A new temporary pavilion has been added to the campus of the Singapore University of Technology & Design as an outdoor extension of the library. The 200 m2 lightweight, catenary structure avoided the use of beams and columns to maximize its available internal space. © City Form Lab, SUTD
Thousands of uniquely shaped members form a temporary timber pavilion created by the City Form Lab at the Singapore University of Technology and Design.
July 2, 2013—When the directors of the library at the Singapore University of Technology and Design became interested in developing an underutilized plot of ground behind their campus building, they decided to try something unusual: they organized a student design competition for the space. They asked students to design a covered outdoor space that could be used for gathering and studying, as well as for showcasing the work of the university community. However, the structure also had to be capable of being dismantled after two years, since the university will be moving to a new location. The winning concept, which was submitted by four freshmen, is a lightweight catenary structure that eschews beams and columns to maximize internal space.
As is true of nearly all modern experimental designs, the creation of the pavilion, which boasts 6,848 uniquely shaped members, would have been impossible without three-dimensional visualization software.
The university constructed the pavilion last month after Andres Sevtsuk, Ph.D., an assistant professor of architecture and planning and an investigator at the City Form Lab, and Raul Kalvo, a researcher, completed the development of the design. (The City Form Lab focuses on urban design and is operated in collaboration with the Massachusetts Institute of Technology’s School of Architecture and Planning.) Once that step was completed, three engineers from the Singapore office of the global engineering firm Arup completed the structural design—namely, Mike King, CEng, MIStructE, a principal; Ben Sitler, an engineer; and Russell Cole, the office leader.
Two integrated systems comprise the pavilion’s roof: the entrance
corridor is a single curvature gridshell vault, while the main
enclosure is a true gridshell with curves in two directions.
© City Form Lab, SUTD
The finished product is a 200 m2 column-free timber canopy that rests atop a 300 m2 deck. Tiles of rolled steel serve as cladding. The pavilion offers three arched openings, providing access to the structure from a nearby path, from a small courtyard overlooking the library, and from outdoor deck space that is uncovered.
Kalvo, who wrote in response to written questions posed by Civil Engineering online, noted that to develop the winning catenary concept into a buildable design, the City Form Lab explored a number of structural systems that would be capable of creating a workable gridshell. “We initially explored a hexagonal structure, which was very efficient in material use, but ultimately shifted to a more stable triangulated structure,” Kalvo said. “The form went through many iterations in response to site constraints, access paths, views, trees, and structural logic.”
Economic considerations also loomed large. “A key consideration from the outset was that the structure needed to be cheap, as all elements had to [be] fabricate[d] on accessible machines in Singapore,” explained Kalvo. “In designing the gridshell we aimed to keep all structural elements flat, and all of their cutting lines perpendicular. These constraints allowed us to fabricate the structure on relatively cheap and accessible 2-D cutting machines, and substantially lowered the cost of the project.”
Each of the project’s 3,008 plywood panels, 3,255 spacer blocks, and 585 steel cladding tiles has a unique shape. “A precise coordination of such a high number of special elements required the use of custom scripts,” Kalvo said. The designers used Rhino, which is produced by Seattle-based Robert McNeel & Associates, along with the program Python, to fine-tune the design. Flexible scripts tailored to this project made it possible to develop the curved lines of the gridshell, Kalvo explained. The scripts enabled the design team not only to fine-tune the design until the very end but also to generate the unique geometries of each piece, along with the required cutting outlines and drilling holes, he said.
The gridshell is composed of 6,848 uniquely shaped pieces: 3,008
plywood panels, 3,255 spacer blocks, and 585 external rolled-steel
cladding tiles. Individual plywood sections—numbered to indicate
their position and neighbors—are connected with a simple hinge
and strap detail. © City Form Lab, SUTD
As it was built, the roof structure can be conceptualized as two integrated systems, according to Arup’s King, who also wrote in response to questions submitted by Civil Engineering online. He explained that the main entrance corridor is a single-curvature gridshell vault, whereas the main enclosure is a “true gridshell” that, like a dome, curves in two directions. During the design phase, King and his team used Arup’s in-house software plug-in Grasshopper to generate the structural analysis models directly from the architect’s model, an approach that minimized geometrical errors and shortened the time required to establish analysis models, he said.
