Photo 1

 DEMOLITION AND FOUNDATION 
The demolition of the old Hilton Hotel started from July 1995 and the contract lasted for about 9 months. The method employed to demolish the 28-storey hotel building was rather traditional. Four 
excavating machines equipped with pneumatic breakers were used for the demolition. Several dumping shafts were formed on the floor slabs for disposal of building debris.

Photo 2

When the building was demolished down to ground level, raking shores using universal steel beams were erected to support the 2-level basement of the old Hilton Hotel before further demolition proceeded. After the shoring was erected, demolition to the upper basement continued. The lower basement remained untouched (see Photo 1). 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 construction of the diaphragm walls and bored pile foundation for the new building. All the diaphragm walls for the tower employed in the project were of 1.2m thick reinforced concrete (see Photo 2). The perimeter wall helped to support and stabilize the ground during construction of the new basement. A 500mm thick in-situ RC wall was later constructed on the exposing face 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 (see Photo 3). This shaft was lined on the sides by 1.2m thick diaphragm wall panels which acted as a temporary structure for the sake of forming the shaft.

Photo 3

Large diameter bored piles were used as foundation for the new building. The bored piles were basically of two standard sizes. Eight of the piles were 6m in diameter and dug mechanically for supporting the super-columns. 21 piles were of 1.5m diameter and dug mechanically using grabs and protected by steel casing during excavation. These piles were for the support of the columns for the 6-level basement structure.

Photo 4

 FORMING A 37m-DIAMETER SHAFT PIT AND 
 THE CONSTRUCTION OF THE CORE WALL    
Before carrying out of 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 forming a pit large enough to house the core structure and its foundation. A pit in the form of a shaft, 37m in diameter, 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 (see Photo 4). This form comprised 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 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.

Photo 5

 CONSTRUCTION OF THE BASEMENT 
Immediately after the completion of the 6m diameter footings, steel columns were erected on top of each footing. While for the smaller diameter bored piles, steel columns were inserted into the concrete while it is still green using the punch-in method. These columns were used as support to the basement slabs during the construction process using a top-down sequence, as well as to act as permanent columns after being encased in concrete at a later stage. 

In order to allow the core wall and the structural frame to proceed at the same time with sufficient working space, a separating distance equivalent to 9 floor levels was introduced. In order to do so, 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 being erected first.

Dwg. 1: Principle of Double Bit method.

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 
was cast and connected to the core wall structure as soon as that stage of excavation was 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. In order to expedite the progress, the basement excavation and construction were done in a "Double Bit" manner (see Photo 5, Dwg. 1). The floor system in the basement was of flat slab design with dropped panel around column heads. Average slab thickness was 350mm (500mm thick for slab on the lowest basement, no ground beam was provided).

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

 SUPERSTRUCTURE 

Photo 6

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 (see Photo 6). 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 sizes mostly in 457 x 191 series. In order to provide an entrance lobby with a more spacious look, there are 8 super-columns, each in size 2.5m in diameter, supporting the entire building, leaving a clearance of two columns at each elevation with a headroom of about 15m above the lobby area.

Dwg. 2: Typical section of the Cheung Kong Center. (Courtesy acknowledgement to Leo A Daly Pacific Ltd.)

Photo 7

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 of 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 (see Photo 7). The tube columns were grouted by pumping concrete upward at an interval of 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 (see Dwg. 2). An anchor steel frame is embedded in the core wall to provide adequate connection to the outrigger frame (see Photo 8). The outriggers and the belt trusses are structurally separated (see Photo 9) in order to allow them to have slight movement during wind, while maintaining sufficient strength and rigidity to support the entire building structure. The tremendous wind pressure acting onto the external walls will eventually transmit the loads through the outrigger and the composite floor to the core wall, thus practically utilized the advantages and balanced the weaknesses of the two structural systems.

Photo 8

Photo 9

Dwg. 3: Section showing the set-up arrangement of the jump for system.

