Structural design of high-rise building with tube-in-tube and over-limit in Lanzhou Guofang Hotel?

The following is the related content of the tube-in-tube over-limit high-rise structure design of Lanzhou Guofang Hotel brought by Zhong Da Consulting for your reference.

This paper mainly introduces the structural layout, seismic calculation and analysis, main seismic measures and related structural conceptual design of the cast-in-place reinforced concrete tube-in-tube high-rise building in Lanzhou Guofang Hotel. At the same time, the treatment of several main problems in this engineering design is also discussed and introduced in detail, which can be used as a reference for similar engineering design.

1 project overview

Lanzhou Guofang Hotel is located in Dongfanghong Square in Lanzhou, which is the second phase project of Lanzhou International Expo Center. It is a large super-high-rise public building with five-star hotels, commercial office buildings and shopping malls. The total construction area is about 90,000 m2, which is divided into two parts: the main building and the podium. Among them, the skirt building is a hotel annex, with two floors underground and nine floors above ground, with a total height of 43.4 meters. There are three floors underground in the main building, which are garages and six-level civil air defense and equipment rooms. There are 39 floors above ground, consisting of offices, guest rooms and public rooms. 1-9 is 4.8m high, the standard floors are 3. 1m and 3. 15m, and the 39th floor is a revolving restaurant with mezzanine, with the height of 18m. There is a three-story tower on the roof, with a total height of 16 1.9m, which is the tallest building in downtown Lanzhou (see figure 1). The seismic fortification intensity of this project is 8 degrees, which belongs to a high-rise building exceeding the current code height. In accordance with the requirements of Decree No.59 of the Ministry of Construction, the national and provincial seismic fortification review committees have successively passed the special review of the preliminary design and construction drawings. Seismic joints are adopted between the main building and the annex building of this project. Due to the limitation of space, this paper only introduces the structural design of the main building.

2 Foundation and foundation

2. 1 Selection of foundation bearing stratum

The stratum structure of the site is simple and stable, and the construction site is classified as Class II. The distribution of each layer from top to bottom is as follows:

1. Miscellaneous fill: 0.9~2.5m thick, mainly silty soil.

2. Pebble layer: the top surface is about 0.9~2.5m deep and 4.7~9.7m thick.

3. Sandstone layer: The buried depth of the top surface is 7. 1~ 12.5m, which is unexposed, and the surface layer is about 9m strongly weathered. The standard value of foundation bearing capacity fk=500kPa, E0=45MPa, the surface layer has a certain slope from west to east, and the lower part is slightly weathered, E0=85Mpa.

There are three underground floors of the main building, and the strongly differentiated sandstone layer will become the basic bearing layer. This kind of sandstone has a great feature of high bearing capacity and high undisturbed deformation modulus, but it is greatly reduced when it meets water in air. It is a very common and successful practice to use this strongly differentiated sandstone layer as the bearing layer in buildings with a height of1000 m in Lanzhou area. However, whether a building with a height of 144m and a design foundation pressure of 800kPa can be loaded on natural foundation has become the most concerned issue in foundation selection. Through the following measures, analysis and calculation, and after many expert argumentation, it is considered that the natural foundation scheme is feasible.

1. Increase the buried depth of foundation, and the elevation of basement is about-15m. On the one hand, it can improve the overturning resistance of the superstructure; On the other hand, the weathered layer with the worst performance of 2~3m on the surface of strongly differentiated sandstone layer can be eliminated, so as to avoid that half of the basement of the main building is placed on the surface of sandstone layer and the other half is already at a certain depth due to the slope of the top surface of strongly differentiated sandstone layer. The foundation should be located on well-differentiated sandstone with uniform and good performance as far as possible.

2. Because of the deep buried depth of foundation, the design value of foundation bearing capacity after depth and width correction can meet the requirements of foundation pressure. The checking calculation of foundation deformation also shows that the final settlement is very small, about 55 mm In addition, because the differential settlement caused by different weathering degrees of sandstone within 2m below the basement is small, there is no overall inclination problem, so the bearing capacity and deformation of foundation meet the requirements.

