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THE PETRONAS TWIN TOWER

The Petronas Twin Tower is 452 metres tall to the tip of its spire and contains approximately 370,000 square metres (or 4 million square feet) of total floor area in 85 occupied floors. The project location is on a 100 acres site at the Junction of Jalan Ampang and Jalan P. Ramlee and geologically lying at the contact zone of Kenny Hill and limestone rock formations.

The challenge put forward to the design team was to come up with an economical and buildable structural frame capable of resisting the extremely large vertical and lateral loads for the Petronas Tower, the tallest building in the world.

The Petronas Tower is subjected to wind forces based upon a design wind speed of 126 km/hr. This is a gust speed lasting for 3 seconds and with probability of occurrence of once in 50 years only. Malaysia is not located within seismically active zone and the local building regulation do not require building to be designed against earthquake load. The wind load condition is anyhow predominant when compared against any minor tremor if any at all.

The combined effects of the vertical and lateral loads produce total building weight of 2,700,000 kN and turning moment of 4,500,000 kN-metre. The challenge is to design the superstructure and the substructure to take these huge forces that would produce acceptable settlements, sway and dynamic acceleration response.

Superstructure

Five (5) different schemes originating from an all steel scheme, all concrete scheme and hybrid of steel and concrete scheme were studied and evaluated for occupant safety and comfort, economic, constructibility and adherence to the architectural constraints.

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The study of the various schemes concluded that the concrete core and concrete cylindrical tube system offered significant advantages over the other schemes. The five (5) major components of the "tube within a tube" structural system are as follows:

1. 23m x 23m core wall which forms the inner tube.

2. Circular cylindrical tube of 46m diameter with 16 concrete columns which ranges in diameter from 2.3m at the base to 1.2m at the top which forms the outer tube

3. Eight (8) outrigger walls linking core to cylindrical tube at the 38th floor and thus enabling the full width of the building to act in resisting lateral forces.

4.Structural steel composite floor system with its inherent strength and speed of construction.

5. 58.4 m span (height = 170 m) 3-pinned arched sky bridge at levels 40 to 43.

Concrete strength for the concrete core and cylindrical tube columns vary from 80 MPa to 40 Mpa. By taking advantage of the mass and stiffness of high strength concrete used for the columns and core walls a superior dynamic response was obtained. The resulted maximum building sway at the top most habitable floor is well within the acceptable building drift index of 560 (720mm vs 880 mm) and the ISO acceptable criteria of dynamic acceleration of 20 mg (15-18 vs 20 mg). These results have been confirmed by the extensive wind tunnel testing carried out in Canada and our own 3-d static and dynamic computer analyses.

Substructure

The project location is at the contact zone of Kenny Hill and Limestone rock formations. The subsurface conditions beneath the two towers, based on about 400 bore-holes may be summarized as alluvium overlying Kenny Hill which in turn is underlain by Limestone. The depth of limestone varies from 20m to more than 200m below the existing ground.

The column loads (service) at the foundation level vary from about 33,000 kN at the bustles to about 11,400kN at the main towers. The foundation loads (service) at the core are of the order of 860,000 kN (vertical) and 4.5 million kN-M (moment due to lateral loads).

Various options for the foundation design of the Petronas Twin Tower including end bearing piles socketed into the limestone and friction piles in the Kenny Hill formation were considered. Due to the extreme variation in the level of limestone rockhead (from 20m to more than 200m) and its karstic nature, end bearing piles socketed into the limestone were found to be impractical and unsuitable. This was due to the following two reasons. Firstly, the extreme variations in pile lengths would cause differential stiffness and consequently differential settlement problems which would be difficult to overcome. Secondly, due to karstic nature of the bed rock it would be impossible even to make an estimate of pile lengths which causes great uncertainty in the costing of the foundation.

In view of the above, the second option of friction piles fully in Kenny Hill formation was adopted. This solution, however required a shift in the tower positions of about 60m towards south-east from the originally planned position.

The final foundation design concept comprises a 4.5m thick piled raft (13,200 cubic metres) founded at 20m below the existing ground and supported on 104 numbers of 1.2 m x 2.8 rectangular friction piles varying in depth from 40m to 105 m. The bearing capacity of the Kenny Hill Formation at the founding level is sufficient for the raft alone to take the foundation loads. The settlement, however would exceed the allowable limits. To control the settlement, friction piles have been introduced which reinforce the Kenny Hill soil increasing its modulus and thus effectively act as settlement reducer.

The estimated total and differential settlements are of the order of 70mm and 20mm respectively which are within the allowable limits of 100mm and 25mm respectively. The factor of safety used in the foundation design against bearing failure exceeds 2.5.

All significant cavities in the limestone formation and slump zones in the Kenny Hill within the tower foot print and a depth of 150m below the existing ground have been grouted. The depth of 150m is arrived at from 10% pressure zone below which stress in the ground is insignificant.

(article by Hamdan & Azam)

Other facts: No. of double-deck high speed lifts = 29

Stainless steel cladding : 55,000 sq m; vision glass : 77,000 sq m.

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