Construction and Performance of Curtain Wall Systems for Super Highrise Buildings – the Hong Kong’s Experience

The construction of super highrise buildings has been very active in Hong Kong for decades. Recent renowned projects like the 50-storey Manulife Tower, the 62-storey Cheung Kong Center, the 80-storey “Center”, the 88-storey International Financial Center, and a number of recent residential buildings exceeding 60-storey tall, are without exception, using curtain wall as their external envelope. Using thin wall as external envelope for tall buildings has always been a challenge to designers and builders, in particular in terms of cost, energy, water-tightness, installation, dimensional and structural stability, interfacing arrangement with other building components and maintenance etc. Making use of the Hong Kong’s experience, the writer wish to highlight some local practices and summarize how such thin wall systems are designed and installed. USING CURTAIN WALL IN BUILDIGNS OF HONG KONG High-rise buildings started to spring up in the skyline of Hong Kong since the 1970’s in parallel with her economic take-off from a traditional manufacturing-based industrial economy and transforming herself into an world-renowned international financial centre. High-rise buildings by that time were concentrated in the commercial districts like Central in the Hong Kong Island and Tsim Sha Tsui on the Kowloon Peninsula side. The first generation of buildings using what-so-called a curtain wall system can hardly be described as a full system which is usually of proprietary design. The pioneer systems were just external façade/walling designed by local architects and with materials supplied by local manufacturers. The common systems used by that time were in majority stick-type, spandrel and cover, or unit-in-frame systems, constructed of aluminum sections, sometimes incorporated with large areas of stone slabs to cover up solid walls. They were popular due to their highly adaptability, low cost, easy to designand-install nature. More deluxe commercial buildings were built in the late 70’s as the economy of Hong Kong growing hotter and hotter. Developers tended to request for systems with higher performance as the external envelop for their buildings, in terms both of appearance, material, construction and maintenance concerns. Due to the relative lack of experience at that stage, the performance of these second generation curtain wall systems (from late 70’s to mid 80’s) could still hardly described as satisfactory. Problems like seepage, staining, deformation, deterioration and rapid aging of the jointing materials, were very common to many walling cases, often to a condition that made repair and maintenance almost impossible. The replacement of these walling systems not only costly, but also created great disruption to the normal utilization of the building users, and at the same time badly lowered the property value of the entire premises. Photo 1 – Commercial centres of Hong Kong where buildings with curtain wall of various era form an excellent showcase (from left, Central in HK and Tsim Sha Tsui on the Kowloon side. The third generation of curtain wall systems roughly started in the mid/late 80’s, by the time Hong Kong was experiencing her economic climax before the handing over of her sovereignty back to China in 1997. Accommodated experience in the application of curtain wall systems in highperformance buildings contributed solidly in the process of perfecting these systems. Throughout the period, the design and production teams, as well as the manufacturers and the engineering supporting teams, were growing more mature in the mastering of the local situation and market. Most problems appeared in the previous cases have been much effectively taken care of. Systems of this generation are in general much more satisfactorily received by most users. COMMON TYPES OF CURTAIN WALL SYSTEMS External wall with large area of glazed portion that carries no superimposed load except wind load is usually termed as curtain wall. Traditionally curtain wall consists of a metal frame system infill with vision or opaque panels, that serves to provide glazing for window openings as well as to cover-up structures like columns, slabs and beams, or sometimes even sections of solid wall. There are many ways to serve the purpose, depending on a number of factors such as the design and budget for the project, layout and shape of the building structure, as well as other architectural or structural requirements. According to the American Architectural Manufacturers Association, curtain wall systems can be classified in five types, namely, the stick system, unit system, unit and mullion system, panel system, and the column-cover-and-spandrel system. However, due to the introduction of high-performance framing/articulation products and high-strength structural glass, some newer forms of curtain wall systems such as large-area glazed wall using spider clips, bow mullions, hangers or cable stiffeners as supports and connections, are new systems that cannot easily be classified using traditional concepts.