R E S P O N S I V E C I T I E S _ D E C A R B O N I Z E 100 101 INTEGRATIVE HYBRIDITY: A Framework for the Co-design of Hybrid Systems 1ICD - University of Stuttgart, 2ITKE - University of Stuttgart, 3IntCDC - University of Stuttgart KEYWORDS Co-design, Design Framework, Hybrid Systems, Biomaterials ABSTRACT Hybrid bio-based materials offer innovative solutions in construction, addressing design challenges single materials cannot meet alone. The integration of material's complementary characteristics enables a balanced demand for resources and allows for the investigation of novel tectonics and typologies in architecture. Despite the advancements in integrative design methods, known as co-design, there is a lack of methodologies and frameworks for designing a hybrid system. This study proposes a method to enhance and expand the co-design process of hybrids by incorporating critical factors integral to their development. The objective is to create a foundation for the development of bio-based hybrids in architecture. The methods extend the co-design process, wherein complementary material properties are considered, material and spatial relationships are established, building system logics are categorized, and guiding inquiries facilitate the evaluation of the system at both material and architectural levels. The methods are demonstrated by two case studies. One involves hybrid systems made of timber and Flax Fiber Polymer Composite (FFPC), and the second evaluates state-of-the-art hybrids demonstrating its broader applicability. The findings (i) demonstrate the potential to align material roles with project-specific criteria and (ii) validate the method as an effective tool to guide the design of hybrid systems. LAURA MARSILLO1,3 laura.marsillo@icd.uni-stuttgart.de SHIRIN SHEVIDI shirin.shevidi@gmail.com JAN KNIPPERS2,3 jan.knippers@itke.uni-stuttgart.de ACHIM MENGES1,3 achim.menges@icd.uni-stuttgart.de REBECA DUQUE ESTRADA1,3 rebeca.duque@icd.uni-stuttgart.de TZU-YING CHEN2,3 tzu-ying.chen@itke.uni-stuttgart.de KALAIVANAN AMUDHAN speculativist@gmail.com SAMUEL LOSI sqamuel.losi@gmail.com 1. INTRODUCTION 1.1. ON THE RELEVANCE OF HYBRIDS The building and construction industries account for over 40% of global carbon dioxide emissions (Ahmed Ali et al., 2020), underscoring the need for sustainable alternatives to traditional materials like concrete and steel. Bio-based materials present an opportunity to reduce dependence on non-renewable resources, but over-reliance on specific biomaterials risks depleting resources and harming biodiversity (Zhang et al., 2022). Diversifying material use and allowing time for natural regeneration become critical in Architectural, Engineering, and Construction (AEC) practices. Bio-based hybrid systems offer a solution by combining materials with complementary properties to create efficient structures with reduced carbon footprints. This approach reduces dependence on single materials while enabling tailored performance, with materials strategically assigned roles based on their strengths and limitations. With over 60% of a building's embodied carbon coming from its primary structure (Zhang, 2020a), it is crucial to expand the use of bio- based materials for structural applications. The concept of hybridity has its roots in botany and genetics, where it initially described organisms resulting from the combination of two distinct species (Jevremovic, 2017). From a material science standpoint, hybridity offers a chance to fill a gap in the material- property space, expanding design possibilities (Ashby & Bréchet, 2003). According to Ashby (2003), an ideal hybrid combines the best qualities of each material to meet challenging design requirements that neither can meet alone. From an architectural perspective, hybridity means pairing different structural systems rather than materials. According to Engel (1997), a hybrid comprises interdependent systems that share their structural responsibilities equally. Their behavior emerges from how systems are connected and organized, while their successes are derived by effectively harnessing their contrasting qualities in an integrated way (Engel, 1997). Building on Ashby’s (2003) framework for hybrid materials and Engel’s (1997) systematization of hybrid structures, this research situates itself at the intersection of these domains by defining hybrid systems as load- bearing architectural solutions within the AEC context. These systems are characterized by: I N T E G R A T I V E H Y B R I D I T Y / R E B E C A D U Q U E E S T R A D A , T Z - Y I N G C H E N , K A L A I V A N A N A M U D H A N , S A M U E L L O S I , L A U R A M A R S I L L O , S H I R I N S H E V I D I , J A N K N I P P E R S A N D A C H I M M E N G E S R E S P O N S I V E C I T I E S _ D E C A R B O N I Z E 102 103 • Materials relying on one another to function effectively. • Complementary properties are combined to enhance overall system efficiency and new capabilities. • Responsibilities are shared across different domains. Besides its environmental relevance, hybrid systems open up new design possibilities with morphologies unattainable by a single material. In fabrication, they can present extended roles, improving material usage, and streamlining assembly. The distribution of responsibilities across domains further encourages interdisciplinarity and innovation in the AEC sector. 