|July 24, 2013|
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|CHICAGO – It seems increasingly evident that 2013 will be remembered as The Year of Prefab in the tall-building community. While YouTube video of China Broad Group’s 15-day construction of the 30-story prefab T30 in 2012 and this year’s feverish preparations for Sky City J220 in Changsha, China, grabbed the lion’s share of media attention, the B2 Tower, part of the Atlantic Yards development in Brooklyn, New York, USA, is also making waves. At 32 stories, B2 promises to become the world’s tallest modular building, however briefly. Perhaps more significantly, B2 will have been built on a highly constrained site in New York City, straddling a rail yard and the entrance to the new Barclays Center arena, after negotiating the brambles of local building code, organized labor, and transport / logistical challenges.|
On July 24, David Farnsworth, CTBUH’s New York City representative and a principal at Arup, traveled to Arup’s Chicago office to present on B2. Though modular construction is not new, the idea of taking it to a height of more than 30 stories is a novel concept for most developers, Farnsworth said. Arup and architect SHoP’s initial proposal to Forest City Ratner, the developers of Atlantic Yards, had been to use modular construction. “We had initially made the pitch for modular construction,” Farnsworth said. “They said, ‘Thanks, we’ll think about it.’ They were quiet for a good year. But after the [Broad Group T30 video] came out, they said, ‘Hm, maybe we should think about it again.’"
Modules for Mass Customization
As a first point of business, Farnsworth cleared the air around how his team uses the word “modular,” which has long been a loaded and multifaceted term for architects and engineers.
“’Modular’ can be anything from prefabricated curtain-wall panels, to precast concrete to bathroom pods, to full-on volumetric, complete apartment fit-outs in modular units,” Farnsworth said, indicating that B2 uses the complete fit-out units, with MEP riser connections made in the corridors. “Where I think that we've taken it to another level is the level of finish they do in the factory.”
|Rendering of the B2 Tower in Brooklyn, New York, all images © SHoP Architects|
Because most modular manufacturers have not worked at this scale before, the team formed a joint venture FCS Modular between Forest City and the contractor, Skanska, to produce the modules. FCS Modular plans to continue producing modular units on a contract basis to other companies’ projects, Farnsworth said.
“Forest City had 16 modular buildings that they needed to build and they had incentive to invest in a process to do it better,” Farnsworth said.
The B2 contains 360 apartments and about 320,000 square feet (29,729 square meters) of residential space. Even though using modular construction will result in substantial savings – most significantly by cutting construction time by a third – the project is no more monolithic than conventionally constructed building. Out of 930 total modules, 225 are unique structure types.
“Much of it is mass customization, rather than taking a standard product and trying to array it in a way that makes it a building,” Farnsworth said. “The massing guidelines required quite a few setbacks and transfers to make the building look like it's not one big monolithic volume, but rather three or four different volumes. And it really helps in the architecture, but again, those create module variations.”
Great effort is taken to make the apartment interiors feel like cohesive entities, even when they consist of more than one module. Farnsworth showed one unit that comprised a kitchen and living room in one module and a bathroom in another. The two modules meet along a “mate line,” which the designers cover with an interior wall or door frame whenever possible. Exposing mate lines in open-plan arrangements does not affect the flooring, but it does require onsite finish work to complete the ceiling.
|Interior rendering of an apartment consisting of two modules|
The structural approach, with a few exceptions, is stunningly simple. Each module has its own self-contained structure and setting pins that allow it to lock into its neighbors above or below, and carries its own gravity load down to the plinth level, a conventionally constructed site-built platform. Lateral loads begin to become an issue once the 30-story range is crested, so a brace frame is laterally connected to all the modules. The brace frames are connected at the top with an out-rigged hat truss.
B2 will have practically no concrete in its superstructure, save housekeeping pads and the mechanical floor, thus making the building unusually light and conducive to shallower transfer beams and foundations than would otherwise be required. The unit floors are two-inch (51mm) metal decking with ¾-inch (19mm) cementitious particle board; ceilings are comprised of separate metal decks. This, combined with the fact that each module is an independent cage of welded tube steel, has the surprising effect of adding more structural material than would be used by a conventionally-framed light-gauge project. Yet it has quality and safety advantages that justify the choice, such as added fire protection and acoustical insulation, due to the double thickness of walls and roofs.
Fit-Out Before Ship-Out
The top benefits of modular construction are speed and quality control, and these are mainly realized off-site. The steel chassis of the modules are fabricated in Virginia, then trucked to the Brooklyn Navy Yard, a few blocks from the site, for fit-out.
|Diagram of the structural system|
|Because the most time-consuming and unpredictable part of any construction job is so-called “wet work” – connecting water pipes and waiting for liquid materials to cure and set – as much work as possible is done on bathrooms and mechanical risers before the modules arrive on site.|
“We prefabricate the bathrooms as bathroom pods, offline, so when the frames show up, the bathroom pod is ready and can be slotted right in,” Farnsworth said. “Then all the factory assemblers have to do is fit out the remainder of the services from the central risers, which are actually constructed as part of the bathroom pod, as well as the fit-out of the finishes.” The units are then shipped out nearly fully assembled, including facades and windows. This minimizes the amount of connection and finish work that has to be done onsite.
The mental picture of an assembly line piecing the units together on a conveyor belt is not quite right, Farnsworth said. In the factory, each module is set into a position as near as possible to its on-site disposition and approached from all sides, so that everyone from crane operators to carpenters can be confident connections will align onsite with extremely tight tolerances.
|Diagram showing the construction sequence of the modules|
|Labor and Logistics|
The logistics of transporting a module to an urban site is no small undertaking, but the prototype testing showed that crane time required to lift a module and place it is surprisingly fast. Though skill is obviously required to execute a module lift, the sheer reduction in number of crane movements and on-site work in general together strike a major victory for safety, speed and quality control.
“When we first started, everybody was worried that it was going to take longer to erect these and connect them together than we'd anticipated,” Farnsworth said. “By the time we were done with the setting sequence, everybody was worried that the factory wouldn't be able to keep up.”
The team also designed the sequencing to be flexible enough to allow for errors, such as modules sustaining damage in transit. So long as it’s not a one-off custom module, a spare can always be substituted.
The unfamiliar nature of modular construction understandably had some labor unions concerned. Unions have not been heavily involved in affordable-housing projects such as B2, and were eager to change that as the building economy recovers from the recession. The fabrication and finishing trades both have struck agreements with FCS Modular so that onsite and offsite work can be unionized, Farnsworth said.
|Overall view of the final Atlantic Yards complex including B2 on left|
|In It for the Long Run|
The project has yet to place its first module, and it hasn’t been hiccup-free. Aside from a large strategy shift from the original Gehry Partners plan to a reduced-scale plan with SHoP as the design lead, the project has drawn the ire of some trade associations, which object to its unconventional use of labor.
In July, the Plumbing Foundation, a trade group representing licensed plumbers, filed suit against the developer for using unlicensed plumbers at the offsite location, which is an arrangement the city’s building department had reached with Forest City in 2011, in recognition of the fact that plumbing connections are made on-site by licensed labor.
Ultimately, however, Forest City has 6,400 apartments to build at Atlantic Yards in the next ten years, in a city known for high construction costs. It does not seem likely the project will run aground on such claims. If B2 succeeds, it seems likely that, at Atlantic Yards, prefab will prevail at a previously unseen scale and height.
|Barclays Center with the Atlantic Yards complex in the background with B2 on right|