Conference Paper

Hybrid Tower, Designing Soft Structures

Abstract and Figures

This paper presents the research project Hybrid Tower, an interdisciplinary collaboration between CITA - Centre for IT and Architecture, KET - Department for Structural Design and Technology, Fibrenamics, Universidade do Minho Guimarães, AFF a. ferreira & filhos, sa, Caldas de Vizela, Portugal and Essener Labor für Leichte Flächentragwerke, Universität Duisburg-Essen. Hybrid Tower is a hybrid structural system combining bending active compression members and tensile members for architectural design. The paper presents two central investigations: (1) the creation of new design methods that embed predictions about the inherent interdependency and material dependent performance of the hybrid structure and (2) the inter-scalar design strategies for specification and fabrication. The first investigation focuses on the design pipelines developed between the implementation of realtime physics and constraint solvers and more rigorous Finite Element methods supporting respectively design analysis and form finding and performance evaluation and verification. The second investigation describes the inter-scalar feedback loops between design at the macro scale (overall structural behaviour), meso scale (membrane reinforcement strategy) and micro scale (design of bespoke textile membrane). The paper concludes with a post construction analysis. Comparing structural and environmental data, the predicted and the actual performance of tower are evaluated and discussed.
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Hybrid Tower, Designing Soft Structures
Mette Ramsgaard Thomsen, Martin Tamke, Anders Holden Deleuran, Ida Katrine Friis Tinning, Henrik Leander Evers The Royal
Danish Academy of Fine Arts, Schools of Architecture, Design and Conservation, Centre for Information Technology and
Architecture (CITA), Copenhagen, Denmark.
Christoph Gengnagel, Michel Schmeck, Department for Structural Design and Technology (KET), University of Arts Berlin
ABSTRACT
This paper presents the research project Hybrid Tower, an interdisciplinary collaboration between CITA - Centre for IT and
Architecture, KET - Department for Structural Design and Technology, Fibrenamics, Universidade do Minho Guimarães, AFF a.
ferreira & filhos, sa, Caldas de Vizela, Portugal and Essener Labor für Leichte Flächentragwerke, Universität Duisburg-Essen.
Hybrid Tower is a hybrid structural system combining bending active compression members and tensile members for architectural
design. The paper presents two central investigations: (1) the creation of new design methods that embed predictions about the
inherent interdependency and material dependent performance of the hybrid structure and (2) the inter-scalar design strategies for
specification and fabrication. The first investigation focuses on the design pipelines developed between the implementation of
realtime physics and constraint solvers and more rigorous Finite Element methods supporting respectively design analysis and form
finding and performance evaluation and verification. The second investigation describes the inter-scalar feedback loops between
design at the macro scale (overall structural behaviour), meso scale (membrane reinforcement strategy) and micro scale (design of
bespoke textile membrane). The paper concludes with a post construction analysis. Comparing structural and environmental data, the
predicted and the actual performance of tower are evaluated and discussed.
Figure 1: The interior of the Hybrid Tower is characterised by the tensioning system and the resulting inward oriented cone-like membranes
INTRODUCTION
Hybrid Tower examines the creation of hybrid structures that combine two or more structural performances to create a stronger
whole (Gengnagel et al. 2013). By merging membrane performance with overlapping actively bent compression rods, the tower’s
load behaviour and structural integrity performs as a grid, whose stability can only be achieved through the interaction of the material
systems. The project explores the utilisation of flexibility on the material and structural level. Where architecture is traditionally
conceived as static, this approach positions adaptability as the idea of reducing stiffness, allowing for deformations and thus
minimizing material use. Similar to the concept of resilience in nature, a system’s survival is understood as a function of its ability to
adjust to environmental change that allows the system to become robust and endure. Hybrid Tower employs its inherent ‘softness’
its flexibility and bending - as resilient measures. Built for the outside courtyard of the Design Museum Denmark it engages
environmental forces of wind and weather.
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Hybrid Tower is part of the Complex Modelling research project (Complex Modelling 2013 - 17). The overall aim is to develop new
interdisciplinary design strategies that enable the integration of material performance as a particular challenge to architectural design,
opening up new horizons for design practice. The ability to design for and with material performance is understood as a core resource
for design innovation being closely tied to material optimisation. The objective of Hybrid Tower is to pilot the forming of these
methods. Compared to static and homogenous systems this involves an increased level of complexity in terms of modelling, analysis,
fabrication and construction.Based on a close collaboration between architecture and structural engineering, technical textiles design
and textile evaluation, the project develops new form finding methods that combine tensile and compressive behaviours and realise
these through the direct interfacing between design and CNC knit fabrication. In Hybrid Tower a central goal has been to prototype
and understand how these pipelines between realtime physics/constraint solvers and FE modelling can be devised.
Structural concept of the Hybrid Tower
Hybrid Tower is constructed from stacked overlapping glass fibre reinforced plastic (GFRP) rods, which are connected and braced by
a bespoke knitted membrane made from high tenacity polyester yarn.The rods are bent into arch like shapes utilizing the material’s
ability to be deformed elastically. The main advantage of using this bending active approach is the simplicity of shaping elements
and the possibility to explore a wide architectural language of shapes. Furthermore it allows the , minimisation of material and
energy consumption (Gengnagel et al. 2013).(1). As the residual stress from the shaping process grows in proportion to bending
curvature and element thickness, actively bent rods have to be very slender and of a high strength to stand the high stresses.