3. The in-situ load test after foundation pit excavation proves that the standard value of foundation bearing capacity adopted in the design is reliable. In the data of five test points, except for two points, the bearing capacity of sandstone under the load plate is extremely low due to poor test conditions, and the bearing capacity of the other three points is very high, and it can continue to be loaded when it is loaded to 1900kPa, and there is no proportional limit on the p~s curve.

2.2 Basic selection

According to the upper load and underground function, the box foundation with greater rigidity (three floors underground) is selected. Between the inner and outer cylinders, a wall with a thickness of 800mm is set at the column grid position, and the thickness of the foundation slab is 1200mm. In order to increase the area of the bottom of the foundation, reduce the pressure on the foundation, and make the axial force of the outer cylindrical column more evenly dispersed on the foundation, the bottom plate and wall of the box foundation are selected at the position of 3 meters from the outer cylindrical column.

The internal force analysis of foundation is influenced by foundation model, foundation model, superstructure stiffness and various parameter conditions, which often makes the results of different calculation methods more or less different. In this project, the simplified manual calculation is combined with the elastic foundation beam method of professional software BOX to calculate the internal forces of the foundation under various models, and the design internal forces are determined after comprehensive analysis.

The main building and the podium are separated by seismic joints on the ground, and the underground is connected into a whole, and post-pouring belt is set to solve the problem of uneven settlement between them.

3 Superstructure

3. 1 structural selection and layout

3. 1. 1 structural selection

The main body of this project is144m high, with regular layout and close to the square. Through technical and economic analysis, the cast-in-place reinforced concrete tube-in-tube structure is adopted. The inner tube is a shear wall (the plane length and width are 2 1m and 12.6m respectively), and the outer tube is composed of high skirt beams with dense columns (the plane length and width are 45m and 36.6m respectively). In addition, combined with the building function, some shear walls are arranged at the four corners of the outer tube to enhance the torsional stiffness of the structure and the overall bending capacity of the outer tube. See Figure 2 for the structural layout of standard floors.

3. 1.2 body shape parameters

In order to ensure the spatial mechanical properties and bearing capacity of the tube-in-tube structure in this project, the parameters in the design should be controlled within a reasonable range as far as possible on the premise of meeting the architectural functions. Among them, the plane aspect ratio of the structure is 1.23, the structural aspect ratio is 3.9 in the short direction and 3.2 in the long direction, and the inner tube aspect ratio is 12.5 in the long direction and 7.5 in the short direction, and the inner tube area accounts for 16% of the building area. The column spacing of the outer tube is 4.2m, the opening rate of the standard layer elevation of the outer tube is 39%, the aspect ratio of the hole is 0.64, and the ratio of the height to the column spacing is 0.74, which is basically close.

3. 1.3 Main structural members

In order to meet the requirements of building entrance, the outer tube column at the bottom floor is11100 mm square column, and the second floor to the top floor is changed from 1400x800mm to 1400x600 flat column.

The outer wall of the inner tube is 500mm thick at the bottom and 400mm thick at the top. The inner partition wall is 300mm up and down. In order to improve the ultimate bearing capacity and ductility of the wall, for shear walls with a thickness of not less than 400mm, concealed beams with a height of 700mm are set on each floor.

The cross section of the 4.8m-high bottom frame tube beam is mostly 500x 1600mm, and that of the 3. 1m-high standard layer is mostly 500x 1300mm. The cross section of the coupling beam of the inner tube at the bottom is mostly x 1600mm, and the cross section of the standard layer is mostly x1000 mm..

The entrance of the south first floor and the entrance of the underground garage need to form a larger column spacing space, so a column is removed from the bottom of this part and a transfer beam with a high floor is set on the second floor. Adjust the height of the beam in the equipment floor with higher height from the top tower and the middle floor to reduce the sudden change of floor stiffness.

