|The atrium of Shanghai's Jin Mao Tower (photos of interior details) is the world's highest at 650 feet.|
The Jin Mao Tower, which was dedicated in 1998, presented Sarkisian with a set of challenges quite distinct from those he would face in Oakland.
The tower – Jin Mao means "Golden Prosperity Building" in Chinese – dominates the skyline of Shanghai from a height of 1,381 feet. Its 88 stories include 50 floors of office space topped by a 38-story Grand Hyatt Hotel. Out of respect for the number eight, which signifies good luck in China, the tower contains an octagonal central reinforced concrete core, eight perimeter mega-columns made of concrete and steel, and eight steel built-up mega-columns, all resting on a 4-meterthick, reinforced-concrete, pile-supported mat.
The immediate test for Sarkisian in Shanghai was to anchor the Jin Mao in soft, clay-filled local soils that had caused much shorter structures to settle 10 inches or more. He overcame this by installing a 3-foot-thick slurry wall, or diaphragm wall, 100 feet deep around the building's half-mile perimeter. Workers drove 429 evenly spaced open steel pipe pilings, each measuring 3 feet in diameter, through the spongy Huang Pu River floodplain and into the stiff sands below. The pilings extend 275 feet underground, a greater depth than any previously attempted in China.
"To overcome the weak soil, we needed, in effect, to create a table with 429 legs for the foundation to rest on," Sarkisian says.
Made of steel and concrete, the Jin Mao is a composite building, a new concept that has since gained considerable popularity. The goal was to create a system that resists winds and earthquakes with the fewest possible structural elements. To connect the concrete core to the composite mega-columns on the perimeter, Sarkisian used levers, or outrigger trusses.
"A composite building allows us to locate materials where they most efficiently resist loads, thus minimizing cost and materials. The concrete core provides excellent stiffness, while the structural steel floor framing allows us to use long, column-free spans with minimal weight. This, in turn, reduces the size of the vertical members and the foundation."
|Sarkisian's (top to bottom) Pin-Fuse Frame, Pin-Fuse Joint and Link-Fuse Joint.|
The use of both steel and concrete in a super-tall tower, however, created a dilemma.
"The mix of materials in the vertical elements shortens when subjected to load," says Sarkisian. "Some deformations occur during construction due to self weight, while some occur over time, in some cases up to 10 years or more. Concrete and steel both deform elastically, but concrete also creeps and shrinks over time.
"For a building as tall as the Jin Mao, vertical displacement at the top could be as much as 12 inches. More significant is the relative movement between neighboring vertical elements, especially the core relative to the composite mega-columns that are interconnected with the stiff steel outrigger trusses. When subjected to large relative displacements, these trusses would attract forces so great that they could be ripped apart.
"To counteract these forces, we introduced pins into the trusses to allow rotation during construction. We did not bolt the connections until after the structure was completely built. After the bolts were installed, the structure was capable of resisting all future design loads."
The solution to the specific challenge of the Jin Mao led Sarkisian to develop the patented Pin-Fuse Joint and the Link-Fuse Joint and Pin-Fuse Frame, for which U.S. patents are pending. All are designed to fuse and dissipate energy during earthquakes; after an event, friction in the joints is restored by high-strength bolts.
"It gets back to the idea that buildings are not static but very dynamic. The joints of buildings have to be considered as potential moving parts, especially during extreme seismic events."
The Jin Mao has won nine national and international awards for engineering and architectural design and has become one of Sarkisian's favorite havens.
"I've stayed in the Grand Hyatt many times now. The combination of living in the building and seeing what an icon it has become, in China and internationally, is really satisfying."