor-like- elements [31-34].
Structural systems, bearing the aforementioned deployable characteristics, have been industrialized but face some particular challenges which do not easily align with the fundamental construction principles normally used in machine design. The hard-mechanical approach prioritizes uniformity, regularity and compatibility over individuality and adaptability. As a result, mechanical devices are usually conceptualized as mono-functional and standardized modules, whose mechanics conform to a grid of orthogonal axes. A mechanical system like this, however, entails many limitations and is difficult to be applied in other than planar and parallel configurations. Here, adaptation can only be achieved at the expense of additional mechanical complexity, which results in heavy and maintenance-intensive structures [35]. An example of a spatially transformable kinetic tensegrity structure is the ‘Kinetic Tower’ [36]. The particular structure forms a realization of the flexible guyed mast vision of Frei Otto [37]. The outrigger system of rhomboid-shaped core units and vertical interconnecting cables provides different spatial bending shapes through integrated dampers on the joints. Related developments, aiming for real form flexibility, identify design strategies of replacing main structural components with actuators. Consequently, depending on the actuators number used and their specific characteristics, the structures’ overall weight and the energy consumption for their kinematic reconfigurations are disadvantageously affected [8].
3.1. Soft-mechanical Approach
Biology is of particular interest, since it provides not only isolated phenomena in the design of structures, but also new technical and methodological strategies. An important characteristic of natural systems is the integral, multi-layered, finely tuned and differentiated combination of basic components that lead to structures with multiple networked functions [38]. Natural structures consist of only a few basic components that are geometrically, physically and chemically differentiated. Softness and body compliance are significant features, often exploited by biological systems, which tend to seek simplicity and show reduced complexity in the interactions with their environment [39]. Consequently, in contrast to the hard-mechanical approach followed thus far by the construction industry, soft-mechanical principles, conceptually derived from the study of natural systems behavior patterns, have shown their potential in succeeding a vast diversity of morphological adaptation, allowing, at the same time, a promising outcome in terms of energy performance, stability, material minimization and aesthetics.
Compared to technical systems, flexible structures in nature replace local hinges by elastic deformations and thus, distribute the acting forces over a wider area where bending takes place. Force is not directly translated into kinetic motion and displacement; instead, it is stored temporarily into the material's molecular structure, i.e. residual force, allowing an incremental distortion of the material. Following this principle, bending-active members carry the imposed loads visually and therefore, reflect directly their load-deformation behavior. During deformation, the residual forces, caused by active bending, increase the residual stresses of the structure and therefore, enable it to be self-stiffening [11]. Within this frame, planar surfaces can form single, or double, curvature surfaces according to the type and magnitude of the force applied.
In elastic structures, rigidity can be increased either by the combination of bending and tension prestress stored within the individual members, or by coupling multiple elements to a hybrid system. In principle, hybrid systems are defined through linkage of different components in parallel and/or in series that are combined to resist forces by developing a specific mechanical behavior due to their different resisting nature [40]. The potential of hybrid systems lies on the synergetic possibilities emanating from exploiting the system disparities: reciprocal compensation of critical stresses, system-transgressing multiple functions of individual components and increase in rigidity through opposite systems deflection [41]. The use of bending principles in hybrid systems enables not only initial complex geometries, and their subsequent stabilization through additional prestress, but also the capability of the members to undergo reversible deformations [42].
Elastic kinematics, as found in complex plant movements, has already acted as role model for the development of new bio-inspired adaptive systems. Out of the broad spectrum of biomimetic products, fiber-reinforced composites represent some of the most successful biomimetic technical applications. The potential of developing fiber-reinforced compound materials is very high because the fiber-matrix structure in plants is comparable to those in technical materials and the complex fiber-matrix structures in plants are organized in at least five hierarchical levels [43, 44], from the molecular scale over to the nanoscale and microscale to the macroscale [45].
