Designers Apply Lessons From World's Tallest Tower To Improve Future 'Megatallest'
After tortured beginnings that included a false start followed by a protracted design competition during which the eventual winner "gambled" $2 million, a shroud of secrecy has enveloped the planned 1-kilometer-tall Kingdom Tower, to be sited north of Jeddah, Saudi Arabia. If built, the estimated $1.2-billion megatower would replace the 828-meter-tall Burj Khalifa as the world's tallest building. It would also owe the burj a major debt. The design team, many of whom worked on the burj, is applying lessons learned in Dubai to the Kingdom Tower in a concerted effort to simplify design and construction of the concrete structure.
Currently, there is no mention of the Kingdom Tower—the pet project of a Saudi prince named Alwaleed Bin Talal Bin Abdulaziz—among the projects listed on the website of the skyscraper's general contractor and part owner, the Saudi Binladin Group. SBG is mute on the subject. The developer, Kingdom Holding Co.'s Jeddah Economic Co., has muzzled the entire design team. The Council on Tall Buildings & Urban Habitat lists the building as proposed, though pile construction was scheduled to begin last month. Reports that the work has begun could not be confirmed, but sources familiar with the situation, who requested anonymity, do confirm that design development continues.
Last summer, the developer unveiled a scheme for a 5.3-sq-km mini-city called Kingdom City, which would cost $20 billion and include the Kingdom Tower. The sleek, tapered tower—designed by Chicago-based Adrian Smith + Gordon Gill Architecture (AS+GG), with the Chicago office of structural engineer Thornton Tomasetti (TT) and mechanical engineer Environmental Systems Design Inc., also of Chicago—would enclose 530,000 sq meters.
AS+GG was formed in 2006, after Smith and Gill left the Chicago office of Skidmore Owings & Merrill. While at SOM, Smith was the lead designer for the Burj Khalifa. Many from the burj team currently work at AS+GG, including Peter A. Weismantle, AS+GG's director of supertall building technology. Robert C. Sinn, currently a TT principal, also worked on the burj while at SOM. And both buildings have the same wind engineer, Guelph, Ontario, Canada-based Rowan Williams Davies & Irwin Inc. (RWDI) and the same elevator consultant, Galveston, Texas-based James A. Fortune.
In retrospect, one could almost consider the Burj Khalifa as a living laboratory for supertower design. During interviews last fall (before the project went quiet), members of the Kingdom Tower design team talked about lessons they learned from the burj, mostly pertaining to the megatower's shape, structure and skin.
The Kingdom Tower's design may be simpler to build, but the project's beginnings were much more complicated than the burj's. For the burj, the developer held a three-week ideas competition, suggested by Smith, to narrow the field from the several architects interviewed. The deal with SOM was soon sealed on a handshake. After the selection, conceptual design matured over one and a half years.
The building now known as the Kingdom Tower was initially called the Mile High Tower. Eventually, the developer abandoned as impractical the idea for a tower a mile tall. After that followed a strung-out design competition, which lasted nine months instead of two, for a shorter building. During the competition phase, representatives of AS+GG made eight trips to Saudi Arabia to present schemes. The firm even built and shipped 10-ft-tall models. "It was really expensive," Smith said. The competition "wasn't for the faint of heart."
The selection was supposed to be made by September 2009. By that time, however, five of the eight initial submissions still had to present to the prince. By December, two teams were left: AS+GG and SOM. Ultimately, AS+GG was selected in February 2010.
Smith calls the selection process "torturous," for both his firm and SOM. "I knew it was a big gamble, but it was something we wanted badly," he said. "Had we not won, we would have lost $2 million and would have had to lay off 30 to 40 people because we were all holding staff for this project," he said.
The team wasn't out of the woods yet. In December 2010, well into design development, the project went on hold for six months. "The owner didn't have its own structure together yet," said Smith.
Design work started again last June. By the end of October, the team had secured a permit for the foundations.
Plans call for a mostly residential, mixed-use tower, including a hotel, office space and condominiums. The tower would also contain the world's highest observation level. Smith calls the architecture an evolution of the Y-in-plan concept used on the burj and first introduced in the early 1920s by architect Mies Van Der Rohe. The evolution mostly relates to the structure and the perimeter wall, he adds.
