Climbing Core Leads Waterside Place Tower Construction
Core-First Construction: Liberty Construction Services Uses Self-Climbing Formwork for Concrete Core of $157M Waterside Place Being Built by Suffolk Construction in Seaport District
Steel framing recently got underway for a $157 million high-rise apartment tower in Boston’s Seaport District using a “core-first” construction method to expedite the project.
Braintree, Massachusetts, concrete contractor Liberty Construction Services built the 255-foot rectangular box-shaped core for Waterside Place Phase 1B, a mixed-use development underway by general contractor Suffolk Construction. Developed by Gables Residential and designed by CBT Architects, the 325,000-square foot apartment building is the second phase of the Waterside Place development on the South Boston Waterfront. It provides 307 luxury apartments together with 3,500 square feet of street-level retail shops and parking for 84 vehicles. The first tower of Waterside Place, located next door, was completed in February 2014 and provides 236 apartments.
Suffolk Construction commenced work on Phase 1B in February 2018 under the supervision of Nicholas Morel, Senior Project Manager. Early work included site excavation and driving piles to support the building’s foundation. Liberty Construction Services, a subsidiary of Suffolk Construction, began working on the concrete core in October 2018 using a PERI automatic self-climbing formwork system.
A Building Within a Building
Concrete cores are incorporated in high-rise structures such as Waterside Place to augment a high-rise building’s resistance to not only gravity forces (vertical live and dead loads), but also to lateral forces presented by wind and seismic events. Considered individually, the core walls behave structurally to resist shear, while collectively, they resist twisting by forces coming from any direction. This is similar to the behavior of hollow structural sections (HSS), which are known for their superior resistance to lateral torsional buckling over other shapes (such as wide-flange or H sections). Concrete cores also enhance safety, providing fire-resistant enclosures for egress stairways, elevators and service utilities.
From an engineering standpoint, Waterside Place can be considered a composite, or mixed, design structure, combining a concrete core with outrigger lateral steel framing – essentially creating a building within a building. The lateral framing is connected to steel “embed” plates cast in the core walls.
Some types of core formwork may be used repetitively, especially self-climbing systems, so in general, projects using them are able to proceed faster than those employing conventional construction methods. And on some high-rise building projects, the concrete core is only partially completed before ironworkers start attaching steel framing, further speeding progress. At Waterside Place, ironworkers started attaching steel framing once the concrete core reached the seventh floor, according to Bob Arcari, Concrete Foreman, who oversees field work under Liberty Construction Services General Superintendent Fred Collins.
A Potain MD-485-M Tower Crane was dedicated to the core construction and was used primarily for hoisting and moving materials around the site, rather than having to lift formwork panels and platforms from level to level. The formwork climbs independently, propelled upward by several integrated 40-ton-capacity hydraulic cylinders that lift the entire unit consisting of formwork and working platforms from one level to the next. The number and sizes of the hydraulic cylinders are such that the system can also carry a concrete placing boom, which eases pouring concrete for the decks around the core. Furthermore, the system design allows the core to climb ahead of the decks without requiring additional brackets on the outside wall.
During each ascent of the PERI system, the trailing climbing brackets re-use the same two anchors from the leading working brackets during the previous climbing process. Prior to each climb, workers strip the inside formwork either hydraulically or manually and roll back the outside formwork on overhead gallows. The hydraulic cylinders push off the climbing brackets to advance the working brackets upward. These working brackets carry the entire system, including formwork and platforms, to the next level. The climbing brackets later re-use the same anchor points as the working brackets. This particular system used PERI’s Maximo panel formwork that allows positioning panels manually with a hand tool such as a pry bar, with all adjustments being made from one side of the wall.
Steel Framing Attaches to Core
In constructing the core, workers initially built 25-foot-tall starter walls using conventional cast-in-place methods, in order to provide a base to support the self-climbing formworks.
The concrete core crew made its first placement of concrete using the climbing-form system on October 10, 2018. Once the core had been formed and cured at the 7th floor, ironworkers started attaching framing steel for the main building to steel embed plates in the core walls. Core construction was essentially complete in March 2019.
According to Arcari, the completed concrete core has inside dimensions of 26 feet, 7 inches by 40 feet and is 255 feet tall, with two opposing 24-inch-thick walls and two opposing 18-inch-thick walls. Its elevated floor slabs are 8-inch-thick, and consist of 5,000 psi concrete placed on PERI Sky Deck forms. The core walls are constructed with high-strength 8,000 psi concrete for the first 14 floors, and 6,000 psi for floors 14 through 24 (top floor is a mechanical mezzanine).
Approximately 3475 cubic yards of ready mix concrete for core slabs and walls was provided by Boston Sand & Gravel, with the material being transferred to the climbing forms by Independent Concrete Pumping of Wakefield, Massachusetts. Company owner Jim Toothaker explained that they utilized a 61-meter Schwing pump with boom for the core’s lower 14 floors. He said they supplied the upper floors using a Schwing 31meter pump as a standpipe pump (pump fed ready-mixed concrete directly to the standpipe – boom was not used).
Safe, Fast, with Fewer Workers Needed
Liberty Construction Services’ Arcari, a veteran of some 25 years in the construction industry, commented on the self-climbing formwork system:
“This was the first time I’ve worked with the PERI system, and I was impressed by it. It’s a safe system, has a fast climbing rate, and took a smaller crew size than I would normally have had to use.”
Construction of the Waterside Place apartment building is scheduled to be completed in February 2020. Located on Congress Street in Boston’s Seaport District, itwill provide 307 luxury apartments, ranging from studio, one-, two- and three bedroom apartments. In addition, there will be 3,500 square feet of street-level retail shops and parking for 84 vehicles.