Rule-Based Design of Prefabricated Hollow Section Plywood Wall Units
Research stage: intermediate doctoral stage
ABSTRACT. The subsequent project investigates to which extent panels can be spaced and simultaneously connected by means of dowels. The relationships between the position and alignment determining parameters are expressed in specific dowel formations. These were made visible and examined in relation to their appearance. The principle is particularly relevant in timber construction and can be considered in the context of Konrad Wachsmann's theories on the influence of tools and industry on contemporary architecture.1 One central aim of the work is to break down the standardization that characterizes timber construction in particular and that industrialization has brought about. In this context it is investigated how the new construction method contributes to reducing the material requirements of timber constructions while increasing its design range.
SETTING. The dwindling availability of resources is one of the central issues in building technology. 2 Another is the continuous increase in building efficiency and complexity that demands faster design processes and therefore the need for digitally-assisted decision-making. 3 4 In order to meet this challenge, new solutions in the field of architecture are urgently required. 5 A promising tool in managing the complexity of various design parameters so far have been integrated design concepts, which can balance priorities or help highlighting design problems. Based on these, the presented work aims to illustrate and analyse the respective advantageous states of various constructive configurations of a new developed form of wood-based construction and thus to establish an architecturally higher-quality and resource-efficient alternative to conventional timber construction.
As the main renewable building material, wood-based construction was always going to be at the core of any further consideration. In the context of building with wood and timber, one of two prevalent construction methods are mainly utilized: The material-intensive solid construction (Mass Timber) method and the timber frame construction method. Both can be prefabricated in factories to increase efficiency within the construction process. The corresponding manufacturing processes are increasingly based on numerically controlled systems that can follow uniform movement patterns as efficiently as non-uniform movement patterns. The latter in particular increase the possibility of implementing heterogeneous, requirement-based wall structures. This was shown by Beyreuther who reduces material requirements of mass timber walls by dispersing their inner layers.6
Figure 1: Construction principle of an IDS element ; image: Felix Schmidt-Kleespies
Based on these statements and on the premise that the consumption of resources in wood-based construction would be further reduced, a third construction method was conceived, mainly derived from architectural observations. The construction method reverses the load-bearing behavior of the established timber frame construction by activating the surface layers (directed Laminated Veneer Lumber panels) of the structure as primary load-bearing structure, and connecting them with a secondary load-bearing layer of cylindrical wooden dowels arranged at skew-whiff angles to each other (fig. 1). The resulting hollow space in between the panels serves as insulation layer that can be filled with diverse materials. Based on the principle of the varying assembly vectors of the dowels restraining each other, the concept was named Interlocking Dowel System (IDS).
Similar research on glueless connections of sawn timber boards and dowels using interference fits was carried out by Thoma. The core of that project was the systematic investigation of the swelling and shrinkage behavior of wood caused by the varying conditions of ambient humidity. 7 While the same connection type is applied here, the IDS is focused on coplanar connections of two-dimensional wall-segments in order to generate an enclosed and insulated architectural space. It makes full use of wood as a material and eliminates the use of any metal connectors, which in turn has the potential to improve recyclability, eco-balance and the thermal insulation properties of the building elements. Interlocking wood-based structures are also investigated on by Robeller et al. and Bucklin et al..8 9 However, the use, distribution and placement of the dowels in an interlocking system is a wholly new approach to constructing wall elements when compared to conventional design and fabrication methods.
From an architectural viewpoint it is important to demonstrate that the proposed construction method allows for a wide range of possibilities when collocating window and door openings of different shapes and sizes in the outer shell of a design. Only proven in theory so far, the system has the novelty of being easily scalable when compared with timber frame construction. Additional advantages of it are that the exposed wood surfaces of the support structure do not require any further boarding and that it demonstrates greater resource efficiency than conventional construction systems due to it not depending on any solid wood cross sections and additional covering, further increasing its social relevance.
