Redevelopment of Hilton Hotel and Nearby Properties - 
Construction of the 62-storey Cheung Kong Center at Central, Hong Kong


Raymond Wong Wai Man  Division of Building Science and Technology, City University of Hong Kong

Introduction
The project situated in a 9,650 m2 site which comprised the previous Hilton Hotel, the Beaconsfield House and Garden Road Carpark in Central. The new building is a 62-storey composite structure, with a 22 m x 27 m reinforced concrete inner core encased in a 47 m x 47 m external steel frame, together with a 6-level basement that constructed over the 2-level old basement of Hilton Hotel.

The site comprised of 3 separate portions and handed over to the contractors at 3 distinctive stages. The former Hilton Hotel portion in which the main building tower situated, was handed to the demolition contractor in mid 1995 and further to one single contractor for the foundation and the general building works in May and December 1996 respectively. The Beaconsfield House portion, in which some open spaces and a public toilet would be built, was handed over in early 1997. While the Garden Road Carpark was handed over at a much later stage in December 1998, by the time the new 6-level basement of Cheung Kong Center, which used as an immediate substitute to the Garden Road carpark, was eventually completed.

With these special constraints and the usual rapid time requirement urging for earliest completion of building by the developer, a very fast track construction schedule was thus unavoidable. Within a contract period of about 105 weeks, the contractor was required to complete and hand over the building in 3 stages. The construction of the basement and the main structure up to the 25th level including all the basic building services provision, should be completed within the first 45 weeks. In the following 25 weeks, the rest of the composite structure should all be completed. The remaining time would be concentrated on the overall finishes of the building.

Demolition and Foundation
The demolition of the old Hilton Hotel started from July 1995 and the contract lasted for about 9 months. Method employed to demolish the 28-storey hotel building was rather traditional. Four excavating machine equipped with pneumatic breaker were used for the demolition. Several dumping shafts were formed on the floor slabs for deposal of building debris. 

When the building was demolished up to ground level, raking shore using universal steel beam was erected to support the 2-level basement of the old Hilton Hotel before further demolition proceeded. After the shoring erected, demolition to the upper basement continued. The lower basement was remaining untouched. It was filled afterward partly with debris obtained from the demolition and partly by imported filling materials to minimize disturbance to the basement structure. When the works were completed, the site was formed and leveled up to the road level.

The works that followed were the construction of the diaphragm walls and the bored foundation for the new building.

All the diaphragm walls employed in the project were of 1.2m thick reinforced concrete. The perimeter walls were permanent structure, which helped to support and stabilize the ground during the construction of the new basement, as well as to act as the permanent basement wall. There was a 37m-diameter shaft pit formed in the middle of the site for the construction of the core wall for the future tower. This shaft was lined on the sides by diaphragm wall panels which acted as a temporary structure for the sake of forming the shaft.

Large diameter bored piles were used as foundation for the new building. The bored piles were basically in two standard size. Eight of the piles were 6m in diameter and dug manually for supporting the super-columns. 20 piles were of 1.5m diameter and dug mechanically using gribs and protected by steel casing during excavation. These piles were for the support of the columns for the 6-level basement structure.

Forming a 37m-diameter shaft pit and the construction of the core wall
Before the carrying out of the basement construction using a top-down method, the first major work below ground was to construct the central core of the main building tower, the foundation of which rested on the bedrock about -28m from existing ground level.

Instead of constructing the central core in a top-down manner, the core was built bottom up. This could be done by the forming of a pit large enough to house the core structure and its foundation. A pit was thus formed with the sides supported by panels of 1.2m-thick RC diaphragm wall. When the pit was excavated down to the required formation level, a 5m-deep RC raft was constructed as foundation for the core. 

On top of the raft foundation, the core wall on the lowest basement level was constructed using traditional timber formwork. Basing on the completed wall section, a jump-form was then erected to construct the core wall, which comprised of the shutter panels for the casting of the entire core wall section, a lifting screw jack system, as well as the work platform and scaffold that attached onto the form system for access to the shutter panels for works. This jump-form system would be used starting from the second lowest basement until it reached the roof on the 62nd level.

Construction of the basement
Immediate after the completion of the bored pile, steel column was erected on top of each pile at its formation level, which would be used as support to the basement slabs during the construction process using a top-down sequence. 

In order to allow the core wall and the structural frame to proceed to a safe separating distance, the first slab of the basement (i.e. the ground floor slab) was cast after the core wall had been completed up to the 9th level, and with the transfer truss on the 2nd and 3rd level basically erected. 

Further excavation downward was relatively smooth. With the temporary diaphragm wall that formed the 37m-diameter shaft gradually being demolished, the basement slab bound by the 8 super-columns was cast and connected to the core wall structure as soon as a stage of excavation completed. This made the basement structure at the centre very rigid and from thereon, excavation to the sides continued, with the central part acting as a base to shore-support the newly excavated sides. At certain points, basically along the bottom of Garden Road and Battery Path, temporary ground anchors were installed as a means to strength the diaphragm wall panels. The floor system in the basement was of flat slab design with dropped panel around column heads. Average slab thickness was 400mm (500mm thick for slab on the lowest basement, no ground beam was provided). 

