Torre KOI, a 276 m tall mixed-use tower under construction in Monterrey, will become the tallest building in Mexico upon its completion. Internacional de Inversiones
Thornton Tomasetti overcomes myriad challenges to design a functional tower that meets an established architectural design as well as the client’s expectations.
November 12, 2013—By the time Thornton Tomasetti joined the team constructing what will be the tallest tower in Mexico, the architectural design was substantially complete and the client had already leased much of the mixed-use space. As a result, the engineering firm was responsible for developing a structural system that would work within strict project parameters. The firm overcame the challenge using a great deal of analysis and collaboration with the client and other team members.
Torre KOI, as the project is known, will be a 276 m tall office and residential tower located in Monterrey, in northern Mexico. A real estate development firm headquartered in Monterrey known as Internacional de Inversiones is constructing the tower, and Mexico City-based architecture firm VFO Arquitectos designed the project. Thornton Tomasetti is providing design development of the building structure, and Stark + Ortiz, a structural engineering firm based in Mexico City, will take the project through to construction.
The 68-story tower is being constructed next to two existing towers in a development known as the VAO Complex, from which it will offer panoramic views of the Cerro de la Silla, the Sierra Madre, and the Cerro de las Mitras mountains. The tower will rise from the sloped site with nine levels of parking, half being only partially below grade as a result of the topography, according to Christopher Crilly, P.E., LEED-AP BD+C, a senior project engineer in Thornton Tomasetti’s Washington, D.C., office. The tower’s 5th to 20th levels will house office space, and its 21st floor will hold mechanical systems. An amenities level—including a swimming pool, a small theater, and a gym—will be located on the 22nd floor, which is also where the tower will step back, becoming more slender. From there, condominiums will occupy all of the levels through the 57th floor. The two floors above that will accommodate high-end office space, and the two above that will have event, meeting, and back-of-house facilities. The top floor will be an open-span ballroom.
Although a preliminary engineering proposal (not developed by Thornton Tomasetti) did not include a structural core, Thornton Tomasetti determined that such a core was necessary for the serviceability of the tower. “We added a structural concrete shear-wall core to stiffen the lateral force-resisting system to meet industry standard deflection and acceleration limits for a building of this height,” Crilly explains. But adding the core consumed a portion of the square footage that had been designated for the office and residential units. As a result, the building had to become larger. “The building needed to grow a bit in plan to pick some of that space back up,” Crilly says. “Through some coordination and design iterations with the architect, MEP engineer, and wind tunnel consultant, the building became larger than originally planned.”
The tower will be founded on a pile-supported 4 m thick mat foundation. The piles will be 1 1/2 m diameter drilled shafts that descend to a depth of 7 m to rock. The core will rise from the foundation with an aspect ratio of 20 as a result of being designed for residential units rather than offices, which typically have more elevators. “You don’t want a large core sized for office elevators taking up space on the residential floors,” Crilly explains. “We have a core sized for a residential building, with office elevators squeezed in.” The tower’s concrete framing will extend from the core. The garage levels will be posttensioned waffle slabs with widely spaced ribs, while the office and residential levels will be flat-plate, two-way posttensioned slabs with drop panels at the longer spans.
Belt walls will tie the perimeter columns to the structural core as
part of the tower’s lateral force-resisting system. Thornton
The governing lateral force on the project is wind. While the concrete core will be the primary lateral force-resisting system, belt walls—floor-to-floor concrete walls at the building’s perimeter—will be located at the 21st and 61st floors to further control wind accelerations. The belt walls will connect to the perimeter columns and tie them back to the core via slabs located above and below the walls. Engineers added the belt walls after working with a wind tunnel consultant to get the wind-induced accelerations within tolerable limits, Crilly says. “In many cases a belt-wall system is not quite as efficient as more traditional outriggers,” he says. “But when you’re close to achieving enough stiffness with a core but not quite there, belt walls can get you over the top; so in our case, it is a little more efficient than direct outriggers, which would have been more than we needed.”
The tower’s foundation piles are already being drilled as engineers continue to work out the details of such elements as the large parapet that will crown the tower and the glass facade surrounding the tower. They are also continuing to analyze the tower’s differential movement to predict how much the columns will compress over time. “We came into this project late, but the owner is very excited to get started on construction,” Crilly says. “We’re concentrating on the lowest couple of levels until we get far enough ahead of the contractor. There will come a point when the rest of our design will develop faster than they can build.”
Although engineers had to increase the size of the tower to accommodate the more robust structural system, they have otherwise developed a design that successfully meets the established project guidelines, Crilly says. “Coming up with a system fast that worked with the architecture, worked with the owner, [and] worked with the wind loads and the wind tunnel analysis has been a challenge,” he says. The tower’s completion is anticipated in the next two to three years.