The strength grade of concrete gradually transits from C60 at the bottom to C35 at the 38th floor and C40 above the revolving restaurant.

3.2 floor selection

The axial span of the inner and outer cylinders in this project is 12m, and the choice of floor has great influence on the use function, structural rationality and economy. Therefore, various schemes are compared:

1. Column is added between the inner and outer tubes to reduce the floor span, and ordinary reinforced concrete beam-slab system is adopted. This scheme is simple in process, light in weight, good in economic index and convenient in design and construction, but its architectural function is poor, so it is rarely used in tube-in-tube and frame-tube structures in China at present.

2. Prestressed beams with a spacing of 4.2m are used between the inner and outer cylinders on the column axis. Because of the small span, ordinary reinforced concrete slab can be used for the floor between beams. The scheme has light floor weight and good economic index, but the height of beam affects the floor clearance, which requires a larger floor height.

3. There is no beam between the inner and outer cylinders, and a "ring" prestressed concrete flat plate is adopted. This scheme has the smallest structural height and large clearance. Under the condition that the total height of the building remains unchanged, it has the largest number of floors and building area, the bottom of the slab is flat and beautiful, which is beneficial to the passage of pipelines and the formwork system is simple. But the floor is heavy, the internal force is complex, and the economic index of the floor is not ideal.

As the owner requires as many floors as possible under the condition of limited total height of the building, after comparison of various schemes, the office and guest room floors of 1 and 10~39 adopt prestressed concrete slab structure (the third scheme mentioned above) with a thickness of 270mm；; 2 ~ 9 floors of public houses are connected with the first phase of the project, and the floors are relatively large. Prestressed beam and ordinary reinforced concrete slab structure (the second scheme mentioned above) are adopted, and the beam section is 400× 650 mm. ..

3.3 Structural Calculation and Analysis

3.3. 1 structural calculation

1. Use SATWE and TAT in PKPM series software for comparative analysis and calculation. Because of their regular structure, the overall calculation results are close. However, the shear wall calculation models of SATWE and TAT are different, and the local internal force and reinforcement of the wall are somewhat different. In the design of construction drawing, the calculation results of SATWE wall element model which is more in line with the actual situation are adopted. The following are the main analysis results considering the translation-torsion coupling effect.

1) Period of spatial vibration mode: t1= 2.41(translation coefficient in Y direction is1.0); T2=2. 10(X translation coefficient1.0); T3= 1.04 (torsion coefficient 1.0).

2) Under the action of earthquake in X direction; The maximum interlayer displacement angle is1/2328; The bottom shear weight ratio is 2.85%.

3) Under the action of Y-direction earthquake, the maximum interlayer displacement angle is11760; The bottom shear weight ratio is 2.74%.

4) The ratio of the maximum interlayer displacement to the average interlayer displacement is within 1.05.

2. The supplementary calculation of elastic time history analysis under frequent earthquakes is carried out. Two groups of measured waves and one group of site simulated waves provided by Seismological Bureau of Gansu Province are selected, and the maximum value of seismic acceleration time-history curve is 70cm/s2. The calculation results show that the average value of all time history analysis results is less than that of CQC method considering coupling effect.

3.3.2 Analysis of calculation results

1. From the above calculation results, it can be seen that the period and displacement of the structure are within a reasonable range, but the shear weight of the bottom of the structure is relatively small, which is mainly determined by the long-period dynamic characteristics of the structure itself and the characteristics of modal decomposition response spectrum method, and the estimation of earthquake action may be low. From the economic point of view, it is unreasonable to increase the stiffness and improve the earthquake action by increasing the size of components. Therefore, the seismic action of this project is multiplied by the amplification factor 1.25, so that the shear-weight ratio at the bottom of the structure is not less than 3.2%. The structural displacement and other parameters after the enlarged earthquake meet the requirements of the code, and the seismic effect of the enlarged structure is basically not less than the elastic time-history analysis results, which shows that the structural design based on the results of modal decomposition response spectrum method is reliable.

