The top occupied floor is at 498 m. Above that, a tapered "lantern," 56.7 meters tall and made from an expressed steel diagrid structure, will cap the building.

Permitting was tough because the code is intended for shorter buildings. "Everything was a breakthrough," says Nemeth. "You have to prove to officials that the building is meeting international standards and is safe," he adds.

The structure has a reinforced concrete core to resist lateral loads, coupled with eight reinforced concrete perimeter megacolumns, two on each side of the building up to the 76th floor (see drawing, left). Megacolumns gradually decrease in size eight times, from 3.3-m squares at the ground floor to 1.4 m x 2.3 m.

At three multistory mechanical levels, four structural steel outrigger trusses connect the core and megacolumns. Each truss level, with chords embedded in core walls, is a different depth. The deepest has diagonals spanning six levels. The system resists overturning moments much like a skier with arms outstretched and ski posts planted in the snow, says SawTeen See, the partner in charge for the structural designer, Leslie E. Robertson Associates (LERA), New York City.

Steel belt trusses, at several mechanical levels and up-to three stories deep, pick up gravity loads from steel perimeter columns and transfer them to megacolumns. Belt trusses also provide redundancy, allowing columns to share loads if one is compromised.

At belt truss levels, axially loaded floor beams emanate from the core to corner columns, where there are no megacolumns. By connecting to the top and bottom chords of the belt truss, beams resist rotation of the belt truss with respect to the megacolumn, allowing the structural system to work efficiently, says LERA.

Megacolumns, perimeter columns, even the belt trusses lean in. The core also slopes and steps in. The taper helps with wind loads because of a smaller "sail," but the various single and double slopes complicate connections, especially in the megacolumns, says See.

Another complication is a "no floor-slab zone" bordering the core in the officetel floors. The walled-off void was required to reduce usable floor area to satisfy the code. Floor beams continue to the core and in-plane bracing transfers diaphragm forces to core walls.

A 6.5-m-thick reinforced concrete mat on rock, reinforced locally with piles, supports the tower. The mat ranks as the world's thickest for a building.

For the frame, LERA had to anticipate differential shrinkage and creep, which will be recalculated during construction by the local engineer of record, Chang Minwoo. Differential movement of megacolumns with respect to core walls creates compressive forces in the smaller steel columns. That creates additional upward load on the uppermost belt truss and downward load on the lowest belt truss, says See.

To minimize compressive forces, LERA specified 40 MPa high-yield-strength steel for columns. Also, the engineer specified long-slotted holes in the bolted connections from the columns to the belt trusses. Bolts are tensioned and reindexed later on, after the tower is completed. "It is safe not to connect these during construction," says See.