Wind can menace skyscrapers, but engineers have a solution for the big one going up in downtown Omaha.
The future Mutual of Omaha headquarters tower will have a crucial wind-resistance feature built in. It’s a common feature in tall buildings, but constructing it in what will be the requires a specialized device.
Rising from anchors deep underground, a concrete box-like structure called a “shear core†is taking shape in the center of the construction site at 15th and Douglas streets.
Made from fortified concrete, strengthened with steel bars and built with the aid of a specialized self-climbing device, this giant rectangular concrete tube 94 feet long and 64 feet wide is on its way up — and up.
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It will eventually reach 650 feet tall before it disappears from view as the new 44-floor building is constructed around it.
Although the shear core will provide some help holding up the building, its primary purpose is resistance to horizontal forces like wind, said Stephen Harris, director of development for Lanoha Real Estate Company, the developer for the project.
“Shear is one of the types of forces that act on a tall building like this,†Harris said. “It’s the one that creates bending in something that’s tall and skinny like a building. The core is that concrete spine, so to speak, that keeps it rigid in the up-and-down direction.â€
The design and engineering of the building will meet or exceed industry standards, including for extreme weather such as tornadoes, a spokesman said.
Building such a massive structure requires a clever bit of engineering.
Most people are familiar with concrete forms, the temporary walls that support and mold wet concrete in the desired shape until it hardens. Those work fine at ground level for pouring a sidewalk or footing.
But for a skyscraper in a confined work space in an existing downtown area, the engineers needed something special.
The device is called the DOKA Shear Core Climber.
The self-climber, identifiable as a yellow encasement around the structure, creates a mobile concrete formwork system surrounding three levels of the structure at a time.
“It is engineered specifically for the building,†Harris said. “So it was designed and engineered based on the specification, size, structural requirements and all of that configuration geometry of our building.â€
After the concrete is poured and sufficiently cured, 18 hydraulic cylinders lift the top two levels of the self-climber 15 feet to its new position. Once they are locked into place, the hydraulics lift the bottom level up to create the new platform position.
Over the next several months, the construction crew will engage in a repetitive cycle of preparing, pouring and erecting the shear core.
Sensors keep the structure going vertical, what’s referred to as plumb in construction lingo.
As the core rises higher, powerful pumps will be needed to pump wet concrete to reach the top levels, Harris said.
The work cycle will repeat to a full height of 650 feet near the top of the tower. When the core work is complete, the tower crane will be used to disassemble the self-climber and lower its components to the ground.
In January 2022, officials of Mutual of Omaha announced plans to develop a corporate headquarters facility in downtown Omaha. Construction was launched in January 2023.
At 677 feet, the building will be the tallest in the region. The project is expected to be completed in 2026.