Glassfiber reinforced plastics (GFRP) combine high strength with a low Young’s modulus and are like timber and similar material
preferred materials for such systems. To stabilise these systems membranes have proven to be very efficient restraining the slender
elements and increase the hybrid system's stiffness dramatically (Alpermann, Gengnagel 2012).
The concept of the textile hybrid tower requires a balance between stiffness and rigidity to withstand external impact and a certain
flexibility and softness to allow the structure to adapt to impact, store energy by deforming elastically and releasing this energy upon
recovery. The arrangement of the actively bent rods forms a grid shell like structure embedded in channels and pockets inside a
bespoke knitted fabric. The structure forms an almost hyperbolic tower. Built for the outdoors courtyard of the Design Museum
Denmark, the structural weakness of tall structures are on the global level: wind compresses the front and creates suction forces at the
sides, which create deformations that lead eventually to buckling and collapse (Fig. 2). Hence, the system needs to be restrained. This
is done through pulling the membrane between the rods of each layer with tension cables to the tower’s central axis. This results in a
spoke wheel effect which provides horizontal stiffness and braces the rods.
Figure 2: Wind distribution according to Danish building code (left) and the deformation of the Hybrid Tower under respective wind load (middle)
and without (right)
The system’ s overall stability relies on the interaction between the GFRP grid and the membrane behaviour. The scale and amount
of detail necessary to realise the membrane system makes the use of woven PVC membranes, which requires pattern cutting,
seeming and application of details, a highly laboursome process. Knit was therefore chosen as a particular textile technique that
allows bespoke detailing and is structurally flexible allowing for a high degree of 3 dimensionality in the membrane design. The
integration of detailing into the textile fabrication alleviates the need for sewn on details such as pockets for receiving the rods, but
also creates the opportunity to design and fabricate a continual connection between rod and membrane enabling the best possible
performance of the hybrid system.
In Hybrid Tower, the micro scale design and specification of the membrane is integral part of the overall design project, and highly
interwoven with the form finding and the analysis of the towers overall structural behaviour.
COMPUTATIONAL MODELLING APPROACH
Challenges
During the design process, extensive physical prototyping was used to ideate a hybrid system that would be structurally performing
and could be resolved at micro, meso and macro scale.
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Figure 3: The design process takes place in three interconnected types of models.
From a computational design modelling perspective the primary development challenge has been how to model the hybrid behaviour
of bending active tensile membrane structures in an interactive form finding and design system. This involved solving several
subproblems including how to model the bending members and membranes in one integrated system, which physics solver to use and
how to implement it. A large part of the project has therefore been focussed on how to generate and manipulate the tower topology
and dimensions, which analysis models to implement as well as define processes by which the form found geometry can be further
processed so as to develop the final fabrication data for the bespoke membranes.
Computational Workflow
To handle the challenges of the Hybrid Tower a computational workflow was developed consisting of a four-step-process (Fig. 4):
Model Variables contain the definition for the tower design, constant and flexible geometric parameters and material properties, the
Generative Model instance produces a sets of geometric possible results based on the parameters set before. The Analytical Model
produces performance data of environmental impact, The Design Instance transfers the model data to the required production data. If
the design doesn’t match the desired requirements, the process is started again.
Figure 4: Flowchart of the developed Modelling Pipeline
Generative Modelling Methods
Our generative design modelling approach extends previous work on actively bent structures (Adriaenssens, Barnes 2001; Deleuran
et al 2011; Quinn et al 2013; Alpermann, Gengnagel 2012; Ahlquist, Menges 2013; Mele et al 2013; Lienhard 2014) by
implementing the new Kangaroo2 real-time physics and goal-driven constraint solver library. This library overcomes the stability
and performance problems, which dynamic relaxation form finding methods have, as it employs methods which have been
popularized as position based dynamics (PBD) (Bender 2014), where instead of using summed forces to calculate accelerations, from
which velocities, then positions are updated, the particles are simply to a position which satisfies the constraints, modifying the
positions directly. Iterating repeatedly over all projected constraints lets the system converge fast, stable, allows it to include hard
constraints and runtime user interaction. The Kangaroo2 library uses a similar approach and treats herein geometric constraints,
elastic materials, applied loads and other energies as different forms of the same type of object - encompassing them under the term
goal.
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Our modelling pipeline is based on the Rhino/Grasshopper CAD environment and implements the central algorithms as Python
components with RhinoCommon and Kangaroo2 libraries. The pipeline is divided into four consecutive stages:
Topology and Geometry: The principle of stacking overlapping bending members around a central vertical axis is used to generate
the fundamental tower topology. Bending members are abstracted to constituent polylines discretised and used to generate meshes
representing the knit. This forms the base for the tension system.
Figure 5: Three instances of tower topologies with 3,5,10 sides and 1,5,9 stories. The local polyline members of the system are highlighted in the
fourth image, followed by the three structural member types (bending rods, membranes, tension wires) and their geometric representations (polylines,
meshes lines)
Form Finding and Dimensioning: Kangaroo2 goals are defined for each member representing its behavior and conditions, as the
exact member dimensions per layer and possible flexibility. These are passed to the solver component which iteratively form finds
the structure and allows the designer to interactively manipulate the system.
Figure 6: Screenshots from the iterative and interactive form finding and dimensioning process, showing the initial shape (left, intermediate states
after 10, 50, 100 iterations and the converged state after 200 iterations(right)
Analysis of Form Found Geometry: A desired property in membrane design is high double curvature as this stabilises the membrane.
