Totaling approximately 980,000 square feet, the Minnesota Life–Phase II office building is one of the largest buildings in St. Paul, Minnesota’s history. The combination office, retail, and parking facility was developed for the expansion of the Minnesota-based insurance company Minnesota Life.
Opened to the public and employees in October 2000, the L-shaped office building engulfs an entire city block and is connected to the heart of downtown St. Paul via four existing skyways. The $108 million, 14-story cast-in-place concrete building consists of 11 stories of office space, two stories of retail space, and four stories of underground parking with 950 spaces.
The design requirements for the project were very diverse. The greatest challenge was keeping to the very aggressive schedule required to meet the owner’s move-in date.
Typical bays in the interior of the tower are 30-by-40-feet, with 40-by-42-feetbays at both ends of the building. The floor system is a combination of 19-inch and 25-inch deep wide module joists supported on post-tensioned girders. Two different joist sizes are used in the entire project: (1) 5-inch thick slabs on 60-inch wide by 14-inch deep pans, with 12-inch wide ribs, spaced 6 feet on center and (2) 5-inch thick slabs on 66-inch wide by 20-inch deep pans, with 14-inch wide ribs, spaced 6 feet 8 inches on center. The post-tensioned girders, which allowed the contractor to strip the formwork earlier and addressed the owner’s request for larger bay dimensions, are 48-in. wide and are 19-in. and 25-in. deep to match the overall joist depth. The columns vary from 24-by-24-inches in the tower to 44-by-44-inches in the parking area.
A combination of shear walls and moment resisting frames is used to resist the lateral forces. The tube-shaped structure above the first floor transfers into an I-shaped structure in the parking garage. Elevator pit walls and thick pit slabs are utilized on the first floor of the parking structure to transfer the lateral loads to the I-shaped core below. Deep moment frames at the exterior portions of the building provide added resistance to the lateral and torsional forces on the building.
A 6,000-psi mix was used for the concrete in all the decks (tower and parking) and in the columns of the tower, and an 8,000-psi mix was used for the columns in the parking area. Early in the construction phase, the design team gathered test results on a new concrete mix that reduces the effects of cracking and shrinkage. A significant strength gain was measured, which allowed the contractor to remove formwork earlier than scheduled and to cut weeks off a very aggressive schedule.
One of the main reasons concrete was chosen for the framing system was that it made possible the move-in date of the owner. The design team was able to submit drawings periodically on this fast-track, design-build project and continued designing while construction was in progress. Also, the owner requested concrete for its perceived quality: all floors contain raised access flooring, and the concrete system provides a stiffer structure that minimizes deflection and vibration problems. The long spans also provided the desired flexibility required by the owner.
Minnesota Life Insurance Company, St. Paul, Minnesota
Architectural Alliance, Minneapolis, Minnesota
Meyer, Borgman and Johnson, Inc., Minneapolis, Minnesota
McGough Construction, Inc., St. Paul, Minnesota
Cemstone, Mendota Heights, Minnesota