China Central Television Headquarter, Beijing

B.ARCH –IV ADVANCE STRUCTUREAalap Patel-35 A Jugal Parekh-34 AKalp Dewani-27 B Anil Changesa-23 A

Project: CCTV Television Station and HeadquartersClient: China Central Television (CCTV)high:- 234 mLocation: Beijing, ChinaSite: 18ha in new central business districtProgram: Total 575,000 sq.m: CCTV building 400,000 sq.m; TVCC building 75,000 sq.m; service building 15,000 sq.m; parking 85,000 sq.m

• Building height – 234m• Building width-156m• Site area-599000m²• Built with landscape and parking site area-1,87,000m²• Height to width ratio- 1.56:1• Structural system floor area-58361 m²• Floor area-389,079 m²• Ratio of floor area tostructural area- 5.6:1• Area of serviceCore- 15,000 sq.m• Excavation depth -24.7m• Foundation-33m long ,1.2 m in dia 1242 piles

 In total, 41 882 steel elements with a combined weight of 125 000 tonnes, including connections, were erected, at a peak rate of 8000 tonnes per month.

The new headquarters of China Central Television contains the entire television-making processwithin a single building. The 234m tall tower redefines the form of the skyscraper, with theprimary system comprised of a continuous structural tube of columns, beams and bracesaround the entire skin of the building. In order to gain structural approval an Expert Panelprocess was necessary, for which a performance-based analysis was carried out to justify thedesign. This made extensive use of finite element analysis and advanced non-linear elastoplastictime history to evaluate the structural behaviour and ensure the building safety underdifferent levels of seismic event.

Public space and circulationThe leaning form and varied programme, including the need toaccommodate large studio spaces, posed additional challenges for the gravity structure, andresulted in the introduction of a large number of transfer trusses throughout the tower. Erectingand connecting the two massive towers presented the structural engineers and contractorswith further design and construction challenges.

The functions and layout within the CCTV building.

Studio and broadcast

The functions and layout within the CCTV building.

Staff and VIP facilities

Structural formSuperstructure – the ‘continuous tube’• Early on, the team determined that the only way to deliver the desired architectural form of

the CCTV building was to engage the entire façade structure ,creating in essence an external continuous tube system. Adopting this approach gave proportion that could resist the huge forces generated by the cranked and leaning form, as well as extreme seismic and wind events. This ‘tube’ is formed by fully bracing all sides of the façade. The planes of bracing are continuous through the building volume in order to reinforce and stiffen the corners. The continuous tube system is ideally suited to deal with the nature and intensity of permanent and temporary loading on the building, and is a versatile, efficient structure which can bridge in bending and torsion between the Towers, provide enough strength and stiffness in the Towers to deliver loads to the Base, and stiff e n up the Base to rein force the lower Tower levels and deliver loads to the foundations in the most favorable possible distribution, given the geometry.

Internal primary structureall the structural support elements in the building are of structural steel, except some external columns are steel-reinforced concrete columns due to the magnitude of loads they are designed to carry. The floors are composite slabs on steel beams. internal elements combine to create unique floor configurations for every floor of the building. That is, the floor span between cores or internal columns and thechanges on every floor. Moving up the building, the floor spans increase on two adjacent sides of the building and decrease on the other two opposite sides. As a consequence,where floor spans decrease on the inward sloping sides of the building, some internal columns can be removed once the distances become structurally manageable in onespan. For the same reason, once the increasing floor spans on the outward leaning sides of the building start to Impact on the finished floor height due to increased beam depth, additional columns are introduced.

External primary structureForm the onset, it was decided to adopt an external skin oF leaning columns, horizontal edge beams and triangulated bracing on a two-storey pattern to form an enclosed tubestructure to support the building. Furthermore, the braced tube structure affords a multitude of alternative load paths. Such a robustness feature is highly desirable, especially in seismically sensitive Beijing. It also provides safety in the event of an extreme design incident, such as blast removal of a major column in the building. The external diagrid structure is also boldly expressed in the building’s façade. It visually expresses the pattern of forces in the external tube, reinforcing the transparency betweenstructure and architecture, a strong philosophy in the building’s design. The unique diagrid pattern in the external structure was arrived at after extensive iteration andoptimization, in close collaboration with the architect

Transfer trussesTransfer trusses are introduced to collect the columns required at intermediate heights in the towers to cope with the increasing floor spans. The transfer trusses span between the internal core and the external tube structure. They are typically located in plant floors in the building so as to be hidden from view and minimize impact to the architecture and floor usage. The large member sizes of the transfer trusses mean a potential for these trusses to act as outriggers as they link up the external tube with the internal steel cores. An outrigger effect would be undesirable because this would then complicate the primary seismic load path. To prevent this condition, the transfer trusses are connected to the internal cores and the external columns at singular pin joint locations only.

