Self-Detailing and Self-Documenting Systems for Wood Fabrication ...

Self-Detailing and Self-Documenting Systems for Wood Fabrication: The Burj Khalifa

Neil MeredithGehry Technologies

James KotronisGehry Technologies

Abstract. This paper tracks the design, fabrication and installation of a complex wood ceiling in the Burj Khalifa office lobby in Dubai, UAE. Self-detailing and self-documenting systems are here understood as the use of parametric tools to capture and reuse design and fabrication know-how. This approach seeks not to supplant the designer or fabricator’s skills, but to create a shared platform for material innova-tion open to all the participants in the design team. By treating the production and detailing of this case study project as a system instead of a discreet design object, the authors also seek to illustrate the changing role of computational design tools and the implications for the practices of detail design and fabrication. Case studies are given on the usage of the CATIA/Digital Project Knowledge-ware environment to capture design intent and fabrication details in a system that allows them to adapt to new geometric configurations given different inputs; mate-rial bending limits and systems used to describe and map their physical constraints; automated tools built around the sorting and managing of wood veneers; and ratio-nalization strategies related to wood bending and fabrication.

1 Introduction

A complex, double-curved wood ceiling for one of the main entrance to the Burj Khalifa (then Burj Dubai) was under construction and it was apparent that the pro-posed material system would not work as designed. Instead of delaying the schedule or scrapping the design, an integrated team was quickly mobilized. This team was based in different design and fabrication domains, with all participants working to-ward the shared goals of redesign, fabrication, delivery and installation of the new ceiling, all within a tight schedule and construction site. Partners included Skidmore Owings and Merrill (SOM), Imperial Woodworking Company (IWA), ICON In-tegrated Construction and Gehry Technologies/New York (GT). Working through the design issues, the new team developed a strategy to strip the design back down to essential geometry and redesign the system from the ground-up while satisfying

N. Meredith and J. Kotronis

design intent and a host of fabrication and constructability constraints through a shared parametric model. The previous system—a stick-built plank system wrapped over a series of ribs—worked within the constraints of a small physical mock-up but did not scale up prop-erly to the required geometries for the finished ceiling. A new system was developed using a pre-fabricated unitized panel approach. Although more risky in terms of on-site adaptability, building the panels offsite in IWA’s Chicago woodshop and then shipping them to Dubai in stages gave the added benefits of quality and speed. Now not only did the panels need to arrive on-time within a very aggressive eight-month schedule, but they needed to be built perfectly, arrive undamaged, and be pre-coordinated for installation and all surrounding building elements (structural, mechanical ductwork, interior finishes, lighting, etc.).

2 Systems Approach

From the beginning the focus was on a high quality digital process so the panels could be manufactured and installed correctly on the first attempt. As is often the case, high-pressure deadlines become the driving force for a more innovative fab-rication and delivery process. Within this new project structure, traditional drawing and submittal processes were adapted to work with digital model files as contract deliverables and fabrication information, not out of an abstract desire for an “in-tegrated delivery process” but out of a very real concern for what was best for the project in terms of speed, quality and cost.

DESIGN INTENT

COMPLETION

PANELIZATION

MATERIAL BENDING LIMITS

SURVEY DATAAS-BUILT

COORDINATIONDRAWINGS

ON-SITEASSEMBLY

CNCTOOLPATHS

VENEERINVENTORY

COORDINATION

SHIPPING

FABRICATION

PLANKING

DETAILING

SYSTEM DESIGN

VENEER MAPPING

SHIPPING & SEQUENCING

KE DESIGN SURFACE

COORDINATIONMODEL

FABRICATIONMODEL

SURFACE &PLANK MODEL

KE PANELIZATION

KE PLANKING

KE PANELASSEMBLY

KE VENEERMAPPING

Figure 1: Project Process Diagram.


A priority for the project team was developing a systems approach. Paramet-ric design tools and automation are nothing new either in fabrication or architec-tural design. The presence of these tools guarantees simply that: their presence. An emerging digital craft culture, one that directly engages the people making and shap-ing materials and processes, points to a new and more considered process. Fundamental to this project was the idea of reuse. Given the iterative nature of working with an involved design and complex set of fabrication constraints, the sys-tem evolved along two lines. The first was a set of drivers, or parametric geometric rigs. Typically made from simple surface and wireframe geometry, they form the adaptable geometric core of the design model. The second system, building on the parametric driver core, is a set of modules related to panel fabrication. By modular-izing the technology and explicitly managing the relationship to the driver geome-try, these modules (built in the Digital Project Knowledgeware environment) depart from a typical scripted or automated approach to model design, and focuses instead on organizing models around specific design and fabrication goals. A script-based approach would typically build in complexity linearly from the base wireframe geometry out to resolved 3D geometry, folding in constraints as you go, growing in complexity. This can often result in a “house of cards” model by where simple changes anywhere can cause things to break or become overly complicated to the point where only one person can control or adapt the script (typically the scripts author). By explicitly managing inputs and outputs and taking a system approach to the design of the model, multiple parties can work in parallel on the same model, ad-vancing complex modules with relatively simple links to the core driver geometry.

