A spectacular, S-shaped design for the high-rise Conjunto Paragon Hotel in Mexico City made the building such an iconic development that several international hotel chains bid to have it under their name. The building is now a modern and elegant landmark structure in Santa Fe, a growing section of Mexico's capital city.

The 414,411-square-foot building has 29 above-ground floors, plus seven underground levels for parking. The building also features 120 luxury hotel suites, shops, offices, and world-class restaurants.

The hotel's complicated geometry, curved panels, intricate medallions, two cubic protruding shapes, and balconies called for an innovative and flexible construction system. The challenging, undulating design also required precise execution during erection.

An architectural precast concrete façade was an ideal solution for these demanding requirements. A harmonic web of precast concrete panels and large windows, the winding, radius design with precise lines visually slims the structure and makes the massive building look homogeneous and artistic.

The precision of precast concrete production and panelized construction were the keys to defining the unique shapes needed. Given the building's 496- foot height and complexity, it would not have been easily or economically possible to complete the wavy design with traditional construction materials. The intricate shapes and radius forms of the precast panels required innovative manufacturing techniques by the precast producer, Pretecsa.

Making waves

"The wavy image expressed in the original design was definitely the main challenge," says Pretecsa's communication manager, Erick Ginard. "Th is shape complicated the manufacturing and installation of precast elements. Developing both concave and convex curved pieces was a challenge."

To solve the problem, the producer used 3D modeling to design the panels and utilized a flexible-form casting system to significantly reduce the number of necessary molds. The cutting-edge forms, says Ginard, needed only a few turns of nuts and bolts to adjust them to the next needed radius, either concave or convex.

"We had to find a way to manufacture inexpensive, disposable molds that did not require so much time to manufacture, or to design a master mold system that we could adjust to the given requirements," says Gervacio Kim, Pretecsa's operations director.

The firm set out to develop a system where one mold could provide all desired shapes. Inspiration came from a sushi restaurant where a "flat" surface was magically twisted to different shapes, Kim says.

"We were intrigued by the idea of individual linear pieces that, joined together, formed a flat panel," the operations director says. "If adjustments were made at the edges of each linear element, the panel could be curved into a three-dimensional surface— concave by tilting the end linear sections up; convex by forcing up the middle linear sections. The idea was that by adjusting the axis, we could obtain all the shapes needed from a unique single surface form that would receive the architectural concrete."

Bending, not breaking

The second issue was to find a surface that could be bent without breaking and that, with daily adjustments, could recover its deformation and correct its curvature without damage. The answer was a simple casting bed made from linear strips of plywood with an epoxy coating on both sides of the wood.

The finished adjustable casting bed consists of a steel frame base and the plywood casting surface. Steel stud supports beneath each axis can be individually adjusted up or down by turning  5/8-inch adjustment bolts to create the curved surface.

Another challenge was keeping the precast panels vertically aligned with the harmonious curves in the façade during construction. "During the erection, the edges of the pieces had to match each other with a high degree of exactitude and perfection, making the panels form the desired shape and making the building look quite artistic," says Ginard.

Workers welded steel sheets on the cast-in-place structural frame to connect the precast panels. Horizontal and vertical level marks were set in the frame at each floor level, and the precast pieces were carefully installed corresponding to these marks.

Wind also presented a challenge on the tall building, so precast erection was completed very carefully and was only conducted during the morning.

Early completion

Precast panels were erected in a horizontal sequence. This led to completing spaces at a very early stage of the project, and released large sections for placing glass and protecting the construction workers who dealt with interior finishes. The early close-in allowed owners to offer the space to hotel chains before the interiors were finished. Th is, in turn, allowed the hotel managers to adapt interiors to their specific needs.

In total, 520 curved and straight precast pieces, both concave and convex, ranging from 43 square feet to 65 square feet, were utilized. Also, 750 3.9-square-foot precast pieces were included in the first levels of the building, covering 43,055 square feet. The panels consist of a white mix, light chiselhammer finish.

The building's foundation, structural system, and floors consist of cast-in-place concrete. Steel extensions fixed on the building structure were used to help incorporate the balcony protrusions.

All precast components were regionally sourced. Recycled materials, such as wood and metal bars, were used to create models and molds for the precast parts.

For more on the precast concrete producer in this story, visit www.pretecsa.com.

Roy Diez is the executive vice president of JOA Inc., a virtual marketing/communication agency specializing in the architectural, building, and construction markets. Email [email protected].