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Selective subtraction

Denne idé er en del af The Circular Construction Challenge – Rethink Waste

mundu
8. oktober 2018

FInding an alternative for facade systems of buildings that replace, aluminium, steel & PVC extrusions and profiles with timber instantly make the majority of material used in building envelopes circular as timber is a cradle to cradle material. An acetylated timber such as Acoya would be suitable for this or a technique such as Yakisugi, allowing for the timber to be left without the need for harmful treatments.

The waste reduced from the mould process when creating a complex geometry is at least 50%. When creating a GRC panel the mould accounts for a minimum of 50% of the material and can only be used up to 3 times and must then be destroyed. By utilising kerfing one can theoretically omit the mould entirely. This then requires the reshaped wood to be locked but the locking method, whether post-tensioned will require dramatically less material than an entire mould. Transportation is also a factor as the cutting pattern can be made off-site and shipped flat and formed on site to save space when transporting. 

Building systems are made of layers of different elements. Embedding the material performance into one element that performs multiple uses saves on layers and layers of materials that perform one particular use.

 

Reshaping Wood

Wood is able to be reshaped and there are two processes available; deep drawing, more common with sheet metals but relatively new method for wood, and traditional bending methods. These bending methods can be divided into two categories: elastic and plastic shaping.

These methods often require specialist equipment, formwork or moulds, which usually end up as waste when creating complex geometric forms. The introduction of digital fabrication methods such as CNC milling, laser cutting and robotic sawing have introduced new bending possibilities such as composite kerfing that do not require formwork or moulds and allow the bending information required of the material to be embedded into the wood itself via a reductive process

For the reasons mentioned above this paper seeks to identify which processes are key to this method of bending and
These methods often require specialist equipment, formwork or moulds, which usually end up as waste when creating complex geometric forms.
seek to move beyond the state of the art examples to try to speculate on a methodology that will enable the exploration for a potential complex free-form timber facade mono-system that could embed structural and thermodynamic requirements into the material during fabrication

 

Kerfing Typologies

Kerfing is a method by which cutting into the material at an angle perpendicular to its surface to create a series of cuts or penetrations enables the wood to bend beyond its elongation ability in tension, compression and depending on the cutting pattern also shear stress. This technique is not new but the accuracy and ability to create cutting patterns that will enable a specific type of bending performance open up new opportunities for this technique. There examples from the past where techniques for cutting kerfs into wood outline the anticipated result based on incision sizes when utilising a table or circular saw. There are a number of geometrical variations of this method, and some that are only possible due to modern CAM technologies, that are outlined above.

Although the most of the variations of kerfing outlined above, bar composite kerfing, are executable without modern computational simulation and fabrication methods it is the added control afforded by computational fabrication techniques that have enabled the novel outcomes as seen with ZipShape and in the Performative Wood Studio at Harvard GSD.

Parallel kerf

This method is the simplest way to apply kerfing to wood. A series of parallel cuts allow the cut area to deform due to the accumulative local weakening and disruption of fibre continuity. The distance between the cuts and the size of the cuts can be adjusted.

Lattice Kerf

A series of parallel overlapping cuts allow the cut area to deform but due to the overlapping pattern, these areas can also twist around the torsional links that are created and allow not only increased bending performance in the direction adjacent to the cut. By offsetting the pattern and cutting it from the opposite side of the sheet material you are able to create a multi-directional twist.

Lattice Variant

A series of overlapping or interlocking cuts where the geometry differs to that of a straight cut. This approach allows the possibility of interlocking patterns such as the swastika. This enables a multi-directional deformation behaviour that allows the sheet to twist in all directions.

Composite
These methods use kerfing in conjunction with other methods to create more novel bending results. These include Schindler Salmon’s ZipShape and the Hyperboloid Structure Constructed from Pre-stressed robotically kerfed elements.

 

Fabrication of kerfed elements & state of the art

In terms of fabrication of these geometric patterns there are a number of tools available;

Table Saw - A table saw is an effective way of creating fast cuts into thick pieces of material. The bottleneck here comes with the human operation. This not only is limited to the operator’s ability to stay awake but there is also an issue when a consistent level of accuracy is required which as we will see later inhibits novel methodologies that require this level of control.

CNC Milling - With computational control accuracy and precision algorithms are able to be utilised to predict the final resultant geometry due to the cutting pattern or the cutting pattern is able to be derived from a developable surface. With 5 axis CNC mills the kerf cut is able to be placed at an angle allowing for mitred cuts. These mitred or stepped cuts add another degree of structural stability as when the cut angles are calculated correctly the need to fill the subtracted material with a glue or glue composite is avoided.

Laser Cutting - Some prime disadvantages with laser cutting is the difficulty in adjusting the depth of the cut, which unlike a CNC mill, is not reliable. Another problem for implementing a 3D dimensional kerf with this process is the lack
Fabrication of kerfed elements & state of the art

of industrial 5 axis laser cutters. On a smaller scale, however, the laser can produce more intricate patterns, these however, are more common on engineered sheet materials of a smaller depth. This type of fabrication of kerfing is more common with the maker community whereby laser cutting is a faster and easier way of prototyping 2D patterns.

Robotic Fabrication - A saw or mill on a robot arm adds greater flexibility in orientation of cuts and the tools used to make them. For example, a robot arm with a saw attachment will be able to tackle material in a faster time than a rotary drill bit.

