The Use of the Cassette System in the Construction of Irregular-shaped Building Facades: A Case Study of SKT Tower in Seoul, Korea

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1 The Use of the Cassette System in the Construction of Irregular-shaped Building Facades: A Case Study of SKT Tower in Seoul, Korea Seonwoo Kim 1, Ghang Lee 2, Min Jeong 3 and Hoonsig Kang 4 1 Building Informatics Group/ Graduate Research Assistant, Department of Architectural Engineering, 2 Building Informatics Group/ Assistant Professor, Department of Architectural Engineering, 3 Hanmi Parsons Co., Ltd., and Building Informatics Group & Department of Architectural Engineering, 4 Building Informatics Group/ Graduate Research Assistant, Department of Architectural Engineering, Yonsei University, Seoul, Korea Abstract: Owing to advancements in curtain wall design and construction technologies, it is not difficult to find examples of twisted, tilted of tapered buildings these days. The SKT Tower in Seoul, Korea, is just such an example. The SKT Tower has a visually interesting design. Curtain wall panels are attached irregularly to a high-rise structure, which is bent at the center. In order to build such irregular-shaped buildings, project teams face unique and complex design and engineering challenges throughout the design and construction processes. Curtain wall systems are one of those challenges. The design of curtain walls must take the manufacturing, delivery, lifting, and assembly process, weather conditions, maintenance issues, and the overall design intent into account. When buildings have irregular shapes, the design requirements become much more challenging to satisfy. This paper reports on the challenges and solutions of a successful case of constructing irregularly aligned curtain walls on the tilted surfaces of a 33-story building. Key words: irregular-shaped building, cassette system, surface, curtain-wall unit Introduction Irregular-shaped buildings are buildings with tapered, tilted, twisted, or curved shapes. The Bilbao Guggenheim Museum, designed by Frank Gehry, is a well-known example of an irregular-shaped building, and the development team faced many challenges during the design and construction processes [4, 7]. The problems were resolved by adopting state-of-theart CAD/CAM (Computer Aided Design and Manufacturing) technologies and developing a new penalization method for the surface of the building [11]. The Chinese National Swimming Center, which was completed recently, is another example. It was designed in the image of water bubbles by PTW Architects. The design and structural problems in this case were solved by repeatedly using a unit object called a water cube and, again, by adopting advanced computer and information technologies commonly known as BIM (Building Information Modeling). These examples show an architectural trend of today. The design of buildings is becoming more complex. Although there are many successful cases of irregular buildings, curtain wall construction of irregular-shaped buildings is still challenging. The SKT Tower, the new main headquarters building of SK Telecom, a telecommunications company located at

2 Ulziro, Seoul, was not an exception. The building shape is slightly sloped and bends forward from the 27th floor to represent the building bowing to customers [13]. This paper reports on the engineering challenges and solutions during the curtain wall construction of the SKT Tower and on the lessons learned. Fig. 1 SKT Tower, Ulziro, Seoul (Photograph by Seonwoo Kim) Fig. 2 Kim) Background SKT Tower Facade (Photograph by Seonwoo Many studies have been conducted on the design process of irregular-shaped buildings, but there have not been many studies on the construction problems of irregular-shaped buildings. For example, many of the previous studies focused on the methods for developing and representing complex forms, such as rapid prototyping and CAD technology, during the design phase [3, 9, 10]. Interference checking using 3D and 4D technologies between buildings and other subbuilding systems was another topic [1, 2, 8]. There have been a number of case studies on Frank Gehry s buildings, but many of them focused on the design process or organizational structures [6, 12, 14]. One of a few exceptions is Denis Sheldon s Ph.D. thesis on the surface panelization method for Frank Gehry s buildings using CATIA [12]. Another one is Yun and Schodek s proposal for the boundary structure. It was an effort to build irregular-shaped building structures in a more efficient way. They showed the possibility of adopting the proposed boundary structure to build irregular-shaped buildings by developing a partial model of the Bilbao Guggenheim Museum [5, 15]. There are a few more studies on the construction aspects of irregular shapes [7, 12], but the studies on construction problems are still rare. This paper reports on the challenges and solutions of the SKT tower, focusing on the curtain wall construction process. Construction Challenges The SKT Tower is a 33-story office building which is designed by Aaron Tan at RAD (Research Architecture Design). SK C&E (Construction and Engineering) was the general contractor. The construction period was from January, 2001 to December There were many problems in detailing, fabricating, lifting, installing, and waterproofing curtain wall units due to their irregular shape. The following sections report on them in detail. 1 Iterative Adjustment of the Large Number of Distinctive Unit Types The SKT Tower was much more complex than now when it was first designed. It had a zigzag facetted skin composed of thousands of crank-type curtain-wall units. The design was economically and practically impossible to realize. The number of unit types was unrealistically large, and it was too expensive to manufacture each of them and assemble them together. The design was rationalized through much iteration of meetings between Aaron Tan, the chief architect, and contractors, to reduce the number of curtain-wall unit types while keeping the original complexity of the design.