1.2. CONTEXT AND RESEARCH PROBLEM Hybrids have long been integral to human innovation, combining materials for enhanced functionality in objects and building components (Aksit and Altstädt, 2020). Recently, hybrids have gained greater attention in AEC research. Examples include Rock Print (Aejmelaeus-Lindström et al., 2017), which employs granular materials and robotic techniques to merge the compressive strength of gravel with the tensile capacity of strings, creating reversible, binder-free structures. Living Prototypes (Tamke et al., 2023) focuses on flax fiber composite and timber, utilizing robotic Coreless Filament Winding (CFW) to produce lightweight, efficient slabs; while Maison Fibre (Dambrosio et al., 2021) integrates carbon and glass fiber composites with timber plates, where the plates provide walkable surfaces and distribute loads in a multi-story building. In co-design methods, research often addresses data exchange and interoperability to enhance interdisciplinary collaboration and streamline the design process (Pérez et al., 2022a). Pérez et al. (2023) propose a method linking fabrication and material parameters in CFW to structural evaluation, improving accuracy and reducing design iterations. Zhang et al. (2020b) introduce a holistic quality model that assesses decisions at different design and construction stages where quality characteristics are defined and measured- ensuring technical, environmental, and social quality in co-design. However, these methods do not address the inherent complexities of combining materials into hybrid systems, lacking a framework to guide architects in designing, evaluating, and managing material interactions during early design phases. This highlights the need for an expanded co- design framework to tackle these challenges. 1.3. RESEARCH OBJECTIVES This research’s objective is to develop a framework to guide the co- design of hybrid systems integrating structural, architectural, and material considerations. The project presents methods to leverage materials' complementary properties tailored to defined goals across domains. It provides tools for early-stage design decisions, enabling designers to navigate the complex interrelations between materials, demonstrating its potential through two case studies. The project seeks to advance hybridity in construction, promoting it as a sustainable and adaptable approach to building design. 2. METHODS This section outlines the methods that form the proposed framework, revealing progressively more details of the intricate material relationships. In 2.1, guiding principles are defined across four key domains relevant to the AEC sector: Form, Architecture, Construction, and Structure. In 2.2, a strategic material assessment is presented, aiming to understand the material's complementarities within the same four domains. In 2.3 the system is categorized by analyzing spatial relationships and linking the design to relevant typologies. In 2.4 an evaluation method is presented at a system and material levels, using empirical knowledge derived from the project to assess qualitative criteria through quantitative measures. These methods provide a framework (Fig. 1) for exploring, systematizing, and refining hybrid systems in constant feedback throughout the early design process. 2.1. GUIDING PRINCIPLES To integrate a hybridity perspective, four domains defining key principles were identified from the co-design method, expanded as questions, at the material and building system level: Form, Architecture, Structure, and Construction. The Form domain explores geometric adaptability and how materials influence and respond to shape, and their capacity to expand in different dimensions. The Architecture domain considers aesthetics, the definition of spatial boundaries, and the translation of a hybrid system into a cohesive building system. The Structure domain examines the interdependence between materials, and strategies to enhance redundancy and load distribution. Lastly, the Construction domain focuses on fabrication and assembly aspects, including material interactions during production and disassembly. Figure 1: (a) Co-design Diagram (Knippers et al., 2021); (b) Integrative Hybridity Framework: Expanding co- design methods for hybrid systems. I N T E G R A T I V E H Y B R I D I T Y / R E B E C A D U Q U E E S T R A D A , T Z - Y I N G C H E N , K A L A I V A N A N A M U D H A N , S A M U E L L O S I , L A U R A M A R S I L L O , S H I R I N S H E V I D I , J A N K N I P P E R S A N D A C H I M M E N G E S R E S P O N S I V E C I T I E S _ D E C A R B O N I Z E 104 105 This approach employs a set of inquiries based on material-specific relationships and parameters to structure the design process. The domain-specific inquiries, outlined in Table 1, serve to break down the complexities of hybrid material systems, fostering critical analysis and cohesive design solutions. 