Photo 10

Core wall
The structural design of the core wall resembles two linked "I" section 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 (see Photo 10, Dwg. 3). The core was constructed using a Jump-form system designed on a working cycle aiming at an average of 3 days per floor (see Photo 11 and 12).

Photo 11

The progress was maintained in principle with the anticipation of certain delay at levels where the outriggers were located, as well as in levels where the thickness of the core had to be reduced.

Floor slabs inside the core were cast in-situ at a deferred stage (see Photo 13). Internal partitions inside the core including walls to lift shaft and staircases were erected using drywall system to eliminate unnecessary formwork or wetwork.

Photo 12	Photo 13
Photo 14	Photo 15

Photo 16

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 caissons which descended 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 (see Photo 14). 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 (see Photo 15). Detailed method statement and proposal showing the erection procedure and sequence was required to be submitted for approval before actual carrying out of the work

The erection of typical floors worked basically under a 3-storey cycle to cope with the length of the circular steel tube (concrete filled columns) on the edges of the building. When the steel tubes were positioned and aligned at the bottom, they were further secured by connecting to the floor beams. Connection was done by tension control bolts with the inner end bolted to gusset plates that were fully embedded in the core wall (see Photo 16).

Photo 17

After the steel beams were put in place and secured, a metal deck was laid on top acting as the permanent shutter for the forming of the composite floor (see Photo 17). Shear studs were also welded on top of the beams to improve the ability to take up shear by the floor membrane. Reinforcing bars were 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 stanchions for the 8 super-columns; the weight of each fabricated section is about 28 tonnes. Since these members were required to be placed into the 6m diameter caissons, crawler cranes working on the ground level were used for the lifting purpose 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 formwork 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 was still very great. To cater for this requirement, two tower cranes with luffing jibs of 600 Tm capacity, and one with fixed jib of about one-third the capacity of the former cranes, were used to assist in the lifting of all the required materials and components during the erection processes. The cranes were hydraulic-lifted and mounted inside the voids of the core wall and supported on temporary I-beams.

Two concrete pumps with approximately 90 m3/hr output and 65 bar working pressure were stationed at ground level. They were used mainly for the placing of concrete for the core wall, concrete filled tube columns and the composite floors. Concrete delivery pipes that were fixed and housed inside the core wall were used conveniently for the purpose. The pipes would then be extended at the same time as the structure was ascended. A two-staged concrete placing arrangement, with an intermediate pump located at 36th floor, was introduced.

 CONCLUSION 
The Cheung Kong Center construction demonstrated a typical combination of modern construction methods. The use of top-down method to construct very deep basement, a composite structure of very large scale and size, the use of mechanical formwork for a particular part of the structure, or even some more sophisticated finishing items such as stainless steel unitized curtain wall system, raised floors and building services provisions which integrated with extensive information and automation technology. These kinds of works are, in fact, not novel to the construction industry nowadays. The art of executing this kind of project depends on 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 is a tribute to the competence and professionalism of the building industry of Hong Kong.

Acknowledgement
The role of the author in the project of Cheung Kong Center is an observer only who studied the project throughout the entire construction period as part of his personal professional developments. Upon the completion of this paper, the author wishes to thank various key parties whose information and assistance have substantially contributed to the successful preparation of this paper. In particular, to Ove Arup & Partner Hong Kong Ltd. for their advice and comment on a more technical basis, to Leo A Daly Pacific Ltd. for their information on the project brief and design, to Paul Y-ITC for their information on the overall construction background, as well as to the owner of the building, Hutchison Whampoa Property, for giving the permission to publish this paper. Their sincere assistance is therefore deeply appreciated.

Reference
David Scott, Goman Ho and Hayden Nuttall (June 1999), Design and Construction of the 62-storey Cheung Kong Center, Symposium on Tall Building Design and Construction Technology, Beijing.