Fabrication

Manual, industrial & computational fabrication

Manual wood-craft - A world of manual wood-craft emerged over generations and by identifying the structural integrity of each log and utilising the grain as the overarching structural backbone, always to be cut along when preservation of structural strength was paramount, as to cut across this highly anisotropic material would result in a structurally weaker element. The heterogeneity of this material and the need to preserve the grain lead to a world of complex and elaborate fabrication methodologies that, although slow, facilitated the precision to cut and bend this material whilst maintaining its structural properties.
Industrial - The industrial revolution brought with it synthetic materials such as iron, steel and concrete that sought to directly challenge wood and its dominance. The homogeneity of these materials allowed the implementation of mass industrial manufacture workflows

that were based on being able to create a standard that allowed a material’s behaviour to be predictable from component to component, such as with plywood sheets. Modernism was born of this “liberation from the in-situ improvisation of the craftsman”.
Computational - New digital technologies are now able to replicate the manual wood crafting techniques that were handed down from master craftsman to apprentice. From computational fabrication techniques to advanced wood scanning techniques. Technology for digitising each log’s makeup is already widely used in contemporary sawmills to create cutting patterns that make the most efficient use out of each log to minimise waste and maximise yield. Yet there is now a well-established normative industrial process and there are those who are utilising these engineered woods with advanced processing and fabrication techniques such as automated assembly and integrated bespoke connection details, which also lead to novel architectural methodologies and outcomes.
New ways of working with wood

CNC fabrication has had a foothold in the contemporary timber industry for a decade now. Partly due to traditional timber construction workflows having always been orientated around pre-fabrication, which made the leap from manual to digital workflows less arduous as the onus was still based around pre-fabrication. The opportunity to utilise CAM for novel architectural possibilities attracts both clients and designers that pursue free-form structures that encourage the development of new fabrication methodologies.
Digital processes enable the production of thousands of individual components, timber or not. A digital model is the most common for a fabrication methodology that requires precision machining for the parts to fit together once delivered on-site. The quality required for fabrication of complex curved geometries requires information relating to the final geometry to be “embedded into mass-customized, digitally fabricated building components”. It should be noted however this is not necessarily true for all building materials, especially when it comes to free-formed unitized facade systems that contain the information regarding their size in each panel unit but may require on-site cold bending to warp the planar panels to achieve the required geometrical positioning for the overall geometry to be accounted for.

Traditional fabrication methods usually cannot accommodate complex curvatures as well as low tolerances and here timber shines. Glue-laminated or reshaped timber blanks can be CNC milled to an appropriate tolerance. This requires every surface that defines the original reference surface be milled also. Steel can be reshaped but rather than take up any geometrical complexity in the joint it is preferable because of the flexibility afforded to reshaping timber blanks. All required drilled holes and connection details are machined into components, even those that are curved. With free-formed surfaces, local on-site systems are employed to verify locations. With digital fabrication, you can embed information into the material itself, whether those be locator notches or other marking indicating
an assembly logic.

Architectural Proposition

Kerfing is a powerful technique when fabrication of curved surfaces necessitates a mouldless approach. There is also potential for kerfing to be used when structural performance is paramount as the strategic accumulative local weakening and disruption of fiber continuity avoids unnecessary loss of structural integrity, especially when required structural loads have been catered for in the initial design process but this can also be augmented on a global scale by the macro-structural system or by using an additive process to minimize the resultant spacing that occurs. The spacing does bring with it another challenge which is the ability for the material to relax into the desired geometry. There are methods that can be utilised to mitigate the spacing created by the cut which involves creating mitred cuts.
Rather than omit the spacings entirely would there be a usefulness in functionally grading the structure? Could the resultant spacing be more than just information that contains geometrical requirements? Could the spacing be graded to perform specification thermodynamic qualities? Could one flat sheet contain enough information for it to be prefabricated off site but to contain enough information for it to act as structure and skin?

Overall Curvature

As the primary objective of this investigation is in regards to geometric complexity due to the inherent ability of kerfing, the ability to replicate a desired geometry within a certain tolerance. By taking standard approaches to complex geometrical penalisation I will be able to compare how well a kerfed alternative would perform.
Curvature Range
The two types of geometry I will aim to test within my 1:1 mock-up are:
Synclastic
(of a surface) curved toward the same side in all directions
(mathematics) of a surface whose Gaussian curvature is positive at all points
Anticlastic
(of a surface) curved in opposite ways in two directions; saddle-shaped
(mathematics) of a surface whose Gaussian curvature is negative at all points
The overall geometry will aim to integrate both conditions to test the geometric extremes, however it should be noted the ability to achieve this depends entirely on the scale of the mock-up and the final approach used to generate the desired curvature.
Wood Machining
Primarily a 6-axis robot will be used for fabrication to allow for the consistency and control that is required to create

Timber Panelling

Timber panels
The primary consideration for the mock-up will be the geometric attributes but secondary is how a number of conventional facade elements could be replaced by wood. The geometric undertaking is large but an investigation into the use of timber panels for complex geometric application is also a concern.

Moisture control
An important aspect for a building envelope but one I will not consider with equal weight in my investigation. However by using kerfing to create penetrations it would be prudent to consider how these penetrations could be mitigated, perhaps with a composite panel, whereby the kerfed wood drives the geometrical considerations and the insulation layers behind offset the resultant thermal breaks.

Fire Safety & Acoustics
As with moisture control fire safety and acoustics hold equal weighting within a building envelope but are not factors I will be investigating in my mock-up.

 

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Indsendt af mundu den 8. oktober 2018

Full abstract document available upon request.