Figure 2: Favorable distribution of a specific dowel variation for the mid-section of a load-bearing wall panel (left); sideview of the specimen (right); image: Felix Schmidt-Kleespies
FOCUS. From a constructive viewpoint the design outcome of IDS elements is so versatile, that it will take a lot of effort to substantiate it in its entirety. Although numerous desirable influencing factors can easily be defined, the correlations between them remain to be verified. For example, the arrangement and orientation of the dowels has a major influence on the natural frequency and thus the structural properties of the wall. Simultaneously, they also affect thermal conductivity, sound transmission and the external appearance of the structure. On the manufacturing side of things, relevant parameters such as maximum and minimum angles of inclination of the CNC-machining influence the alignment of the dowels (fig. 6). In accordance with constructive aspiration of the project, individual influencing factors were categorised into three groups with the following order of importance (1) structural, (2) manufactural, (3) aesthetical.
By taking full advantage of current technological capabilities, variable composite constructions can be fabricated. It was assumed that denser, heterogeneous and non-linear dowel distributions would perform better than homogeneous distributions in terms of load distribution. If so, that would facilitate a random, non-linear arrangement of wall openings as well as scalability of IDS. The following describes an approach to develop a concept for a stress-dependent element-related distribution of the dowels.
METHODS. First, a basic model was created, in which multiple dowels with three varying kinds of longitudinal orientations were evenly distributed using an orthogonal grid with 10cm spacing. A first demonstrator was implemented to take any restrictions due to limitations of the production line into account (fig. 2). A dowel inclination angle of 14 degrees and the wood species (beech) were specified. During the simulation (fig. 3) several models with random homogeneous dowel arrangements applying the Perlin noise function and using the same grid were generated and then compared to the base model. It was determined that the natural frequency of the wall with an ordered clustered arrangement of dowels was higher than with random arrangement.
Figure 3: Force field lines in a simulation of a unilaterally fixed wall element with window opening on the basis of randomly arranged dowels (top) and of a clustered and directed dowel distribution (bottom); illustration: Felix Schmidt-Kleespies
Following that, global factors such as the influence of loads occurring due to different wall positions (in the overall structure) were examined on the model. In order to generate different reasonable dowel patterns, different variations of edge elements as well as those located in the middle of an overall structure were initially modeled with a significant amount of varying heterogeneously oriented dowel positions and their performance was compared with that of a regular clustered distribution. Favorable dowel patterns for certain load-cases were documented and provide the basis for training an AI in further processing.
In order to examine convenient dowel distributions within IDS elements that contain wall openings a new wall with a rectangular cut-out in an asymmetric position was modelled. Then a previously simulated load case was applied to a randomised dowel distribution and its corresponding force fields were generated (fig. 5). The next step was to arrange the dowel distribution in a way that clusters of 3-4 differently oriented dowels were moved from lower stressed areas to ones with higher values. Dowel distributions adjusted in this aspect performed marginally better than the rigid random patterns even when the overall number of dowels was noticeably reduced. The resulting model was then constructed to ensure its manufacturability (fig. 4).
Figure 4: Collage of favorable distribution of a specific dowel variation for the mid-section of a load-bearing wall panel with window opening; image: Felix Schmidt-Kleespies
Figure 5: Force field lines in a simulation of a unilaterally fixed wall element with window opening on the basis of randomly arranged dowels (top) and of a clustered and directed dowel distribution (bottom); illustration: Felix Schmidt-Kleespies
CONCLUSION. Using the example of the innovative, interlocking construction principle of wooden panels, it is possible to consider different dowel patterns and implement them based on the different design requirements. It was ascertained that a heterogeneous dowel distribution in the elements can positively impact their performance and properties. To this end, dowel connections that are particularly suitable for prototypical construction as well as different dowel patterns programmed for specific load cases were documented (fig. 2). Applying load-customised dowel patterns enables the uncomplicated retroactive adjustment of existing wall openings without to revisit any detailed planning processes. This enables a wholly new approach to the design of the outer shell of timber-based constructions.
From an architectural point of view, the greatest requirement is to now further develop the concept in order to incorporate as many of the influencing factors as possible and to define a rule-based design framework incorporating those factors in order to help planners discover other unforeseen dowel patterns. However, an optimal arrangement of dowel positions taking into account several parameters, has been arduous to specify so far. A possible solution would be to specify density zones in which the minimum number of dowels is specified but their exact arrangement is left to the designers. Therefore, further research focusing on an algorithm based mathematical model, that determines correlations between these influencing factors and combines them in enhanced dowel patterns is necessary.