3 Drivers

Comprised of simple surface and line geometries and built in a way that is fully adaptable, these drivers form the flexible geometric core of the system. Inputs and parameters can be adjusted or swapped for other inputs, and the outputs from these modules can form the basis for other, more sophisticated downstream processes, as we will see later in this paper. At any point the system can respond to updates or changes.

3.1 Physical Testing

The first step in developing the drivers was mapping the physical constraints of the desired materials. Before we even approached the computer, IWA performed some quick bending studies with a series of plank samples (MDF cores with veneers on each outside plank face) over different radii circles. Though quick and simple, these tests formed the basis of an understanding of maximum bending stresses allowable within the surface design. These were the first of many “physical computers” that guided and shaped the work being done behind the screen.


Figure 2: Physical Testing of Wood Bending Limits.

3.2 Surface Rationalization

The original design surface was then rationalized to maximize geometric simplicity while still meeting the design criteria and minimum bending radii of tested plank materials. As with any project, a certain vocabulary developed around some of the more abstract geometries to allow for quick reference. Moving from the North to South on the design surface we have the beach area, so called for the way that it flat-tens out from a vertical to horizontal surface. The southern end, or the boat consists of two swept profiles trimmed about a central intersecting plane. Connecting these two halves are the arms, two bridge-like geometries linking the disparate halves. The first general move was to flatten out the underside profile of the beach. This simple change created a whole zone of like panel types while not compromising the overall feel of the design surface as you approach it from the escalators. Another key step was substituting blended or lofted surfaces with more rational surface types in both the beach and the boat areas. Panels made from torus sections and swept surfaces (as opposed to more free-form surfaces) have the key benefit of being able to use the same repeated unit while still allowing for double curvature within the panelized surface [Peters 2008][Pottmann et al. 2007]. The arm surfaces created the greatest challenge in the driver surface design. A series of free-form surfaces were eventually developed that maintained surface tangency to the beach and the boat, while still falling within the various bending constraints for fabrication.


PRIMARY DRIVERS

BL END WIREFRAME

BLEND SURFACES

DRIVER SURFACEFigure 3: Surface Drivers.

COMPLETED SURFACE DRIVER

M AJOR PLANKING DIVISIONS

M AJOR PANEL DIVISIONS

COMPOSITE PLANKING/PANELS

Figure 4: Planking/Panel Drivers.

Working with the design team, the design intent was maintained throughout this design and detailing process without either trying to simply “build the rendering” or losing major features of the design. With all the parties engaged in the process and assessing the results in a single shared parametric model, the team was able to move forward with confidence in both how the design would look and how it would be built.

3.3 Panelization and Planking

With a controlling surface in place, many iterations of planking and panelization options were then tested. From the underside of the ceiling, the surface is a continu-ous field of planks. Looking at the back surface, rectangular panel divisions simply follow the same divisions as the planking, picking up lines as necessary. Having a parametric rig for the development of both the planking and the paneling was key as this was not a simple linear process, but instead an iterative back-and-forth conver-sation between maximum panel sizes, constructability, changing planking widths, and various design options for the planking itself. More prosaic constraints such as the size of the freight elevator available on site were worked along side more design-oriented issues such as the typical width of the plank. Due to the varied topology of the design surface, geodesic curves—or curves that follows the shortest distance along a surface between two points—are used to simulate the natural bending properties of wood planks as they are laid upon a double-curved surface [Pottmann et al. 2007]. Other geometric techniques such as


projecting curves on a surface or using surface intersection have the problem of geo-metric overdeterminancy. When you project a line or do any other geometric trans-formations to produce a curve, you introduce an orientation or a visible geometric approach. The curve reads as something applied to surface, revealing the hand of a strict geometric system. Alternately when the lines resolve along a surface as with geodesic lines, the resulting curves maintain a natural relationship to that underlying surface curvature [Pottmann et al. 2010].

