Office building in Beijing (China Zun Tower)

Office building in Beijing (China Zun Tower)

Building use

Office building

Country/Region

China

Overview

The structural system adopts a mega brace-frame, reinforced concrete core structural system. The outer mega frame and the inner core are vertical load-bearing members, which bear the structural gravity and transmit it to the foundation. They also serve as two structural lateral members, bearing the role of the entire structure under earthquake and wind loads.

Basic information (construction date, number of stories, gross floor area, adopted design code, engineer(s), Contractor(s), etc.)

Construction date: 2018
Number of stories: 108
Gross floor area: 437,000m2
Adopted design code: GB 50009-2012; GB 50011-2010; GB 50017-2003; GB50010-2010; JGJ 3-2010; JGJ 99-1998; JGJ 138-2001; CECS 159-2004; CECS 28:2012
Engineer(s): Arup; Beijing Institute of Architectural Design
Contractor(s): China Construction Third Engineering Bureau Group Co., Ltd.

Issue and/or innovation

A variety of steel concrete composite components are strategically widely used in steel and concrete components to achieve cost effectiveness in earthquake resistance and increase the available floor area. The use of steel plates embedded in the bottom concrete core wall improves the ductility of traditional concrete walls, significantly improving seismic performance, and the bottom wall thickness is only 1.2 meters. Cost effectiveness is achieved without compromising safety. Giant frames utilize the entire building size to maximize lateral stiffness.”. In order to achieve efficient mega structures, one of the largest column sections used in buildings has been innovatively designed. The outer frame giant columns adopt a box shaped section, which bears most of the floor gravity loads and lateral resistance. The bottom section of the giant column is 60.8 m2, which cannot be achieved with traditional composite structures. After careful design and indoor tests with a 1:12 physical model, it has been proven that the “multi cavity” concrete filled steel tube section is reliable for such large and important columns. Welding tie bars on vertical stiffeners controls the local buckling of the steel plate to achieve a reasonable reinforcement ratio. During construction, the steel structure box can be used as a construction platform without the need for reinforcement fixation and formwork installation. The filling of concrete inside the giant column can reduce the thickness requirements of the fireproof layer to save costs. The maximum size of the column is controlled at 4 meters, reducing the workload of welding length and cross-section. “Multi cavity” column and details have been incorporated into Chinese composite structure design codes and national standards.

Reason for composite solution

China Zun Tower is located in a fortification intensity zone of 8 degrees, with a height of 528m, and is the highest building in the same earthquake intensity zone. The structural system adopts a mega brace-frame, reinforced concrete core structural system. The outer mega frame and the inner core are vertical load-bearing members, which bear the structural gravity and transmit it to the foundation. They also serve as two structural lateral members, bearing the role of the entire structure under earthquake and wind loads. Due to its high fortification intensity and high structural height, the internal force of lateral members under the action of gravity and horizontal forces is very large. Conventional reinforced concrete members need to provide such high bearing capacity and sufficient lateral stiffness, and their cross-sectional dimensions will be far greater than ordinary buildings, which will have a significant impact on the layout of the building, and even from the perspective of functional use is unacceptable. Therefore, in order to take into account the structural stiffness, bearing capacity requirements, and building usage requirements, and to consider the impact of many factors such as engineering cost, late stage enclosure, and construction feasibility, and to achieve the maximum comprehensive economic and technical performance, composite structural components are used for mega columns and core as main lateral resistance components, making full use of the advantages of high compressive strength, high stiffness, and good fire resistance of concrete, As the main material for mega columns and core walls. At the same time, concrete and section steel are combined to form a composite component. Using the characteristics of high tensile strength and good ductility of steel, the overall bearing capacity and ductility of the lateral resistant component are effectively improved, enabling it to obtain higher structural bearing capacity and stiffness with a smaller cross-section, meeting design requirements.

