The China Central Television Headquarters in Beijing, designed by acclaimed architect Rem Koolhaas, was recently named by the Council on Tall Buildings and Urban Habitat (CTBUH) as the world’s best tall building. While undoubtedly an architectural marvel, it is also a stunning engineering accomplishment.
In describing the building, which was completed last year, Koolhaas said "CCTV defies the skyscraper's typical quest for ultimate height." Rising from a common platform, twin towers – one 234 metres in height, the other 194 metres – lean toward each other and eventually merge in a perpendicular, 75-metre, 15-storey cantilever. It is an intriguing design that posed a challenge for structural engineer Cecil Balmond and his team at Arup.
Early on, the team determined that the only way to deliver this incredible architectural form was to utilise the entire façade structure, creating an external continuous tube system.
The provision of primary support for the gargantuan 540,000 square metre building is actually achieved via the irregular grid on its surface. This diagonal pattern is a visual manifestation of the forces travelling through the "three-dimensional cranked loop." The smaller the diagonal shapes, the stronger the load and greater the support. With earthquakes a frequent occurrence in the region, the braced tube structure provides additional robustness and protection to withstand any seismic activity and enhance safety.
The building challenged the boundaries of prescriptive Chinese construction codes and required unprecedented performance-based analysis to finally gain the approval of a panel of structural and seismic experts appointed by Beijing.
The key to the final engineering solution, and the ultimately successful construction outcome, was understanding the way in which the two towers behaved before they were linked together. In their partially-constructed form, seismic activity and strong winds were not the only potential issues. Beijing’s extreme hot and cold weather conditions also caused the structures to expand and contract.
For five days, the engineers monitored both the global and relative movements of each of the towers to accurately predict the correct dimensions of the linking elements. The team found that early morning before sunrise was the optimum time to attempt the join, as this was when the steel in both towers was at a uniform temperature and movements caused by the environment were at a minimum.
Final adjustments were made to the length of the linking elements before installation, as it was vital that the towers could be fixed together within a few minutes.
"The CCTV headquarters is an unusual take on the skyscraper typology. Instead of competing in the race for ultimate height and style through a traditional two-dimensional tower soaring skyward, CCTV’s loop poses a truly three-dimensional experience,” the CTBUH said. “As a piece of structural engineering, CCTV is also an object lesson for those who wish to push the boundaries and sweep aside the received notions of skyscraper design. The building’s design violates conventions, while validating and rewarding intensive and focused collaboration and study.”
Past winners of the CTBUH Award for Best Tall Building include The Doha Tower in Qatar, The Burj Khalifa in Dubai and the Shanghai World Financial Centre.