Wednesday, October 1, 2008

Central Plaza, Hong Kong

Central Plaza is the second tallest skyscraper in Hong Kong. With a height of 374 m , Central Plaza is only surpassed by in . The building is located at 18 Harbour Road, in Wan Chai on Hong Kong Island. It was the tallest building in Asia from 1992 to 1996, until the Shun Hing Square in Shenzhen, People's Republic of China, was built. The 78-storey building was completed in August 1995. The building surpassed the as the tallest building in Hong Kong until the completion of .

Central Plaza was also the tallest reinforced concrete building in the world, until it was surpassed by CITIC Plaza, Guangzhou. The building uses a triangular floor plan. On the top of the tower is a four-bar neon clock that indicates the time by displaying different colors in 15 minute intervals, blinking at the change of the quarter.

An anemometer is installed on the tip of the building's mast; the anemometer sits at 378 m above sea level. The mast has a height of 102 m . It also houses the world's highest --Hong Kong City Church.

Design



Central Plaza is made up of two principal components: a free standing 368 m high office tower and a 30.5 m high podium block attached to it. The tower is made up of three sections, a 30.5 m high tower base forming the main entrance and public circulation spaces, a 235.4 m tall tower body containing 57 office floors, a skylobby and five mechanical plant floors and the tower top consist of six mechanical plant floors and a 102 m tall tower mast.

The ground level public area along with the public sitting out area forming an 8,400 m? landscaped garden with richly ornate fountain, trees and artificial stone paving dedicated for public enjoyment. No commercial element is included in the podium. The first level is a public thoroughfare for three pedestrian bridges linking the Mass Transit Railway, the Convention and Exhibition Center and the China Resource Building. By turning these space to public use, the building got 20% plot ratio more as bonus. The triangular building shape of the tower is not truly triangular but with its three corners cut off to provide better internal office spaces.

Design constraints



Triangular shaped floor plan


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


Super high-rise building


As the building is situated opposite to the HKCEC, the only way to get more sea view for the building and not to be obstructed by the neighbouring high-rise buildings is to build it tall enough. However, tall building would bring 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 if not properly resolved will 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 , a floor-to-floor height of 3.9 to 4.0 m 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 . Therefore a comprehensive study was conducted and finally a floor height of 3.6 m 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 in office area could still be achieved with careful coordination and dedicated integration.

Structural constraints



*The site is a newly reclaimed area with a maximum water table rises to about 2 meters 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 BLWT at UWO.

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 centres and 1.1 m deep 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 the tallest reinforced concrete building 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.

Current tenants


*Nvidia
*ExxonMobil
*CB Richard Ellis
* 41st Floor & 42nd floor
*Sun Microsystems
*Hong Kong City Church 75th Floor



Gallery



Image:Hong Kong Central Plaza.jpeg|Central Plaza, 20 April 2003
Image:HK_Wan_Chai_Platform_Great_Eagle_Centre_n_Central_Plaza.JPG|Central Plaza and the Great Eagle Centre, 19 May 2007
image:central-plaza2.jpg|Triangular shaped floor plan of Central Plaza, 20 April 2003
image:central-plaza3.jpg|Vertical shot of Central Plaza, 20 April 2003
Image:HKWanchaiwaterfront.jpg|A view of Wanchai waterfront with Hong Kong Convention and Exhibition Centre in the foreground and Central Plaza behind it, August 2005
image:Central Pier 9.jpg|A view of Wanchai waterfront as seen from Central Pier 9, Nov 2007

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