Despite not following directly the abstraction of a plant movement, two recent architectural examples of a building envelope design with kinetic elements were inspired and derived from the observation and analysis of natural role models. The first example is the development of the shading system of the biomimetic media façade of the Thematic Pavilion at Expo 2012 in Yeosu, South Korea, which was strongly influenced by the prototype Flectofin, derived, in turn, from the biomimetic principle of the valvular pollination mechanism in the Streliziareginae flowers [35, 46]. In principle, the backbone element of the flower is made of a flat section that is attached perpendicularly to the fin; a thin shell element. The sideways bending of the fin is a failure mode initiated by torsional buckling, when bending of the backbone develops. Consequently, a recurved surface is formed in the fin, which provides the entire system higher stiffness [47]. Following this kinematics principle, the facade elements are made of slightly curved plates of glass-fiber reinforced polymers, supported by two hinged corners at the top and bottom. In the other two corners, a small compressive force is applied by an actuator in the plane of the lamella, which leads to a controlled buckling. Thus, the structural effect, responsible for the members’ kinematics, is initiated by lateral torsional buckling and continues in a non-symmetrical bending mode. The second example is the SoftHouse Project’s adaptive façade. The development originated from a kinematics investigation in stripe-like configurations to conclude to a highly adjustable, multifunctional adaptive skin. The stripes’ membrane covering consists of both glass-fiber mesh and elastic tensile material depending on the elongation tolerance in each case individually. The façade components provide three kinds of actuating force; linear actuating, responsible for swivelling the element at mid-span locations; rotating actuating supports to form twisting reactions and; sliding roof-end supports to execute bending deformations [48].
Technically, the use of bending principles in hybrid systems enables initial complex geometries, subsequent stabilization through additional pretension, reversible deformability and sustainable controlled kinematics of the members. The flexible guyed mast model, originally proposed by Frei Otto [36], consists of a central flexible curved element that may obtain different configurations through three sets of longitudinal cables, and provides such an adaptive hybrid bending-active system. Another example is the hybrid prototype structure of three polyethylene terephthalate glycol, PETG lamellas interconnected by struts of variable length [49]. Further developments of adaptive hybrid structures, consisting of primary bending-active members and a secondary system of struts and cables with closed circuit and variable length, have been proposed in [11, 50, 51]. In these prototype developments, the cables have a dual function: to stabilize the primary members and to provide the structure with controlled transformability, in order to obtain different configuration states throughout its kinematics.
3.2. Simulation Approaches
The design development and simulation of the physical behavior of complex kinetic structures based on soft-mechanical principles, including the implementation of form-finding techniques, were traditionally achieved using physical models; a well-known direction of investigation pioneered in the work of Antoni Gaudi and Frei Otto [52]. Although, the experimental form-finding allowed the visualization of results, two main disadvantages have been discussed in [53]; namely, the limited number of variants and the measurement of physical models. In addition, conventional model-making and analysis might lead, in most cases, to time consuming and geometrical inaccuracies during the investigation, and to results that prevent the establishment of feasible modelling and simulation procedures. On the contrary, recent advances in computational design and simulation open up new directions of research, providing a methodological shift towards more effective implementation of tools and mechanisms, enabling their involvement in various stages of the process, from the conceptual design, to the simulation of their physical behavior, and then to the analysis of structural capacities. The aim is precisely to capture their behavior and achieve an active integration between geometrical development and physics-based simulation of modules and systems, as well as an efficient numerical analysis of the results.
In the first case, the preliminary typological investigation of morphologies and the load-deformation analysis of systems are examined. In examples of conventional kinetic systems, the implementation of computer simulation, as mechanism for visualization of motion behavior, is required using advanced 3D modelling tools with animation and rendering capabilities [54]. In this category, the simulation approaches might be established around the idea that complex morphologies can be decoded into manageable components in the rationale of a bottom-up procedure [55]. Once the shape and motion behavior of each unit are determined, by synthesizing mass customized structural elements together, the overall morphology of the system can be achieved [56]. The application of parametric design and control approaches enables the visualization of the design solutions, close to the actual animation of their motion behavior; an implementation that can be found either in conceptual or detailed design stages.
