Burj Battles Wind High Above Dubai
Event: “Supertallest: Designing Structure.” World’s Tallest Building: Burj Dubai Lecture Series
Location: New York Academy of Sciences, 05.23.2007
Speakers: William F. Baker, PE, CE, SE, FASCE — partner, Skidmore, Owings & Merrill, Chicago, and chief structural engineer, Burj Dubai
Introduction: Carol Willis — Director, Skyscraper Museum
Organizers: Skyscraper Museum; New York Academy of Sciences
©Skidmore, Owings & Merrill LLP
The tallest skyscrapers of the 21st century are likely to face obstacles beyond what we can now imagine. Principally residential, concrete-framed, and Middle Eastern or Asian — as opposed to commercial, steel-framed, and North American like their 20th-century predecessors — Carol Willis observed one challenge that’s certain to remain in effect is wind. The higher a tower extends, the stronger the wind, and the more unpredictable. How do you strengthen a structure against wind forces in a place where no one has ever ventured up to measure them?
The portfolio of engineer William Baker, PE, CE, SE, FASCE, already includes one building temporarily considered the world’s tallest, the Petronas Towers in Kuala Lumpur, Malaysia. The Burj Dubai recently passed Petronas, reaching the 128th floor, and in September it will pass Taipei 101 assuming world leadership in height, at least among freestanding land-based structures (offshore oil rigs excluded). Going where no architect or engineer has gone before, Baker recognizes, means confronting unprecedented torsion stresses, wind vortices, stack effects, and other demands. Aeroelastic studies with models and wind tunnels allowed for extrapolation to actual conditions and ultimately to “tuning” of the building, like a musical instrument. Skidmore, Owings & Merrill’s design for the Burj uses spiraling sequenced setbacks, turbulence-enhancing textured cladding, an orientation that reduces the site’s most problematic airflow, and numerous other strategies to “confuse the wind,” manage the periodic rhythms of oscillating vortices, and maintain stability amid the forces encountered above 2,000 feet.
The Burj does not compete with bulky buildings like the Sears Tower in area, measuring roughly 3 million square feet (the Sears has 4.4 million), since its largely residential program calls for a smaller leafspan than a predominantly commercial building requires. (The extremely wealthy tenants who will occupy the Burj’s boutique office spaces also tend to have relatively small staffs.) Express and local elevators are stacked to minimize the proportion of floor space devoted to shafts. The Y-shaped triangular floorplates create greater torsional stability than a square or rectangular design would allow; a buttressed hexagonal core with webs of interior concrete walls throughout the three wings functions as a concrete axle. “Every piece of vertical concrete,” Baker explained, “is part of this giant beam” enlisting gravity for stability. “Gravity is amazingly reliable. If you’re resisting a load with rebar, that’s pretty reliable, but resisting with gravity is about as good as you’re gonna get.”
The exact height of the Burj remains a carefully guarded secret; all published figures Baker has seen are wrong. He pointed out that measuring building height is far from an exact science, citing debate within the Council on Tall Buildings and Urban Habitat over four types of measurement (a new standard may emerge from the International Height Meeting in Chicago, under way at this writing). Regardless of whether spires, antennas, occupied floors, or other factors determine official height, the Burj will stand well beyond its projected competitors for years, at least 2,300 feet — nearly halfway to matching Frank Lloyd Wright’s hypothetical, once-fanciful Mile-High Tower.
The unusually close focus afforded by a three-lecture series on a single building promises to reveal many fascinating aspects of the Burj. If the controversies it has already generated in the socioeconomic realm inspire analyses anywhere near as sophisticated as Baker’s technical presentation, look for some spectacular debates as the discussion moves from how it’s being built to why, and for whom.
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