2.2. STRATEGIC MATERIAL ASSESSMENT Selecting materials for a hybrid involves evaluating their complementary qualities and the opportunities their combination creates across domains. In the strategic material assessment phase, the materials are analyzed based on the domains defined in 2.1. In the Form domain, it is determined if a material possesses formative potential properties or if it rather conforms or adapts. The Architecture domain focuses on how each material contributes to aesthetics, functionality, and spatial boundaries. The Construction domain assesses joining methods and the materials' roles in fabrication. In the Structure domain, the materials' strengths and weaknesses are evaluated to ensure one can compensate for the other's limitations. All characteristics are gathered and compared, and complementary features are considered to inform the design process. Table 1: Strategic inquiries for different domains and sub- categories on material and building system levels. Domain Subcategory System Level Subcategory Material Level Fo rm Geometric Freedom How well can the system adapt to different shapes? Geometric Influence How much does each material define the system's form? System Expansion How well can the system extend from 1D/2D to 3D? Formability Who shapes whom? A rc h it ec tu re Architectural Potential How easily can the system logic evolve into a building system? Aesthetic Hierarchy How much does each of the materials impact the aesthetics of the system? Embedded Enclosure How integrated is the enclosure in the system? Spatial Boundary How much does one material define spatial boundaries? C on st ru ct io n Fabricability How well do materials support one another during fabrication? Fabrication How much does one material influence the other during fabrication? Separability How easily can the materials be separated at the end of life? Joinery How much does each material contribute to joining the components? St ru ct u re Redundancy What is the level of structural redundancy in the system? Interdependency To what extent are the materials interdependent? Complementary Struct. Roles How much do the materials interact synergistically to enhance the system? Hierarchy How does each of the materialscontribute to the primary structure system? 2.3. SYSTEM CATEGORIZATION This phase introduces a logic to systematize and categorize hybrid systems, supporting the transition from abstract to more defined designs. It streamlines decision-making, enables clear exploration of material combinations, clarifies material allocation and roles, and identifies critical factors that impact structure and fabrication. This phase starts by analyzing how the materials are related to each other spatially. It includes three arrangements in which materials can be side by side or enclose each other (Fig. 2). Next, the hybrid system is categorized by the roles and relationships of materials, linking the design to building system typologies. Within this categorization, materials either shape or hold one another: shaping involves modifying or influencing a material's geometry during fabrication while holding refers to maintaining a specific position and orientation. No fixed typologies are proposed to maintain design flexibility, considering the open possibilities hybrids can bring. This step is crucial in bridging the gap between experimental exploration and practical solutions, ensuring the conceptual framework leads to achievable outcomes. 2.4. EVALUATION As the hybrid system takes shape, the roles of each material become more clear, and inter-domain connections emerge. Meanwhile, the complexity and aspects to cover increase. To assist and improve this process, an evaluation method is proposed, allowing designers to filter the hybrid design through the lenses defined by the inquiries presented in Table 1. Each domain occupies a quarter of the quadrant chart (Fig. 3), organized by subcategories and respective guiding questions. This layout facilitates the visual identification of strengths by area and helps refine strategies for the project. The evaluation is inherently empirical, based on the designer's knowledge and qualitative judgment, as quantitative data on materials is often unavailable in the early design phase. By translating qualitative criteria into a quantitative scale (0 to 1), the hybrid can be evaluated on system and material levels. The evaluation supports designers by providing a measurable comparison, not universal standards. It produces scores that reflect the qualitative aspect of each system in a quantifiable, thus comparable way. Figure 2: Material/Spatial Relationships: define how both materials in the hybrid system are related spatially to one another. I N T E G R A T I V E H Y B R I D I T Y / R E B E C A D U Q U E E S T R A D A , T Z - Y I N G C H E N , K A L A I V A N A N A M U D H A N , S A M U E L L O S I , L A U R A M A R S I L L O , S H I R I N S H E V I D I , J A N K N I P P E R S A N D A C H I M M E N G E S R E S P O N S I V E C I T I E S _ D E C A R B O N I Z E 106 107 2.4.1. SYSTEM LEVEL Design iterations are assessed at the architectural system level, focusing on the four established domains, which are further divided into eight subcategories relevant to building systems (Fig. 3 left). Each iteration is evaluated based on the guiding questions outlined in section 2.1, with well-integrated solutions scoring closer to 1.0. This evaluation measures the degree of integration between materials within each subcategory, highlighting areas of higher and lower synergy and facilitating the assessment of how each design iteration meets the architectural system requirements. 2.4.2. MATERIAL LEVEL At the material level, the four domains are also subdivided into eight subcategories, each addressing the inquiries defined in 2.1 (Fig. 3 right). This evaluation assesses each material's contribution to the system, with ratings summing to 1. A balanced rating indicates a higher level of interdependence and a balanced distribution of roles, while a larger disparity suggests one material dominates. This evaluation helps clarify the relationship between materials and their contributions to specific goals in the hybrid system. 