Office building in Beijing (Li Ze SOHO)

Office building in Beijing (Li Ze SOHO)

Building use

Office building

Country/Region

China

Overview

Due to the large number of floors and spans and the load-bearing area of the columns, circular steel pipe concrete columns are used in several layers of the main structure.

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

Construction date: 2019
Number of stories: 45
Gross floor area: 172,800m2
Adopted design code: GB 50009-2012; GB 50011-2010; JGJ 3-2010; GB50017-2003; CECS 28:2012; CECS 230-2008; GB50045－95(2005); GB 50010-2010; GB 50007-2011
Engineer(s): Beijing Institute of Architectural Design Co., Ltd. and China Academy of Building Research
Contractor(s): China Construction Eighth Engineering Division Corp., Ltd.

Issue and/or innovation

In this project, the floors on both sides of the atrium spirally cantilever outward gradually. The angle between each floor slab beam and the inner steel of the core at the corner node position is different. Instead of setting studs directly on the steel, the connection between the profiled steel section in the core wall and the floor steel beam is realized by connecting the embedded steel plate with the profiled steel through a connecting plate, and then connecting the floor steel beam to the embedded steel plate. Using an embedded steel plate can solve the connection problem between the steel beams and the core tube at different angles in a unified way, without affecting the climbing formwork construction of the core tube. The use of connecting plates conflicts with the horizontal hoop reinforcement of the core tube, so embedded steel plates are used instead of the original broken hoop reinforcement in the core area to enhance the overall integrity of the joint in the core area. Considering the fact that the core tube and connecting beams are poured as a whole during actual construction, the steel skeleton of the connecting beam is directly inserted into the steel column of the core tube, with a depth not less than 1 times the height of the connecting beam steel skeleton, and the inserted part is anchored by bolts at the web and flange. Since the steel skeleton mainly bears the shear force on the web, bolts are only set on the web within the range of the connecting beam. The steel flanges of the connecting beam are gradually and inwardly recessed, and the width of the flange is ensured to allow the longitudinal steel bars to pass through after entering the hidden column to avoid opening holes. This method, while satisfying the force transmission requirements at the node, is economical, simple in detailing and easy to construct.

Reason for composite solution

This project is a high-rise structure with irregular shape, and the steel frame-concrete core-tube system is generally used. Due to the large number of floors and spans and the load-bearing area of the columns, circular steel pipe concrete columns are used in several layers of the main structure. The performance target of the core tube’s shear wall is unyielding under intermediate earthquakes, and the wall is heavily reinforced with steel bars. In addition, profiled steels are arranged within the edge components. Some of the core tube’s shear walls and beams exceed the shear limit in the code, requiring the arrangement of profiled steels.

Specific solution/technical details

The subway linkage line runs through the entire project site from northwest to southeast. Two streamlined towers are located on either side of the subway linkage line, spiraling up like DNA double helix structures. Each level rotates gradually from the first floor to 45 degrees. The two towers surround an indoor atrium approximately 200 meters high, forming a complex streamlined architectural form. The structural establishment of this project must address two inherent weaknesses of single towers: 1. The insufficient Y-axis stiffness due to single-side frame columns; 2. The structural horizontal torsion caused by plane rotation and large cantilevers. The solution to problem 1 is to use a series of curved steel bridges and waist trusses with a span of 9-38m at levels 13, 24, 35 (equipment floors), and the top floors, installed between the two single-tower arc frames, to bind the two towers together. The solution to problem 2 mainly involves comparing the circular steel pipe concrete frame-reinforced concrete core tube structural system and the steel frame-support core tube structural system in the scheme phase. Although the latter can reduce the structural self-weight and the impact of large cantilevers, compared to the former, the reinforced concrete core tube has greater stiffness, and better torsional displacements and displacement angle results. Therefore, this project adopted the circular steel pipe concrete frame-reinforced concrete core tube structural system. The full-height atrium allows the two single towers to be connected into one whole structure with only four bridge links. The bridges play a key role in ensuring the integrity of the entire structure, and their seismic performance target was raised to withstand a severe earthquake. Nine analytical models were established to analyze the impact of different bridge locations on the overall structure. To ensure the structural integrity, closed ring-girder systems were formed with the bridges’ outside surfaces and waist trusses at the floors with steel bridges. Considering the functional requirements for the building space, no trusses were added after connecting the inside faces of the steel bridges with the frame columns. Instead, the horizontal forces were transferred to the corner of the core tube through the floor beams, which were designed with performance target of maintain elastic under intermediate earthquakes and not collapse under severe earthquakes. The cantilever at the top of the building affects approximately 20 floors, with the cantilevered part accounting for approximately 5% of the total floor area on those levels. Since the total mass and stiffness of the floors were concentrated in the core tube of the single tower, the ratio of the total mass of the cantilevered part to the total mass of the floors was lower. Therefore, this giant cantilever can be seen as a “giant bracket” extending from the core tube system of the single tower, mainly supported by the forked column at the corners. The horizontal force is borne by the ring girder system composed of the floor beams, bridge links, and waist trusses. In the design of the cantilevered structure, the tension that may occur in the cantilevered part was calculated based on a zero-stiffness floor model, and the vertical seismic effect was taken into account to examine the force on the related steel beams, supporting frame columns, and wall limbs. To avoid progressive collapse of the cantilevered part, leading to extensive chain destruction, the analysis of anti-continuous collapse was conducted. The irregular grid mixed structure requires significant effort in joint processing, covering issues such as multi-angle oblique crossing and small-angle intersection. In this project, the bridge links the two single towers together to ensure that they work in coordination. The forked column (Y-shaped columns) bears the vertical transmission force of the top 20m large cantilevered structure. Both of them are considered to be key components that have a significant impact on the overall safety of the structure. Their joint detail is complex and withstand significant force, which therefore, requiring a specialized node force analysis. Unlike conventional projects, this project has anti-symmetric structures, and the plane deformation of the structure occurs under its own weight. In the design process, construction simulation were conducted to analyze the possible torsional displacement and its impact on structural safety when conventional construction measures are taken. Since the subway linkage line has few daily vehicles and a low impact on the upper structure’s vibration, and the method of combined construction is simple and reliable, the main structure of the project is integrated with the subway linkage tunnel. To ensure the normal operation of the subway, strict control of the later-stage settlement of the main structure was necessary. In the foundation design, a detailed settlement control analysis was conducted, and the settlement monitoring was carried out throughout the construction, which showed good consistency.

Impact or effectiveness

Li Ze SOHO is a complex twin-tower structure, with each individual tower’s levels spiraling around the atrium and the outer frame columns all being curved, forming a symmetrical and complex twin-tower structural system with a height of 191.5 meters. The structural design team utilized their abundant experience in designing ultra-high-rise structures to perfectly realize the architectural intent. The BIAD design team creatively optimized the geometric and structural logic in the ZHA concept through their high level of control over non-linear complex architectural forms and rich experience in original projects. Facing of extremely special site foundation conditions and demanding spatial requirements, they tackled technical challenges one by one through comprehensive coordination of multiple disciplines such as structure and MEP, transforming the building into a new public space in the city and turning “creativity” into “reality.” Through precise design in aspects such as building function, structure, electro-mechanics, fire safety, and energy control, this building is not only a city landmark with an “eyeball effect,” but also an important urban functional anchor that adapts to Beijing’s future high-quality urban development, supports the positioning of Fengtai Financial Business District, and effectively connects the city center with the Daxing Airport area.