DOUBLE-CURVED SURFACE

GEODESIC LINE

FLAT PLANK LAID-UP ON SURFACE

UNFOLDED PLANK (STRAIGHT EDGES)

Figure 5: Geodesic Line on Double-Curved Surface.

4 Fabrication

4.1 Wood Plank System

Taking the wireframe planking curves and surfaces as inputs, a series of flexible components were designed to parametrically reconfigure to various detail condi-tions. The first module to use this approach was the three-dimensional wood planks forming the visible side of the panel. Due to the desire for variegated veneers across the surface, individual planks with different veneers needed to be laid-up on the panels. Complicating this arrangement, many of the panels had an anticlastic surface topology, requiring individually cut and finished planks for many of the panels. In areas where the panels had one-degree curvature across the design surface (aided by the rationalization of the input surface in to toroids, planar, and swept surfaces with evenly distributed geodesic plank lines) a single plank profile could be used for the entire panel. In panels with compound curvature, individual profiles would need to be modeled in three-dimensional space, unfolded and then individu-ally CNC cut for placement back on the panel. Therefore these planks were not true planks, but were instead unfolded developable surfaces [Pottmann et al. 2008]. Automating the modeling and unfolding of these surfaces accurately was a complex challenge, especially when factoring in material thickness, the topology of the input curvature (concave vs. convex surfaces produce slightly different un-folded surfaces when factoring in the thickness of the board) and the true fabri-


cation and bending limits of the materials [Kensek et al. 2000]. A CATIA/Digital Project Knowledge Pattern aided the quick prototyping of the geometrical solution, providing a framework for future geometry development without having to recode the entire automation routine. Utilizing this catalog-based approach, different plank configurations could be swapped in to the Knowledge Pattern without having to alter any of the instantiation scripts or input driver geometry (samples are available at http://www.sheetd.com).

Figure 6: Planking System Knowledge Pattern.

4.2 Veneer Mapping

Another challenge with prefabricated units was predicting the distribution of the wood grain across the entire ceiling surface. In a typical plank-based design such as a floor, veneers are sorted for aesthetic criteria and then selected by craftsman to ensure the even distribution across the surface, avoiding dark and light patches or clustered areas. This ceiling was built as a unitized system of panels, fabricated offsite and out of sequence. (Due to the schedule, some panels were still being built in Chicago while the initial grouping was being installed in the UAE.) Often called veneer “randomization” the eventual solution was hardly random, but instead sorted and mapped veneers to discreet planks in an automated system. The tool also al-lowed for multiple scenarios to be tested in an iterative digital design environment, giving the craftsman a tool to visually asses the veneer placement while still allow-ing for a new level of control during fabrication. In the end, every veneer for the entire project was tagged and managed across the entire ceiling surface through this method. In building the veneer tool—developed by Victor Keto in the GT New York office—we looked at the existing technologies that the fabricator worked with. Following their typically process, the shop would purchase logs of veneers. These


would then be hand-sorted by various aesthetic categories down to parts, bundles and leafs. In this system the fabricator sorts the veneers by the same craft-based visual characteristics that they are accustomed too, but what is introduced is a new way of managing and applying the sorted veneers. The craft remains, but a new automated system extends this knowledge to a new scale and level of control. Given a database of veneers and a plank geometry target model, the automated software routine maps the veneers on to the geometry, filtered through a set of rules that govern the adjacencies of the various veneers. Again the idea was to extend the craft-based approach to veneer sorting, but capture this intelligence in a set of rules or system that can be applied on future projects. So using the adjacency matrix—or a diagram indicating what veneers can or cannot be next to each other—the automated routine grabs a veneer from the database, checks that it fits geometrically, and if it has a match, color codes the plank model and removes the plank from the database. This stochastically executes across the entire design surface till all the planks have a corresponding veneer. Each time the tool is run a new pattern emerges. Once com-pleted, all the veneers are accounted for both visually and within the database. One unanticipated outcome of the tool was the addition of another log of veneers for the project. Initial trials of the planking module did not produce random enough variation across the surface. After adding an additional log to the database, the dis-tribution loosened and visual randomness increased across the surface, revealing a gap in the initial rule-of-thumb calculations for veneers. Only after taking the step to capture the design intent and fabrication means-and-methods in to a collaborative design model, did a coordinated result emerge. Although developed as a specific tool for the use in this project, it has some general portability and usefulness outside the confines of this single use, extending the system as a technology and the reach of the fabricators “design intelligence” on to other pre-fabricated veneer panel applica-tions [Speaks 2003].

