The mixed-use Fortune Center, in Quingdao, China, will transition from an orthogonal floor plan to a curved structure that seems to embrace the sea. Pei Partnership Architects
A gracefully curved landmark structure in a key industrial city in China required a robust structural engineering solution to reach new heights.
October 1, 2013—Although, facade work continues, the structure has been completed for the Fortune Tower in Qingdao, China, a dramatic landmark structure that stretches 242 m to the sky. The tower is a hybrid structure—the first 25 stories are orthogonal, while the top 39 are elegantly arced and angled to “embrace” the nearby Yellow Sea.
The building was designed by C.C. Pei of Pei Partnership Architects, in New York City, and will be one of the tallest buildings in the city of 8.7 million residents in eastern China. Qingdao boasts a large port that supports a robust industrial economy. On the basis of successful collaboration on past projects, Weidlinger Associates, Inc., of New York City, was called upon to develop the structural engineering solutions for the complex tower.
“They asked if we could do it,” recalls Tian-Fang Jing, P.E., a principal of Weidlinger Associates. “I looked at it. It’s a very interesting project; it’s a difficult project. But as an engineer, we always say, ‘Yes, we can do it.’ ”
The tower’s stunning transition from a square stone-facade base to the gleaming upper, curved floors address the developer’s need for a mixed-use structure. The lower two floors will house retail spaces. Office space is planned for floors 3 through 23. The top floors are slated for a luxury hotel or apartments, each two levels high.
“All the units face the ocean and the beach,” Jing says. “The corridor is on the back side.”
The unique shape of the tower presented a daunting structural engineering challenge. Because the cast-in-place concrete core doesn’t extend throughout the full height of the tower, the engineers had to design a transfer structure on the 25th floor to handle the gravity loads and the lateral forces from the floors above.
“If you look at the shape, it’s an eccentric structure,” Jing says. “The torsional effect is so [great]. We did wind tunnel studies. It’s a very complicated shape. At the corner in the back side, you have very large wind forces. In U.S. standards, at the top it’s 60 psf at the corner.”
The transfer floor is massive and robust. The floor is 3.74 m tall, with interior main ribs 500 mm thick from top to bottom. Between the main ribs are secondary ribs that are 300 mm thick. The ribs match the structural elements both above and below the floor. All of this is sandwiched between a pair of 250 mm thick transfer slabs.
The stone facade of the more traditionally designed lower floors
gives way to gleaming metal and glass at the upper, parabolic
levels. Pei Partnership Architects
The transfer floor added 20 percent to the cost of the structure, and even once the solution was developed, engineers still had a significant hurdle in their path. Chinese building standards allow transfer floors only up to floor 7 in the region’s seismic zone 6. Because the transfer floor in the Fortune Tower was on floor 25, the team faced a stringent design review.
“If you exceed code limits, then they always have special review groups—experts from design firms, professors from universities, experts in certain fields, construction experts,” Jing says. “They will review your design. Once they review, they give comments or they say, ‘No, this is no good. You have to change it.’ The review board asks you to do a very detailed analysis for this transfer floor. A detailed finite element model was prepared for this special floor to show where the stress concentrations are and how you solved these stress concentrations.”
Jing says that because the eccentric shape of the building produces uneven loads, some of the corner columns in the base are under heavier loads than others. Those columns were bolstered from 1.1 m square to 1.4 m square and made from a composite of steel and concrete. The largest columns have steel wide-flange shapes approximately 500 mm by 500 mm and as thick as 76 mm.
At the top of the tower, strength is provided by smaller cast-in-place concrete elevator cores on either side of the wing.
The building employs a mat foundation on a rock formation, with a five-level basement for parking.
Weidlinger was the design structural engineer on the project, with the local design institute in Qingdao serving as the engineer of record. Construction began in 2009, and current plans call for the building to be complete in late 2014.