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Cover Story Design and installation of sky bridges The 12 sky bridges, connecting the seven blocks, at the 26 th and 50 th storeys, form part of outdoor sky gardens which are equipped with amenities for recreational purposes. The bridges are made of steel with concrete slabs on top. The lengths of the bridges vary, with the longest spanning 48 m and weighing 327 t. The widths and heights of the bridges are 20 m and 3.9 m, respectively. The design of the sky bridges was carried out by T.Y.Lin International Pte Ltd, HDB’s consultant. The design adopted a 3-dimensional triangular truss layout which is stable without lateral support and could be erected independently. The bridges are designed to withstand wind forces in all directions. The side faces are tapered to reduce the obstruction to wind flow, thus minimising the wind pressure on the face. One end of each bridge was designed to be fixed to the building, to improve the natural frequency of the bridge and reduce vibrations from walking and jogging, thus enhancing comfort levels for people. Owing to the tight site conditions, there were many challenges in the erection of the bridges, relating to the installation method and procedures, availability of space at site, and duration of the installation. Owing to the sizes, the bridges were fabricated in segments off-site, at a factory (Fig 7), transported to the site, and assembled onto the complete structure. To overcome the space constraints, the 50 th and 26 th storey bridges were stacked on top of each other (Fig 8). This also facilitated the subsequent lifting operation. In the factory, as well as during the onsite assembly of the sky bridge members, the fitting up, welding, testing, and trial assembly of the sky bridge trusses were supervised by an independent checker. The progress of the work was closely supervised by the Resident Engineers and Resident Technical Officers who were stationed on-site and at the fabrication yard. After fabrication, the sky bridges were lifted to their respective heights using the strand jack system which was used instead of cranes, due to the height of the buildings. In addition, this system Fig. 7: Welding, testing, and trial assembly, of sky bridge steel trusses at the factory. Fig 8: Assembly of the sky bridge steel trusses on site. The 50 th and 26 th sky bridges are stacked on top of each other. allowed the bridges to be assembled at a lower level before hoisting the whole assembly, thus enhancing safety on site. Four strand jacks were required for each lifting. The jacks were installed and positioned at four corners between the two buildings at the 50 th storey roof top position (Fig 9). Prior to the lifting, a trial jacking was done, that raised the skybridge 300 mm off the ground, to ensure that the jacks were functioning properly. The bridges are fixed to the core walls of the residential buildings. To fix the bridges safely and securely to the core walls, base plates and sleeves for tension bars were cast together with the core walls. Great care was taken to ensure that the cast-in items aligned accurately with the tower blocks. When the bridges were lifted to their final positions at the 50 th and 26 th storeys, and adjusted, the main trusses were connected to the building by high strength bars and locked in-place by casting the concrete slab which forms the floor of the bridge. 14 · THE SINGAPORE ENGINEER Jun 2010
Cover Story The sky bridge structural system Main structure Diaphragm Connecting member to member Connection to building core wall Main truss Three-dimensional steel truss. Concrete topping 125 mm thick. Full penetration butt weld. Using Macalloy posttensioned bars. Consists of 1 top chord and 2 bottom chords. The top chord has a 125 mm thick concrete topping. The combination acts as a composite. The concrete topping at the top deck level and mezzanine level ties with the building floor slab and, together with the tie beam between two core walls, acts as a diaphragm to resist the lateral and vertical loads. PROJECT CREDITS Client Housing & Development Board Project Management SIPM Consultants Design Architect ARC Studio Architecture + Urbanism Project Architect RSP Architects Planners & Engineers C&S Consultant Surbana International Sky Bridge Consultant T.Y.Lin International M & E Consultant Surbana International Cost Management Surbana International Main Contractor Chip Eng Seng Contractors (1988) Pte Ltd. All images by HDB. Fig 9: Lifting of sky bridges to the 50 th storey using the strand jack system. THE SINGAPORE ENGINEER Jun 2010 · 15
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