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Diagonal Tower Turns a Corner

Night exterior rendering of the Diagonal Tower, in Seoul, South Korea
Diagonal Tower, in Seoul, South Korea, will employ a faceted diagonal megabrace at the perimeter and supplemental steel columns integrated into the skin to carry the structural loads. © SOM/Crystal CG

A dazzling tower slated for a new business district in Seoul, South Korea, employs diagonal columns for strength and efficiency.

March 19, 2013—The Yongsan International Business District under development in the heart of Seoul, South Korea, is an amazingly ambitious project that brings together some of the world’s top architecture and engineering firms to redevelop a nearly 140-acre site on the Han River. When complete, the center is planned to feature approximately 25 new buildings, the tallest at 665 m.

Architect Daniel Libeskind, BDA, AIA, prepared the master plan for the development. One of the key buildings in the project is the Diagonal Tower, designed and engineered by the New York City office of Skidmore, Owings & Merrill (SOM). The 343 m office tower is located at a prominent location at the site, near busy Yongsan Station.

“This project is the entry portal to the development,” says Charles Besjak, S.E., P.E., AIA, M.ASCE, the director in charge of structural engineering in SOM's New York office. “And because of that, it [will be] the first thing you see. Behind it will be the taller tower. The Diagonal Tower definitely provides an iconic image as the front door of the overall development.”

Besjak says SOM’s architecture and engineering staff worked together “hand in glove” to develop the dramatic tower, in which the floor plates are rotated 45 degrees at a point one-third of the way up tower’s height, and again at the two-thirds point. This rotation creates a dramatic, diagonal façade that maximizes views of the river and mountains and between the other tall towers that are being developed as part of the master plan. 

The tower will employ a faceted diagonal megabrace at its perimeter and supplemental steel columns integrated into the skin to carry loads so efficiently that SOM was able to use 25 percent less structural steel than a conventional moment-frame tower—and eliminate interior columns while they were at it.

“The basic footprint is 48 meters by 48 meters,” Besjak says. “There are columns at 12 meters on center to create four equal bays. Each of the panel points in the diagonal megacolumn coincides at the vertical columns and the floor line. That’s really important because it eliminates eccentricities in the perimeter connections and allows all the members to remain in compression or tension rather than bending, which is the key to its efficiency.”

 Rendering of Sky Lobby Cafeteria

 The tower was designed with no interior columns to maximize the
floor plans and the views of the Han River nearby and the
mountains in the distance. © SOM

The tower employs a central octagonal concrete core, floor spans averaging 12.5 m in length from the exterior to that core. In order to maximize the aspect ratio and stiffness of the tower, the perimeter diagonal megabrace incorporates the full width of the tower. There are no load transfers in the structure.

By effectively distributing loads to the building’s exterior, the design team was able to manage gravity loads to minimize uplift on the tower. This enabled the team to use a 4 m thick mat foundation, located six levels below grade on solid rock, with no additional deep foundations required. 

To create an inviting lobby space that provides transparency to the rest of the development at the street level, the steel columns on the building’s faces come down only to the second floor, where they are redirected at a 45-degree angle back into the tower’s concrete core. The megabrace perimeter columns at the corners do come all the way to the ground, but a cable net wall around the lobby areas creates an open appearance at the street level.

Moving the gravity loads to the exterior of the structure proved to be a benefit when the design team began to address wind loads at the site. Because Diagonal Tower will be in close proximity to other tall buildings, the wind tunnel models revealed that it would be subjected to additional forces generated by the wake effect of the adjacent towers.

“The close proximity of the adjacent supertall tower increased the magnitude of wind forces on Diagonal Tower,” Besjak says. “The building geometry and rigorous, disciplined structural system we designed mitigate these loads and limit uplift on the tower while also maximizing the structural efficiency.

“This is probably one of the most interesting developments I have ever been involved in,” says Besjak. “At Yongsan International Business District, each individual [architecture/engineering] firm was allocated a building within the master plan. It was a who’s who of architecture and engineering. I think it brought out the best in everybody and generated some truly innovative structures, like the Diagonal Tower.”



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