In designing the Confluence on Third luxury apartment complex in Des Moines, Iowa, designers were looking to provide market-rate housing with noteworthy amenities while also creating an efficient, cost-effective design.
The project features 211 studio, junior-suite, one-bedroom, and two-bedroom apartment homes. Outdoor amenities include a fire pit, pool and hot tub, dog run and dog-wash station, and a bocce ball court.
Underlying these features for the 209,000-square-foot, four-story building would be an underground level of parking that had to be built efficiently while providing a fire-rated separation between the two spaces. It also had to meet a schedule that would include winter construction.
Designers at Urbanworks Architecture LLC specified a total-precast concrete system for the parking level to meet these needs. “Precast concrete provided an efficient system to transfer housing loads while providing the clear fire-rated separation we needed,” says John Seppanen, senior project architect. “It also allowed for simpler construction sequencing during colder seasons.”
The precast system was able to easily transfer the loads from the wood-framed upper levels and handle the weight of the green-roofed amenities and hot tub.
The system, fabricated and erected by Mid-States Concrete, consisted of columns, beams, hollowcore, and solid slabs. Precast components also were used for the building’s podium and elevations in the courtyard. Deeper-than-usual double-tees were used on the steps to accommodate the courtyard’s green-roof assembly.
The building took up a full city block, opening access from all sides for construction. But there still were challenges, says Justin Webster, project manager for Frana Companies Inc. General Contractors.
“The site was essentially a zero lot-line project, so there were many challenges in setting the precast,” he says. “However, the efficiency in time to erect the precast from the interior of the project far exceeded the time needed for a post-tensioned cast-in-place design or a similar system. It also fit our winter erection schedule better.”
Erection continued smoothly through the winter and spring months. “We wanted to cap the deck as quickly as possible,” Webster says. “Precast allowed us a shorter timeline in adverse conditions.”
Precast also benefited the construction due to the project’s multiple elevations for concrete decks. “Precast had a larger than normal advantage, allowing us to raise and lower elevations as needed for the building’s courtyard to allow for the pool, spa, and landscaping amenities.”