Two International Finance Centre, Hong Kong, China

Statistics

In-Depth Analysis by Michael W. Su

At the moment of this writing, Two International Finance Center stands as the tallest building in Hong Kong. This slender, graceful tower was designed by Cesar Pelli & Associates in collaboration with Rocco Design Limited and engineered by Ove Arup & Partners to reach 416m at the top of its distinctive crown of curved spikes, 407m at its covered roof, and 402m at the highest habitable floor. In Hong Kong, extreme lateral loads due to typhoons and earthquakes are a particular consideration. As such, designers for this locale often find themselves delicately balancing structural requirements and usable floor plates in order to achieve necessary structural stiffnesses without yielding too much area to services or columns. For 2 IFC, in particular, the designers opted for the combination of a massive reinforced concrete core of square cross section with eight rectangular composite “mega-columns” of steel and concrete and eight much smaller steel columns. Like the Jin Mao Tower completed five years earlier, this ensemble of structural elements is arranged into a cruciform configuration with mega-columns in line with the core to counter major gravity and lateral loads and small corner columns to resist minor gravity loads from the extremities of the floor plate. Specifically, the high strength steel and concrete, combination of three 3-story tall core-embedded (called “retro-cast”) outrigger trusses and belt trusses, exceptionally lightweight curtain wall system, and wide spacing (24m) between the mega-columns on each façade all combine to provide sufficient gravity, lateral, and torsional resistance so as to render interior or more-substantial corner columns unnecessary.

      From the foundation to the sixth floor above grade, the mega-columns are formed from six 90mm thick high strength steel sections enclosed with high strength concrete. With rising height, the number of steel sections is reduced from six to three to two and, finally, to one on the upper floors as the size of the column and the strength of concrete is also diminished. These changes in the steel-to-concrete ratio are accompanied by a change in the coupling between steel and concrete such that from the foundation to the 26^th floor, the mega-columns are essentially reinforced concrete columns, while beyond the 26^th floor, the mega-columns effectively act as steel columns with concrete cladding and fireproofing. The result is a structure that testifies to the advances in engineering and material science achieved in just the few years since the Jin Mao Tower was completed. In fact, among myriad other accomplishments, the contractors for 2 IFC set records when they poured concrete for the world’s largest “cofferdam”, erected the world’s largest outrigger, and pumped concrete over 28m higher than the previous record holder ­– the Petronas Towers’ record-setting 380m.

      Besides the remarkable simplicity of its structural system – and the resulting expanse of column-free spaces, 2 IFC was also erected in record time. Due to the relative accessibility of bedrock at the site, a highly unusual – and record-setting – circular “cofferdam” 61.5m in diameter with 1.5m thick walls and averaging 40m deep extends from grade to bedrock. The tower’s 6.5m thick concrete raft foundation is poured directly atop this exposed bedrock. Then, a sufficient quantity of basement slabs is formed solely to facilitate the erection of the tower. This simultaneous bottom-up and top-down approach permitted construction to begin on the core and column structure of the tower well before the completion of the basement, as much as 18 months. Also, each mega-column is formed by its own hydraulic, self-climbing steel formwork. The reinforced concrete core with an approximately 3x3 internal column grid is also raised with its own formwork. Finally, flooring and façade systems were designed with minimum number of components by relying on long, deep perimeter girders and simply connected beams for the floors. In turn, floorslabs are formed from concrete poured on steel decks on these simple beams so that the pours did not even require temporary supports before fully curing. As a result of these time-saving innovations, the contractors were able to attain the elusive goal of raising one floor every three days even as they simultaneously worked on other sections of the tower.