Structural design plays aesthetic role
Structurally accommodating several unusual design features of the 19-story, 1.1 million-sq.-ft. Union Pacific headquarters building in downtown Omaha, Neb., required close collaboration between design architect Gensler, San Francisco, and structural engineer Walter P. Moore and Associates Inc., Houston. The result was creative solutions that incorporate structural elements into the architecture.
Most notable, says David Landis, principal with Walter P. Moore, is the structural system that will frame the 120-ft.-tall glass atrium.
"Gensler's design breaks away from the typical tall building layout, where all the elevator and stair cores are in one location," says Landis. "So we lost what we would normally use for bracing an atrium like this."
To meet both the structural and architectural design criteria, the design team developed an X-bracing system that forms the walls of the atrium.
"It's essentially a series of four-story-tall X braces that extend the height of the atrium," says Landis. "These braces will be covered by a glass wall and left exposed on three sides of the atrium to act as a design feature as well."
Accommodating a 90-ft. span over the building's south entry was also a challenge, says Landis. "Gensler does not want columns or trusses to intrude into that space, so we will utilize a Vierendeel truss, which essentially eliminates the diagonal bracing," he says. "It's not as efficient as a conventional truss, but it accomplished what the architect wanted — a long span over the entry without having a truss look to it."
To support a 12-in. raised-floor system specified for the office space, Landis says an additional 12 pounds per square foot floor loading was required. The office floors are designed for a total of 82 pounds per square foot, except in certain core areas, where 200 pounds per square foot loading is needed to accommodate high-density file systems.
"We designed these areas with shorter structural steel spans — 30 feet instead of 45 feet — to help take the heavier loads," says Landis.
Foundations forge ahead
General contractor Holder Construction Corp., Atlanta, along with its local partner Hawkins Construction Co., Omaha, have started to drill to accommodate the concrete piers that will form the base of the foundation, which will span 250 by 270 feet.
"The foundation utilizes 105 drilled shafts, ranging from 30 inches to 60 inches in diameter," says Landis. "They are drilled approximately 60 feet down and socketed 3 feet into rock."
Once the piers are set, column base plates and steel columns will sit directly on top, and the slab on grade of the basement level will be poured around the top of the pier and the column base plate.
"We've seen a number of RFIs [requests for information] for the structural steel frame, and we expect to receive shop drawings any day," says Landis.
He says the design and development team considered a concrete frame, but ultimately chose steel because of its faster erection time and the availability of labor.
"Hines [the development manager] was concerned with the availability of labor for the concrete for work for a building of this size in this market," he says. "Both systems were priced pretty evenly, so cost wasn't the driver."