For the bending members a key geometric property with structural implications is the local and maximum bending radii. These
properties are analysed and visualised in the viewport in order to allow for informed design decisions.
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Figure 7: Comparative bending radii analysis of differently dimensioned towers. Note the relationship between macro shape and bending radii.
Unrolling Membranes: Developing the knit meshes in the plane for fabrication is problematic as they are double curved. A
constraint-based approach was developed which forces a 2D mesh to have the same edge lengths as the 3D knit mesh. This in-plane
mesh can constantly be checked against the maximum dimension of the fabrication machines.
Figure 8: Developing a membrane in the XY-plane. The values indicate differences between the form found and the in-plane meshes (MAD = Mesh
Area Difference, TELD = Total Edge Length Difference).
Analytical Modelling Methods
Bending stress utilization can only be passively determined in the generative model by evaluating beam bending curvature, unlike FE
models which generate accurate and complex stress matrices describing all possible stress components (bending, torsion, shear).
The generative model can approximate realistic material behaviour for certain physical phenomena; for example beam bending is
simulated accurately following the Barnes and Adriaenssens model (Adriaenssens 2001). The generative model currently falls short
on its ability to accurately simulate phenomena such as beam torsion, and shell/plate elements. A validation in an advanced FE
environment for structural systems is hence necessary in order guarantee precision of physical behaviour in the generative model for
the phenomena in question - bending, torsion and membrane behaviour.
Large deformations are generally unproblematic in mass spring system (MSS) environments, however in order to achieve the same in
an FE environment, which is inherently founded on the concept of small deformations, extra measures are necessary (such as load
step iterations, non-linear solvers and 3rd order differential equations).
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In the SOFiSTiK model, beams are usually “pulled” to their target positions using single element cables with reduced stiffness and
incrementally increasing levels of prestress, i.e. the elastic cable method (Lienhard 2014).
Ideally, all physical shaping and application of membrane pre-stresses in the tower system are to be simulated in the FE model. This
falls however short, when confronted with the amount of bending active elements of the tower. For the sake of speed and
computational simplicity, stresses from beam bending based on the bending radii found in the generative model, were superimposed
with stress results under pre-stress and wind loading on the model. Due to the high curvatures of the beams, their utilization after
forming alone ranged from (60% - 80%). This meant the stress reserves in the beams were a precious resource under additional
loading from membrane pre-stress and external loads. Subsequently form-stabilization from the spoke wheels played an integral role
in the maintenance of global stiffness in this hybrid tower.
The FE simulation was realised in 3 steps:
Form finding: the stiffness of membrane elements and cables are reduced by a factor of 100-10.000 and a prestress is
applied to the spoke cables. The resulting shortening of the spoke cables pulls the membrane midpoints towards the
central axis of the tower giving the membrane strong anticlastic curvature. The form finding process was concluded
with updating node coordinates reverting the stiffness of all elements back to 100%.
Loading: wind loads were applied according to building codes and projected on to the local z-axis of the membrane
quad elements.
Analysis: stresses and deformation
FABRICATION AND CONSTRUCTION
Designing the knitted membrane material
Designing the hybrid interaction of membrane and bending active system necessitates a finely to balanced correlation between the
membrane and the bending active rods. To develop this system it is important to understand and specify the precise behaviour of the
used materials. While the GFRP rod is a tested material for bending active structures and available off the shelf with different
diameters and properties, knit is a process by which bespoke materials can be developed in direct response to application driven
design criteria (Ramsgaard Thomsen 2008) is always made bespoke to the application. A central part of the project has therefore
been the dual task of eliciting design criteria from the design process and creating methods for knitting the bespoke membranes,
while at the same time developing means by which these can be formally tested so as to be simulated within the design system.
The use of knit as a restraining membrane results in a set of structural requirements, which knit usually does not fulfill:
high strength
near isotropic material behaviour under load
limited elasticity
The membrane design develops methods for four strategic details. It includes the positioning and detailing of diagonal pockets and
channels where the textile structure splices and re-joins into a double layered protrusion. It contains structurally supported
perforations preparing seam lines. It is knitted to shape alleviating the need for pattern cutting. And it develops methods for local
reinforcement strengthening edges as well as central radial pulling points.
Figure 9: Defining three of the four details: the pocket, channel, the seam line and the reinforcement
The development of knitting patterns and the choice of fibres took place in collaboration with Fibrenamics, Universidade do Minho,
Guimarães, Portugal. Through an iterative process of prototyping small scale samples were developed and tested on a uni-axial force
gage. Initial tests showed that 110 and 55tex High Tenacity Polymer in a Piquet-lacoste knit structure demonstrated high stability in
structure, high strength and were relatively isotropic. In order to get insights into the material behaviour the Japanese MSAJ/M-02-
1995 standard for the bi-axial testing of weave, was adopted to our knit by the University of Duisburg Essen. The tests measures in
intervals in both material directions and the approach allows to determine fictitious material constants, suitable for application in FE,
for highly non-linearly behaving materials, such as knit. In our knit the tensile stiffness is extremely low with Young’s moduli of
approximately E = 5 kN/m for wales and course for the full stress interval and Ewales = 10 kN/m and Ecourse = 26 kN/m in stress
interval of 1 - 3 kN/m, which is the one determined relevant for the setup in Copenhagen. In contrast, the Poisson’s ratios are very
high with minor n = 0.83 and 0.66, respectively. Strains are large with up to 70 % under uniaxial loading.