The primary gridshell members are discrete plywood sections connected with a simple hinge and strap detail that together form the triangular grid modules, according to King.
The plywood sections were cut and turned on their side to create the gridshell so that the stresses along the longest axis would act in the plane, King explained. “Since these are discrete members, typically with six edges joining at each connection, a detail was developed to transfer any moments due to asymmetric loading and to provide a more direct load path,” he said. “This differs from the classical wood gridshell system, which employs long continuous laths, but introduces a different set of construction complexities.”
Plywood, while inexpensive, is not typically used in the tropical climate of Singapore, and it had to be protected from the elements, according to King. The engineers also had to devise a connection detail that served three purposes: provide axial load and moment continuity between the primary grid members, preserve the architecture’s aesthetic character, and integrate the necessary structural requirements for the openings. The openings, King explained, required stiff members for the framing and ties to resolve the pavilion’s base thrust.
The pavilion offers three arched openings that provide access to
the structure from a nearby path, a small courtyard overlooking
the library, and uncovered, outdoor deck space. © City Form
The team came up with an “elegant solution,” as King described it. The plywood units are bolted to the external steel cladding tiles of the structure, making the cladding an integral part of the structure. Steel straps located on the lower face of the pavilion work with the cladding to resolve moments from push-pull forces and provide a direct load path for the axial forces, he said. Out-of-plane shear forces are transmitted through the connection nodes—inexpensive stainless steel hinge connections from a local hardware store—and steel plates frame the access points to the pavilion and the base of the structure.
The steel and timber deck that supports the roof rests atop strip footings. Steel floor joists are used to resolve the horizontal thrust at the base of the gridshell, tying the opposite sides of the gridshell together and preventing them from spreading, according to King.
The project offered a significant learning opportunity for students, who were involved in every aspect of the design and construction. Sevtsuk, who also wrote in response to written questions submitted by Civil Engineering online, put it this way: “This was a wonderful opportunity to provide students an overview of a real building project from A to Z.”
More than 100 students participated in the final construction of the project, sorting the precut panels and creating the triangular modules prior to their delivery to the site for construction, which was led by the Asian construction firm Arina International Hogan. “Fabricating a large quantity of unique elements may always produce an unexpected cumulative error that is invisible in one or two joints, but problematic across hundreds of joints,” Sevtsuk said. “Such a
risk haunted us [until we installed] the last piece in the pavilion.” However, the site work “consisted of an orderly assembly of prefabricated elements, [and] the elements fortunately fit with each other quite well,” he noted.
For clarity, each plywood and sheet metal element was engraved with an identification number during the milling process that dictated where the element was to be located within the structure and which pieces would be its immediate neighbors, Sevtsuk said. An assembly map was made available to the contractor that illustrated the order in which the modules were to be created and delivered to the site. “A relatively complex gridshell, composed of thousands of
unique pieces, could thus be assembled by hundreds of hands using a single assembly drawing,” Sevtsuk said.
Not only has the project provided outdoor work and study space—albeit temporary—for students; it has also served as an experiment in design and construction for the City Form Lab. “There is certainly room for further research and experimentation in this area,” Sevtsuk said. “Well-planned and easily understood assembly systems could potentially allow much more complicated structures to be built without highly specialized labor.”
“The geometric algorithms we developed actually allow any structural pattern—triangular, hexagonal, quads, or irregular—to be achieved using a similar system of double-wall beams on a double-curved surface,” Kalvo added. “We hope to explore these other approaches further in the future.”
When the Singapore University of Technology and Design campus relocates, the S$200,000 (U.S.$157,600) pavilion will be dismantled and recycled. The students who conceived the pavilion—Dexter Chew, Goh Yi Qian, Avery Khoo, and Soh En Wei—participated in modeling, structural testing, and assembly, along with numerous other students who helped out.