To facilitate the removal of large volume of excavated materials, several temporary openings were formed on the basement slab so that the excavated soil could be removed by lifting gribs, buckets of excavating machines (in stages) or partly by dumper truck entering into the basement through temporary ramp.

Superstructure
Structural system
The Cheung Kong Center is a composite structure with the inner core (measured approx. 22m x 27m) constructed of Grade 60 concrete and the external envelope in concrete filled steel tubes. The size of the floor plate measured about 47m x 47m. The external frame and inner core is tied with steel beams which topped with a composite deck of 130mm thick. The span of the steel beams varies from about 10m to 14m and of size in 457 x 191 series. In order to provide an entrance lobby with a more spacious look, there are only 8 super-columns, each in size 2.5m in diameter, supporting the entire building, leaving a clearance of two columns at each elevation.

To economize the structure, a transfer truss system is provided on the 2nd and 3rd level so that closer spaced columns can be used in the design for the upper floors. These columns are in the form of concrete filled steel tube with section in uniform thickness (12.7mm) and ranging from 1.42m diameter for the lower floors to 0.96m for the top floors. The tube columns will be grouted by pumping concrete upward for every 3 floors.

In order to minimize the effect of deflection due to wind load, 3 sets of outrigger/belt truss systems are provided at the 22nd/23rd, 41st/42nd and 61st/62nd levels. An anchor frame is embedded in the core wall to provide adequate connection to the outrigger frame. The outriggers and the belt trusses are structurally separated in order to allow it to have limited movement during wind, while maintaining sufficient strength and rigidity to support the entire building structure.

Core wall
The structural design of the core wall resembles two linked "I" section (refer to Drawing ) with flange thickness ranging from 1500mm for the lower floors and reduced gradually to 400mm for floors above the 44th level, and web thickness from 600mm to 400mm respectively. The core was constructed using a Jump-form system designed on a working cycle aiming at an average of 3 days per floor (4.2m floor to floor). The progress was in principle maintained with the anticipation of certain delay at levels where the outriggers located, as well as in levels where the thickness of core had to be reduced. Floor slabs inside the core were cast in-situ at a deferred stage. Internal partitions inside the core including walls to lift shaft and staircases were erected using drywall system to eliminate unnecessary formwork or wetwork.

Erection of the steel frame
The first lot of structural elements to be erected for the superstructure were the eight super-columns, that embedded and stood on the top of the 6m diameter caisson which was about 25m below ground level. The transfer truss frame was then erected on top of the super-columns. To enable the truss be erected safely and firmly at this level, a temporary support structure was first erected onto the head of the super-columns as a working base. Since this 8.75m high transfer truss was leaning outward from the building line by about 1.8m, the procedure and sequence to erect the entire transfer structure would be quite crucial. Detail method statement and proposal showing the erection procedure and sequence was required to submit for approval before actual carrying out of work.

The erection of the typical floors worked basically under a 3-storey cycle to cope with the length of the circular steel tube (concrete filled columns) on the exterior of the building. When the steel tubes were positioned and aligned, they were further secured by the connection of the steel beams. Connection was done by tension control bolts with the inner end bolted to gusset plates that fully embedded in the core wall.

After the steel beams were put in place and secured, metal deck was laid on top acting as the permanent shutter for the forming of the composite floor. Shear studs were also welded on top of the beams to improve the ability to take up shear by the floor membrane. Reinforcing bar was then fixed on top of the deck before the placing of concrete.

Provision of Plant and Equipment
The heaviest members used in the project were the steel stanchion for the 8 super-columns, the weight of each fabricated section is about 25 tonnes. Since these members were required to place into to 6m diameter caissons, crawler mounted crane could be used for the lifting purposed in this case.

Since the formwork system used for the construction of the core wall was of self climbing type, no additional cranage requirement was thus needed to facilitate the operation of the form system. However, the cranage demand for the erection of the structural steel frame for the 62-level superstructure as well as for the laying of the composite floor deck were still very great. To cater for this requirement, two tower crane with luffing jib with 600 Tm, one with fixed jib about half the capacity, were used to assist in the lifting of all the required materials, members and components during the erection process. The cranes were hydraulic-lifted and mounted inside the voids of the core wall and supported on temporary I-beams.

Two concrete pumps were stationed at ground level. They were used mainly for the placing of concrete for the core wall, concrete filled tube that used as columns, and the composite floors. Concrete delivery pipe that fixed and housed inside the core wall were used conveniently for the purpose. The pipe would then be extended at the same time as the structure ascended. 

Conclusion
The Cheung Kong Center is a typical combination of modern construction techniques and methods. The use of top-down method to construct a deep basement, composite structure of very large scale and size, the use of certain kind of mechanical formwork for a particular part of the structure, or even some more sophisticated finishing items such as curtain wall, raised floor, or building services provisions which integrated with large amount of information and automation technology. These are not new to the construction industry nowadays. The art of executing this kind of project is that, whether the works can be done in a cost effective, punctual, orderly and safe manner. In this respect, Cheung Kong Center is undoubtedly a remarkable project that justify the competence and professionalism of the building industry of Hong Kong.

Acknowledgement
This paper has used some of the information the source of which are from the following organization. The writer wishes to acknowledge and thank for their kind support.

Ove Arup & Partner Hong Kong Ltd.
Nippon Steel Corporation
Paul Y-ITC
Leo A Daly Pacific Ltd.


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