1 project overview

this project is located in xiangluowan business district, the central business district of Tianjin jinbin new district, between changbei road, Xichang road and Binhe road. The seismic fortification intensity is 7 degrees, and the design basic seismic acceleration is 1.15g, which is Class III site soil. This project includes 177.3m(46 floors) of Block A and 99.6m((25 floors) of Block B, with 3 floors of basement. Blocks A and B are completely separated above the ground (floor 1) and the basements are connected together. Because the size of Block B of this project is regular, and the height is 99.71m, Block B of this project is within the limit. This article only introduces the related situation of Block A.. Tower A of this building has 46 floors, of which floors 2-4 are lobby, office, a small number of businesses and restaurants, floors 5-15 are hotels, floors 16 and 32 are refuge floors, floors 17-39 are hotel-style apartments, and floors 41-46 are offices. Among them, the height of floor 1 is 5.5m, that of floors 2 and 4 is 4.15m, that of floors 3 and 5-25 is 3.9m, that of floors 26-39 is 3.6m, and that of floors 41 and above is 3.9m..

2 structural layout and selection

The plane of Block A of this project is an inverted trapezoid with hypotenuse, and the height-width ratio of the building is about 5.96. A large core tube is arranged in the middle of the whole plane, one core tube is arranged on the left and right sides of the large core tube, a hollowed atrium is arranged between the two core tubes, and a local shear wall (small tube) is arranged between the two core tubes and the lower part of the core tube to enhance the overall structural rigidity. According to the building plan, the steel tube composite column frame-core tube structure is proposed for this project. The core tube is the first line of defense of lateral force resisting system. In order to enhance the seismic performance of the core tube, I-beams are installed in the key shear walls and the edge members of some shear walls of the core tube. The steel tube composite column frame forms the second lateral force resisting system of the structure. After comprehensive comparison, it is proposed to adopt ordinary reinforced concrete beams in the selection of frame beams.

the structural design is based on the principle of simplicity first, and on the premise of meeting the expected performance target, the sudden change of stiffness and strength of local floors is avoided as much as possible to avoid the generation of structural weak layers [1]. Through analysis and comparison, there is no rigid strengthening layer (Outriggers) in this project, and the so-called finite stiffness strengthening layer is formed only in the refuge layer by increasing the beam section. The building height of Block A of this project is 177.3m According to Article 5.1.6 of Technical Specification for Concrete-Filled Steel Tubular Composite Column Structures (CECS188:2115), the height of this building is less than the frame-core tube height limit of 181m. Therefore, Building A does not belong to the height overrun. According to the calculation, the buildings A and B in this project belong to torsion irregularity after considering accidental eccentricity, but there is no torsion overrun (the ratio of torsional displacement of floors is less than 1.41), and there is no plane irregularity and vertical irregularity in this project. There is an atrium in the middle of the standard floor, in which the atrium of Block A is about 112m2, and the atrium of Block B is about 125m2, and the comprehensive opening area is less than 31%. The opening area and effective floor width meet the requirements of the high code for floor opening, and there are no other irregularities.

3 Performance-based seismic analysis of structures

According to the characteristics of this project, when calculating and analyzing the structure, in addition to considering the conventional vertical load, small earthquake action and wind load along the wind direction, the performance-based seismic design method is adopted to analyze the yield of the structure under the action of moderate earthquake and the elastic-plastic time history analysis under the action of rare earthquake, so that the structure can meet the requirements of the code.

3.1 overall structural analysis

this project uses the structural analysis program SATWE》(2116/16 edition), which is compiled by PK.PMCAD Engineering Department of China Institute of Building Research, for structural analysis. In order to reflect the rationality of the index, Block A and Block B are calculated separately, and the overall calculation is used for internal force and reinforcement calculation. Because of the high height of Building A, as a comparative analysis, MIDAS (version 731) software from South Korea is used as a supplement.

3.2 time history analysis of frequent earthquakes

according to the requirements of article 5.1.2 of the code for seismic design of buildings (GB 51111-2111), the time history analysis of frequent earthquakes should be carried out in Block A of this project. The earthquake time history of this project adopts 2 natural waves and 1 artificial waves (7 degree, Class III site), and the maximum acceleration of frequent earthquakes is 55cm/s2. The calculation results show that all three waves meet the requirements of not less than 65% of the results calculated by the mode decomposition reflection spectrum method, and the average values of the three waves are 84.8% (1) and 87.7% (91) of the results calculated by the mode decomposition reflection spectrum method. It basically meets the requirements of wave selection in seismic code. According to the results of time-history analysis, the seismic shear force in about 1/3 of the upper part of the structure is slightly larger than the calculation result of mode decomposition reflection spectrum method. In the construction drawing design, the calculation result of response spectrum in this range should be appropriately enlarged.

