At the pinnacle of the 56-meter Sky 56 tower, located in Lyon’s bustling Part-Dieu district, the 14th and final floor was constructed using Ytong autoclaved aerated concrete. Bouygues Bâtiment, the project manager, chose this material for its exceptional lightness and insulating properties.
With its striking design and proportions, the Sky 56 tower is a defining feature of the district. This curved, modern building offers 30,000 m² of services, retail spaces, and offices. The 14th floor, an attic with a height of approximately 3 meters and an area of 800 m², caught particular attention. Unlike the rest of the structure, which was built with reinforced concrete, Bouygues Bâtiment selected Ytong AAC for this level due to the demanding specifications. "We originally considered reinforced concrete but chose AAC for its lightness and superior insulation," explains Benjamin Gathier, the structural work manager at Bouygues Bâtiment.

AAC: up to four times lighter than conventional concrete

For enhanced thermal performance, Ytong Verti 25 blocks were used on this level. Gathier notes that the biggest challenge was that the 14th floor's footprint didn’t align with the structure below. Thanks to the lightweight nature of cellular concrete, the team was able to place walls on floors rather than load-bearing elements—a feat that would have been far more complex with traditional concrete. Grégory Gnidach, Head of Country Key Account Management, underscores that AAC, which is up to four times lighter than concrete, greatly simplified the design and avoided the need for structural reinforcement. The material's fire-resistant properties also addressed the project’s safety concerns.

The project "Sky 56"

Gregory Gnidach explains how the project developed from his point of view: “It was the versatility of Ytong blocks that convinced Bouygues Construction that our building solution was the best technical and economic compromise for constructing this penthouse. I remember being contacted directly by Benjamin Gathier, who was the project group leader for Sky 56 at the time. He had thought of our solution himself but needed technical information to ensure that our solution could meet all his constraints. The equation to be solved was to impose the lowest possible load on the top floor slab in order to avoid costly structural constraints on the rest of the building while maintaining good thermal performance and ensuring fast execution.
Through initial discussions and listening to his needs, we were able to establish contact and provide the first set of responses. Soon, the conversations became increasingly technical, and that's when I sought support from our Technical Office. Together, we eventually solved the equation by providing all the technical and regulatory answers, which led Benjamin to approve our solution."

Overcoming obstacles

But that wasn’t the end of the project. Gregory Gnidach reminisces: "The construction was scheduled for August, during the summer holidays. Benjamin was used to outsourcing the work, but no subcontractors were available. That's when I suggested training his teams at our installation school, which he did with the support of our technical trainer-demonstrator. If I were to summarize this experience, I would say that we worked as a team, knowing how to showcase our solutions and services. Everyone played their part, and it was a collective success!"

Despite the summer timeframe, the installation of the 14th floor was completed in just one month.

"Completely Satisfied"

Today, the Sky 56 tower proudly stands with its 14th floor overlooking a stunning rooftop. Gathier is more than satisfied with the decision to use Ytong: "This choice allowed us to meet all the challenges we faced, and there were many given the building's configuration!"