Zifeng Tower: Unique Shape And Facade

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Zifeng Tower (formerly known as Nanjing Greenland Financial Center) in Nanjing, China, is a mixed-use complex comprising of several buildings resting on two sites, parcels A1 and A2. Parcel A1 contains a podium connecting two towers. The taller 450m (1,476ft) tower consists of offices and hotels, and the 100m (328ft) tower contains purely office space. The shape and placement of the buildings are designed to echo the geometry of the existing roads and maximize exterior views of the city. The buildings’ composition maintains the east-west viewing corridors along East Beijing Road and provides a visual link to the nearby historical drum and bell towers. At the top of the tower there are several restaurants and a 72nd-floor observatory. The observatory offers views of the Ningzheng Ridge Mountains, Xuanwu Lake, the Yangtze River and urban landscape of Nanjing and beyond. According to skyscraper. com, the core features “two interlocking forms that resemble dancing dragons from Chinese culture”. There are landscaped glass atria throughout the building as well as a garden and outdoor swimming pool on tower’s podium roof. Floors 70 through 88 are the mechanical floors located in the building’s spire.
Design And Structural System
The unique triangular shape of the Nanjing Greenland Financial Centre was designed to take full advantage of the site’s shape, size, and orientation. The building’s form takes inspiration from the local landscape: the tower is oriented to maximize views of the nearby mountains, lake and historic Nanjing buildings, while the two forms that encircle the central core of the tower mimic “dancing dragons,” a common motif in Chinese iconography. These “dragons” are composed of angled panes of glass staggered for a scale-like appearance. The seams that run between the two forms reference the Yangtze River, which flows through the city. Nanjing’s triangular form relates to its site, providing stunning views of the nearby mountains, lake, and historic buildings. The spire of the building reaches 458 metres. The spire and tower atria are lit at night, making the Nanjing Greenland Financial Centre an icon on the city’s skyline.
Structural System
The structural system of the tower is composed of a central concrete core supported through external columns. Clad in a glass curtain wall, with fins of glass extending past the corners of the ovular building, the tower offers a dramatic sight, particularly when illuminated at night. The structure is made from steel as well as reinforced concrete and steel. The building’s exterior walls consist of panes of glass set at angles within staggered modules, resembling the scales of a dragon. Horizontal and vertical seams work to separate the building’s textured glass surfaces, evoking images of the Yangtze River.

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Landscaping is a significant part of the complex as the buildings are set back from the street with several large landscaped, public open spaces. South of parcel A1, is a sunken garden that will connect to the future subway. There is also a roof garden at the top of the podium in parcel A1 to reduce the heat island effect. In addition to the ground level and podium garden spaces, the design incorporates sky gardens that wind their way up the façade, bringing the green all the way up the tower.
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Lateral Load Resisting Structural System
The main tower consists of a composite system utilizing both structural steel and reinforced concrete elements to resist both gravity and lateral loads. Typical floor to floor heights are 6m to 7m in the podium zone, 4.2m in the office zone and 3.8m in the hotel zone. Mechanically floors are generally double-height spaces at 8.4m tall. The lateral load resisting structural system provides resistance to both seismic and wind loading. The primary lateral system is comprised of an interior reinforced concrete ‘super-core’ shear wall system and exterior composite columns. Shear wall thickness range from 300mm to 1500mm over the height of the building with reinforced concrete link beams joining adjacent sections of shear wall around door openings and major mechanical penetrations. The closed form of the super-core’s perimeter provides large amount of the overall torsional stiffness of the building. The core wall thicknesses were optimized in order to better balance the triangular shaped core for both bending stiffness and torsional rigidity. This resulted in thicker walls near the ‘tip’ of the core for the trapezoid- shaped closed form and slightly thinner walls for the rest of the core. The exterior composite columns are linked to the super-core by structural steel outrigger trusses at the 8.4m tall mechanical floors at levels 10, 35 and 60. Outrigger trusses typically align with the web walls in the core and extend from the perimeter column through the core to the other perimeter column on the opposite side of the building. Embedded steel columns near the edges of the core walls were extended for a minimum of three floors above and below the outrigger trusses to aid in transferring the force couples developed under lateral loading. The exterior composite columns at these levels are linked together by a structural steel belt truss system at the perimeter to provide more uniform load distribution in the columns. Composite column sizes range from 900mm diameter to 1750mm diameter over the height of the building. From level 63 to 67 a portion of the reinforced concrete core continues up in combination
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with a braced steel frame to form the lateral system. Above level 67 to the roof at 381m, the lateral system consists of small reinforced concrete core and a perimeter moment from structure. A structural steel spire continues to 450m. The secondary lateral system for the Main Tower consists of a moment- resisting frame at the perimeter of the building. The perimeter moment frame system provided additional torsional stiffness, structural integrity, and redundancy for the overall building. The gravity load- resisting structural system consists of structural steel floor framing supporting a 155mm thick composite metal deck floor slab. Typical floor framing is spaced at 3 metres on and welded; headed shear studs are used to provide composite behaviour between the slab and supporting beams. Floor framing inside the “super core” consists of reinforced concrete beams supporting a reinforced concrete oneway slab. The central reinforced concrete “super-core” and the exterior composite columns then transmit the floor framing loads to the foundations. The below grade levels were constructed of reinforced concrete using a temporary, internally-braced slurry wall retention system. A permanent reinforced concrete foundation wall was then constructed inside of the slurry wall system. The foundation system for the Main Tower consists of a 350mm thick cast-in-place reinforced concrete belled caissons in the underlying rock.