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The process of creating solid test data for simulation and design was however compromised during the production process. Moving
from the production of samples to the final membrane patches necessitated a larger knitting machine in turn resulting in changing
material properties. The final fabric was therefore more elastic than anticipated. This had a severe influence on the construction,
geometry and behaviour of the tower.
Generating Fabrication data
The fabrication data was directly derived from the generative model, after the design was verified by the FE model.The model
generates the final length of the GFRP rods and the in-plane mesh of the membrane. This was further processed into the CNC knit
pattern for the fabricator (Fig. 10). The process of developing the CNC knitting pattern necessitated the incorporation of the stretch
of the material through the active bending system, as well as the anisotropy of the knit and the specifics of the fabrication process.
The necessary compensation factors were established through thebi-axial testing of samples and some tests with the final assembly
system.
Figure 10: A pixel image with details for pockets, channels, reinforcements and seam holes informs the CNC knitting machines on a loop by loop
base. The image shows the pattern before the compensation of knit for the machines.
Assembling the tower
The assembly strategies for the tower were tested on 1:5 and 1:1 prototypes with membrane surfaces made from weave and partly
with CNC knit. Though more prototyping would have been necessary these physical tests allowed us to assess the behaviour of the
assembly and the functioning of inserting the rods in the textile through channels and pockets.
The aim for the assembly was to create a strategy, which would not necessitate scaffolding or other expensive supports. Assembly
therefore took place on the ground, starting with the top layer, and lifting the tower up, constructing each consecutive ‘storey’
beneath it. The Tower was planned to be 8 storeys high. The low stiffness values of the bespoke knitted fabric resulted in an increase
of material stretch and hence increase in size of the tower, so that the final height was already reached with 6 stories (Fig. XX). A
simulation with the material properties of the finally produced knit, showed as well a 80% decrease in load capacity under wind load.
Figure 11: The Tower in the courtyard of the Danish Design Museum - April 2015.
POST CONSTRUCTION ANALYSIS
During the exhibition we conducted a 10-day evaluation monitoring of the structure with two time lapse cameras to the front and side
of the tower as well as a synchronised wind measurement device. These evaluation data were used to determine the differences
between the behaviour of the built demonstrator and the predicted behaviour of our computational design models.
The simulation was conducted in Sofistik and uses the developed modelling pipeline, with the material dimensions and properties
from the built tower. The rods are defined as beam elements with diameters of 12, 10 and 8mm. Tension cables at the naked edges
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and inside the structure are defined as ideal cables elements with a diameter of 8mm of Steel S355. Supports were fixed and the
tower height was 6.90m.
Simulating the behaviour of the tower under wind loads in the Sofistik FE environment was challenging, as the softness of the
structure lead to numerical problems and diverging simulations. The knitted fabric was approximately 50 times softer than a usual
pvc-type-I-membrane, and convergence could only achieved for 20% of the initially predicted wind loads, which were taken from the
Danish code for temporary structures.
The overlay of the videos with the weather data allows for correlation of wind (gust) speed and deflection of the tower. Up to wind
speed of 6.5 m/s (which equals Beaufort 3) the tower stayed in the elastic domain. The measured peak wind of 11m/s lead to local
damage and a kink in the shape of the tower
The monitoring showed that the tower does have some flexibility to store energy by bending, tensioning and bouncing back. The
possible deformation is actually a lot higher than calculated (table 1). This is due to differences between the real structure and the
simulated model, especially in detailing:
The connection between the rods and the membrane can not be modelled correctly, as the friction can not be considered in
the simulation to sufficient precision.
When under tension the rods can slide within the tunnels and pockets, increasing deformation and local stress.
The ends of the rods fitted into pockets and induce point loads into the membrane that enlarge deformation even further and
produce wrinkles in the membrane.
Max. Windspeed Beaufort
Wind
Pressure Deflection
[km/h]
[m/s]
[kN/m2]
FE-Model
Temp. Structure
107,8
30,0
11
0,54
Temp Structure. 20%
48,2
13,4
0,108
Monitored
23,4
6,5
3
0,0254
110mm
Table 1: wind speed used in the simulation of the tower. Temp struct. shows the basic wind speed for temporary structures, below 20% of that speed
and in the bottom the monitored wind speed from the monitoring.
The evaluation of the collected weather data resulted in a maximum wind (gust) speed of 6.5m/s. The associated deflection was
measured from the video footage and compared to the deflection of a FE model simulating the same load level.
Figure 12: horizontal deflection observed in the video foo tage is approximately 200mm (wind from right). Figures from left to right: undeformed
tower, deformed tower, comparison of deformed/undeformed tower (top section)
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Figure 13: Horizontal deflection in the simulated model occurred mostly local (top right) with a maximum of 110 mm (wind from right). Figures from
left to right: undeformed tower with deformed grid (deformation scaled by factor 3.0), comparison of deformed/undeformed tower (top section)
(deformation scale factor 1.0), deformation in plan (deformation scale factor 1.0).
As expected the biggest deformation occurs at the top of Tower. In the simulated model the deformation occurs only in a small
region where compressive wind occurs, while the whole top is shifted more evenly to the side on the prototype. The maximum
deflection of the model is 110mm, while the prototype moves ca.200mm. These deviations can be explained by differences in the
stiffness of the jointing. This observation is in line with earlier research (Quinn et al. 2013), which indicated, that the modelling of
details is crucial for a realistic simulation. In the digital model, these are defined as rigid nodes with direct load transfer, in the
prototype the forces are transferred from one rod to the other via the membrane, that stretches and slides to a certain amount. This
local behavior of (elastic) stretch and the friction at the transition from rod to membrane to rod has not been considered in the
simulation.