2. The period of the first torsional vibration mode is less than 0.5 times that of the first and second translational vibration modes, and the translation-torsion coupling effect of the structure is small. In addition, the ratio of the maximum inter-story displacement to the average inter-story displacement is within 1.05, which shows that the structure is regular and symmetrical and has high torsional stiffness.

3. In the design, because the outer tube has a more rigid skirt beam and some shear walls are arranged at the corners, the overall bending resistance of the outer tube is greatly enhanced, and the working characteristics of the outer tube as a three-dimensional component are obvious and the shear lag is small. Under the action of horizontal force, the overall bending moment formed by the axial force of the outer tube column and the wall bears about 65% of the total overturning moment at the bottom, the ratio of the stress of the corner wall to the stress of the column in the flange frame is less than 2, and the inner tube wall and the outer tube corner wall bear about 90% of the total shear force on average, so there is basically no situation that the tendons of each component exceed the limit.

4. Under the action of horizontal force, the floor displacement curve of tube-in-tube structure with uniform vertical stiffness is generally inverted S-shaped, and the inflection point of the maximum story displacement angle is generally in the middle and upper part of the building height. This project conforms to this law under the revolving restaurant, but there is another anti-bending point with large interlayer displacement angle above the revolving restaurant, which shows that the interruption of the 39-story outer frame tube greatly weakens the structural stiffness and forms a weak position.

3.4 Main seismic measures

As mentioned above, the height of tube-in-tube in this project is 144m, which has exceeded 20% of the maximum applicable height specified in the code, and there are many problems of height exceeding the limit. In addition, due to the functional requirements of the building, there is a weak layer problem caused by the sudden change of vertical stiffness. In view of these two outstanding problems, this project pays attention to structural conceptual design and adopts some comprehensive seismic strengthening measures. The analysis of seismic calculation results also shows that these measures are necessary and effective.

1. On the premise of meeting the functional requirements of the building, the main building and the podium should be stitched separately. That is, it avoids the sudden change of vertical stiffness of the structure caused by the integration of the main podium, and also avoids the huge eccentricity of the super-high main building to the nine-story podium and the complexity of the structure after the integration. Make the plane layout and vertical layout of the tube-in-tube structure of the main building meet the requirements of regularity as much as possible.

2. Combined with the architectural layout of the four corners of the building, some walls are set to increase the structural stiffness, so that it can bear the corresponding internal force under the horizontal force and reduce the maximum internal force of the inner tube and the outer column. This effect has been analyzed in 3.3.2.

3. Choose reliable bearing stratum and increase the buried depth of foundation, so as to ensure the overturning resistance of super high-rise buildings.

4. Carry out special site safety evaluation to ensure the reliability of site ground motion parameters.

5. In the seismic checking calculation of elastic stage, two different calculation models are used for comparative analysis, and at the same time, two groups of measured waves and one group of site artificial waves are selected for supplementary calculation of elastic dynamic time history analysis.

6. Increase the ground floor shear-weight ratio under earthquake action, improve the structural reliability, and make the vertical and horizontal shear-weight ratio not less than 3.2%.

7. Take more stringent seismic measures than the design code to further improve the deformation capacity and energy dissipation performance of the structure.

8. Major weaknesses have been strengthened:

1), after the local columns at the bottom layer are extracted to form a space with large column spacing, measures are taken to strictly control the axial compression ratio of the wall columns at the bottom layer, improve the reinforcement ratio of the wall columns, and adopt core columns for the outer tube columns. At the same time, the frame-supported beam is calculated and analyzed according to the deep beam model, and the reinforcement ratio and stirrup ratio of the frame-supported beam are improved, and the structural measures are strengthened to ensure its sufficient safety.

2) The measures to strengthen the weak layer of the top-floor revolving restaurant are introduced in detail in 4.2 of this paper.