Seismic Safety
In order to obtain seismic review approval for the Nanjing State- Owned Assets & Greenland Financial Centre’s Main Tower, one of the tallest structures in the world to date, enhanced design measures and performance-based evaluations were utilized. The critical parts of the lateral system were designed for earthquake forces between two and six times that typically required by Chinese code. In addition a full 3-Dimensional Non-Linear Elasto-Plastic analysis for a 2500-year earthquake was completed to determine the structures’ response and serviceability. A multistage axial shortening, creep and shrinkage analysis was also performed to evaluate the long-term load sharing between the central core and the perimeter of the Tower via the outrigger truss system.
Façade And Construction Materials
The tower features a unique façade system, rather than a typical flush-glass curtain wall system; it is composed of offset modular panels that protrude in plan to create a distinctive texture to the building’s elevations. Each curtain wall unit is a triangle in plan and shifts a half module between every two floors. It creates a scaled effect and has a very unique visual appeal in catching the light and reflections of the city. The small edge of the triangle unit is a fixed perforated metal panel with a hidden operable panel behind it for natural ventilation and smoke exhaust. This will help to reduce mechanical ventilation energy usage during some transitional seasons. The long edge of each triangle unit consists of a high performance insulated low-E glass panel, to help to cut down the heat gain through the building façade.The construction materials of the building consist of reinforced concrete and steel. The triangulated, serrated façade treatment is interesting, adding depth and texture to the imposing form while allowing natural ventilation throughout. Carefully sited and planned, the tower creates a “flatiron” effect-stretched to its incredible height on one of Nanjing’s busiest intersections.
Elevators And Fire Protection System
The office tower is served by two banks of seven elevators, one bank for the low-rise office levels, and one for the high-rise office levels. Two service elevators are designated as fire elevators and serve all floors of the office tower (three separate service elevators will serve all floors of the hotel). The hotel portion of the main tower will be served by three express passenger elevators from the ground floor entrance lobby to the level 36 sky lobby, where five local passenger lifts will take guests up to the highest hotel level on 65. The office tower is served by floor-by-floor, variable volume, supply air handling units with variable speed drive. Four pipe fan coil units are provided for hotel guest rooms. Gas fired steam boilers and chillers, and associated water pumps are located on the first basement level. Gas service is brought into the building for the boilers and for kitchens. The fire protection system for the building consists of a zoned combination automatic sprinkler and standpipe system in accordance with regulatory requirements for a fully sprinklered high-rise building in China. Several key features of the building design were not fully addressed by Chinese codes and needed to follow a performance-based design approach. These features were: the primary tower, which exceeds the 250-metre height limit of the code; high-occupancy spaces in the upper levels of the primary tower and the podium building, instead of below the third floor as permitted by code; area of refuge floors in the tower serving up to 23 floors rather than 15 as code specifies; and a single fire compartment for each office floor of the tower. The design of the atrium space in the podium building also posed a challenge, because it called for a single fire compartment connecting all levels of the podium building, a design of the atrium mechanical smoke exhaust system based on performance measures, and a reduction of fire resistance rating for certain structural elements of the atrium roof structure, as well as circulation walkway bridges and escalators within the atrium. Fire strategies included providing: adequately sized and welldistributed exit paths; structural fireproofing and fire barriers that limit fire spread and protect buildings from collapse during occupant exit; various fire detection and notification systems; active fire suppression and smoke extraction systems that limit fire size and spread of smoke to enable occupants to safely exit and to minimize building damage; and infrastructure elements that promote safe and efficient fire fighting operations.For building features that met Chinese codes, RJA evaluated them for code compliance in the normal course of plan review and approvals. Features that didn’t meet code, however, needed to be evaluated using a performance-based design analysis consisting of computer fire and egress modelling. Specifically, the analysis evaluated representative high-rise and retail atrium areas to validate the proposed fire-protection design solution. This analysis included: heat effects and smoke flow from a design fire, activation of automatic fire detection and suppression systems, tenability conditions of egress routes, evacuation times of representative areas, and performance and reliability of fire safety features. Fire and egress modelling verified that proposed building design features that did not meet Chinese code and were required to undergo performance-based design analysis did, in fact, meet the established performance criteria and were equal to the fire safety intended by the code. Fire growth scenarios, based both on accepted theoretical models and on existing fire test data, aptly assessed the effects of a fire within representative areas of the building and validated the appropriateness of proposed fire safety features. This analysis formed the basis for design acceptance by the authorities having jurisdiction, and the development was completed in 2009.
Landscaping
Landscaping is a significant part of the complex as the buildings are set back from the street with several large landscaped, public open spaces. South of parcel A1, is a sunken garden that will connect to the future subway. There is also a roof garden at the top of the podium in parcel A1 to reduce the heat island effect. In addition to the ground level and podium garden spaces, the design incorporates sky gardens that wind their way up the façade, bringing the green all the way up the tower.
Conclusion
The Nanjing Greenland Financial Centre is another impressive skyscraper along with others such as the Shanghai World Financial Centre, Jin Mao Tower, and Two International Finance Centre. Other skyscrapers such as the Shanghai Tower, Guangzhou TV & Sightseeing Tower, and the China 117 Tower will be completed in the next few years as China continues to modernize and build some of the most impressive skyscrapers in the world today.