Fig. 14: differences in the deatiling and modelling of the intersecting and overlapping profiles. From left to right: physical prototype (1:4) with zip-
tied connections, digital model in Rhino/Grasshopper environment displaying bending radii with rigidly modelled node, digital FE-model displaying
utilisation of rigidly modeled node, physical model (1:1) displaying rods integrated in tubes and pockets - load transfer between the rods via
membrane only.
CONCLUSION
The Hybrid Tower is a complex construct of multiple material behaviours interacting at different scales and exposed to
environmental impact. The project raises the question about the setup and depth of digital design models in order to predict a
structures behaviour and specify the fabrication and materials with a sufficient accuracy. The Hybrid Tower is an intermediate stage
of ongoing research and did not yet match all of the expectations.
Despite the challenges encountered in the design and development process the project successfully developed a digital workflow
(Modelling Pipeline), creating a stable environment for generative constraint based form finding (Generative Model), processing data
to an FE environment (Analytical Model) allowing for the necessary structural feedback. Though the feedback loop from the
analytical FE model to the generative model was not automated the overall process was sufficient to drive the specification and
digital production of materials and elements of the structure.
The deviation in the behaviours of the simulated model and the prototype confirm the impact of connections, intersections and joints
on the structures behaviour, as the need for proper testing and evaluation of materials and assemblies. The question remains, how to
improve the implementation of analysis, model building, detailing and testing in the design process, to deliver an increased level of
accurate feedback in early design stage and to shorten the specification, development and evaluation cycles of material and systems.
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Key is to embed the digital design pipeline in a dynamic process of constant calibration between the digital and physical instances.
Where precise definitions are not yet possible sensible simplifications have to be estimated and tested for validity. The focus in the
ongoing research will be to improve the exchange of data between the instances of the pipeline as well as to parallely investigate
methods of implementing the analysis directly into the process.
ACKNOWLEDGEMENT
The project is funded by The Danish Council for Independent Research (DFF). Membranes were developed by Fibrenamics,
Universidade do Minho, Guimarães, Portugal. Further development and fabrication took place with AFF a. ferreira & filhos, sa,
Caldas de Vizela, Portugal. Mechanical testing of the knit was conducted by University Duisburg-Essen, Laboratory for Lightweight
Structures. The Tower was exhibited at Designmuseum Danmark. We wish to thank: Daniel Piker for generously involving us in
testing Kangaroo2. Dongil Kim, Esben Clausen Nørgaard and the students of CITA.studio for their tireless support. Photos by
Anders Ingvartsen.
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Ramsgard Thomsen M, Hicks T (2008) To build a Knitted Wall, Proceedings, Ambience, smart textiles conference, Gøteborg, 2008
... Our research is a simultaneous inquiry into the design and analysis tools and processes for the conception, specification and production of membrane structures made from the CNC knit, as well as a speculative inquiry into the new spatial and structural possibilities that come with the new material. The inquiry takes place through built demonstrators, namely Hybrid Tower (Thomsen et al., 2015), Tower Guimaraes and Isoropia for the Danish Pavilion at the Architectural Biennale 2018 . A special structural, as well as aesthetic and spatial focus, has been set on the strategic use and control of the stretch of the material. ...
... New opportunities for efficient methods to create non-standard logics for new spatial and tactile experiences emerge through the integration of design, analysis and fabrication combined with the shift of the textile system from weave to CNC knit (Ahlquist & Menges, 2013;Popescu et al., 2018;Sabin, 2013;Thomsen et al., 2015). The inherent flexibility of knit, the ability to integrate shaping and detailing in the textile fabrication process open opportunities especially for designing membranes at the smaller scale of state-of-the-art membrane architecture, as in Ron Herrons Imagination Building (Lyall, 1992). ...
... They did however not include building scale objects or devised knitted textiles a role as structural member. This is however necessary, when constructing Form Active Hybrid Structures of Active Bend and Tensile members (Thomsen et al., 2015). ...
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In this paper, we are presenting a design to fabrication system, which allows to produce efficiently and highly automated customised knitted textile elements for architectural application on industrial computer-controlled knitting machines (Computer Numerical Control (CNC) knitting machines). These textile elements can, in this way, be individual in both geometry, detailing and material behaviour. This work extends recent work on CNC knitted tensile members and presents a set of innovations in design and manufacturing, which together allow to build structural systems, in which highly individualised membrane members allow a structure to take on multiple structural states. Underlying these innovations is a shift from the focus on geometry and homogeneity in material and behaviour, expressed in current state-of-the-art membrane structures and materials. Instead our research lays the foundation for a new class of membrane materials with varying bespoke local material properties. In this paper we present the underlying digital tools and processes for design, analysis and manufacturing of these hyper specified textile membranes. We showcase and evaluate the potentials of Computational Knit for novel structural membrane systems through the large-scale installation Isoropia designed and built for the Danish Pavilion in the 2018 Venice Architectural Biennale.
... New opportunities for spatial expression and tactile experiences emerge through the integration of design, analysis and fabrication combined with the shift of the textile system from weave to CNC-knit (Ahlquist & Menges, 2013;Popescu et al., 2018;Sabin, 2013;Thomsen et al., 2015). The inherent flexibility of knit, the ability to integrate shaping and detailing in the textile fabrication process opens especially opportunities for designing membranes at the smaller scale of state of the art membrane architecture, as in Ron Herrons Imagination Building (Lyall & Herron Associates, 1992). ...
