13 Case Study CCTV Building Headquarters Cultural Center
13 Case Study CCTV Building Headquarters Cultural Center
13 Case Study CCTV Building Headquarters Cultural Center
org/papers
Title:
Authors:
Subjects:
Architectural/Design
Building Case Study
Structural Engineering
Keywords:
Construction
Design Process
Form
Foundation
Performance Based Design
Structure
Publication Date:
2008
Original Publication:
Paper Type:
1.
2.
3.
4.
5.
6.
Council on Tall Buildings and Urban Habitat / Chris Carroll; Craig Gibbons; Goman Wai-Ming Ho; Michael
Kwok; Paul Cross; Xiaonian Duan; Alexis Lee; Ronald Li; Andrew Luong; Rory McGowan; Chas Pope
Arup
13 Fitzroy Street
London
W1T 4BQ
t: (+44) 020 7636 1531
www.arup.com
The new headquarters of China Central Television contains the entire television-making process
within a single building. The 234m tall tower redefines the form of the skyscraper, with the
primary system comprised of a continuous structural tube of columns, beams and braces
around the entire skin of the building. In order to gain structural approval an Expert Panel
process was necessary, for which a performance-based analysis was carried out to justify the
design. This made extensive use of finite element analysis and advanced non-linear elastoplastic time history to evaluate the structural behaviour and ensure the building safety under
different levels of seismic event. The leaning form and varied programme, including the need to
accommodate large studio spaces, posed additional challenges for the gravity structure, and
resulted in the introduction of a large number of transfer trusses throughout the tower. Erecting
and connecting the two massive towers presented the structural engineers and contractors
with further design and construction challenges.
Introduction
This article describes the structural design and
construction of the CCTV Building in Beijing,
including development of the structural concept, performance-based seismic design and
Expert Panel Review process.
Architectural Concept
China Central Television (CCTV), the countrys
state broadcaster, plans to expand from 18
to 200 channels and compete globally in the
coming years. To accommodate this expansion, they organized an international design
competition early in 2002 to design a new
headquarters building. This was won by OMA
(Office of Metropolitan Architecture) and Arup,
which subsequently allied with the East China
Design Institute (ECADI) to act as the essential
local design institute (LDI) for both architecture
and engineering.
The unusual brief, in television terms, was that
all the functions for production, management,
and administration would be contained on the
chosen site in the new Beijing Central Business
District (CBD), but not necessarily in one build-
CCTV Building | 15
collaboration with the architect, since the pattern of visually expressed diagonals was a key
aesthetic aspect of the cladding system.
The braced tube structure gives the leaning
Towers ample stiffness during construction,
allowing them to be built safely within tight
tolerances before they are connected and
propped off each other. The tube system also
suits the construction of the Overhang, allowing its two halves to cantilever temporarily
from the Towers.
The continuous tube has a high degree of
inherent robustness and redundancy, and offers the potential for adopting alternative load
paths in the unlikely event that key elements
are removed.
Gravity loads are also carried by vertical
columns around the buildings central service
cores, whilst a number of steel transfer trusses
are introduced to support the floors in the
Overhang, at high levels in the sloping towers,
and over large studios in the Podium area.
Each tower sits on a piled raft foundation. The
rafts vary in thickness up to 7metres, and extend beyond the footprint of the Towers to act
as a toe, distributing forces more favourably
into the ground. The foundation system is arranged so that the centre of the raft is close to
the centre of load at the bottom of each tower,
and no permanent tension is allowed in the
33m long piles. Limited tensions in some piles
are only permitted in major seismic events.
Performance-based design approach
The legal framework in China governing building design practice is similar to those of Japan
and some continental European countries
where the design codes are legal documents
published and enforced by the state government. Design engineers must comply with
the codes when designing buildings and
structures covered by their scope, but equally
the codes provide legal protection to the
16 | CCTV Building
Level 2
Description
Minor
Moderate Severe
Peak ground
acceleration
0.07g
0.20g
0.40g
1 in 475
years
1 in 2475
years
Average Return 1 in 50
Period
years
Level 3
Probability of
exceedance
63% in 50 10% in 50 2% in 50
years
years
years
Fortification
Criteria
No
damage
(remain
elastic)
Repairable
damage
No collapse
...structure CCTV
The analysis iteratively modelled the redistribution of load between piles when their safe
working load was reached. The analysis was
repeated for each load case until the results
converged and all piles were within the allowable capacities. Finally, the envelope of these
analyses was then used to design the raft
reinforcement.
Foundation design
The design of the foundations required that
the applied superstructure loads be redistributed across the raft so as to engage enough piles
to provide adequate strength and stiffness. To
validate the load spread to the pile group, an
iterative analysis process was used adopting a
non-linear soil model coupled with a discrete
model of the piled raft system (see Figure 4).
Several hundred directional load case combinations were automated in a spreadsheet
controlling the GSRaft soil-structure interaction
solver.
CCTV Building | 17
Connection Design
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. The connections must
resist the maximum probable load delivered
to them from the braces with minimal yielding
and a relatively low degree of stress concentration. High stress concentrations could lead to
brittle fracture at the welds under cyclic seismic loading, a common cause of failure in connections observed after the 1994 Northridge
earthquake in Los Angeles. Two connections,
representing the typical and the largest cases,
were modelled using powerful finite element
analysis software such as MSC/NASTRAN (see
Figure 5).