3. RESULTS To illustrate the proposed methods, two case studies were conducted. The first applies the full design framework to a collection of timber- flax Fiber Polymer Composite (FFPC) hybrid systems. In this case, design iterations are designed, systematized, and evaluated following the principles outlined in this research. The second case study analyzes two built hybrid projects using parts of the framework, showcasing its broader applicability. Figure 3: Left: Example of Hybrid System Evaluation scoring the design across four domains and eight subcategories, with orange points indicating results (rated 0–1) for each subcategory. Right: Example of Hybrid Material Evaluation showing each material's contribution, with dark green for one material (M1) and light green for the other, summing to 1 per subcategory. 3.1. CASE STUDY 1 The first case study was conducted together with a group of students within a master's design studio focused on developing timber-FFPC hybrids using CFW (Prado et al., 2014) as a fabrication method. Through extensive physical and digital prototyping, conceptual models evolved into detailed architectural structures following the methods proposed. Out of forty-five proposals, thirty-five were selected for this study. The study began by gathering the material properties of FFPC and timber according to the categories defined in 2.1, with the main findings presented in Table 2. Several complementary attributes were identified. Given that FFPC performs optimally under tension while timber excels in compression, the hybrid systems allocated structural roles accordingly. Timber's inherent rigidity also proved valuable for stabilizing fibers during fabrication and as an enclosure, enhancing system versatility. Additionally, environmental aspects from both materials also presented complementary relevance. Timber, a renewable material, has significant potential to reduce carbon emissions in construction (Tupenaite et al., 2023). However, the rising demand for timber may increase wood harvests by 54% between 2010 and 2050 (Peng et al., 2023), risking overexploitation, land degradation, and biodiversity loss (Beck-O’Brien et al., 2022). Natural fibers, such as flax, offer a promising alternative with growth cycles of about 120 days (Goudenhooft et al., 2019), compared to timber's 40–100 years (O’Donoghue et al., 2024). When used in FFPC, these fibers can form lightweight and high-performance structures (Gil Pérez et al., 2022b). Hybridizing timber and FFPC leverage their strengths, creating lightweight systems with reduced environmental impact while providing sustainable solutions beyond sole reliance on timber. Domain Properties FFPC Timber Form Morphological Linear, enabling continuous and flexible placement Volumetric, inherent rigidity Structure Structural High strength-to-weight ratio; optimal under tensile forces; less effective under compression. Anisotropic Excels in supporting compressive loads. Anisotropic Construction Fabrication Additive manufacturing; highly formable but requires temporary support until cured Subtractive manufacturing; less formable; available as boards and profiles, can serve as support for fibers Architecture Functional Light-filtering, space-defining Walkable surfaces, enclosure Table 2: Complementary properties of FFPC and Timber following key criteria for hybrids. I N T E G R A T I V E H Y B R I D I T Y / R E B E C A D U Q U E E S T R A D A , T Z - Y I N G C H E N , K A L A I V A N A N A M U D H A N , S A M U E L L O S I , L A U R A M A R S I L L O , S H I R I N S H E V I D I , J A N K N I P P E R S A N D A C H I M M E N G E S R E S P O N S I V E C I T I E S _ D E C A R B O N I Z E 108 109 Informed by the preceding analysis, thirty-five hybrid proposals were developed combining timber and FFPC in different ways. They were systematized based on material-spatial relationships and then categorized by building system logic. The categorization analyzed material roles in each proposal, identifying a total of six building system typologies (Fig. 4). Thumbnails representing each proposal are showcased in a chronological progression from abstract explorations to more defined typologies and systems (Fig. 5). Each proposal was assessed using the evaluation methods (2.4). The evaluation gives the opportunity to see all projects through the same lenses defined in 2.1. With the quadrant format, the evaluation graph enables a quick visual assessment within each domain, informing the design process on the direction and level of integration in each hybrid solution. A map with simplified graphics showcases the results of all iterations (Fig. 7), and a histogram compares the average results, overlapping system and material levels chronologically (Fig. 8). Proposals in the Spatial category (Fig. 6) demonstrated strong material interdependency and balanced role distribution, whereas others, like Corrugated Plates and Tensegrity, remained at early stages. Figure 4: Design Categorization: Material/Spatial Relationships define how materials relate spatially to each other, and System Categorization defines roles and interdependency between materials within different typologies. The scores assigned to each iteration are relative, making collective evaluations and periodic score reviews essential. The results presented in Figure 8 demonstrate that early iterations favored timber (light green), with FFPC (dark green) receiving lower scores. However, later iterations showed increased reliance on FFPC, resulting in a more balanced material distribution and higher integration levels, as indicated by the purple graph. This shift likely stems from the designer’s initial bias toward timber due to greater familiarity