Sculpting the Skyline: Architects, engineers, and contractors tackle a challenging geometry to build a supertall tower with a striking silhouette for a desert city.
It is in the nature of tall buildings that rankings are short-lived, but at least for the moment, the 1,354-foot-tall, 77-story Al Hamra Firdous Tower, by Skidmore, Owings & Merrill (SOM), is the tallest building in Kuwait City. It is the tallest all-office building and the tallest skyscraper with a concrete structure in the region.
- Concrete formwork: Peri, Munich, Germany
- Stone supplier: Jura Limestone Suppliers, Eichstat, Germany
- Glass supplier: Guardian Industries, Dudelange, Luxembourg
Other tall buildings now sprout from the sand in Kuwait, including the lipsticklike NBK Tower, by Foster + Partners, in the early stages of construction just across the street, and the hourglass-shaped United Towers by Kohn Pedersen Fox, almost complete on a site a few blocks away. At 984 and 787 feet tall, these are shorter than SOM's tower and don't qualify as “supertall.” But if, or when, Al Hamra's height is superseded, its contribution to the city's skyline shouldn't be diminished. It possesses both a geometric rigor and a graceful asymmetry since it is mostly glass-skinned and rectilinear, but seemingly wears a flowing cloak of concrete.
A supertall tower was not the original plan for the site, which sits at the center of a promontory jutting out into the Arabian Gulf (also known as the Persian Gulf). The consortium that owns the land, Al Hamra Real Estate, initially planned a 50-story office building and an adjoining 4-story shopping mall—both designed by a local firm, Al Jazera Consultants. But in 2005, soon after starting construction on the mall and beginning excavation for the tower, Kuwaiti officials changed the zoning regulations to allow for a much taller structure. The clients decided to move forward with the retail portion of the project. For the architecture and engineering of the tower, however, they called in SOM, whose tall-building experience stretches back at least as far as the 60-story One Chase Manhattan Plaza, completed in Lower Manhattan in 1961, and now includes Dubai's Burj Khalifa, the world's tallest skyscraper.
FORM FOLLOWS PROGRAM
The tower was completed late last year, though work on tenant spaces and some of the public areas continues. But even before completion, some observers compared its striking silhouette to a figure wrapped in a cloak or a dishdasha—the floor-length robe worn by Kuwaiti men. The tower's project team, however, says any such associations are purely coincidental. “It's the product of parametric study,” says Gary Haney, SOM design partner, referring to the computational process used to generate Al Hamra's form. The tower's geometry is based on a set of criteria that took into account the clients' leasing strategy as well as environmental factors such as solar exposure and wind loading. “The tower responds to its context and cannot be repeated elsewhere,” confirms Farid Abou Arraj, projects development manager for Ajial Real Estate & Entertainment, the owner's representative.
Among the clients' programmatic needs was office space of a certain size and configuration: The developers wanted floor plates each with a gross area of about 25,000 square feet—a size they decided would appeal to tenants interested in leasing a single floor. They also desired a narrow core-to-curtain wall span of no more than 40 feet and office space oriented to take advantage of views of the gulf to the north, east, and west.
To meet these requirements, SOM found it would have to reduce the maximum-allowable floor plate by about 25 percent on every floor. The desire to make the most of the views of the water suggested a floor plan without south-facing office space. However, solar analyses conducted with the aim of reducing heat gain from the brutal desert sun supported the removal of the quadrant at the southwest corner of every floor. Meanwhile, computational fluid dynamic (CFD) studies and the subsequent wind-tunnel testing of physical models demonstrated that a tower with a slightly irregular profile would be the most effective in mitigating vortex shedding—a phenomenon that creates wind eddies and induces side-to-side movement—an obviously undesirable feature in a supertall building. “If the shape of the tower changes as it rises, the formation of organized vortices is disrupted,” explains Mark Sarkisian, SOM director of seismic and structural engineering.
SQUARE BUT SINUOUS
From the process of balancing the various criteria, a tower with a nearly conventional plan emerged: It has a central shear wall core surrounded by a perimeter moment-resisting frame. However, the building appears to have been vertically sliced, with a chiseled-out section equal to about a quarter of every floor plate that gradually travels from the southwest corner near the building's base, where it meets the retail podium, to the southeast corner at the tower's apex. A pair of hyperbolic paraboloid, reinforced-concrete “flare walls” delineate the edges of the incrementally shifting void. And set within the resulting recess is an almost 5-foot-thick reinforced-concrete wall with punched openings angled to control penetration of the sun. On every office floor behind this hefty facade is a circulation corridor that provides a vantage point for occupants to take in framed views of the city's developing skyline.
The building is divided vertically into three stacked office-floor zones. Visitors and tenants reach the upper two by taking express elevators to sky lobbies that offer meeting space and other amenities, and then travel to intervening floors via local elevators. Eventually, by way of a set of VIP elevators, they will be able to travel from the lobby directly to the crown, where developers plan a restaurant or sky lounge. It is not yet leased or fitted out, but has a dramatic sloping ceiling, almost 100 feet tall at its highest point, and affords sweeping views over the gulf. SOM wisely preserved this potentially valuable real estate as habitable space by choosing to locate Al Hamra's cooling towers on top of the retail podium instead of the tower roof.