Figure 7: Veneer Mapping.


4.3 Panel Types

While the field of computational design has opened up new possibilities for the infinite variation of parts in an assembly, a very real-world constraint that is often ignored amongst the technological exuberance is the actual desire for some types of standardization. Standardization was pursued along two tracks in the Burj ceil-ing, through creating reusable panel types, and the capture and reuse of detail types within the panels themselves.

Figure 8: Panel Types.

Maximizing the reusability of panel types ultimately allowed for a simplified approach to fabrication without sacrificing the design intent. In addition to cost and time savings, quality also increases when you reduce the number of unique detail conditions. Standardization was obviously not the first project concern given the complex and topologically varied nature of the design, but within the framework of the design, certain panels could be repeated without affecting the outward vi-sual nature of the finished ceiling. The fabrication modeling sought to capture this approach, making subtle tweaks to the driving surface and the planking/paneling scheme to produce like panels in certain areas, all the while maintaining a smooth and continuous curved ceiling surface. In total, 76 unique panel types were devel-oped that were able to work for all 195 panels across the entire ceiling. Several factors contributed to this. Because all the pieces were fabricated using 3-axis CNC manufacturing for both the backing frame and the finished plank pieces, panels that were three-dimensional mirrors of other panels could be fabricated by simply mir-roring the cutting profiles. Additionally, due to some of the early stage surface ratio-nalization there were more panels built on flat, swept, or toroid geometry, allowing for additional panel reuse within these zones.

4.4 Detail Reusability

A second type of efficiency, standardizing typical details and material configurations within the panels themselves, was pursued through another Knowledgeware tech-nology called Document Templates. Within a Document Template, a set of input ge-ometries and parameters create the foundation for a parametric rig. This model can


then reconfigure itself to a new geometric context while still maintaining established relationships and rules (minimum edge distances, minimum part size, wood bending within tolerance, etc). This is similar to the planking Knowledge Pattern described above, but Document Templates depart from more typical automated approaches in that in addition to creating unique parts, they can also contain standard parts gath-ered in an assembly model. Furthermore all the geometry in this assembly can be linked to automated drawing outputs. Though not simple to set-up and debug, these Document Templates can accelerate a design process through the use of rule-driven self-detailing and self-documenting assemblies. Detail modules within the assemblies can exist not only as stand alone parts but can include a sequence of operations or actions. To take one example, the slotted “dog bone” connection detail used to snap together and register all the frame compo-nents is geometrically unique to every frame member, but the module that produces the cut is stored in a catalog and can be applied any number of similar connections.

Figure 9: Document Template Driver Geometry.

Our general approach was to use the templates to establish the general topology of the panel (number of planks, rough placement and divisions of frame members, etc.), which could then be individually detailed and tweaked right up until release for fabrication. A typical workflow was to instantiate a panel assembly, asses how the parts worked together, make dimensional adjustments (such as the spacing of the backing frame), and then run an update to sync all the elements in the assembly and update the drawing outputs. Although technically possible to create a template that utilizes a series of rules to completely detail itself, we took a softer approach by where the template creates all the necessary parts and relationships, but the final positioning of pieces were adjusted visually within the parametric framework. This provided a nice balance, by where we automated the repetitive geometry creation but still allowed for enough slack to make sure details actually worked or there were not any strange or fussy panel configurations.


Developed in concert with the fabrication team, the Document Template evolved to support direct panel fabrication from the digital files. Using the surface and plank driver geometry as inputs, this template produced a number of outputs for both visu-alization and fabrication. Each of the separate material layers in the panel assembly was either unfolded or laid flat for eventual cutting on a CNC mill. Moving from the finish face of the panel to the interior, the panel is composed of: planks made of an MDF core with a layer of veneer on both faces of the plank to avoid delamination, two layers of bending plywood in alternate grain directions, and then a plywood “waffle frame” system. The “waffle frame” system is comprised of a interlocking rib members that create a 3D assembly. Individual pieces were snapped together using the slotted grooves pre-cut in the pieces. The entire assembly is then either compressed under pressure with various glues to create composite strength (in the case of the thin bending ply layers) or is mechanically attached (in the case of the frame). Once all the pieces are sanded and assembled, the entire panel is sealed as a unit to create a water resistant finish. After crating and shipping, each of the panels were then supported from the ceiling using a suspended rod system.

PLANKS

BENDING PLY 1

BENDING PLY 2

FRAME BORDER

FRAME INFILL HORIZONTAL

FRAME INFILL VERTICAL

Figure 10: Panel Assembly.