with it when compared to FFPC. Figure 5: Categorization of design iterations. System categories received letters from A to F and chronological order numbers from 01 to 07, identifying each design iteration. Figure 6: Example of chronological evolution of design iterations within the Spatial Category, going from abstract explorations towards more defined building systems. I N T E G R A T I V E H Y B R I D I T Y / R E B E C A D U Q U E E S T R A D A , T Z - Y I N G C H E N , K A L A I V A N A N A M U D H A N , S A M U E L L O S I , L A U R A M A R S I L L O , S H I R I N S H E V I D I , J A N K N I P P E R S A N D A C H I M M E N G E S R E S P O N S I V E C I T I E S _ D E C A R B O N I Z E 110 111 3.2. CASE STUDY 2 The second case study evaluated two built projects. The first, Maison Fibre (Dambrosio et al., 2021), combines timber panels with carbon and glass FPC slabs in a modular multi-story building, representing an early effort to use timber-FPC hybrids at a building scale. The second project, the ITECH Research Pavilion 2024, builds upon the design exploration presented in Case Study 1. This pavilion advanced the A7 proposal (Fig. 6) into a full-scale built structure, including vertical and horizontal structural timber-FFPC hybrid elements. The pavilion’s logic combines timber struts and plates connected and embraced by FFPC, assigning mostly compression forces to timber and tension forces to FFPC. Timber, in this case, extrapolates its structural role, acting also as an embedded frame for the fibers during fabrication and as roof enclosure. Figure 7: Evaluation Map. Each block shows the evaluation of a design iteration containing the naming and the simplified versions of the evaluation quadrant graphs. Iteration A7 is presented on a bigger scale to facilitate readability. Figure 8: Overview of evaluation values showing the average score for each material in each design iteration and their average hybrid system score. Fig. 9 shows that in the first project , FPC dominated, with timber playing a secondary role. While hybridity scores showed limited fabrication integration and reduced structural complementarity, the project reached high architectural scores, using timber to create a walkable surface, expanding the architectural potential of fiber structures. The second project, in contrast, implemented integrative hybridity from the outset, resulting in a morphology that reflects a high level of material interdependence and presents a balanced material role distribution. The system presents high scores across different domains but it fell short in terms of redundancy and separability. 4. CONCLUSION The application of the methods in Case Study 1 demonstrated their potential to guide the co-design of hybrid systems. By using the complete framework on a collection of design iterations, diverse proposals emerged, showcasing the expanding design space of hybrid systems. The chronological appearance of improved system scores highlighted that hybrids can be tailored to meet specific criteria and achieve balanced material roles. Case Study 2 evaluated two built projects, one developed outside the proposed framework, and the other fully designed by the methods proposed in this research. The results highlight the effectiveness of the methods in assessing and visualizing the distribution of material roles and integration across different domains, providing valuable insights for designers in the early design stages. The study also found that high hybridity system scores don't always represent better performance in all areas. Figure 9: Hybridity evaluation on system and material levels of two built projects. Left: Maison Fibre (Dambrosio et al., 2021) combining carbon and glass FPC with timber plates in a multi-story building. Right: ITECH RP 2024 integrating timber-FFPC in a lightweight integrative hybrid system. I N T E G R A T I V E H Y B R I D I T Y / R E B E C A D U Q U E E S T R A D A , T Z - Y I N G C H E N , K A L A I V A N A N A M U D H A N , S A M U E L L O S I , L A U R A M A R S I L L O , S H I R I N S H E V I D I , J A N K N I P P E R S A N D A C H I M M E N G E S R E S P O N S I V E C I T I E S _ D E C A R B O N I Z E 112 113 Systems with low fabrication integration sometimes excelled architecturally, while highly integrated hybrids faced fabrication or assembly challenges. Future developments could extend the methods for later design stages, reducing reliance on empirical input, and incorporating quantitative data, such as material amount, fabrication time, environmental impact, and structural performance. Additionally, the relationship between the degree of hybrid system integration and its correlation with these quantitative metrics warrants further investigation. This research introduces a framework to assist designers in the early design phases of hybrid systems, enabling the navigation of complex interrelations across domains. It offers a methodology to support informed decision-making, balancing material roles and system integration. It guides the development of hybrids tailored to respect material availability, leading to increased awareness of material ratio and applicability. Although developed specifically for this study, the framework’s principles are versatile and can be adapted to different materials and contexts, fostering the integration of bio-based hybrid construction practices in the AEC industry. 5. ACKNOWLEDGMENTS This research was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC 2120/1 – 390831618. 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