Torre Reforma, a 57-story tower that opens its doors next month in Mexico City, may at first glance look slightly out of place. It's not because of its shape. The triangular form, positioned so one face has a clear view of Chapultepec Park, the Central Park of the Mexican capital, was designed by architect L. Benjamin Romano of LBR&A Arquitectos to resemble an "open book," all while conforming to an adjacent historic home on the building site that couldn't be demolished. It looks odd because Mexico's tallest building is also the world's tallest exposed concrete structure, a soaring gray mass standing in stark contrast to the steel-and-glass towers that dominate most city skylines. In an area that faces regular earthquakes, concrete seems like a bit of a risk. But ask Romano to explain the physics and features of the structure on Paseo de la Reforma, and he'll tell any doubters to simply look at the surrounding landscape.
"Look at the historical architecture all across Mexico, the temples that have stood for hundreds of years," he says. "They're all stone, and they all work because they can be flexible during seismic events."
While the solid concrete facade may seem anything but pliable, the design by LBR&A, with engineering assistance by Arup, offers a new model for tall concrete towers. This building may look Brutalist, but when necessary, it can bend.
Romano decided to pursue a concrete facade for a few reasons. One the the most important was environmental. The designer and developer wanted to aim for LEED Platinum status, and by swapping out glass curtain walls with concrete that blocks the sun and naturally cools the interior, Romano estimates they'll save 25 percent on heating costs.
But the structural advantages of concrete walls also pushed Romano to make an atypical choice. With solid, concrete outer walls, the building can support itself without interior steel columns, which add to the final material cost of a construction project. No columns also means open floor plans, which makes the building even more attractive to tenants and the hot commercial real estate market (Romano says 60 percent of the building has already been leased).
But building those concrete walls, as well as a strong enough foundation, also created a few engineering challenges. At 240 meters (800 feet) tall, the structure needed a strong foundation, so the walls continue 60 meters underground, creating a 10-story basement that supports the entire structure. But those relatively heavy walls, if they were built as continuous slabs of concrete, would risk breaking and cracking under any serious seismic event. The solution, according to Tabitha Tavolaro, an assistant principal at Arup and structural engineer for this project, was redundancies and resiliency. A series of double-V hangers and irregularly spaced gaps was added to the concrete facade. Resembling a series of windows that move up the side of the building, they allow the concrete walls to bend when under stress without breaking.
Tavolaro also noted that the atypical design was put through rigorous testing by Arup to make sure it would stand up to shaking ground. The firm ran an advanced series of computer simulations, testing the building against virtual earthquakes and the full range of seismic activity predicted for a 2,500-year span. It's a state-of-the-art analysis that goes way beyond the code requirements.
"Hopefully, these tests allow tenants to take some solace," she says.
Concrete tends to discolor when poured in large volumes, and Romano obviously wanted to avoid a splotchy exterior. His solution turned this material weakness into a strength; instead of pouring large, one-story walls, 4.2-meter lengths which would streak during construction, he asked the construction company to pour single, 70 centimeter-tall rings of concrete to create a deliberate, spiral-like design, akin to the coloring of a wood floor. While pouring concrete at that height is far from unheard of—for this project, telescopic arm with a pump went up the length of the building as it rose throughout construction—the end result is singular.
"It's a beautiful form of a right angle," according to Tavolaro. "The architecture of the building expresses so much structure, it was really important to develop a system that could be both aesthetically compelling and offer the robustness required of a region with high seismic demands."