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 shatter the sacred rule of the skyscraper: the tradition of 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.

The Duel of Geniuses: Where "Delirium" met Logic

The structure of the CCTV was not born from software, but from a unique intellectual symbiosis between two brilliant minds who challenged architectural tradition: architect Rem Koolhaas (OMA) and structural engineer Cecil Balmond (Arup).

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

Koolhaas wanted to put an end to the tradition of the skyscraper as an isolated and predictable "parallelepiped." Balmond, for his part, understood that to achieve this, the building should not be a shape, but an interconnected system of 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.

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.

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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.

Technical Specifications & Team: The Icon’s Blueprint

Project	CCTV Headquarters (China Central Television)
Location	Beijing, China (CBD District)
Architecture	Rem Koolhaas & Ole Scheeren (OMA)
Structural Engineering	Cecil Balmond (@Arup)
Height / Floors	234 m / 51 levels (75m "Linked Cantilever" overhang)
Typology	Media & Broadcasting Hub | Deconstructivism / High-tech
Structural System	Continuous External Steel Diagrid ("The Web")
Awards	CTBUH Best Tall Building Worldwide & 10-Year Award.

Industrial Specs & Solutions | CCTV Headquarters

AECO VERIFIED

Component	Partner / Brand	Technical Execution Detailed
Structural Steel	Baosteel Group	Supply of heavy-duty steel plates for variable-geometry diagrid nodes and high-stress load transfers.
Facade Systems	Permasteelisa Group	Advanced unitized curtain wall custom-engineered to follow the non-planar and inclined geometry of the towers.
Vertical Transport	Schindler Group	Custom S-7001 lift systems specifically designed to operate within inclined and angled elevator shafts.
Fire Protection	Promat	High-performance passive fireproofing solutions for critical structural steel nodes and external connections.
Anchoring Systems	Hilti	Seismic-grade fastening solutions for the high-performance building envelope and MEP service integration.

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Want to dive deeper into structural design?

If you are passionate about cutting-edge engineering, discover all the secrets of structural design in my bilingual book (ES - EN). 144 pages and 109 illustrations explaining how architecture defies the limits of the possible.

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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.

AECO Architecture & Engineering Glossary | CCTV Tower, Beijing

Thermal Equilibrium: The physical state achieved at dawn when both tower sections reach an identical temperature. This was the critical factor for welding the cantilever, preventing metric deviations caused by asymmetric solar expansion.

Linked Cantilever: A 75-meter spanning overhang. Unlike a simple cantilever, it closes a structural circuit, merging two independent towers into a single interconnected force system.

Variable Diagrid (The Web): A continuous steel exoskeleton where mesh density is non-uniform; cells are compressed in high-stress areas (cantilever nodes) and expanded where load requirements are lower.

Giant Rigid Frame: A structural typology that replaces pure compression with a system that absorbs bending moments and converts them into axial forces distributed across the entire ring surface.

Symbiotic Structure: A design concept where global stiffness does not rely on the verticality of individual elements, but on the mutual support of inclined volumes closing a 3D loop.

Seismic Performance: The capacity of the perimeter diagrid to dissipate energy and resist seismic loads through the redundancy of high-strength steel connections.

Series: Avant-Garde Constructions | jmhdezhdez.com

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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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