... The inherent flexibility of knit, the ability to integrate shaping and detailing in the textile fabrication process opens especially opportunities for designing membranes at the smaller scale of state of the art membrane architecture, as in Ron Herrons Imagination Building (Lyall & Herron Associates, 1992). Current application of CNC knitting in the field have focused on small scale prototypes, but didn't engage in building scale or devised knitted textiles a role as structural member, as needed in Form Active Hybrid Structures of Active Bend and Tensile members (Thomsen et al., 2015). ...
... In Isoropia we further develop the inter-scalar approach first suggested in the Hybrid Tower projects (Holden Deleuran et al., 2015;Thomsen et al., 2015). Here, design takes place at multiple scales from the overall structural system, to the single patch, the implemented knit structure and down to fibre selection and fibre surface. ...
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Extending recent work on Form Active Hybrid Structures of Active Bend and CNC knitted (Computer Numerical Control) tensile members we present a set of innovations in design and manufacturing, which together allow to build structural systems, that morph across multiple structural states. While state of the art tools and fabrications methods in textile hybrid structures provide architects and engineers with means to adopt the geometry of a chosen textile system to the requirements of a given site, constraints in design thinking, tools and manufacturing however still limit the ability to change the spatial and structural qualities and expressions within a textile object. The potentials of our developments to create new spatial expressions and atmospheres in textiles structures are demonstrated and evaluated through the large-scale installation Isoropia designed and built for the Danish Pavillion in the 2018 Venice Architectural Biennale.
... The first demonstrator "Hybrid Tower" was realised in April 2015 [8] (Fig. 1). The project initialised research into the design and specification of hybrid structural systems that combine membrane structures with bending active members and bespoke knit. ...
... In order to enable fast design iteration a tight coupling of a generative design toolset (K2 based) - stiffness-independent, purely force and vector based -for formfindung and design explorations and a capable FE analysis (SOFISTIK -implicit Direct Stiffness FE approach), for the verification of the found shapes under realistic load conditions -was developed [8]. And although this interface had only a small degree of automatisation and was hence not very effective, the overall design workflow and especially the lightweight design tool were precise enough to provide sufficient specification for shape and detail of the final knit. ...
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Membrane architecture uses currently off the shelf materials and produces the shapes and details through cutting and laborsome joining of textile patterns. This paper discusses investigations into an alternative material practice - knit - which engages bespoke membrane materials. A practice which allows for customised and graded material properties, the direct fabrication of shaped patterns and the integration of detailing directly into the membrane material. Based on two demonstrators built as hybrids of bespoke CNC knit and bending active GFRP rods this paper discusses the affordances and procedures, which this new practice of digital fabrication of membrane material requires. The central focus is set on the interaction between the involved disciplines and the emerging iterative process of design, material specification, prototyping, evaluation, (re-)design and (re-)specification. We discuss how design and engineering practices change, when material properties move from given and constant into the area of design and gradient.
... Mette Ramsgaard Thomsen et al. explored computational approaches of form-finding, simulation, material specification, and digital fabrication for knits and textile hybrids and built a digital chain interfacing the different design models of textile hybrids (Thomsen et al., 2016). The Hybrid Tower and Isoropia projects by CITA further expanded the computational approaches, scale, and structural innovation, and took the prototypes into the real world of mass fabrication and certification (Thomsen et al., 2015(Thomsen et al., , 2019. Mariana Popescu et al. proposed structural design, digital fabrication, and construction approaches for ultralightweight knitted formworks and successfully applied the computational techniques of form-finding, structural analysis, geometry division, and pattern generation to the creation of complex concrete structures, including the award-winning KnitCandela concrete shell (Popescu, 2019;Popescu et al., 2020). ...
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Knitted composites are textile composite materials that consist of knitted textile reinforcement and polymer matrix. Knitted composites exhibit great design flexibility by allowing the customization of shapes, textures, and material properties. These features facilitate the optimization of buildings’ material systems and the creation of buildings with light weight and high material efficiency. To achieve such a lightweight, material-efficient building structure with knitted composites, this research investigates the material properties of knitted composites and proposes a design process for building-scale knitted composite systems. In the material study, this research examines certain mechanical properties of the material and the effects of additional design elements. In the design exploration, this research explores the design workflow of the structural form, element arrangement, and knit distribution of the material system at the macro-, meso-, and microscales. The project of MeiTing serves as proof of the concept and the design workflow.
... In the last years, for example, various research teams have investigated the design possibilities of bending-active structures and applied their knowledge to the construction of small to mid-size art installations, pavilions, and other architectural interventions. A good example is, for instance, the work of Ahlquist and Lienhard [5] or Thomsen et al. [6] and their hybrid structures made from fiberglass rods and stretchable membranes or the prototypical plywood structures, which were recently built by the author [7]. ...
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With this paper, the author aims to contribute to the exploration of bending-active structures by investigating the promising role that additive manufacturing technologies may play in the production of mass-customized joints and other practical connection elements. After a general introduction to the basic concept of bending-active structures, the author will discuss the particular opportunities and challenges related to the joinery for this structural system. Engineering nodes and connections for bent components can be a rather difficult task since the structure's exact geometry and loading condition is often unknown at the beginning of the design process and can only be ascertained through a series of form-finding and analysis steps. It is not unusual that this process results in the need for highly complex and difficult to produce connection details. 3D printing could be a potential remedy to this problem as it simplifies the fabrication of complex parts and renders the possibility to fine-tune joints to the structure's specific requirements. On the example of three early-stage design experiments, the author will share the gained experience and thereby conclude the paper by showcasing an integrated approach to the design and fabrication of bending-active structures that takes full advantage of 3D printed joinery.
