CCTV（大裤衩）结构设计全程记录（pic+text）

Engineering CCTV

by Arup*

 

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Proposed headquarters for CCTV in Beijing, designed by Rem Koolhaas, engineered byArup. 
Image: ? OMA

Public space and circulation. 
Image: Arup

 

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To support the rapid expansion of China Central Television (CCTV), an international design competition was launched in 2002 for a centralized headquarters building in Beijing. Winning the commission was Rem Koolhaas (Office for Metropolitan Architecture, OMA), teamed with engineering firm Arup and the East China Architecture and Design Institute as both architect and engineer of record. Koolhaas imagined a building whose three dimensional form brings CCTV's staff and functions into a "continuous tube." This is part of the story of the engineering challenge. ? Editor

Because the seismic design of theCCTV building lay outside the scope of the prescriptive Chinese codes of practice, Arup proposed a performance-based design approach from the outset, adopting first principles and state-of-the-art methods and guidelines to achieve set performance targets at different levels of seismic event. Explicit and quantitative design checks using appropriate linear and nonlinear seismic analysis were made to verify the performance for all three levels of design earthquake.

The basic qualitative performance objectives were:

·  no structural damage when subjected to a level 1 earthquake with an average return period of 50 years.

·  repairable structural damage when subjected to a level 2 earthquake with an average return period of 475 years.

·  severe structural damage permitted but collapse prevented when subjected to a level 3 earthquake with an average return period of 2500 years.

 

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Studio and broadcast spaces within the CCTV headquarters, Beijing, designed by Rem Koolhaas, engineered byArup. 
Image: Arup

Staff and VIP facilities. 
Image: Arup

Part of the tube structure: regular grid of columns and edge beams. 
Image: Arup

Part of the tube structure: patterned diagonal bracing. 
Image: Arup

The CCTV's tube structure is a composite of a regular grid of columns and edge beams plus patterned diagonal bracing. 
Image: Arup

Internal columns starting from pilecap level. 
Image: Arup

Internal columns supported on transfer structures. 
Image: Arup

The CCTV foundation system. 
Image: Arup

 

 

 

 

 

 

 

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For the CCTV development site, the peak horizontal ground acceleration values associated with the three levels of design earthquake are 7 , 20, and 40 percent of gravity respectively.

Elastic Superstructure Design

With the structural bracing pattern determined from the initial concept work, a full set of linear elastic verification analyses were performed, covering all loading combinations including level 1 seismic loading, for which modal response spectrum analyses were used.

All individual elements were extensively checked and the building's global performance verified. Selected elements were also initially assessed under a level 2 earthquake by elastic analysis, thus ensuring that key elements such as columns remained elastic.

The elastic analysis and design was principally performed using SAP2000(limited nonlinear structural analysis and design with static, dynamic, and push-over capability) and a custom-written postprocessor for the Chinese steelwork code, which automatically combined the individual load cases applied to the building for the limit-state design.

Capacity ratios were then visually displayed, allowing detailed inspection of the critical cases for each member. Due to the vast number of elements in the model ? 10,060 elements representing nearly 300,000 feet (90,000 meters) of steel and steel-reinforced concrete (SRC) sections ? and the multitude of load cases, four postprocessors were run in parallel, one for steel columns, one for SRC columns, one for braces, and another for the edge beams that together form the continuous tube.

The SRC columns used a modified postprocessor to account for the differences between the steel and SRC codes; section properties of these columns were determined using Xtract(nonlinear large strain composite cross-section analysis), which also computed the properties for the subsequent nonlinear analyses.

The postprocessor provided a revised element list which was imported back into SAP2000, and the analysis and postprocessing repeated until all the design criteria were met. As the structure is highly indeterminate and the load paths are heavily influenced by stiffness, each small change in element property moves load around locally.

Optimizing the elements only for capacity would result in the entire load gradually being attracted to the inside corner columns, making them prohibitively large, so careful control had to be made of when an element's section size could be reduced and when there was a minimum size required to maintain the stiffness of the tube at the back face.