To understand the evolution, it is necessary to compare the burj and the Kingdom Tower. The burj consists of a 585-m-tall, structural-concrete superstructure, with setbacks, topped by a 200-m-plus structural steel spire that culminates in a steel-pipe pinnacle. Through level 156, the burj is a high-performance concrete tower with a Y-shaped footprint. A hexagonal, shear-wall core is at the nexus of the Y’s wings. Every seven floors, an outer bay "peels away," creating a setback and 27 different floor plate sizes. Outrigger shear walls for stability are in three-floor mechanical levels, every 30 stories or so. Slabs are generally two-way, reinforced-concrete flat plates. Core slabs have beams. The tower is founded on a high-performance, reinforced-concrete raft on piles.
The Kingdom Tower's design also calls for a three-legged building. But gone are the setbacks, the outriggers and the transfers, which wreaked havoc with the construction pace. Instead, the legs are sloped and connected to each other by three tilted planes, which form the tower's outer walls. Each wall, or face, is a single plane, tilted at a slightly different angle, so that each wall reaches the tower's top at different heights. The only asymmetry is the three tilted planes, which form the gradual taper.
In plan, the three-winged building looks like three hammerheads because, near the tips of the wings, there are "serious walls" that cut across, said Sinn.
Inside, throughout each floor plate, a series of concrete walls would contain 12,000-psi concrete and be interconnected by link beams.
Avoiding setbacks and transfers translates to minimizing formwork resetting time. On the burj, the contractor lost three weeks at each setback redoing formwork for walls and slabs, said Smith. On the Kingdom Tower, a day, at most, will be lost to resetting forms, he added.
Thanks to the burj's shape, the terraces at the setbacks are very windy at the higher elevations. For the Kingdom Tower, the terraces are between the legs in so-called quiet zones, where there is not a lot of wind pressure. "That should be much more accommodating," said Smith. It also helps to increase the value of the rental units.
Another lesson from the burj relates to its 3.2-m floor-to-floor height. "It was a coordination nightmare to get the utilities, structure and headroom within the 3.2 meters," said Smith. That required too many beam penetrations, which were difficult within the robust structure, he added.
The Kingdom Tower has 4-m floor-to-floor heights. That eases coordination and means fewer beam penetrations.
The Kingdom Tower's foundation design is also simpler than the burj's. There is no bedrock at the site along the Red Sea, which contains a form of limestone made from decomposed coral. The foundation design calls for 270 augured piles that reach 50 to 70 m below grade, topped by a mat.
Once the piles are in, there is no excavation. The 4-m-deep mat is one level above grade. That means no digging down into the water table—which is close to the surface because of the proximity to the Red Sea—and no dewatering. "We couldn't get big enough pumps to dewater the site. It would have been like trying to dewater the sea," said Peter Weismantle.
The designers also learned that a simpler skin would be beneficial. "It's not anywhere near as complex as the burj," said Smith.
The design of the Kingdom Tower may be paying heed to issues of constructibility, but that in no way means the project is easy.
For example, during the time-out period, the developer made a decision to make every unit bigger and increase their number. The design team then went back to the drawing board and added one meter to the slabs around the perimeter and 20 more floors to the occupied space. "That reversed a lot of what we had done to make things more compact," said Smith.
As of last fall, the plan was for a building topped by a structural concrete spire. No one has ever pumped concrete up one kilometer. "The issue will be the mix design and labor at the top," said Weismantle. "To have a mix that can be pumped and placed at these heights [is a big challenge]," he added.
Wind loads also required attention. Early on, there was concern because the tower's edges were so sharp. But wind-tunnel tests showed no need to round the corners. "It's a well-behaved building," said Sinn. "The experience of Adrian and his team paid off," he added.
Peter A. Irwin, RWDI's chairman, agrees. For the burj, RWDI tested many iterations on the shape and "a lot was learned," said Irwin. "On Kingdom Tower, we started with a good shape and didn't do as many iterations."
John Galsworthy, RWDI's project director, agreed, saying, "In tests, the tapering form helped to mitigate cross-wind excitation, known as vortex shedding."
In terms of the building's technology, everything is pushed a step rather than being a radical departure—except for the elevator technology, said Weismantle.
Plans call for 59 elevators, including 54 single-deck and five double-deck cabs. The dedicated elevator serving the observatory is designed to travel 10 m per second in both directions.
The top occupied floor is more than 600 m high. For the elevators, the key issue is building sway. The elevator design incorporates "followers," cables that keep the main cables from hitting the shaft walls. The followers are more than half the height of the occupied levels of the building—more than 300 m.
Though the tower's form is inspired by the folded fronds of young desert plant growth, the three-petal footprint is ideal for residential units and the tapering wings produce an aerodynamic shape that helps minimize structural loads due to vortex shedding, said Smith. He is satisfied that the lessons learned from the burj construction will result in a better building that is much quicker to build.