3 2 Detailing and Fabrication Issues of Curtain Wall Units The design, modified through many cycles of adjustments, still had problems when it was sent for curtain-wall fabrication. The fabricator felt that there were still too many aluminum bar types to manufacture them within budget. Since manufacturing of custom aluminum bars includes multiple steps and takes a long time, it becomes exponentially complex and expensive as the number of bar types increases. A typical manufacturing process of custom aluminum bars includes design and engineering, shop drawing generation, aluminum die casting, pressing out, coating, insulating, metalworking, water-proofing, glazing, and installation. Also, the proposed design had a 600mmthick unit, which caused a structural fraction at the connection. The meetings between the architect and contractors continued. The design was modified through the seemingly endless number of meetings and experiments. First, it was agreed that there should be a straight element that could safely support an inclined element. A 580mm-thick element was designed to hold a 200mm-thick straight element and a 380mm-thick inclined element. However, the shelves created, due to the difference between the extrusion depths of curtain wall units in this design, were regarded as problematic for maintenance and repair. At the end, considering all these factors, the average thickness of the curtain units was adjusted to 450mm. Also, a certain degree of regularity in design was obtained through the design iteration process, which was kept until the end of the (a) A cassette system (b) Assembled cassette systems Fig. 3 Cassette systems (By courtesy of Taeoung Kwon, BL Space) construction, although the design was changed several more times after this phase. The next challenge was the manufacturing of the curtain wall units. Even though the number of curtainwall unit types was cut through numerous design iteration cycles, a new manufacturing method was required to engineer and manufacture the previously non-existing curtain wall types. In order to engineer and fabricate a new curtain wall system, Visionwall Corporation in Edmonton, Canada, participated in the project as an energy and thermal environment consultant, Texas TWS Corporation in Dallas, U.S., participated as a fabrication process consultant, and the ESCO Corporation in Ditzingen, Germany, took part as a hardware consultant. Bob Johnston, a curtain wall expert at CDC (Curtain wall Design Consulting) led the new curtain wall development process. The basic idea for simplifying the installation and assembly process of the sloped units was to attach independently manufactured sloped units to straight structural units by pushing the sloped units into the pre-installed structural units like cassette tapes. The two external and internal units were joined through a bulb gasket. A bulb gasket is a mechanical seal between two objects for blocking air flow and for water-proofing. This cassette system and the separation of the structural elements from the sloped elements allowed manufacturers to reduce the number of cast dies for aluminum bars. The number of distinctive unit types was reduced to almost half (from 18 to 10), although the total number of units was still 4,500. The thickness of the structural unit was reduced from 200mm to 175mm. Also, the installation process became simpler because the structural units could be installed like normal curtain wall units. The design was refined by considering insulation, noise, wind loads based on the location and the height of the building, and other structural loads. Local contractors including BL Space, Il Jin Aluminum, and others played significant roles in this and in the construction phases. Additionally, interferences between units were checked using 3-dimensional models. Finally the refined solution was tested through

4 a mock-up test. Fig. 4 A curtain wall mock-up (By courtesy of Taeoung Kwon, BL Space) 3 Ventilation and Water-proofing Problems Ventilation is one of typical problems for any highrise building due to strong winds. It was a more difficult problem for the SKT Tower because of its uneven surface. After considering various options, a window that could be pushed out from the inside to the outside was selected, after considering various factors including wind loads and waterproofness. Water and air were designed to be blocked by three layers. 4 Construction Issues Since the SKT Tower is tilted, accuracy and safety were the most important considerations in lifting, plumbing, and installing the 4,500 curtain wall units. Especially, the reversed surface from the 27th story to the top floor was a problem. The unitized curtain-wall construction method was selected. Prefabricated curtain wall units were delivered to the site, lifted, and fixed to pre-installed anchors. Since it was difficult to distinguish one unit from the others because the differences were minor, units were categorized and numbered. Each unit received a unique serial number when it was designed for fabrication. The serial number was used throughout the manufacturing and construction processes. In order to make the construction process simple and flexible, units of the same unit type received a sticker, which indicated that the units were of the same unit type, so that they could be used interchangeably. For lifting and installation of the curtain wall units, a modified building maintenance unit (BMU) was used. A BMU is a lifting system that is typically used for cleaning and maintaining the surface of high-rise buildings. Since the cassette units had to be pushed in at the inclined surface and pulled out at the reversed surface, a relatively stable BMU was used as a work platform for most places. The BMU was modified so that it could access sloped parts and corners of the building with ease, and so that it could also carry heavy curtain-wall units. However, there were still areas where the BMU could not access. For those places, curtain wall units were lifted using a winch at the roof, and were installed directly from the outside. Despite the irregularity of the building shape, the straightness of the curtain wall units was checked using traditional plumbing methods. Still, the construction errors were within 30-40mm. 5 Considerations for Maintenance An initial façade design contained inversed triangular areas, which birds preferred as a location for their nests. The inversed triangular shapes were removed in the final design. Also, there was a concern about the glass being quickly polluted due to the slope. Consequently, this concern was dealt with by applying oxidized titanium self-cleaning glass and by adopting a robotized cleaning system in addition to the BMU system for periodic manual cleaning. Lessons Learned The SKT Tower seems relatively simple as an irregular-shaped building compared to other irregular shaped buildings. However, it had various complex problems, even when we only considered the curtain wall construction. Through the case study of the SKT Tower, several lessons were learned. First, the project could not have been realized if there had not been active collaboration and communication between the participants. Second, considering the fact that numerous designs were abandoned due to constructability issues in the SKT Tower project, the importance of the early