To further validate the multidirectional modal response spectrum analyses, level 1 time-history checks were also made using real and artificially generated seismic records.

Design and Performance Verification

For the performance-based design, a set of project-specific "design rules" were proposed by the design team and reviewed and approved by the Chinese Ministry of Construction's expert review panel, creating a "road map" to achieve the stated seismic performance objectives.

Appropriate linear and nonlinear seismic response simulation methods were selected to verify the performance of the building under all three levels of design earthquake. Seismic force and deformation demands were compared with the acceptance limits established earlier to rigorously demonstrate that all three qualitative performance objectives were achieved.

Inelastic deformation acceptance limits for the key structural brace members in the continuous tube were determined by nonlinear numerical simulation of the postbuckling behavior. LS-DYNA (software for nonlinear explicit time history analysis), commonly used to simulate car crash behavior, was used for this work.

The braces are critical to both the lateral and the gravity systems of the building and are also the primary sources of ductility and seismic energy dissipation. Nonlinear numerical simulation of the braces was needed to establish the postbuckling axial force/ axial deformation degradation relationship to be used in the global 3D nonlinear simulation model.

This simulation was also used to determine the inelastic deformation (axial shortening) acceptance limit in relation to the stated performance criteria. Postbuckling inelastic degradation relationship curves illustrate the strength degradation as the axial shortening increases under cyclic axial displacement time history loading.

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Models illustrating the development of the CCTV headquarters' facade pattern, designed by Rem Koolhaas, engineered byArup. 
Image: © OMA

Brace stresses for a uniform grid. 
Image: Arup

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Unfolded view of the structure showing areas to densify or rarefy the mesh. 
Image: Arup

Nonlinear finite element simulation model showing local buckling of a typical steel brace. 
Image: Arup

GSRaft model of the piled foundation. 
Image: Arup

The von Mises stress distribution of a large connection plate under the most unfavorable loading combination. 
Image: Arup

 

 

 

 

 

 

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The acceptable inelastic deformation was then determined from the strength degradation "backbone" curve to ensure that there was sufficient residual strength to support the gravity loads after a severe earthquake event.

Having established the inelastic global structure and local member deformation acceptance limits, the next step was to carry out nonlinear numerical seismic response simulation of the entire 3D building subjected to level 2 and level 3 design earthquakes. Both the nonlinear static pushover analysis method and the nonlinear dynamic time history analysis method were used to determine the seismic deformation demands in terms of the maximum inelastic inter-story drifts and the maximum inelastic member deformation.

These deformation demands were compared against the structure's deformation capacities story-by-story and member-by-member to verify the seismic performance of the entire building. All global and local seismic deformation demands were shown to be within their respective acceptance limits, demonstrating that the building achieves the quantitative and hence qualitative performance objectives when subjected to level 2 or level 3 earthquakes.

Foundation Design

The design of the foundations required that the applied superstructure loads be redistributed across the pilecap (raft) so as to engage enough piles to provide adequate strength and stiffness. To validate the load spread to the pile group, a complex iterative analysis process was used adopting a nonlinear soil model.

The superstructure loads were applied to a discrete model of the piled raft system. Several hundred directional load case combinations were automated in a spreadsheet controlling GSRaft, iterative nonlinear soil-structure interaction analysis software.

This procedure iteratively changed the input data in response to the analysis results to model the redistribution of load between piles when their safe working load was reached. The analysis was then repeated until the results converged and all piles were within the allowable capacities. The envelope of these several-hundred analyses was then used to design the reinforcement in the raft itself.

Connections

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. To simplify the detailing and construction of the concrete around the steel section, no connection is made to the web of the column.

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 modeled from the original AutoCAD drawings using MSC/NASTRAN, a heavy-duty finite element analysis package. The models were analyzed, subjected to the full range of forces that can be developed before the braces buckle or yield ? assuming the maximum probable material properties ? to evaluate the stress magnitude and degree of stress concentration in the joints.

