http://www.gostructural.com/article.asp?id=3054
Pittsburghs green mixed-use skyscraper: The 26-story, Three PNC Plaza project
Kimberly Kayler
When construction is complete early next year on Three PNC Plaza—a 26-story, 780,000-square-foot tower in downtown Pittsburgh—the structure will hold the distinction of being the first skyscraper constructed in the city in 20 years. But that's not the most notable distinction it will receive. As a home for office space, condominiums, and a high-end hotel, the building will also rank as one of the largest environmentally sustainable mixed-use structures in the United States.
Green features
PNC's ongoing commitment to building green is evidenced by the bank's current list of 36 LEED-certified branches in five states with plans to build 11 more. Three PNC Plaza is no different. PNC has consulted with Paladino and Company in Seattle for its green building design needs.
The building has contributed to urban redevelopment in downtown Pittsburgh from its very inception. Materials from the buildings that were demolished to make way for the new structure will be recycled to use during construction.
The building will also make use of high-performance glazing, energy-efficient HVAC systems and low-emitting materials to create an environmentally friendly interior for its occupants. All of the building's tenants ― including the Fairmont Hotel, which will manage the top 10 floors ― will be required to adhere to green design standards.
The building's exterior ― designed by San Francisco-based Gensler in association with architectural/engineering firm Astorino of Pittsburgh ― will pay homage to the city's past while looking toward the future. Its street-level granite/limestone façade gives way to a sleek glass and aluminum curtain wall. A three-story, 100,000-square-foot underground parking provision will offer 330 parking spaces.
"We want this building to be one that both responds to the vernacular history and culture of Pittsburgh, referencing the scale of the city, and speaks about the future," architect Doug Gensler told the Pittsburgh Post-Gazette.
Building features demand superior formwork solution
The building consists of eight concrete cores including large steel embeds, 32-foot high foundation walls, and below-ground parking decks. According to Micheal Brennan, structural engineer with Astorino, the Three PNC tower uses a hybrid steel frame with a concrete core wall. Because the 14-story cores change size at the eighth floor, pre-planning was necessary to accommodate the shift.
Ground was broken on the project in August 2006, and in order to meet the ambitious construction schedule, PNC assembled a top-notch construction team that included Pittsburgh-based General Contractor P.J. Dick Incorporated. To assist with the concrete construction on this complex project, P.J. Dick turned to Doka to supplement its existing 6,000 square feet of Framax formwork with an additional 16,000 square feet, as well as Doka's MF 240 platforms.
According to P.J. Dick's Project Engineer Sarah Haynes, when they were selecting the type of formwork to use on the project, some of the key considerations were the following:
* whether to use self-climbing forms or jumping the forms by tower crane,
* what weight and size of forms to use,
* what amount of pressure the forms should withstand,
* what ease is desired for assembling and jumping the forms,
* what availability is required for the formwork, and
* what the cost of renting, delivering, and returning the forms will be.
Brian Katanick, P.J. Dick concrete superintendent, stated that one of the biggest challenges for selecting the right formwork for the project was making sure that the formwork system was adaptable. Several alterations were made to the structural design of the concrete shear walls, such as wall thickness changes, adding or deleting wing walls, and switching to concrete corbels/columns with anchor bolts. Fortunately, the use of the Doka formwork could be easily modified to adapt to the many changes.
"Revisions to the structural drawings were made so frequently it was important that the weight of the whole formwork configuration fell within the allowable parameters of the lifting load charts of the tower crane while still providing a significantly high pour rate," said Katanick.
As a time-saving measure, the MF 240 platforms were pre-assembled by Doka off-site. MF 240 platforms ensure safety and accommodate any inclination of the structure (up to 15 degrees). Pressure spindles ensured the horizontal stability of the MF 240 main operating platform and the adjustment of the climbing formwork to the structure with an angle of inclination. Both the pouring and suspended platforms were made using just one type of bracket, allowing for great flexibility with only a small number of separate parts.
The MF 240 brackets were spaced at desired intervals, thus adapting to varying wall widths easily. Assembly was fast and straightforward because the brackets were modular and assembly required only six pins. When combined with the wall formwork systems, the climbing brackets had a high load-bearing capacity of 11,250 pounds each. The climbing anchor parts, lightweight and easy-to-install, saved time and labor. The lightweight climbing cone screw was attached directly to the embedded insert. The working platform brackets, with multiple holes for adjustment, were used for pouring, intermediate, and suspension.
Work progressed safely with Doka's large 8-foot-wide work deck. Roll-back mechanisms were added after the deck platforms were assembled. In the high isolated work environments of Three PNC Plaza, the deck allowed easy movement for workers. When rolled back and locked off the formwork, it sheltered workers from the windy weather conditions of downtown Pittsburgh.
According to Dave Single, Doka's account manager, the cycle application was different for each core because each had different dimensions, steel embeds, and opening locations. Doka's engineering team met with the customer and pre-planned every detail of each core with multiple drawings showing the different phases of the construction sequence. Utilizing the framed formwork Framax Xlife and MF 240 on each core for the best possible fit, the systems ensured the contractor would stay on schedule. Construction of the multi-shaped cores, together with the 32-foot foundation walls, made the Framax's flexibility invaluable on a project of this scope.
"The versatility of Framax in conjunction with the MF 240 platforms has made it easy to take several multi-shaped concrete structures vertical with a quick cycle time," said Katanick. "Doka's ability to deliver the MF 240 platforms already assembled [which saved space on a tight job site] and respond promptly to daily changes also has helped to keep the project on track."
From a structural engineering perspective, the formwork system performed admirably as the core walls were advanced ahead of the floor diaphragms, said Brennan.
"The professionals at Doka developed an innovative form system to hang off the flat face of the wall so as to construct the story-high corbel elements," he said. "It was a remarkable feat."
Katanick concurred, stating that the design of Three PNC's shear walls posed a challenge from the start of the project.
"There were three main areas of difficulty on this project—the cantilevered walls at the garage levels, the steps in wall thickness on the eighth floor, and the concrete corbels with anchor bolts on the 13th floor. Doka's formwork and staff made each area a smooth transition so that the project could keep moving forward," he said.
Conclusion
Already being touted as a catalyst for downtown renaissance in Pittsburgh, the construction of Three PNC Plaza coincides with the city's 250th birthday. As the city celebrates its past, this forward-pointing structure will serve as a harbinger of what the next 250 years might hold.
Kimberly Kayler, CPSM, CSI, is president of Constructive Communication, Inc., located in Dublin, Ohio. She can be reached at 614-873-6706 or via e-mail at
[email protected]. For more information about Doka USA, Ltd., visit
www.dokausa.com.
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