Specific solution/technical details

The mega columns adopts a multi-cavity giant concrete filled steel tube column, with a rectangular cross-sectional dimension. The bottom adopts polygons according to the architectural modeling requirements. Steel plates are arranged inside the giant column to form a multi-cavity cross-section, meeting the overall and local stability requirements of the giant column. The size of the cavity shall fully consider the requirements of component processing and transportation, on-site splicing, steel bar binding, and concrete pouring, as well as the connection details and corresponding structural requirements with the transfer truss and mega brace support. The steel plates of the internal section steel are connected as a whole, and the periphery is wrapped by concrete and steel bars, obtaining enormous tensile, compressive, bending, and shear torsional bearing capacity, and resisting vertical loads and lateral loads generated by wind and earthquake. The core of the tower adopts a steel reinforced concrete shear wall structure containing steel ribs (steel plates). The full height of the core tube concrete material is made of C60 high-strength concrete, which improves the compressive and shear bearing capacity of the components and effectively reduces the dead weight and seismic quality of the structure, while ensuring a certain degree of ductility. In order to further improve the seismic performance of the structure, steel plates are installed in the wall limb at the bottom of the structure to form a composite steel plate shear wall, effectively reducing the thickness of the wall section and the dead weight of the structure, while greatly alleviating the design pressure on the wall due to the requirements of the axial compression ratio limit value; At the top of the structure, with the closure of the outer frame and the mega brace, combined with the results of the elastoplastic analysis, steel plates are also set in the surrounding wall limbs of the part of the core out of the large roof, to improve the shear resistance of the structure and the floor deformation ability under rare earthquakes. In order to resist the tensile action of the concrete core in the upper and middle floors under large and moderate earthquakes and effectively improve the ductility of the wall, shaped steel concealed columns are uniformly arranged in each wall limb. Based on the feedback results from the elastoplastic analysis process, additional steel concealed braces are added between the steel concealed columns within the wall limb around the core in the waist area of the tower, effectively enhancing the seismic performance of the relatively weak parts of the tower under rare earthquakes.

Impact or effectiveness

The composite section of rectangular concrete filled steel tubular columns with multiple cavities is adopted for the frame mega columns, which greatly improves the bearing capacity and stiffness compared to traditional component types. On the premise of meeting the reasonable use function and layout of the building, the design goal of seismic elasticity in compression bending, tension bending, and shear resistance under the action of horizontal loads at angles of 45 degrees and 90 degrees, taking into account adverse working conditions such as vertical earthquakes, has been achieved. The core adopts three combined sectional forms: steel plate concrete shear wall, steel reinforced concrete composite shear wall, and embedded steel brace shear wall, giving full play to the advantages of steel and concrete, making the thickness of the wall at the bottom of the core tube only 1.2m, which can control the design requirement that the axial compression ratio of the bottom reinforcement area does not exceed 0.45. At the same time, the reinforced zone at the bottom of the core wall is designed according to the elasticity of compression bending and tension bending during moderate earthquakes, and the remaining walls are designed according to the non yielding during moderate earthquakes. The shear section condition meets the performance-based design goal of non yielding during large earthquakes. Due to the selection of highly efficient composite structural components, in such a high intensity area, even though the dead weight and horizontal force of super high-rise structures are far higher than those of conventional structures, the section steel components still use more conventional Q345 grade steel, with a maximum steel plate thickness of not more than 60mm, greatly reducing the adverse impact of using special high-strength steel plates and ultra-thick plates on the difficulty of material procurement and the long ordering cycle during the construction stage, and facilitating construction.

References / Technical Papers Content

Cheng, Y., Liu, P., Citerne, D., Liu, Y. b., Ma, C., Yin, C. 2014. “structural design and research of Beijing CBD Core Area Z15 plot China Zun Tower ;Structural parametric design applied in Beijing CBD Core Area Z15 plot China Zun Tower” J. Building Structures. Vol. 44(24): 9-14.