The shape of the butterfly plate was then adapted by smoothing out corners and notches until potential regions of yielding were minimized and the degree of stress concentration reduced to levels typically permitted in civil and mechanical engineering practice. CAD files of the resulting geometry of the joints were exported from the finite element models and used for further drawing production.

The structural design of CCTV posed many other technical challenges to the large international team which delivered the design through global collaboration, transcending time zones, physical distance, cultures, cost centers, and even the SARS outbreak. In the end, the team delivered a complex design on time and won approval from the Chinese construction ministry's expert panel.

A longer version of this article first appeared in The Arup Journal, 2/2005, and is excerpted here with permission.

 * The Arup author team: Chris Carroll, Paul Cross, Xiaonian Duan, Craig Gibbons, Goman Ho, Michael Kwok, Richard Lawson, Alexis Lee, Andrew Luong, Rory McGowan, and Chas Pope.

source:http://www./2008/0827/tools_1-1.html

Addition:

Metal guru: Arup’s Chris Caroll on CCTV Headquarters, Beijing

 

25 July 2008
By Will Hunter 

Will Hunter discovers how OMA and Arup used metal structures in the astonishing design for the CCTV HQ in Beijing

Like those architects who become such celebrities that they can go by a single name, the ultimate status symbol for a skyscraper is a nickname. Foster has notched up two, with the Gherkin for the Swiss Re Tower and the unprintable but no less accurate sobriquet favoured by cabbies for City Hall. Rogers’ Cheese Grater and Piano’s Shard are both forthcoming.

But OMA’s vision for client China Central Television (CCTV) — a 450,000sq m “loop” comprising two 200m towers connected by a nine-storey base and a 13-storey, 70m overhang — is of such singularity that naming it after a similar-looking object wasn’t an option. The inhabitants of earthquake-prone Beijing simply dubbed it Wei Fang — the Dangerous Building. 

Arup, the project engineer, was more sanguine. “It was beyond anything we’d thought of before, but I didn’t fall off my chair,” says engineer Chris Carroll of his first view of OMA’s proposal in 2002. The practice’s giant loop concept, which links up the compartmentalised tasks involved in making TV programmes, will allow the state-run broadcaster to expand from 13 channels to over 200 worldwide.
Arup had to convince a dozen eminent Chinese engineers and academics that although it didn’t meet most of China’s building codes, the project was viable. “We had three months to prove from first principles that it was safe, buildable, and not ridiculously expensive,” says Carroll. 
As the project approaches its 2009 completion date, I caught up with Carroll to find out how the engineering solution evolved from OMA’s concept and — since it will actually be one the safest buildings in the country — why the locals may need to come up with a new nickname.

Expressing the structure on the facade


Sectional view through facade structure 

“Making the loop form work was the most precious thing to OMA,” says Carroll. Generally, a 250m high-rise could be supported from its core, but Arup soon realised CCTV would have to engage the towers’ entire cross-section. Both towers, which naturally want to fall over in the direction of the overhang, are further challenged by a net slope of 10 degrees. The whole form became a tube where every external face is a structural diagonal grid (diagrid) in a regular two-storey pattern to coincide with the towers’ double-height studios. 

“Wasting resources in China is a capital offence,” says Carroll, “so we were very keen to make this perform optimally.” This base pattern was investigated using various computer simulation models, including those developed by Arup’s Advanced Technology Group for calculating, for example, waste flasks falling off moving trains. 

“We developed a grammar of interventions based on the stresses in the building, doubling the number of beams around heavy-loaded zones, and halving around light-loaded zones,” he explains.

The 3D model was tested to ensure the building would withstand heavy earthquakes — a problem for Beijing — as well as hold up during construction since, unlike a normal straight-up tower, it needed to be connected at the top. 

An unfolded elevational diagram was sent back and forth between Arup and OMA, mapping various investigations until the best solution was found. This dialogue produced some interesting results: for example, the highly unusual appearance where lines of the diagrid appear to be broken. 
Here, unreadable on the facade, the floor plates make the triangulation. This was mostly the result of the optimisation process, but OMA included one or two as moments of architectural playfulness.

