Pouring Virginia's Tallest Building
General contractor Armada Hoffler Construction Company is close to topping out what will be Virginia's tallest building. The Westin Hotel & Residences is part of the $500-million development known as The Town Center of Virginia Beach. The 903,000-square-foot Westin will consist of a 39-story tower with five levels of parking, first-floor retail space, a 236-key Westin Hotel on 10 floors and the Westin Virginia Beach Residences - 21 floors of upscale condominiums.
The tower presents the most interesting challenge. Century Concrete Inc., subcontractor, has a 50-week schedule for pouring the tower. To accomplish this, Century Concrete is relying on four key elements: a hydraulically operated self-climbing wall system, a Peri formwork system for the decks, a concrete placing boom on the building's center core elevator shaft wall, and a tower crane with dedicated hook time.
Gary Manning, Century Concrete's project manager, explains that they determined they needed to turn over a floor a week on the typical floors to stay on schedule. "What we're actually doing is producing a lift in the building - all of the columns and walls and floors - in four days," said Manning.
A hydraulically operated self-climbing formwork system is on the building's center core elevator shaft wall. The entire system is raised without the use of a crane and is connected to the structure at all times during the climbing process. The system is designed to carry heavy loads that include the placing boom for the concrete pump.
The climbing sequence begins by collapsing an interior can, a specialized form, and rolling away the exterior from the previously cast lift. The system is bolted to anchors in the previous lift, and then the hydraulic system is engaged to raise the self-climbing wall system and work platforms to the next level where it is secured in place. The rebar is installed, and the forms are rolled to wall size and secured in place. The concrete is then ready to be poured.
"Once you collapse the can and pull away the outside walls, it takes about 20 minutes to move up to the next section," adds Manning. "It takes the platforms and everything right along with it."
Manning explains that using this hydraulic self-climbing wall system was a first for Century Concrete, so there was a learning curve. "The drawings for the steel in the building were behind the shop drawings for the walls. We had to make a lot of major changes to accommodate the steel. Had it not been the first time (we used the system), we would have had fewer modifications."
Manning adds, though, that once his crew overcame the learning curve on the first few lifts, the labor costs decreased dramatically. He estimates that now labor costs are approximately 10 percent of what they were at the beginning of process.
Century uses a Symons® handset system for the columns on each floor. The columns are in halves, and Century uses the tower crane's hook to fly them in bundles up to the deck. The columns are then placed and assembled by hand, eliminating the need to use the hook to fly each column and freeing the crane for other purposes.
"The wall system is the absolute key to making this thing fast," says Joe Paciga, project superintendent with Century Concrete. "Not having to raise the placing boom with the crane saves so much time. It frees the hook up for everything else"
A Putzmeister model BSA 2109 trailer pump gets the concrete from its ground floor location to each deck. A Schwing boom and mast in the center core wall places the concrete. Hydracrete Pumping Ltd. is supplying the boom, pump and experienced operators for the project's concrete pours. "(This equipment) is not something you just want anybody running when you're moving concrete that far up the center of a building," says Eric Wright, general manager with Hydracrete. "We're out here on every pour."
With this setup, Century is able to pour the deck, pour the three center shafts, and set and pour the columns all in the same day. "With the wall system, the center pour is always a couple of pours ahead," says Manning. "The deck ties into it. The other two shafts go up with the columns, and the deck pours over top. We do this typically without putting in a bunch of hours."
Because of the self-climbing wall system and the concrete placing boom, the crane's hook can be dedicated to flying the rebar for the rod busters and the forms for the deck. The deck forms, also called tables, from German manufacturer Peri are another key factor in completing 39 floors in 50 weeks.
Using this Peri® form system was another first for Century Concrete, and Manning explained that his crew worked through the process on the lower floors of the tower. On the south side of the building, the tower's lower floors go down to the precast garage. Despite the change in elevations and heights of 20 feet on these lower floors, Century initially opted to forego conventional shoring on the decks and use the form system. They constructed a "contraption" for the end of a Lull forklift to cradle the tables during placement and removal. Although the system worked, the first six floors of the tower were too broken up to efficiently use the forms. Century returned to conventional shoring for these elevations, using the Peri forms only sporadically.
Nevertheless, this early phase of the project allowed Century crews to work through the learning curve and by the seventh floor, which was the first typical floor in the tower, their speed increased dramatically. Now, says Manning, Century can strip and fly an entire floor in 12 hours.
Pouring a tower deck is a carefully orchestrated process. The night before a pour, Armada Hoffler coordinates the work of the mechanical, electrical and plumbing subcontractors to ensure their sleeves are installed prior to concrete placement. Because the project is design-build, Phil Sheridan, site manager with Armada Hoffler, states that planning and coordinating this work has been a challenge, but it is essential to maintaining the project's schedule.
Once the MEP subcontractors complete their installations, Joe Paciga and his crew move in to pour the deck. Forty-eight Peri tables are needed to pour one deck, and two floors are under construction at a time - one floor curing and one floor being prepared for a pour - for a total of 96 tables. Keith Sanders, project superintendent with Armada Hoffler, explains that each deck is composed of 5,000-psi concrete; however, the concrete only needs to reach 3,500 psi to move the tables to the next level.
The tower crane operator uses a C-hook to fly the tables between levels. To remove the Peri tables from the interior on each tower level, Century Concrete lowers each table onto a movable cart. The legs of the table can then be retracted just enough to spin them out of the way and allow the cart to move the table to the edge of the deck. Operating much like a forklift, the C-hook is then lowered and a stripping crew guides it into place with tag lines. The weight of the table keeps it on the hook, which leap-frogs it up to the next level where it is placed and locked into position for the upcoming pour.
For the tower crane operator, this process is essentially accomplished by voice communication only. "The tower crane operator relies on communication from the crew to guide him," says Gary Manning. "You can't just have anybody down there doing this because the (tower crane) operator is running blind. You definitely have a foreman on the stripping crew who you know you can rely on and t