However, he writes that the building will be "completed later this year," though developer Forest City Ratner, which is building the tower via the FC Skanska joint venture, has said it's one year behind schedule, expected to open in late 2015.
The lift process can take as little as ten minutes per module. Considering trucking restrictions, delivery logistics, and rigging, it would be entirely feasible to erect 12 modules and associated braced frames in a normal workday. With 36 modules per floor, this could equate to a floor every three days.That would be fast. The most recent pace, however, was 24 modules delivered and installed over two five-day weeks, an average of 2.4 per business day. (Update: the pace has picked up, according to the alert delivered later on July 21, with 57 modules delivered in ten days.)
At this point in time, construction speed is limited by factory production rates rather than erection speed. When modular construction becomes more common and economies of scale are created, this should be quickly overcome.
Modular design has grown increasingly popular for low- and mid-rise building projects over the past decade, amply demonstrating its viability and value. Based on our experience with B2, we believe that the technology can be pushed further — and, in the process, potentially revolutionize urban housing around the world.
Farnsworth writes that structural engineering was a challenge for the 32-story tower, given "substantial wind, gravity, and seismic loads." They considered but rejected a core based on reinforced concrete, but decided that "concrete would require too much work onsite." By contrast, structural steel is lighter and can be built in the factory:
One of the key breakthroughs came when we decided to use the roof of each module as the lateral diaphragm (used to stabilize a given structure by transferring loads from wind and earthquakes to a lateral-resisting braced frame system). This allowed workers to make the primary structural connections between modules from outside the apartment units, avoiding the risks associated with doing heavy steelwork inside the finished modules after they were delivered to the site.
In the final design, the basic building block of the modular system is a factory-welded steel-framed chassis. The sides of each module act as trusses, and the corner columns in the modules carry the weight of all others above. The building’s base was constructed conventionally, providing a level steel platform for stacking modules.
The lack of concrete in the modules makes the structure as a whole very light compared to typical construction. This, combined with the building’s orientation and massing, meant that the structure would sway more in high wind than conventional buildings of similar height. We therefore incorporated two 100-ton tuned mass dampers (not typically found in buildings of under 40 stories) into the design to reduce wind-induced motions to acceptable levels.
Mass customization, not mass production
Rather than stack a single module template, the partners developed what he calls mass customization rather than mass production:
In the final design, although details and methodologies are consistent throughout, each module is very much constructed to its own design. In fact, given the building massing variation along the height and the client’s desire for a wide range of unit types, the final building has 225 separate module structure types. With variations in piping and façade, many of the 930 modules are completely unique.
|Typical floor plan, with 36 modules per floor. (Source: Arup)|
Rentals > condos?
We took shipping requirements into consideration from the project’s outset. Most modules fit within maximum dimensions of 15-foot width, 50-foot length, and 10-foot, 6-inch height, allowing for wide load shipping on New York’s public roads. These limits work quite well for rental apartments, where living rooms are typically 13 feet wide and bedrooms 11 to 12 feet wide.
Also, it the limits work well for rental apartments, do they work less well for condos? It's certainly possible to pair modules to create larger rooms, but the implication is that modular doesn't work as well for condos. Note that Greenland has decided that the next few Atlantic Yards towers will be built conventionally, and will include condos.
The crane at work
Crane requirements also influenced module size. Since B2′s tower crane can lift a maximum of 26.5 tons, we ensured that the heaviest weighed no more than approximately 24 tons. (The smallest weigh only seven.) Modules farthest from the crane were made smaller in order to fall within the specific lift limits at that radius.