Petronas Towers

Statistics

In-Depth Analysis by Michael W. Su

When construction on the twin Petronas Towers in Kuala Lampur, Malaysia was completed in 1998, they were quickly declared to be the two tallest buildings in the world. Although the validity of this claim was immediately disputed, recognition for the unprecedented achievement of reaching these heights in a mostly reinforced concrete building could not be withheld: 452m at the top of the “architectural” spires, 403m at the roofs, and 375m at the highest habitable floors. The architect, Cesar Pelli & Associates, and structural engineers, Thornton-Tomasetti Inc. and Ranhill Bersekutu Sdn. Bhd., chose to work with this material for very specific reasons. To begin with, concrete has always been more familiar to the regional contractors than steel. In Asia, more generally, high-rise concrete structures are much more common than steel buildings. For the islands in the South China Sea, in particular, concrete is preferred because the cost of importing steel is prohibitive. Of course, the extreme seismic and wind loads particular to this region already render the much-higher rigidity of concrete more suitable for many applications. Since the ratio of rentable space to service core was not as critical for the principal tenant, the Malaysian oil conglomerate Petronas, as for most clients and real estate markets, the towers could economically tolerate the necessarily larger concrete cores and columns in comparison to a steel structure. As well, because the towers are to be joined by a two-story high bridge at the 41^st floor, each tower’s staircase could serve as the other’s second means of egress. The size of the service core in each tower could thereby be reduced to incorporate only one staircase. Similarly, additional floor space would be recovered by locating the mechanical services for each of the towers externally in the two 43 story high buildings, or “bustles”, adjacent to the towers. Last, concrete structural members could be minimized by selectively specifying the newest, high-strength mixture with a target strength of 80MPa (11,600psi) – a first for Malaysia. Interestingly, Cesar Pelli has characterized the resultant concrete core and perimeter column structure as a “soft tube”.

      The two “soft tubes” of the Petronas Towers rest on a concrete mat foundation over 3m thick. Unusually for such a tall structure, because the bedrock below the site is too deep to access, this mat is supported principally by the lateral friction resulting from driving 208 massive (3m x 1m) rectangular section barrette piles with indented sides as much as 115m into the underlying layer of firm sand. Of these barrettes, 85 directly support each tower, while the remainder support the bustles. Rising from the foundation at 25m below grade are the 13m x 13m concrete cores of each tower. The core walls have thicknesses tapering from 76cm at the base to 36cm at the top. At the perimeter of each tower is a concentric ring of 16 reinforced concrete “super-columns” with diameters that range from 2.4m at the foundation to 1.2m at the roof. Although at grade level these super-columns are spaced as far apart as 10m, center to center, the columns incline towards the core with rising height to yield the towers’ characteristic shape. The resultant convergence of columns reduce their separation to just 6.7m at the top. These columns are tied to each other at every level by slightly-arched, slightly-arced moment beams to form a complete and independent perimeter wall system. Due to the inherent rigidity of the reinforced concrete, the core and super-columns are tied together with just one layer of outriggers: two-story tall concrete Vierendeel-trusses at floor 38. Braced thusly, the core and super-columns form the “soft tube” that is so resistant to lateral forces that no dampers are required. Specifically, the requisite flexibility under wind loads is achieved by not tying the core and perimeter columns past mid-height, while the high strength concrete members already have the requisite rigidity to sustain seismic loads. Casting so much concrete so high above the ground was itself a tall order, as the contractors broke a world record when they pumped concrete to the height of 380m.

      The structural innovations pioneered by the Petronas Towers were augmented by the specification of rapid-fab and customizable composite metal decking for the floor plates, cantilevered stainless steel – and self-cleaning! – façade elements and sunshades, self-climbing steel forms for casting the super-columns, and 29 of the ever more common high speed, double-decked elevators in each tower. Most visibly, the 60m long, 10m tall, 653,000kg sky bridge connecting the two towers was prefabricated in South Korea, assembled on site, and jacked up as a single unit to a height of 184m for installation at the 41^st floor just above the tower outriggers. This bridge bears on joints that permit lateral displacements of just 25cm – a remarkable precision made possible by the stiffness of concrete.