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Machine Learning (ML) is opening new perspectives for architectural fabrication, as it holds the potential for the profession to shortcut the currently tedious and costly setup of digital integrated design to fabrication workflows and make these more adaptable. To establish and alter these workflows rapidly becomes a main concern with the advent of Industry 4.0 in building industry. In this article we present two projects, which presents how ML can lead to radical changes in generation of fabrication data and linking these directly to design intent. We investigate two different moments of implementation: linking performance to the generation of fabrication data (KnitCone) and integrating the ability to adapt fabrication data in realtime as response to fabrication processes (Neural-Network Steered Robotic Fabrication). Together they examine how models can employ design information as training data and be trained to by step processes within the digital chain. We detail the advantages and limitations of each experiment, we reflect on core questions and perspectives of ML for architectural fabrication: the nature of data to be used, the capacity of these algorithms to encode complexity and generalize results, their task-specificness versus their adaptability and the tradeoffs of using them with respect to conventional explicit analytical modelling.
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Highly specified digital control tools for the customization of knitted fabrics is an existing technology, widely used industrially by trained knitters. Some fabrication challenges inhibit the extent of use of digital customization for knitted products on a mass scale, specifically when complex multiple structured knits are involved. This is due, in part, to physical changes that occur in the overall fabric dimensions, when stitch combinations with different physical attributes are combined within the same fabric. Existing computational tools fail to assist in the simulation and prediction of these overall deformations and the industry as a whole relies on the expertise of highly trained individuals. The outline shape of the fabric is of specific importance as it is commonly preconfigured to a specific shape and dimension that must be reproducible. In this work, we propose a computational parametric tool for digitally designing and industrially producing knitted fabrics, creating a direct link between design and manufacturability. The tool includes an integrated physical simulation component for estimating the fabric deformation. It allows users to visualize and control compensations in the fabric structure, to better match between the initial graphic intent of the design and the actual physical knitted fabric outcome. Our approach aims to reduce the number of iteration cycles for knitting material samples, especially when knitting highly varied designs.
Chapter
Knitting offers the possibility of creating 3D geometries, including non-developable surfaces, within a single piece of fabric without the necessity of tailoring or stitching. To create a CNC-knitted fabric, a knitting pattern is needed in the form of 2D line-by-line instructions. Currently, these knitting patterns are designed directly in 2D based on developed surfaces, primitives or rationalised schemes for non-developable geometries. Creating such patterns is time-consuming and very difficult for geometries not based on known primitives. This paper presents an approach for the automated generation of knitting patterns for a given 3D geometry. Starting from a 3D mesh, the user defines a knitting direction and the desired loop parameters corresponding to a given machine. The mesh geometry is contoured and subsequently sampled using the defined loop height. Based on the sampling of the contours the corresponding courses are generated and the so-called short-rows are included. The courses are then sampled with the defined loop width for creating the final topology. This is turned into a 2D knitting pattern in the form of squares representing loops course by course. The paper shows two examples of the approach applied to non-developable surfaces: a quarter sphere and a four-valent node.
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his thesis aims to provide general insight into form-finding and structural analysis of bending-active structures. The work is based on a case study approach, in which findings from prototypes and commercial building structures become the basis for generalised theoretical investigations. Information is continuously fed back from these case study structures into theoretical research, which creates the basis for overall working methods. The behaviour of five investigated structures is found to be independent of clearly predictable load bearing categories. Their load bearing mechanisms are largely dependent on the boundless variety of topologies and geometrical expressions that may be generated. The work therefore understands active bending as an approach to generating new structural forms, in which common load bearing behaviour is found due to the structures inherently large elasticity and inner stress state. Based on engineering and historical background, methodological, mechanical and material fundamentals of active-bending are discussed in Chapter B and C. The case study structures introduced in Chapter D open a wide field of active-bending applications, in lightweight building structures. Whether the conclusions drawn from case studies, are generally viable for bending-active structures is then discussed in the core of the work presented in two chapters on Form-Finding (Chapter E) and Structural Behaviour (Chapter F). The chapter on form-finding introduces the working methods and modelling environments developed for the present work. The chapter on structural behaviour is concerned with the influence of residual bending stress on the stiffness, scaling and stability of bending-active structures. Based on these findings, generalised design rules for bendingactive structures are highlighted in a concluding chapter.
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In actively-bent elements the curved end-geometry is achieved by elastic bending. The shape depends of the boundary conditions and its curvature- and stress-distribution is normally not constant. Local stress maxima restrict the design-load of the elastically bent element. Restraining systems can be used to optimise the shape of the elastically-bent elements to get a continuous curvature and therefore a homogeneous stress-distribution. In addition they improve the stiffness of the overall hybrid system significantly. Beside a structural optimization restraining systems can as well be used to shape the bent element for architectural reasons.