5 involvement of engineers has to be emphasized, especially in relation to irregular-shaped buildings. For ordinary buildings, it is a matter of whether a building can be built at a lower price or not, but for irregularshaped buildings, it is a matter of whether a building can actually be built or not. Third, the SKT Tower shows that even a simple building like the SKT Tower becomes very complex when a building design goes outside of a typical shape, and there must be continuous efforts to accumulate and develop advanced technologies for the construction process of irregular-shaped buildings. These construction technologies and experiences will make possible the building of more complex designs. Acknowledgment We are truly grateful to Taeoung Kwon, BL Space, Seoul, Korea for his generous assistance, and also for his passion. This work was supported by the Korean Institute of Construction & Transportation Technology Evaluation and Planning (KICTEP) with the program number of "06-Unified and Advanced Construction Technology Program-E01. Structural Engineer 78 (12) (2000) [8] L. Khemlani, Revit Structure 2008, Review, AECbytes, (2007). [9] J. Park, A Study on Digital Technology for the Construction of Curved Forms, Architectural Institute of Korea 21 (12) (2005) [10] G. Ryder, B. Ion, G. Green, D. Harrison, B. Wood, Rapid Design and Manufacture Tools in Architecture, Automation in Construction 11 (3) (2002) [11] F. Scheurer, Getting Complexity Organised: Using Self- Organisation in Architectural Construction, Automation in Construction 16 (1) (2007) [12] D. R. Shelden, Digital Surface Representation and the Constructibility of Gehry s Architecture, Architecture: De-sign and Computation, The Masachusetts Institute of Technology (2002). [13] SK C&E, SKT Tower Construction Handbook, SK Telecom Co. Ltd., Seoul (2005). [14] Y. Yoo, R. J. Boland, Jr., K. Lyytinen, From Organization Design to Organization Designing, Organization Science 17 (2) (2006) [15] Y. G. Yun, D. L. Schodek, Development of Boundary Structures for Complex-Shaped Buildings, Journal of Architectural Engineering 9 (1) (2003) References [1] M. M. Boryslawski, Building Owners Driving BIM: The Letterman Digital Arts Center Story, Building the Future, AECbytes, (2006). [2] D. Bouchlaghem, H. Shang, J. Whyte, A. Ganah, Visualisation in Architecture, Engineering and Construction (AEC), Automation in Construction 14 (3) (2005) [3] T. Fischer, M. Burry, J. Frazer, Triangulation of Generative Form for Parametric Design and Rapid Prototyping, Automation in Construction 14 (2) (2005) [4] F. O. Gehry, B. Colomina, M. Friedman, W. J. Mitchell, J. F. Ragheb, J. L. Cohen, S. R. G. Museum, M. G. Bilbao, Frank Gehry, architect, Guggenheim Museum, New York (2001). [5] H. Hal Iyengar, F. Robert Sinn, F. John Zils, Discussion of ``Development of Boundary Structures for Complex- Shaped Buildings'' by Yong Gib Yun and Daniel L. Schodek, Journal of Architectural Engineering 10 (2) (2004) [6] H. Hal Iyengar, F. Robert Sinn, F. John Zils, Unique Steel Structures In Spain: The Guggenheim Museum Bilbao and the Hotel Arts, Barcelona, Journal of Constructional Steel Research 46 (1998) [7] H. Iyengar, L. Novak, R. Sinn, J. Zils, The Structural Design of the Guggenheim Museum, Bilbao, Spain