Steel Dawn: The Structural Challenge of Joining Two Towers in the Void (CCTV Beijing)

Series: Avant-Garde Constructions

Masterpieces of Architecture and Engineering: #02 CCTV Tower, Beijing

Editions: English Español Deutsch Français Italiano Português

Can a 473,000 m² colossus float over 160 meters above the ground with no visible supports?

In Beijing, the CCTV Tower headquarters decided to break the sacred rule of the skyscraper: verticality as the only path to success. It is not a tower; it is a continuous three-dimensional loop that folds upon itself at impossible angles, creating a 75-meter cantilever that defies gravity.

A Duel of Geniuses: Where "Delirium" Found Logic

The structure of the CCTV wasn't born from software, but from a unique intellectual symbiosis between two brilliant minds: architect Rem Koolhaas (OMA) and structural engineer Cecil Balmond (Arup).
Koolhaas wanted to end the skyscraper typology as an isolated, predictable "parallelepiped." Balmond, for his part, understood that to achieve this, the building should not be a shape, but a system of interconnected forces. Together, they made the structure the absolute protagonist: it is not a building with a structure; it is the structure made into a building.

We are not designing a shape, we are designing a system of forces. — Cecil Balmond.

The Invisible Enemy: Thermal Expansion

The greatest challenge was not just supporting the cantilever, but joining it. The two leaning towers were built separately. Due to high solar exposure in Beijing, the steel of each tower expanded differently depending on the time of day. Attempting to weld them at noon would have been a fatal error; the structures would have been "out of alignment" by several dozen centimeters.





The Epic Solution: The final connection was carried out at dawn—the only moment when both towers were in thermal equilibrium and their geometric positions were identical. It was the instant when the two cantilevers touched for the first time, culminating in an unprecedented engineering maneuver that closed the circuit of forces in December 2007.

A Challenge of Precision: The operation was so sensitive that hours before the closing, four key corner columns had to be temporarily removed. This allowed the structure to gain the necessary flexibility to absorb stresses uniformly before being sealed forever. A precision surgery at an urban scale that transformed two independent towers into a single symbiotic organism.

The Facade as a "Stress Map" (The Web)

If you look closely at the building, you will notice a network of steel diamonds (diagrid) that is not uniform. This is not an aesthetic whim; it is pure physics.

• High-density zones: Where the diamonds multiply and the diagonals tighten, the structure endures the greatest tension and compression forces (especially at the cantilever nodes).
• Low-density zones: Where the stress is lower, the mesh opens up to allow more natural light.

The facade is, literally, a full-scale load diagram. The building is "telling" you exactly how the forces travel down to the ground.

Technical Comparison: Stability through Geometry

As I analyze in my book "TURNING TORSO - SANTIAGO CALATRAVA", engineering tackles iconic landmarks in radically different ways:

Turning Torso (190 m): Relies on a rigid cylindrical concrete core and a steel exoskeleton that "holds" the 90° twist.

CCTV Tower Beijing (234 m): Relies on interconnectivity. Being a closed loop, the towers support each other. It is a symbiotic structure where the rigidity of the whole compensates for the lack of verticality.

The tower is a bourgeois typology. We have made a building that questions the tower; it is a loop, a circuit, a building that moves. — Rem Koolhaas.

Why doesn't it collapse?

Unlike a conventional skyscraper that works primarily in compression, the CCTV functions as a giant rigid frame. The 75-meter cantilever is supported thanks to the rigidity of the nodes in its external diagrid — diagonal load-bearing beams, which convert bending into axial forces distributed throughout the ring. It is a challenge to logic that only advanced engineering can solve.

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Frequently Asked Questions:

Is a leaning building safe?
Yes. Through the diagrid system, loads are distributed across the "skin" of the building, making it extremely resistant to earthquakes and lateral winds.

What is a Linked Cantilever?
It is a cantilever that is not "loose" but connects two structures, closing a circuit of forces that provides a global stability unattainable for an isolated tower.

Why were the towers joined at dawn?
To prevent thermal expansion caused by the sun from asymmetricaly deforming the steel. At dawn, both towers share the same temperature, and their geometric positions are perfect for assembly.

What does "The Web" mean in this building?
It is the technical term used to describe the steel diagrid wrapping the building. It acts as an active exoskeleton that distributes stresses across the entire facade, making the building function as a living organism.

How does it compare to the Turning Torso?
While the Turning Torso relies on a rigid reinforced concrete core to master the twist through an external spinal cord that runs along the full length of the facade, transmitting loads to the foundation, the CCTV Tower in Beijing employs a closed-loop symbiotic structure where both towers support each other, optimizing steel through geometry rather than mass.

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José Miguel Hernández Hernández

International reference in the technical analysis of iconic and sculptural architecture. Specialist in the intersection between engineering, aesthetics, and vanguard design. Author of the bilingual technical books Turning Torso – Santiago Calatrava and Famous Constructions.

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