Tensegrity, tensile and bending-active systems necessitate more sophisticated techniques than previous approaches, aiming not only at their visual animation, but also their digital form-finding. Such mathematical approaches derive from force-density [53] and dynamic relaxation methods [56, 57]. The force-density method describes the equilibrium state of any general network structure composed of nodes and edges under suspension or stretch between given fixed points [53]. The method is based on the force-length ratio or force-density that is applied in every single structural element. The loading in each element prompts the lengthening and deformation of the overall structural system. The process continues until the sum of all forces in each node is zero, leading to an equilibrium state of the system [58]. The dynamic relaxation approach is another form-finding technique, aiming at the geometrical definition of network structures consisting of nodes and cables. According to [57], the method aims at gradually finding the motion of each structural node until it reaches a static equilibrium state. Through an iterative process, the system comes to rest under the influence of a ‘damping’ behavior that reflects the time-based simulation of a pseudo-dynamic process, following viscous damping principles [59].
Recently, computational methods in the area of form-finding concentrate on simplifying procedures, opening new directions in the development of tools that can be widely used in other disciplines, like in architecture, for the geometrical development of structures. The intention is to implement such tools in the early stage of the design process, while experimental investigations are taking place, incorporating material and loading characteristics of structural systems. A well-known computational approach is the particle-spring technique on the basis of Hooke’s Law of elastic stress-strain behavior [60, 61], which may be applied to capture the optimum shape of both hard and soft-mechanical systems. Specifically, this method involves a structure that is translated into a network of particles and springs that represent nodes and edges, respectively. The springs are assigned according to their stiffness and damping properties and the particles are influenced by gravity and self-loading conditions. During the form-finding process, the loads cause the displacement of the particles and hence, the deformation of the overall system. The springs oscillate until their velocity is minimized, resulting in an equilibrium position of the geometry under investigation [62]. Among others, examples of particle-spring tools include the CADenary [63], the Spring FORΜ software [64] and the parametrically controlled physics-based engine Kangaroo (plug-in for Grasshopper) [65]. The latter assigns a spring force behavior based on the length of springs, which is defined by ‘start’ and ‘rest’ length values; a metric control approach that, in turn, influences the tension behavior of the linear structural members.
The methods described above primarily support a geometry-based design approach of kinetic systems, within the preliminary design stage. The system’s geometry is predefined based on analytical geometry or experimental form-finding methods, both of which are used as a controlled means to approximate the actual form-finding. Material limitations are considered analytically. Furthermore, in numerical form-finding through FEA, full control of the material behavior-based geometry is enabled. Material characteristics and limitations are included in the numerical analysis model. In particular, form-finding techniques in bending-active members based on FEA are presented [62]. FEA proves to be a valid form-finding calculation method [66], and takes into account both material and geometrical nonlinearities that occur during the members’ large deformations and displacements.
Furthermore, the conventional form-finding methodology of coupled bending-active structures examined [67], points to the need for all stresses developed in each deformation step to be accordingly stored inside the updated geometry coordinates of the model before calculating the next step. For the erection, and step-wise deformation activation of bending-active elements, Lienhard suggests the so-called elastic cable approach [68]. Contracting cable elements are used to pull together nodes from both individual members until they reach the required deformed configuration. The cable elements are assigned to constant pretension and their elastic stiffness is regulated to a lower level for each static analysis that follows. This process enables cable components to shrink in length and the two end-assigned nodes of the structural members to join. For the form-finding of coupled bending-active members, this technique allows complete freedom of the equilibrium paths followed during the deformation process.
4. BIO-PULSE OSCILLATIONS
It is well acknowledged that dynamic structural forms derived from nature, as well as their relation to the mechanics of moving fluids, can lead to highly effective kinematics [69]. The design of organisms responding to drag, flow and lift offers kinetic efficiency on one hand, while physical adaptations observed in natural forms offer solutions for form-flexible architecture that exists in changing environments, on the other.
The motion strategy derived through pulse oscillations resembles, for example, the process of pumping blood through the body (Fig. 1) [70]. The heart acts like a four-chamber pump, with two atria and two ventricles. De-oxygenated blood returns to the right side of the heart via