Figure 6: Test specimen with homogeneous dowel arrangement during production
DESIGN DRIVEN APPROACH. The current investigation on dowel patterns originates within the confines of design driven research (DDr). Although the end result will be covered due to fire safety regulations, the pattern created by the dowel ends were initially conceptualised out of an artistic interest and the potential they offered. From an aesthetic point of view, however, it would be desirable to make them visible and readable. The project may lay the groundwork for yet unknown future applications of the construction principle, in which the patterns might remain uncovered.
In contrast to classical research in applied science, this project does not try to solve an observable, yet known problem, but to change a future situation by exploring an assumed design problem and putting it to the test. Conjecture is carried out in practice and the research is executed through delineation of unexplored design patterns or interrelations between them. During the research process the design problem and its solution is co-evolving as described by Marchesi.10
In graphic terms the DDr process applied here equals a spiral movement, a term introduced by Roozenburg and Cross.11 The subject approaches an assumed problem by adding information to a design and then reconsiders the resulting design under the former criteria, to check whether it still matches the initial concept or if the initial concept of the design problem needs a reformulation in order to concretise an actual possible objective of the study. The researcher is passing through various cycles of cognitive processes here instead of moving forward in a linear fashion by dividing an existing problem into several sub-problems until the matter seems solvable.
- Wachsmann, Konrad (1962): Wendepunkt im Bauen, [1.-20. Tsd.]. ed. Rowohlt.
- Weidner, Stefanie/Mrzigod, Alexandra/Bechmann, Roland/Sobek, Werner (2021): »Graue Emissionen im Bauwesen – Bestandsaufnahme und Optimierungsstrategien«, in: Beton‐ und Stahlbetonbau 116, pp. 969–977.
- Sandberg, Marcus/Johnsson, Helena/Larsson, Tobias (2008): »Knowledge-Based Engineering in Construction: The Prefabricated Timber Housing Case«, in: ITcon Vol. 13, pp. 408-420.
- Hensel, Michael/Menges, Achim/Weinstock, Michael (2013): Emergent Technologies and Design: Towards a Biological Paradigm for Architecture, 0 ed. Routledge.
- Schwinn, Tobias (2021): A systematic approach for developing agent-based architectural design models of segmented shells: towards autonomously learned goal-oriented agent behaviors.
- Beyreuther, Todd (2018): Next Generation CLT: Mass-Customization of Hybrid Composite Panels Emerging Workflows/Techniques/Design Protocols for Carbon-Neutral Buildings.
- Thoma, Adreas/David, Jenny/Helmreich, Matthias/Gandia, Augusto/Gramazio, Fabio/Kohler, Matthias (2019): »Cooperative Robotic Fabrication of Timber Dowel Assemblies«, in: Research Culture in Architecture: Cross-Disciplinary Collaboration, pp. 77–87.
- Robeller, Christopher/Weinand, Yves (2017): »Constraint-aware design for assembly, for the first integrally attached Timber Folded Plate lightweight structure, covering a column free span of 20 meters with only 45 millimeter thick CLT plates«, in: ACADIA 2017 | Disciplines + Disruption.
- Bucklin, Oliver/Menges, Achim/Amtsberg, Felix/Drexler, Hans/Rohr, Angela/Krieg, Oliver David (2022): »Mono-material wood wall: Novel building envelope using subtractive manufacturing of timber profiles to improve thermal performance and airtightness of solid wood construction«, in: Energy and Buildings 254.
- Marchesi, Maria/ Kim, Sang-Gook/Matt, Dominik T. (2015): »Assessing the Design Innovation Potential of Timber Prefabricated Housing Through Axiomatic Design«, in: Proceedings of the ASME 2015 International Mechanical Engineering Congress and Exposition.
- Roozenburg, N.F.M., Cross, N.G., (1991): Models of the design process: integrating. DESIGN STUDIES.