Computer and physical testing


The 1:20 shake-table model is now in the site’s car park

Arup has made a structure with a high degree of redundancy. Carroll says: “The authorities specified that you could blow the corner columns out from under the armpits, for want of a better expression, and the cantilever wouldn’t collapse progressively .” 

But Arup took no chances and proved the loads could be redistributed even if a three-column failure took place in this area. The authorities weren't concerned about an attack on the building, despite the proposal being made months after 9/11, because they said China "doesn't suffer from terrorism".

Resisting earthquakes was also imperative. The seismic design was outside of prescriptive Chinese codes of practice so, given he national prominence of the CCTV building, Arup proposed a performance-based design approach with three objectives: no structural damage when subjected to a level one earthquake (with an average return period of 50 years); repairable structural damage when subjected to a level two earthquake (475 years); and severe structural damage permitted but collapse prevented when subjected to a level three earthquake (2,500 years).

The detailing of how the floor plate meets the elevational structure was critical. “The main test here is to ensure the brace yields first, and in places that are ductile; in other words not across the weld, where it might form a brittle failure mechanism,” says Carroll. This was tested on full-scale mock-up, and shown to perform under extreme earthquake conditions.

A 1:20 shake-table model (shown here in the site car park) was also made, says Carroll, “as a bit of a deal with a couple of the Chinese professors who really wanted to stay involved and build a proper scale model”. It was shaken under various levels of earthquake and survived intact.


The detailing of the diagrid enables the vertical and horizontal elements to remain comparatively unstressed in an earthquake.

Constructing the cantilevers


Arup explored three options to connect the 70m cantilever

As the structure nears completion, the loading changes considerably. Before the cantilevers are joined together, the towers have to take additional load; after connection, the highest stresses are in the overhang structure as the towers are propped together. Between the two stages, three construction options were investigated: jacking (lifting whole sections up); corner column (supporting the corner with scaffolding); and cantilever (simply building out bit by bit). The latter option was selected. 

It was a challenge to tie the transfer level together while both sides were moving around at such a height. Soft connections, where the beam is hinged, were made in 12 locations. One morning before the sun came up and started to heat up the elements, the army of construction workers welded these together simultaneously. Before this was done, the extreme corner columns of the towers were omitted to throw more of the load back into the building and prevent overstressing of the corner member. 

Internal structure


The piled raft foundation (top) and, in red, the uninterrupted columns that meet it (bottom)



Arup looked at having a sloping core to follow the outline of the building, but this was rejected primarily because of the lifts. Diagonally moving lifts, if travelling at the speeds necessary for a high-rise, throw passengers against one side. The core thus starts in a corner at the bottom and finishes at the opposite corner at the top. To do this within the towers’ footprint, the shaft helpfully steps in — fewer lifts are needed for higher floors.

Internal columns are concrete-encased structural steel, where the concrete adds three-hour fire protection and an additional 30% strength. 

These columns are vertical, and to stop them smashing through the facade there is a transfer structure around half-way up the towers, where a single floor — also a building services floor — is heavily braced to transfer the loads across. For the cantilever, the transfer structure is over two floors, onto which the seven storeys of office space are built.

“The foundations were a study in their own right,” says Carroll. The main towers stand on piled raft foundations, where piles are typically 1.2m in diameter and about 52m long. Due to the magnitude and distribution of the forces, the concrete raft is up to 7.5m thick — around the same height as a Victorian townhouse — and extends beyond the towers’ footprints to distribute the forces more favourably into the ground. 

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 piles; limited tension in some piles is only permitted in major seismic events.




Starting with a regular two-storey diagonal grid the design developed by doubling or halving the structure.


The darker areas on this unfolded elevational drawing show higher stressed zones based on the building’s unique loading patterns.

source:http://www./buildings/technical/metal-guru-arup%E2%80%99s-chris-caroll-on-cctv-headquarters-beijing/3119008.article