4 elastic analysis under moderate earthquake

Considering that the vertical member of this project is a very important member in the earthquake resistance of the whole structure, the elastic checking calculation under the action of moderate earthquake is carried out to determine whether it has reached the seismic performance target of moderate earthquake elasticity. That is, on the basis of the calculation of non-yielding under moderate earthquake, the partial load coefficient is restored to the normal value, the strength of materials is taken as the design value, and the adjustment coefficient of seismic bearing capacity is taken as 1.1, regardless of the internal force amplification adjustment under earthquake action and wind load. At this time, the combined effect of seismic action of members is not greater than the seismic bearing capacity calculated according to the strength design value. The checking results show that the vertical members do not yield under the action of moderate earthquake, and are basically in the elastic stage, and there will be no plastic damage.

5 dynamic elastoplastic time-history analysis under the action of rare earthquakes

EPDA program of SATWE series software is adopted for nonlinear time-history analysis of this project, and 1 artificial waves and 2 artificial waves * * 3 seismic waves provided by the First Monitoring Center of China Seismological Bureau are used for ground motion input of this project. The maximum acceleration is 311cm/s2(1.31g), and the damping ratio is 1.14. Figures 2 and 3 give the story displacement angles in X and Y directions in elastic-plastic time history analysis. The calculation results show that under the action of rare earthquakes, the maximum story drift angle in X direction is 1/188, and the maximum story drift angle in Y direction is 1/123. All meet the limit requirements of 1/111. The calculation results show that the maximum harmful displacement angle between floors 21 and 39 is relatively large, which indicates that this part is a weak position. It is planned to strengthen this part in the design. Under the rare earthquake, the maximum bottom shear force in X direction is 126164kN, which is about 1.1% of the representative value of gravity load. The maximum bottom shear force in Y direction is 131127kN, which is about 1.6% of the representative value of gravity load, and 4.85 times and 4.62 times of the bottom shear force calculated by frequent earthquake response spectrum respectively.

6 structural seismic strengthening measures

Through the elastic and plastic analysis of this project, the structural system of this project is reasonable, the stiffness and bearing capacity are evenly distributed, and it has multiple lines of defense, which can meet the expected goals of performance design. In view of the fact that this project is close to the height overrun, it is proposed to adopt the following technical conditions and strengthening measures in the construction drawing design to meet the expected objectives of performance design. 1) Strengthening measures for frame columns and shear walls. Because the height of this project is close to the code limit, and the seismic fortification intensity is 7.5 degrees, which belongs to Class III soil area, the structure design of frame columns and shear walls is controlled according to the special level. In order to strengthen the seismic performance of the core tube, especially the improvement of the shear bearing capacity, and avoid the shear failure of the shear wall, the core tube shear wall adopts the steel tube composite shear wall. For the relevant specifications, see Technical Specification for Concrete Filled Steel Tube Composite Column Structure, the axial compression ratio of the frame column is not more than 1.65, and the axial compression ratio of the shear wall under the representative value of gravity load is ≤1.45.

the vertical members are designed according to the principle that they will not yield under moderate earthquakes; The shear capacity of horizontal members is also designed according to the design of not yielding under moderate earthquake. Strengthen the reinforcement of walls and columns appropriately, and control the structures of walls and columns according to the special level. The minimum reinforcement ratio of the distribution reinforcement of the shear wall in the bottom strengthening area is 1.4, and the minimum reinforcement ratio of the shear wall in other parts is 1.3, so as to ensure that the shear wall does not have shear hinge under the rare earthquake and has good ductility. The hoop index of concrete filled steel tubular column is ≥1.61, and the tube content is ≥4%. 2) Enhance the structural rigidity. Increase the height of the main girder in the refuge floor to form a limited strengthening layer, enhance the overall stiffness of the whole building, and appropriately thicken the floor slab and slab reinforcement to strengthen the reliable transmission of horizontal shear force under earthquake action. 3) Due to local opening, check the tensile and shear strength of the weak position of the floor slab under the rare earthquake and ensure that it meets the strength requirements (the partial load coefficient is 1.1 when checking, and the material strength is the standard value), so as to ensure that the floor slab can still be used as a rigid partition to reliably transfer horizontal shear under the rare earthquake.

7 Conclusion

Based on the structural design of a super B-level building, this paper systematically discusses the structural layout of the project and the corresponding calculation and analysis results of moderate and strong earthquakes. The analysis results show that the structural system of this project is reasonable, the stiffness and bearing capacity are evenly distributed, and there are many lines of defense, which can meet the expected goals of performance design. At the same time, in view of the fact that this project is close to the height overrun, the strengthening measures of the overrun structure are put forward for the key components of the structure, which can provide reference examples for similar projects.

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