Triangular shaped floor plan

The building was designed to be triangular in shape because it would allow 20% more of the office area to enjoy the harbour view as compared with a square or rectangular shaped buildings. From an architectural point of view, this arrangement provides better floor area utilisation, offering an internal column-free office area with a clear depth of 9 to 13.4 m (30 to 44 ft) and an overall usable floor area efficiency of 81%. Nonetheless, the triangular building plan causes the air handling unit (AHU) room in the internal core to also assume a triangular configuration. With only limited space, this makes the adoption of a standard AHU not feasible. Furthermore, all air-conditioning ducting, electrical trunking and piping gathered inside the core area has to be squeezed into a very narrow and congested corridor ceiling void.

Super high-rise building

As the building is situated opposite to the Hong Kong Convention and Exhibition Centre, the only way to get more sea view for the building and not be obstructed by the neighbouring high-rise buildings is to build it tall enough. However, a tall building brings a lot of difficulties to structural and building services design, for example, excessive system static pressure for water systems, high line voltage drop and long distance of vertical transportation. All these problems can increase the capital cost of the building systems and impair the safety operation of the building.

Maximum clear ceiling height

As a general practice, for achieving a clear height of 2.6 to 2.7 m (8.5 to 8.9 ft), a floor-to-floor height of 3.9 to 4.0 m (12.8 to 13.1 ft) would be required. However, because of high windload in Hong Kong for such a super high-rise building, every increase in building height by a metre would increase the structural cost by more than HK$1 million (HK$304,800 per ft). Therefore, a comprehensive study was conducted and finally a floor height of 3.6 m (12 ft) was adopted. With this issue alone, an estimated construction cost saving for a total of 58 office floors, would be around HK$30 million. Yet at the same time, a maximum ceiling height of 2.6 m (8 ft 6 in) in office area could still be achieved with careful coordination and dedicated integration.

Structural constraints

Sky City Church lounge area in the Apex

• The site is a newly reclaimed area with a maximum water table rises to about 2 m (6 ft 7 in) below ground level. In the original brief, a 6-storey basement is required, therefore a diaphragm wall design came out.
• The keyword to this project is time. With a briefing in a limited detail, the structural engineer needed to start work The diaphragm wall design allowed for the basement to be constructed by the top-down method. It allows the superstructure to be constructed at the same time as the basement, thereby removing time-consuming basement construction period from the critical path.
• Wind loading is another major design criterion in Hong Kong as it is situated in an area influenced by typhoons. Not only must the structure be able to resist the loads generally and the cladding system and its fixings resist higher local loads, but the building must also perform dynamically in an acceptable manner such that predicted movements lie within acceptable standards of occupant comfort criteria. To ensure that all aspects of the building's performance in strong winds will be acceptable, a detailed wind tunnel study was carried out by Professor Alan Davenport at the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario.

Steel structure vs reinforced concrete

Steel structure is more commonly adopted in high-rise building. In the original scheme, an externally cross-braced framed tube was applied with primary/secondary beams carrying metal decking with reinforced concrete slab. The core was also of steelwork, designed to carry vertical load only. Later after a financial review by the developer, they decided to reduce the height of the superstructure by increasing the size of the floor plate so as to reduce the complex architectural requirements of the tower base which means a highstrength concrete solution became possible.

In the final scheme, columns at 4.6 m (15 ft) centres and 1.1-metre-deep (3.6 ft) floor edge beams were used to replace the large steel corner columns. As climbing form and table form construction method and efficient construction management are used in this project which make this reinforced concrete structure take no longer construction time than the steel structure. And the most attractive point is that the reinforced concrete scheme can save HK$230 million compared to that of steel structure. Hence the reinforced concrete structure was adopted and Central Plaza is now one of the tallest reinforced concrete buildings in the world.

In the reinforced concrete structure scheme, the core has a similar arrangement to the steel scheme and the wind shear is taken out from the core at the lowest basement level and transferred to the perimeter diaphragm walls. In order to reduce large shear reversals in the core walls in the basement, and at the top of the tower base level, the ground floor, basement levels 1 and 2 and the 5th and 6th floors, the floor slabs and beams are separated horizontally from the core walls.

Another advantage of using reinforced concrete structure is that it is more flexible to cope with changes in structural layout, sizes and height according to the site conditions by using table form system.