Brett Ryder

There’s a detectable hint of dismay in the voice of Christopher Sharples, AIA, founding principal of New York’s SHoP Architects. “Everyone thinks we’re just stacking boxes,” he says about SHoP’s 32-story modular residential tower that is now under construction as part of Brooklyn’s Atlantic Yards project. When complete, the tower, known as B2, will be the tallest modular building in the world, made up of more than 910 prefabricated elements and comprising 363 units.

“We’re basically designing these buildings like we’re designing an aircraft. We’re modeling every component, we understand what the quantities are, we understand exactly how big they are, and you can get these things fabricated precisely,” says Sharples.

While it may be an oversimplification to compare prefab construction to Legos, there probably isn’t a more apt analogy for modular construction. Factory-constructed, essentially complete units are stacked or connected to create a larger structure. It’s an evolution of prefabricated buildings—those built onsite with a suite of pre-made parts—and it’s taking the idea of pre-manufactured architecture to new heights. With B2 and two other modular towers to follow, SHoP and Atlantic Yards developer Forest City Ratner Companies are bringing modular construction into a taller, more complex 21st century.

Though modular architecture has historically been relatively low-rise, that’s changing rapidly. The squat and boxy modular office buildings of the past are being replaced with mid- and high-rise modular buildings, especially in Europe and Asia. Scaling up has required new thinking and different materials, according to Michael Hammond, editor-in-chief of the website World Architecture News.

“I think what we’re seeing now is this trend turning into different materials that are being pioneered in China, particularly the structural frame,” says Hammond.

The structural frame is part of what’s enabling architects and developers to stack their Lego blocks even higher. Most modular elements tend to be built out of wood, and fire codes limit their usage to about four or five stories. Replacing the wood frame of modules with cold-rolled steel means that architects can stack the boxes up to about 10 stories. Modular frames of stronger structural steel can push the height even higher.

The addition of an internal steel or concrete core within a building structure is what’s allowing prefabricated modular design to penetrate high-rise construction, according to James B. Guthrie, AIA, founder of the Miletus Group, a modular and prefab architectural design firm with headquarters in Rochester, Ind., and a manufacturing facility in Chicago. Guthrie says that once modular buildings rise higher than 10 stories, lateral load- and seismic-related issues become major concerns, and bracing the stacked modules to a steel or concrete core vastly improves the structural integrity, even under earthquake or high-wind conditions. “With the aid of a secondary frame, I believe there is no limit to how high modular-constructed buildings can soar,” Guthrie says.

In England, Manchester-based O’Connell East Architects used a concrete core to provide the stability for the 25-story modular dormitory Victoria Hall at the University of Wolverhampton, completed in 2009. It’s currently the world’s tallest prefab modular building (although, technically, since the ground floor was constructed on-site, only 24 floors count as modular).

B2 is, in some ways, an evolution of this project. According to Sharples, B2 utilizes two internal steel bracing frames that rise like ladders in the center of the building. Designed by engineering firm Arup, these frames create an armature that supports and manages the lateral forces of all the modular cells stacked on top of one another. The stair and elevator cores also provide lateral stability. Sharples says that the design of B2 (seen below) allows the structure to meet seismic codes, wind-load and lateral-bracing requirements, and even New York City’s stringent building code.

For the modules themselves, Sharples says there’s basically no difference between designing them for a low-rise or for a high-rise. The internal frame is what matters in terms of height. “That’s the key,” Sharples says. “Without that, you couldn’t build tall.”