Engineering Sidra Trees
Isozaki created his sidra tree forms using an optimization program, starting with a virtual block of metal. "Topology optimization" works out what the optimum, or lowest-weight, structure will look like if certain loads are applied to it. As the program iterates, it removes material from places in the model where there is no stress. Through this process, Isozaki arrived at the tree form to support the roof.
But such an exercise does not account for the engineering and fabrication challenges associated with a structure of this size and complexity. The task of the design and build team was therefore to navigate the technical realities in order to realize Isozaki's architectural vision.
Enter the SMART team
The entire structure of the trees and roof was modeled, analyzed, and optimized by Buro Happold's SMART (Software Modeling Analysis Research Technologies) team, a group focused on solving complex engineering problems such as those presented by unusual structures.
Dr. Shrikant Sharma, the SMART technical director, describes his group's role in the project as "resolving the geometry and the structure inside to make sure it will keep its organic form while being structurally efficient and buildable." The end result of the SMART team's work is electronic data sent to the contractor, Victor Buyck Construction, based in Belgium. The fabrication takes place in Malaysia, with final assembly in Doha.
The tree structure has two main components. The visible exterior skin is a complex curved form. Underneath it is a simplified (though still complex) structural core of octagonal tubes, each tube itself composed of flat steel plates. Sharma says his group worked to ensure that the core would be "as close to the skin of the structure as possible." The core structure was also designed to facilitate the shipping of parts, in longest-possible straight sections, from the Buyck facility in Malaysia to Doha.
This two-part geometric rationalization provided an efficient arrangement, with a solid regular structure that supports the more free-form exterior skin. It also enabled the structural engineering team, led by Brian Cole, to perform detailed design of the understructure using conventional design codes.
A complex backing frame stiffens each of the exterior plates. The frames are composed of individual strips, the outlines of which match the outlines of the plates to which they are to be attached. In addition to their unusual shapes, the frames have specific cutouts for alignment and provisions for water drainage. The specifications for each frame are defined electronically and provided to the fabrication contractor as digital information.
Managing the interface between trees and building required extra attention. "The facade basically splits the tree into two parts," describes Sharma, "and this imposes several challenges in terms of movement tolerances and thermal gradient control between inside and outside of the building."
Analysis was done to account for the difference in temperature between the air-conditioned concourse interior and the hot Qatari climate outside. Insulation in the two halves of the tree structure helps prevent temperature-related stresses. Strategic terminal expansion joints allow the skin to move differently than the underlying structure, further helping the structure withstand temperature differences and changes.
The Model Tree
Competing architectural, engineering, and fabrication requirements made a digital 3D model imperative. The complex interrelationship between the understructure and exterior skin demanded a parametric model in which all components could be individually identified and interrogated by analysis software. The SMART team also modeled the design digitally so that it could be interpreted and transmitted electronically.
Some conventional 2D engineering drawings were produced from the 3D models for the understructure. Except for some general overview drawings, no 2D drawings were used for the skin.
The SMART team has written a lot of software, some running within the drawing and some externally being called from Rhino®, the 3D modeling software from Robert McNeel & Associates. Rhino was used in nearly every stage of design of this structure, including preprocessing (tree form analysis, structural core mapping, and interface with ANSYS®), surface smoothing, stiffener-frame mapping, flattening, full electronic scheduling, and digital fabrication interface. However, the final file being sent for fabrication is a DXF of the profile that goes straight into a cutting machine.
Over a thousand panels compose the tree form. Initially each shape is cut from a flat plate and flattened, then bent and rolled into a 3D shape. "We need to work on both," Sharma says — "the 2D flat shape and the 3D shape that is put together on the tree." To that end, the SMART team used its own digital prototyping software, SmartForm, which communicates with both 3D CAD software, such as Rhino, and finite-element analysis software, such as ANSYS.
"Rhino is the closest to what you might want a 3D CAD software to be able to do," Sharma continued. "Structures like sidra trees are so unusual, you probably need a specific CAD modeling software just to do sidra trees, it is so unlike anything else. You can't really use standard CAD modeling software to do it all. You could use the interface that's provided to do a part of the job manually but then must actually do some programming to get the full effect."
In Rhino, with a set of tools, you can begin with a solid shape and start cutting it into joinable pieces. Because it is a "very light application," Sharma says it can easily handle a massive model, such as the model of the sidra tree.
Designing for Fabrication
Sharma reports that one of the biggest challenges of the project was to minimize the number of double-curvature panels in the tree structure. Some sections — such as a simple, straight part of a branch — could be formed from flat plates that had been bent or rolled in only one direction, creating a cylinder or a conical barrel. But other parts of the tree required plates with two curvatures, with forms like helmets or saddles.
The process of giving double curvature to steel plates is more costly and time-consuming than single-curvature formations, so the SMART team was charged with maximizing the use of the simpler panels. "That is a challenge when you're trying to make something which in the end needs to look like a smooth geometry, like a tree, which has change of direction and change of cross section everywhere," says Sharma.
The team used SmartForm to draw and analyze the geometry of the trees and determine a set of curves to subdivide the structure into regions that are easily approximated to single curvature. Where possible, the dividing lines occur where the curvature of the tree structure changes, so that neighboring plates can each be bent in just one direction and still form a smooth curve together.
"The whole tree is actually made up of 70 percent single curvature panels and 30 percent double curvature panels," reports Sharma, "but when you see it in 3D you can hardly tell the difference."
"We put double curvature panels only where they are absolutely needed, which is mostly towards the 'nodes,' where more than two branches meet," he continues. "The difference is very subtle, but that was a programming challenge."
Another challenge faced during the project was to be able to identify every single plate, and every single strip that connects to each plate, and to understand and communicate how they're all connected — both during the design process and during fabrication and installation.
"Every single bolt had to have its own identification, its own geometry and its own connectivity with the neighboring panel, so you can actually interrogate them, check for clashes, check for the relationship between them," Sharma explains.
"The information that's been fabricated in isolation also needs to come together and they need to know exactly what goes where. Every plate is cut with a clear idea of what direction it will go into the final jigsaw."
Each backing strip, for example, sports notches and edge markings that communicate which direction that piece should be installed. An ID and numbering system describe how each piece fits into the overall tree system.
The Trees Grow
The sidra tree structure is currently under construction. The roof has been lifted up and the foundation completed. Underway now is the first level of the trees, a series of massive plate girders or I-beams running in two directions. As the roof is lifted to its final height, about 20 meters (66 feet) above the ground, the tree forms are being built underneath it. The whole tree structure is slated for completion by October 2008.
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Susan Smith is the editor of AECCafe, an online news portal for the architecture, engineering, and construction industry, as well as GISCafe and GISWeekly, an online portal and weekly magazine for the geographic information systems industry. She has been writing about architecture and technology for over 15 years and resides in Santa Fe, New Mexico.