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tt reston275The trophy-class office building, Reston Station OB1, in Reston, Virginia, topped out last month. Thornton Tomasetti provided structural engineering services for the 620,000-square-foot, 16-story project designed by JAHN architects. The project is part of the larger Reston Station mixed-use, transit-oriented development, which includes residential and retail buildings located at a metro stop on the Silver Line leading to Washington, D.C. In addition to offices, the tower will have a restaurant and eight levels of below-grade parking.

Our previous collaboration with architect Helmut Jahn includes the Chicago O’Hare International Airport’s United Airlines Terminal One, the “Terminal of Tomorrow,” in the 1980s. The project was a combined effort between Thornton Tomasetti’s D.C. and Mumbai staff.

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“The project was a true trans-Atlantic collaboration of engineers and modelers,” Associate Principal Zach Kates, who served as project manager, said. The tower features a unique, diagonalized, concrete exoskeleton that functions as both the gravity and lateral system.

The tower features a unique concrete exoskeleton and 15-foot cantilevers at each floor at its southern and northern ends. Photo by Wayne Stocks.The building is constructed of two-way, post-tensioned concrete flat slabs with 40-foot interior spans and 15-foot cantilevers at each level. Its north end features an open plaza surrounded by six five-story, sloping, concrete-and-steel composite super columns.

The post-tensioned concrete was configured with the uniform tendon direction perpendicular to the concrete exoskeleton. This easily accommodated the varying column locations on each floor resulting from the sloping exoskeleton column configuration.

Uniform tendons were crossed within the slab such that dead-ends were located within the core of the exposed exoskeleton columns and stressing ends were always located outside of the columns in the slab edge.

The interior concrete columns match the slope of the exoskeleton. In addition to accommodating the large slab spans, the banded direction of the PT acts to resolve the horizontal force component of the interior sloping columns and drag load into the floor diaphragm and exoskeleton.

Staged, post-tensioned concrete transfer beams were used at the roof to support the mechanical penthouse and to maximize headroom on the highest occupied floor level. Construction is scheduled for completion in July 2017.

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