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This project navigates the increasingly common challenges and conflicts which occur at the interface between parametrically designed architectural spaces and the efficiency and resourcefulness of the simulations required in order to build them. Computationally efficient methods of structural simulation for a geometrically irregular and complex elastic timber gridshell were investigated using finite element spring models which were calibrated by means of empirical testing. Streamlined and versatile parametric tools were also developed for the geometry definition and production file generation of the system. In this paper, the streamlined computational form-finding and production tools are described and the structural analysis of a geometrically complex plywood gridshell (4m x 12m x 4.5m) is explored in detail. It was established that on top of assembly and material wastage issues, the structural behaviour of the timber gridshell was limited. In order to address the structural weaknesses of the system, new ideas and improvements were explored including: spatial hybrid structures, custom knitted restraining membranes and integrating assembly logic into earlier phases of design scripting. Various non-linear structural simulation methods were investigated including finite element beam and spring models with their merits being assessed in terms of accuracy, speed and appropriateness within the project’s constraints. Numeric simulations were compared and calibrated with physical test data. It was found that the system’s structural behaviour and simulation accuracy varied greatly for in-plane and out-of-plane loading. While the results of this project are somewhat limited within the context of industrial architecture and engineering, they do contribute to the ongoing debate of parametric design and offer a template for the resourceful and efficient generation and simulation of complex systems. It was established that the complexity of simulation placed disproportionately high demands on resources leading to a note of caution against the risks of over-parameterisation and the value of considered and insightful design. In general the authors believe that there is value in the exploration of softer structures for architectural application. While softer structures may not withstand the rigour of modern building codes, the authors argue that it is right to question the level of redundancy in the structures that we build today.
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Active Bending describes curved beams or surfaces that base their geometry on the elastic deformation of initially straight elements. The bending stress in the bent elements depends on the Young‘s modulus of the material, the height of the cross section and the curvature. To achieve sufficient curvature the height of the cross section and the Young‘s modulus need to be small. Therefore, the stiffness of the lastically-bent elements is small. Additional elements offer the opportunity to create a hybrid structure, whose stiffness is significantly higher than the stiffness of the elastically-bent elements alone. The general influence of Active Bending on stiffness and stress is discussed and numerically analysed. The advantages of and different ways to use elastically bent elements are presented in five case studies. The differences in their structural behaviour compared to curved elements are analysed. It can be shown that, despite the initial stresses and the limitation on the profile’s choice, the structural capacity of a given construction is hardly affected by the elastic bending, which is therefore advantageous over the use of straight elements in terms of fabrication, transport and/or assembling.
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This paper considers a class of tensegrity structures with continuous tubular compression booms forming curved splines, which may be deployed from straight by prestressing a cable bracing system. A free-form arch structure for the support of prestressed membranes is reviewed and the concepts are extended to a two-way spanning system for double layer grid shell structures. A numerical analysis based on the Dynamic Relaxation (DR) method is developed which caters specifically for the form-finding and load analysis of this type of structure; a particular feature of the analysis is that bending components are treated in a finite difference form with three degrees of freedom per node rather than six. This simplifies the treatment of sliding collar nodes which may be used along the continuous compression booms of deployable systems.
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The recent development of material performance as a key driver of architectural design is currently challenging the role of representation and prototyping. This paper shares findings from a research project exploring the potential of a digital-material prototype capable of addressing this challenge. The project examines the possibility of incorporating material properties into digital models using respectively an analytical and a dynamics-based approach. The paper will present three design experiments with different material properties all attempting to deliberately embrace deformation as a key principle of design. This exploration of actively deforming structures is carried out using light weight dynamics simulation producing flexible and intuitive models for sketching material behaviour in the early design stages.
Article
To achieve sufficient anticlastic (negative) curvature, membrane structures are tensioned between high and low anchor points, attached to the ground, buildings or poles. By integrating flexible bending elements in the membrane surface, an internal support and shape-defining system is created that provides more freedom in design and allows reducing the amount of external supports compared to traditional membrane structures. This paper presents a computational framework for form finding of tension structures with integrated, elastically bent, linear elements, based on three-dimensional bending moment vectors and a mixed force density formulation. With an implementation of this framework in CAD modelling software, users can control form and forces by prescribing any combination of force densities, forces, stiffness or lengths to the spline and cable-net elements. Sparse matrix operations are used to compute the resulting equilibrium shapes. The shape-defining possibilities of integrating ‘bending-active’ elements in tension structures are demonstrated through a series of design studies with various boundary conditions and spline configurations. The presented framework and implementation provide a straightforward method for the design of this hybrid structural system, and, therefore, facilitate its further exploration.
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The dynamic simulation of mechanical effects has a long history in computer graphics. The classical methods in this field discretize Newton's second law in a variety of Lagrangian or Eulerian ways, and formulate forces appropriate for each mechanical effect: joints for rigid bodies; stretching, shearing or bending for deformable bodies and pressure, or viscosity for fluids, to mention just a few. In the last years, the class of position-based methods has become popular in the graphics community. These kinds of methods are fast, stable and controllable which make them well-suited for use in interactive environments. Position-based methods are not as accurate as force-based methods in general but they provide visual plausibility. Therefore, the main application areas of these approaches are virtual reality, computer games and special effects in movies. This state-of-the-art report covers the large variety of position-based methods that were developed in the field of physically based simulation. We will introduce the concept of position-based dynamics, present dynamic simulation based on shape matching and discuss data-driven upsampling approaches. Furthermore, we will present several applications for these methods.
To build a Knitted Wall
  • M R Thomsen
  • T Hicks
Ramsgard Thomsen M, Hicks T (2008) To build a Knitted Wall, Proceedings, Ambience, smart textiles conference, Gøteborg, 2008