Remarkably the entire panel fabrication was done with 2D cutting profiles. That is not to say that the fabricator was not working in 3D, but the understanding of 3D had more to do with assembly and fabrication of actual materials in space instead of digital models. The template utilized a 3D solid model, but the Document Template CAM outputs were completely 2D in nature. Instead of relying on robotic fabrica-tion or multi-axis machining, a series of registration elements were output from the model for use during assembly. Looking at one piece of the assembly, the “waffle frame” is a well known and fairly easy-to-assemble construction given the preva-lence of CNC fabrication, but to ensure the larger project goals such as continuity between adjacent panels and tolerancing, the edges of the frame were projected on to a single plane to create a “tabling” jig at 1:1 scale and CNC cut out of ¼" MDF. In addition to the direct-to-fabrication cutting of the plank and frame pieces, various other jigs and templates aided panel continuity and assembly.


Figure 11: Automated 2D Outputs.

Here was another instance where the know-how of the fabricator—by commu-nicating their working methods and working out what they actually need from the model to accurately build the form—was integrated in the model as direct outputs. Complex sets of dimensioned drawings were supplanted by minimal sets of 2D 1:1 files. This was only enabled through iterative design and detailing cycles and trust in the resolution of the geometry within a 3D project database. (Files were central-ized and versioned through a Subversion (SVN) server which was used to sync and manage files across the different geographic locations.)

Figure 12: Detail Design.

After gathering all the pieces together in this “master model” [Shelden 2002] de-tails were then refined and expanded in the 3D environment with all the appropriate links to the upstream driver geometry and the downstream panel fabrication models. While representing an investment in time, effort and infrastructure, the flexibility of the system allowed for rapid update cycles late in the design process. To take one ex-ample, 3D survey data of as-built conditions was continually crosschecked against the developing fabrication model. Fairly late in the design process it was discovered that some of the steel interfacing with the boat section was not within the assumed design tolerances and needed to be updated. Instead of remodeling these parts, the driver surface geometry was slightly tweaked, producing a ripple effect of changes though the fabrication model. These panels were updated to the new surface and reissued just prior to actual panel fabrication.


Figure 13: Panels During Assembly.

5 Conclusions

Through the iterative design and analysis cycles enabled by a structured paramet-ric model, the Burj ceiling was conceived more as an approach to wood and wood bending in complex architectural design, as opposed to the execution of a singular, signature design. This system relied on a controlled set of geometric drivers and reusable geometric modules related to fabrication, allowing for multiple iterations and models within a shared parametric design environment. Though CATIA/Digital Project and the respective Knowledgeware based toolset were relied on heavily throughout the process, the general approach to creating a system for design has general portability to many other building projects. In proj-ects since the Burj we have relied on up-front diagramming of the model structure as a way to communicate this approach to the project team in the early stages of the design process. Likewise in developing approaches to driver geometry and the as-sociated modules. Future research looks to extend this systems based approach to other projects as well as generalize and reuse the design intelligence developed within certain mod-ules, finding new uses for typical geometric modules (veneer sorting, automated panel detailing, batch unfolding of developed surfaces, etc.) as well as a more uni-fied approach to driver design and rationalization.


Figure 14: Completed Ceiling.

References

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Pottmann, H., asPerl, a., Hofer, m., and Kilian, a. (2007). Architectural Geom-etry. Exton, PA: Bentley Institute Press, 289–304.

Ibid, p558.Pottmann, H., Huang, Q., deng, B., scHiftner, a., Killian, m., guiBas, l., and

Wallner, J. (2010). Geodesic Patterns, ACM Transactions in Graphics 29/4, 43:1–43:10 (Proc. SIGGRAPH).

Pottmann, H., scHiftner, a., Bo, P., scHmiedHofer, H., Wang, W., Baldassini, n., and Wallner, J. (2008). Freeform Surfaces From Single Curved Panels, ACM Transactions in Graphics 27/3, 76:1–76:10 (Proc. SIGGRAPH).

KenseK, K., leuPPi, J. and noBle, d. (2000) Plank Lines of Ribbed Timber Shell Structures, Eternity, Infinity and Virtuality in Architecture [Proceedings of the 22nd Annual Conference of the Association for Computer-Aided Design in Ar-chitecture / 1-880250-09-8], 261–266.

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sHelden, d. (2002). Digital Surface Representation and the Constructibility of Gehry’s Architecture. Ph.D thesis in Department of Architecture, Massachusetts Institute of Technology, Cambridge MA.

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