The Center

This 80-storey commercial building, located at 99 Queen's Road Central, covers a site area of 8,816 sq m. The Center is largely devoted to office units with some retail at ground levels. The building has a splendid entrance hall on the sixth floor, while the 42nd floor is a skylobby which doubles as a viewing area. Other facilities include a day-nursery, public open spaces, landscaping areas and at least 350 parking spaces in the underground carpark.

The project used four 24 m caissons as the principal foundation. Each caisson was formed by 30 vertical slurry wall panels arranged in a circular section. Excavation was carried out in the middle, which transformed each caisson into a very big shaft. The slurry walls were excavated by a trench cutting machine known as the "hydrofraise", which uses bentonite slurry as the soil supporting agent as does normal diaphragm wall construction. The hydrofraise removes the excavated materials automatically from the trench and reuse the slurry afterward through the "desanding" process. The average depth of the 24 m diameter caissons was about 45 m. After excavation was completed to the required depth, a 4.5 m deep reinforced concrete raft resting on bedrock was constructed to take up the future loads of the building through a composite column cluster. Some of the parameter walls required for the basement excavation were constructed in the form of bore pile walls by rotary drilling method.

The basement and podium used reinforced concrete and composite columns, while the office tower used a structural steel frame; the best combination for the fast-track project - which originally had a 730-day contract period. The structural steel members for the tower came from Taiwan, the Philippines and Japan, and weighed about 32,000 tonnes altogether. It is also the first building of this size in Hong Kong constructed entirely in structural steel without any reinforced concrete inner core. In order to compensate for the loss in rigidity, the addition of complicated bracing members were required.

Some Challenges

Due to the building's size and the congested urban environment, material delivery had been one of the major challenges in construction. As a result, a well-planned material delivery schedule prepared and updated under close coordination with the major suppliers and subcontractors had to be made from time to time. The project had also made use of four lifts, two material hoists, and four tower cranes, so that internal transportation can be maintained quickly and conveniently within the entire construction period.

foundation and superstructure
Paul Y-ITC Construction & Engineering Co Ltd 

 

Highrise Steel Structure


April 1995 ,when the contractor was doing the foundation work. On the left hand side of the photo is Jubilee Street and on top is Queens Road Central. In the middle, several machines are digging the vertical trench for the construction of the diaphragm wall.

A series of 1.2-metre thick diaphragm wall panels were joined to form a 24-metre diameter caisson shaft. Four of these caissons were built to provide a sound base for the foundation of the main structure of the building tower. The photo shows the excavation work using typical excavating machines inside one of the caisson shafts.
Close up showing the twin drum cutter set. A 150-millimetre suction pipe is placed between the drum cutters. The excavated soil or gravel isbeing sucked out from the trench together with the bentonite slurry. The slurry is then pumped to the bentonite treatment plant where the soil and gravel are removed by a filtration process.

Detailed close-up of a typical caisson shaft. At this stage, the structural steel column cluster has essentially been completed up to the ground level. The scaffold inside the shaft was erected as further support for the construction of the podium structure of the building.
The main frame of the building tower rising from the ground. The four column clusters that support the building, each composed of three 2.5m x 2.5m and one 1.2m x 0.8m steel columns, can be clearly seen here. One special feature in the structural design of this building is that it does not consist of a reinforced concrete central core, which is quite unusual in buildings of similar type. The whole building is based on the design concept of a gigantic space frame in order to achieve the necessary rigidity in structure.

Overall view of the automatic circulation trench cutter.
The overall layout of the site on June 1996. At this stage, the basic structure of the podium had been formed. Since most of the basement structure was constructed using a top-down technique, there was no activity until a later stage, when the podium structure was completed and access to the ground was made available for the basement excavation.

A work platform attached to a corner column to provide access for workers to carry out the necessary welding work.
An overall view of the four caisson shafts which form the major part of foundation system of the building. A typical work stage can be seen here, starting from the excavation, construction of the bottom raft, erection of the structural steel column clusters and the onward fabrication of the column structure respectively.

A 5-metre reinforced concrete raft was constructed in the bottom of the caisson to form the foundation for the column clusters of the building tower.
At this level - some 180 metres above the ground - a worker prepares a lifting rope for the placing of a connecting member.

The core frame at the centre of the building. This core measures roughly 25m x 25m, and will accommodate a pair of stairs and the required escape passageways, lavatories, air handling unit rooms and 26 passenger lifts.
The composite floor, which acts as a diaphragm structure which provides rigidity to the vertical steel members, is constructed of reinforced concrete laid on a ribbed steel deck as a permanent shutter. The one shown here is the deck prior to placementof the floor reinforcement.

A complicated con-nection arrangement at the junction of the inner columns and the floor joists.
A detailed look at one of the four corner gusset blocks which connect and provide the required rigidity for the pyramid structure and the mast.

The dimension of the main frame is reduced and set back to form the "pyramid" top of the tower. The details of the structural arrangement for the housing of the 80-metre mast can also be seen.
The temporary tack rail system provided at 30-storey intervals for the manoeuvring of the gondola used for the erection of the curtain wall panels.

Close-up of the partially completed steel mast. Due to the height of the free-standing mast, a resonant wind effect which creates low frequency vibration must be considered. A special resilient design is used to stop the vibration from transferring to the main structure below. 
The curtain wall system used in The Center has a simple design due to a unitised system combined with the relatively simple and plain form of the building exterior. The only exception is the "half-pyramid" located at the top and bottom of the four cantilevered attached portions. The one here is the inverted half-pyramid - with the bracket for the suspending scaffold partially erected in preparation for the erection of the backing frame and curtain wall at a later stage.

One of the prominent designs employed in The Center is the automatic curtain wall cleaning system which occupies almost the entire area of the 80th floor. With a retractable gate door on each side of the facade to launch the gondola, the cleaning system can cover all the wall surfaces of the building exterior. The photo shows the gate door opened with the rail-mounted crane-car equipped with a telescopic arm launching the window cleaning gondola during a trial operation.
Workers inside the gondola carry out a commissioning test before the handover of the system. A control panel inside the gondola can steer the gondola to any position on the exterior surface of the building.

The operation of the curtain wall cleaning gondola and the crane-car as seen from a distance.
typical floor plan
January 1997
 May 1997