Developing and optimizing the bracing pattern

Performance Based Seismic Design Approach

Non-Linear Superstructure Design And Performance Verification

Having established the inelastic global structure and local member deformation acceptance limits, the next step was to carry out non-linear numerical seismic response simulation of the CCTV building subjected to the Level 2 and the Level 3 design earthquakes. Both the non-linear static pushover analysis method and the non-linear dynamic time history analysis method were employed toperform this task. Finally, the seismic deformation demands were compared against the structure’s deformation capacities to verify seismic performance. This verification was carriedout on a storey-by-storey and a member-by-member basis. For the CCTV building, all global and local seismic deformation demands are shown to fall within their respective acceptance limits, confidently demonstrating that the building achieves the quantitative performance objectives when subjected to both the design intermediate (Level 2) and the design rare (Level 3) earthquakes.

This large-scale shaking table test was of particular interest. In China it is the norm for buildings that fall outside the code to be thus studied, and the CCTV model was the largest and most complex tested to date. The nature of the testing required the primary structural elements to be made from copper (to replicate as much as possible in a scale sense the ductility of steel). The model also included concrete floors (approximately 8mm thick) to represent the 150mm thick composite floor slabs. In all cases, the physical tests correlated closely with the analysis.

1. Construction sequencing and its effecton the final stress in the structural elements2. Ensuring the building and elements areconstructed to the designed setting outand positions, within allowed constructiontolerance3. Construction and linking of the overhang

APPLICATION OF PRESSURE INTEGRATION METHOD TO CCTV

The coordinate system and dynamic model of the CCTV’s structural design was adopted for the wind engineering study, so that the predicted wind loads could be readily used by the structural engineers. This dynamic model consisted of four main axes, Tower 1 axis, Tower 2 axis, Top Link axis, and Bottom Link axis, The element properties were calculated based on these axes. The first 9 modes of vibration were considered in the analysis. The first two modes represented the sway modes in northeast-southwest direction, and northwest-southeast direction, respectively. The third mode showed a twist motion between the two main towers. A significant vertical motion of the top link was noticed in Modes 4 and 8.

In total, 285 pressure taps were installed on the 1:500 scale model to simultaneously measure wind pressures during wind tunnel testing. To consider the near-field terrain effects, a proximity model, which simulated the surroundingbuildings and structures in details within 600m radius from the site, was included in the wind tunnel model. The farfield terrain effects were simulated using spires androughness elements to duplicate the representative wind profile and turbulence properties in the area.

The critical load patterns that needed to be examined in detail were discussed and determined by the structural engineers and the wind engineers from viewpoints of structural design as well as wind response. These load patterns included the worst overturning moments and shears about various principle axes, the worst differential loadsbetween the two towers, the worst torsional loads, the worst loads on the top links, and the worst loads on each tower, etc.

Structural formSubstructure and foundations• The main Towers stand on piled raft foundations. The piles are typically 1.2m in diameter,

and about 52m long. Given the magnitude and distribution of the forces to be transferred to the ground, the raft is up to 7.5m thick in places and extends beyond the footprint of the Towers to act as a toe, distributing forces more favorably into the ground.

• The foundation system is arranged so that the center of the raft is close to the center of load at the bottom of each Tower, and no permanent tension is allowed in the piles. Limited tensions in some piles are only

permitted in major seismic events. For the Base plus three e - storey basement, • a traditional raft foundation is used, with tension piles between column locations to resist

uplift from water pressure acting on the deep basement. 15-20m long, 600mm diameter tension piles will be arranged under the raft with additional 1.2m diameter piles under secondary core s and columns supporting large transfer trusses from the studio areas.

Preparation of foundation raft.Cutting down piles by hand. Column embedded in raft.

construction

One of several construction sequence loading arrangements considered.

construction

Overhang construction• Construction of the Overhang began after the steelwork for the two Towers was completed

to roof level. • the structure was cantilevered out piece-by piece from each Tower over the course of the

next five months.• This was the most critical construction stage, not only in terms of temporary stability but

also because its presence and the way it was built would change the behavior of those parts of the Tower already constructed.

• The forces from the two halves of the partly constructed Overhang would be concentrated in the Towers until such time as the two halves were linked and the building became a single continuous form, when the loads would start being shared between all of the permanent structure.

The completed Overhang structure, showing the three 3m diameter circles punched in the deckto create glazed viewing platforms for the public viewing gallery. The seven initial connection

elements.

The Overhang before connection.Installation of first connectionelement.

The force from the braces and edge-beams must be transferred through and into the column sections with minimal disruption to the stresses already present in the column. The connection is formed by replacing the flanges of the steel column with large ‘butterfly’ plates, which pass through the face of the column and then connect with the braces and the edge-beams. No connection is made to the web of the column to simplify the detailing and construction. The joints are required to behave with the braces, beams, and columns as ‘strong joint/ weak component’