The models were analyzed, subjected to the
full range of forces that can be developed
18 | CCTV Building
Further transfer trusses are introduced to support internal columns within the Overhang,
and to support floors above the large studios
in the Base (see Figure 6). As with the butterfly
plates, forces in the truss diagonals are carried
only by the flanges at connections, with the
webs stopping short of the chords to simplify
construction.
Physical Testing
As part of the expert panel approval process,
there was a requirement for three physical
tests to be carried out, in order to verify the
analytical calculations:
1. Joint Test (butterfly plate): Beijings
Tsinghua University tested a 1:5 scale
model of the column-brace joint to confirm
CCTV Building | 19
Construction sequencing
The final stresses in the building are linked
to its construction sequence. In addition to
regular gravity and lateral forces acting on the
structure, there are significant additional construction stage forces due to the fact that the
building comprises two separate leaning Towers with cantilever up until the point at which
they are joined to become one structure. The
additional bending and overturning stresses
that get locked into the Towers and foundations prior to joining depend on the amount
of structure and faade completed at the time
of connection.
In essence, the greater the construction load
applied to the building prior to connecting
the two Towers, the more this would manifest
itself as increased locked-in base moments in
the Towers. After the connection was made,
any added weight would result in a thrust
between the two Towers via the Overhang.
Construction team
CSCEC, a state-owned enterprise under the
administration of the central government,
was established in 1982 and is Chinas largest
construction and engineering group. CSCEC
now enjoys an international reputation, having
completed an increasing number of projects
abroad including the Middle East, South
America and Africa. The steelwork fabricators
were Grand Tower, part of the Bao Steel group
based in Shanghai (Chinas largest steel manufacturer), and Jiangsu Huning Steel, based in
Jixing, Jiangsu Province.
Other members of the team were Turner Construction (USA), providing support to CSCEC
on construction logistics, China Academy of
Building Research (CABR), one of the major
design institutes in Beijing, and Tsinghua University, which carried out the presetting analy-
20 | CCTV Building
Steelwork construction
The first column element was placed on
13 February 2006. In total, 41 882 steel elements with a combined weight of 125 000
tonnes, including connections, were erected
over the next 26 months, at a peak rate of 8000
tonnes per month.
During the design it was thought that some
high-grade steel elements would need to be
imported, but in the end all the steel came
from China, reflecting the rapid advances of
the countrys steelwork industry. Steel sections
were fabricated at the yards of Grand Tower
in Shanghai and Huning in Jiangsu, and then
delivered to site by road (see Figure 10), with
a size limit of either the tower crane capacity
(80 tonnes) or the maximum physical dimensions that could be transported (18m length).
The geometrical complexity made construction slower than for other steel-framed buildings. Although the rate of erection increased
as the contractor became more familiar with
the process, CCTV has no typical floors.
Nevertheless, up to six storeys per month
was achieved for the relatively uniform
CCTV Building | 21
The presetting process was further complicated by the fact that when completed,
almost all the columns have different stresses,
depending on the ratio of gravity to seismic
loads, unlike in a conventional building
where all perimeter elements will be similarly
stressed. As a result, different presets were
required on different sides of the Towers,
the exact values also depending on the final
construction sequence. In practical terms,
this meant fabricating the columns longer on
one side of each Tower, so that they would
eventually shorten to the correct geometry
under load.
Presetting was in two stages: at the fabrication yard, based on the results of the analytical modelling, and then at installation, if
required, to suit the actual building deformation as monitored during the course of construction. Progress of floor plate concreting
was also controlled to suit the assumptions
made in the presetting estimation.
It does not take a NIST report or a rocket scientist to figure out that requiring additional exit stairs
will improve overall occupant evacuation times The bigger question that needs to be answered is at
what economic cost to society?
David Frable, a General Services Administration fire safety engineer, asks the International Code Council to repeal stronger safety requirements for new
skyscrapers that were added to the countrys most widely used building code last year, arguing that they would be too expensive to meet. From
Agency Fights Building Code Born of 9/11, The New York Times, September 7th, 2008.
22 | CCTV Building
Overhang erection commenced, there was already much movement data from the Tower
construction that could be used to calibrate
the analysis.
Overhang construction
Construction of the Overhang began after the
steelwork for the two Towers was completed
to roof level. Tower 2 Overhang began first, in
August 2007, and the structure was cantilevered out piece-by-piece from each Tower
over the course of the next five months (see
Figure 12). 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 behaviour
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 sufficiently linked and the building
became a single continuous form, when the
loads would start being shared between all of
the permanent structure.
The bottom two levels of the Overhang
contain 15 transfer trusses that support the internal columns and transfer their loads into the
external tube. In the corner of the Overhang,
these trusses are two-way, resulting in some
complex 3-D nodes with up to 13 connecting
elements, weighing approximately 33 tonnes
each.
CCTV Building | 23
CONCLUSIONS
The project demonstrated that a building
with many complex technical challenges
could be delivered successfully within a
tight programme. An international team
was mobilized to make best use of the firms
experience and knowledge, which required
seamless co-ordination between a number of
locations and cultures.
Credits
Client: China Central Television
Architect: OMA Stedebouw BV, Ole Scheeren and Rem
Koolhaas
Engineer: Arup
Local Design Institute: East China Architectural Design and
Research Institute Co Ltd (ECADI)
Illustrations
All Arup except Figure 1 ( OMA), Figure 13 ( CSCEC)
24 | CCTV Building