The building’s second floor is set back from its facade on three sides, creating the double height lobby. The third floor dwarfs and shades the levels below it; the fourth level extends beyond the third. The fifth and largest level is rotated about 16 degrees clockwise from the building’s western face. The sixth floor follows the axis of the fifth and includes a small pop-up for the elevator over-run and roof access.
Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
This new corporate headquarters building in downtown Des Moines provides a modern and highly collaborative work environment with flexibility for growth. The conceptual motif of the building is its set of four vast planes, which emphasize lightness and transparency and are augmented by outdoor terraces and glass on all sides. Occupants enjoy column-free workspaces and an abundance of natural light that offers stunning views to the surrounding streetscape and park. With energy efficient features such as a green roof and generous roof overhangs, the building has earned LEED Gold certification.
Photo credit: Michel Denancé.
Photo credit: Michel Denancé.
Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
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The building’s second floor is set back from its facade on three sides, creating the double height lobby. The third floor dwarfs and shades the levels below it; the fourth level extends beyond the third. The fifth and largest level is rotated about 16 degrees clockwise from the building’s western face. The sixth floor follows the axis of the fifth and includes a small pop-up for the elevator over-run and roof access.
Silman chose structural steel for its ability to solve many different problems with one trade. The building’s long interior spans, exaggerated cantilevers, and thin floor plates necessitated a careful, integrated structural design process. The lateral framing for the building is a hybrid of moment frames, braced frames, and cable-tied super columns. The double height space between the first and third floors creates 32-foot-tall moment frames, complicating an already inherently flexible lateral load resisting system.
Photo credit: Michel Denancé.
Photo credit: Michel Denancé.
Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
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Super columns
The moment and braced frame system was not enough to meet code-stipulated seismic drift criteria and project-desired wind drift criteria, so the design team used cable ties at the exterior 62-foot-tall super columns to help control drift.
Super columns
The moment and braced frame system was not enough to meet code-stipulated seismic drift criteria and project-desired wind drift criteria, so the design team used cable ties at the exterior 62-foot-tall super columns to help control drift.
The showpiece of the Krause Gateway Center is its perimeter exterior cantilevered framing, unofficially known by the design and construction team as the “nosing”.
Photo credit: Michel Denancé.
- Photo credit: Michel Denancé.
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Extreme cantilevers
The nose is a two-level exterior cantilever built using two channels, 3/8-inch-thick plate, C5x9 vertical channels, and two HSS tubes spanning between wide flange beam outriggers. The wide flange beam outriggers cantilever up to 19 feet out from their main support points and support the 7-foot-long cantilevered nosing.
Extreme cantilevers
The nose is a two-level exterior cantilever built using two channels, 3/8-inch-thick plate, C5x9 vertical channels, and two HSS tubes spanning between wide flange beam outriggers. The wide flange beam outriggers cantilever up to 19 feet out from their main support points and support the 7-foot-long cantilevered nosing.
The extreme cantilevers produced significant deflections and torsion at the outrigger end point, which Silman mitigated by kinking the outriggers up at their support point anywhere from 3/4 inch to 5 inches.
The key to the nosing system was the need to attach (in plan) each 60-foot section of nose to the adjacent section, so that the entire system acted together and the outriggers did not differentially deflect. To accomplish this, the design team worked closely with the construction team to develop a pre-loading program which pre-deflected the outriggers to their expected fully loaded levels prior to the nosing being attached and leveled.