UNDERNEATH IT ALL
In tandem with the development of the scheme for the tower's superstructure, Sarkisian's team worked on the design of the foundation, starting with the assumption that the building would be around 70 stories tall and built of cast-in-place concrete. But as the tower concept took shape, it became evident that the spiraling form would concentrate gravity loads on the west side of the building footprint below the southwest flare wall, while very little load would be applied to the north and southeast edges. In response to this load differential, engineers devised a 13-foot-thick reinforced-concrete raft supported on 289 piles, each between 66 and 89 feet long, with the deeper piles located densely around the areas of greatest stress.
The roughly 200-by-230-foot raft, which required almost 30,000 cubic yards of concrete, was poured in 15 separate sections over a period of four months. This segmented approach was dictated largely by local production capacity, but it also helped contractors control the heat generated during concrete hydration—the chemical reaction that occurs when cement is mixed with water. If the material gets too hot—a particular concern given the desert environment—its strength can be compromised. Performing the work at night, along with the use of a concrete mix containing a high percentage of fly ash (a byproduct of coal combustion) also helped keep temperatures in check, says Ali Asfour, construction manager for Ahmadiah Contracting & Trading. The company is part of the client consortium and is the project's general contractor.
Construction of the beefy south-facing wall and the ribbonlike flare walls, which play an integral role in the building's lateral- and gravity-load-resisting systems, was also tricky. As part of a so-called “construction correction program” devised by SOM, the contractors adjusted the self-climbing formwork with each pour to compensate for displacement caused by the counterclockwise-torqued geometry. The process accounted for the elastic movement of the concrete under its own weight during construction and for long-term movement from shrinkage and creep. “Loaded concrete can hydrate for up to 10 years,” says Sarkisian, explaining the latter phenomenon. “Its properties can continue to change during that period,” he says.
Arguably, the tower's base presented an even tougher design and construction challenge than its sculpted superstructure. “How a supertall building meets the ground is always problematic,” says Aybars Asci, an SOM director. Compared to a lower-rise building, a supertall tower has a much smaller footprint relative to its height, but with many more people coming and going, he explains. It is also where gravity loads are greatest and where the structural elements tend to be the largest, he points out.
At Al Hamra, the architects created ground-floor space that could handle the tower's anticipated foot traffic by canting the perimeter columns on the building's north face, increasing the lobby's depth. Designers also made Al Hamra's nearly 80-foot-tall entry hall almost Gothic by devising a system of lamellae—a series of reinforced-concrete weblike vaults—that transfer the tower's gravity load to the foundations. Developed through nonlinear buckling analysis, the system works by reducing the unbraced length of the lobby columns and by decreasing the structural demand on each of them through load sharing with parallel members, explains Sarkisian. The lamellae's primary members are about 4 feet square where they meet the lobby floor. But without use of the bracing technique, the space would have required perimeter columns almost three times as large, he estimates.
The lamellae, which Asci describes as “structurally sensible but spatially interesting” were built with fiberglass formwork fabricated from shop drawings generated from SOM's 3-D model. Even so, constructing the lamellae was a slow process, requiring nearly 100 days to complete. In the meantime, work advanced on the rest of the tower, with floor framing on the north side catching up to the other sections of the building at the 52nd floor, according to Asfour.
White paint covers the lamellae, enhancing their filigree quality, but somewhat diminishing the brute power evident in construction photographs. “No one likes exposed concrete other than architects,” says Asci. Aesthetic concerns aside, however, exposed concrete was never a practical option, especially on the exterior, due to Kuwait City's salty gulf air and its tendency to corrode rebar. In part to prevent such deterioration, the architects chose an especially durable type of limestone cladding, covering the south-facing planar wall in 2.5-foot-by-4.5-foot panels. They clad the serpentine flare walls in the same stone, but with trencadis—a mosaic of shardlike pieces.
The treatment of the flare walls lends them a handcrafted character, especially evident up close. Their texture contrasts with the silvery smoothness of the insulated glazing units (IGUs) cladding the east, north, and west facades. The IGUs include a low-E coating that imparts just enough reflectivity to catch the sky, points out Haney. This coating proved to be one of the key curtain wall challenges, since the architects needed to make sure it would be compatible with the heating and bending process required to fabricate the glass that wraps the corners. These curved units make up 30 percent of the building's glazing.
The reflectivity that Haney is so fond of was evident on a sunny day in mid-March, even though the curtain wall was still covered with construction grime, as well as a coating of dust from the region's frequent sand storms. After an initial cleaning, the glazing will be cleaned once every three months by workers suspended from a maintenance unit that encircles the building on a track cleverly concealed within the steeply sloping parapet.
The owners are in the process of completing their own 26th-floor offices. However, it isn't clear how many other office tenants have committed to taking space in the Al Hamra. The retail anchor tenant, Hermes, opened in December, and as of early this spring, about 86 percent of the shopping mall had been leased—information that Abou Arraj readily volunteers. However, of the tower office floors he says only that there is “considerable interest.” Despite this evasiveness, and an undisclosed budget—even the architects say they don't know the building's total cost—it would be inappropriate to gauge the success of the tower from the vantage point of American developers, who are generally focused on quick financial returns. The tower is undeniably an iconic addition to Kuwait City's skyline, and Abou Arraj seems confident that tenants will materialize—eventually. “We are building for the future,” he says.
Completion Date: December 2011
Size: 1.9 million square feet
Skidmore, Owings & Merrill LLP
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