Project Profile
By Ascent Staff Spring 2022
Charlotte’s new 597 foot tall 40-story Duke Energy Plaza tower is the second tallest building in the world to incorporate precast/prestressed double tees. The new office tower features 1,000,000 square feet of space, on a 2.1-acre uptown site, and will act as the corporate headquarters for over 4,000 employees and contractors beginning in 2023. Featuring retail space, seven levels of parking, and office levels, the project provides a consolidated solution and reduced real estate footprint as a new flexible workplace model and alternative to the multiple locations currently occupied by the company.
Duke Energy’s unique project features a tube structure design, combining cast-in-place (CIP) with precast concrete double tees. The use of double tees in high rise construction is not common practice, but after evaluating other structural systems, and using the existing Duke Energy Center as a successful model, this option was selected. The current Duke Energy tower is the tallest double tee high-rise in the world and showcases the success of this unique building method.
The project team selected double tees as the preferred flooring material due to ease of acquisition and the inherent benefits including fabrication off site, speed of erection, improved loading and structural efficiency, and safety during the build process. Using double tees for flooring and shoring is a faster process than cast-in-place concrete construction. The double tee design also facilitates other onsite construction activities to begin sooner.
Using double tees also served a secondary build purpose, as the shoring requirements were reduced. The team was able to fully load the installed floor levels with construction materials to continue upward work at the next level with no stops or delays. This flooring design provides a high-quality finished product, and pouring topping slab on the double tees provides floor flatness (F/F)/floor leveling (F/L) numbers that are higher than normally achieved with shored construction. Precast flooring also provides the inherent benefits of versatility, fire resistance, and vibration control.
The CIP perimeter was constructed throughout the project utilizing a climbing form work system pouring perimeter columns and beams. The system climbed with the structure one quadrant at a time. The day following the pour, the climbing system was opened and raised, double tees were placed and welded for the floor of the same level, and the system rose up the frame to the next level to prepare for the next form. The climbing core took five days to circle an entire floor, while the building perimeter and double tees were on a four-day cycle per floor, allowing the team to meet an established 25,000 sq. ft. floor each week. This form work system improved product efficiency, reduced time spent lifting the frame at each level, and improved safety. The only structural steel in this project is used for the 60 ft. tall parapet “crown” of the building, and the conference center on top of the podium. In addition to the tower, a nine-story podium extends out and supports a conference center.
Lateral wind and seismic loads are resisted primarily by perimeter columns on 15 ft. centers with a 4 ft. deep perimeter beam, while the core is designed for gravity loads. There are no shear walls in the project, which allows for a smaller core and more flexibility in modification. High strength 16000 psi concrete was used in the lower levels of the CIP frame.
The coordination of the precast and cast-in-place parts provides several inherent advantages. The combined effort allows production concurrently to keep up with the timeline of the project. The team completed the entire project, from design phase through erection, in just over five and a half months, utilizing heavy collaborative and coordination efforts from all involved project partners, with precast production beginning in May 2020 and finishing in June 2021. The overall project began in early May 2019, with double tee installation beginning in September 2020, and the team topped out the concrete structure in the tower in September 2021.
The project process started as the team developed a strategy focusing on an initial modeling phase, which led to full BIM and 3D coordination throughout the process to ensure the CIP and prefabricated double tees would assemble seamlessly. The 3D modeling benefits both internal and external project coordination, as well as pre-planning the erection sequence. The General Contractor, Batson-Cook, utilizes Lean and Design-Build methods in planning and scheduling projects, so all trade partners were brought in early in the process. An extremely collaborative process then ensued, as Batson-Cook developed multiple intensive pulls plans and hour-by-hour schedules to track production throughout the process, identifying strategies for achieving and meeting a fast-paced project schedule. The team also held daily huddle check in meetings and weekly planning meetings to ensure the project stayed on the projected timeline.
Extensive coordination among the project teams was necessary to ensure that project requirements were met as the teams worked through unprecedented times with pandemic safety precautions. With two separate concrete teams and systems and on-site cranes for erection and installation, the project activities had to avoid any potential disruption to the surrounding urban environment. Project challenges also included rock blasting, an infill site sandwiched between a museum and a 100+ year old church, as well as the logistics and material staging of the site. Due to limited space, the project required just in time delivery materials with nightly double tee loads.
Atlanta Structural Concrete’s scope consisted of 1,617 pieces of 42‘by 12DT19 (12’ wide and 19” stem) double tees, composing 762,000 square feet. The ASC team engineered the double tees to fit this unique interrelated system, and each of the precast/prestressed components was manufacturing in their 70-acre facility in Buchanan, Ga., Manufactured to PCI MNL 116, the precast is used in this project as load bearing flooring, shoring, and structural support for the facility. The entire precast scope, from design to erection and finishing, was a swift turnaround over one year. This process reduces trade coordination, shortens construction schedules, and minimizes risk for all involved trades.
ASC’s participation in this project positions precast as an ideal high-performance material and reinforces the need and desire for precast concrete for convenience, speed of turnaround, high quality building materials. The ASC team worked jointly with Charlotte Metro – Concrete, CK Metro, LLC a NC LLC, TVS-Design, Batson-Cook Company on this impressive project. The building meets LEED Gold certification standards.