9. In cooperation with Lanzhou Railway Institute, the elastic-plastic time history analysis of the project is being carried out to check the displacement value under rare earthquake, further determine the weak parts, and summarize the mechanical characteristics of tube-in-tube structure under rare earthquake.

4 Discussion and handling of several problems

4. 1 Influence of floor between inner and outer tube on structural stiffness

In this project, in order to reduce the structural height of the floor as much as possible, the flat structure of 12m is adopted between the inner and outer tubes without beams, so the influence of beams and no beams between the inner and outer tubes on the structural stiffness becomes a problem that needs to be paid attention to in the design. To this end, a lot of calculations have been made when determining the scheme. The results show that, under the premise of the same floor structure weight, the influence of setting beams and not setting beams on the structural stiffness is very small, about 2%~5%. This is mainly because in the tube-in-tube structure, the inner and outer tubes are the main lateral force resisting members, and the stiffness is relatively large, so the restraint effect of the beam on the inner and outer tubes is relatively small. For the frame-tube structure, only four frames can be formed by sparse columns on the periphery, and the stiffness is relatively small. At this time, the cross beam connecting the inner tube and the outer frame has great constraints on the inner tube and the outer frame. The analysis results of several typical plane frame tubes show that the influence of setting beams and not setting beams on the lateral displacement of the structure can reach 20~30%.

4.2 Calculation and analysis of prestressed floor slab

Because there is no beam between the inner tube and the outer tube, the standard layer forms a "ring" plate, which is not a simple one-way plate, and the internal force analysis is complicated. In this project, simplified one-way bar (equivalent frame) model and plane finite element (plate element) model are used to compare and analyze the floor. Among them, the equivalent frame can directly use PREC professional prestressed concrete structure design software. In order to ensure that the internal force calculation is not affected by secondary factors and the design process is clearly controlled, the layered method is used for analysis under vertical load and prestress load, and the spatial analysis method is used for calculation under horizontal wind load and earthquake, and then the combination is made. The prestressed tendons are in the shape of four parabolas, and the corners are obliquely arranged, and the balanced load is about 80% of the dead load.

Through the finite element analysis of the floor, the stress peak will appear at the joint of the floor and the column under the vertical and horizontal loads for the tube-in-tube floor without beam. Therefore, in the design, in addition to the uniform discharge of prestressed steel bars, ordinary steel bars are concentrated on the axis of the column, forming a hidden beam. This arrangement has the following advantages: 1) It is convenient for single anchor tensioning of prestressed tendons at the end, and reduces the adverse influence of the cavity mode at the end of tensioning on the column joint area; 2) ordinary steel bars can be used to effectively control the cracks at the stress peak; 3) The bending moment under horizontal earthquake can be mainly borne by ordinary steel bars with good ductility.

4.3 Design Features of Top Rotating Restaurant

The 39th floor of the main building is an annular cantilever revolving restaurant with mezzanine, with a height of 10m (see Figure 3). Due to the requirements of building function, the structural design of this floor is difficult and has certain characteristics.

1. In order to form a wide view of the dining room, the outer frame tube was completely interrupted on the 39th floor, and only eight pillars were extended at four corners to support the roof. For the whole structure, due to the interruption of the peripheral frame tube, the bearing capacity and stiffness of this floor are greatly reduced, and the deformation of the floor is increased, forming a weak floor. This is analyzed in Section 3.3.2 of this paper. Therefore, the following strengthening measures are taken in the design to improve the ultimate bearing capacity and deformation capacity of this layer: 1) The strength grade of this layer of concrete is increased from C35 to C40；; 2) Increase the distribution reinforcement of inner tube wall and reinforcement of concealed column and concealed beam, which is about 70% higher than that of the lower layer. 3) Eight columns at the periphery adopt steel reinforced concrete columns.

2. The revolving dining room adopts cantilever structure, and the maximum length of the floor beam is 8m from the outer tube, and a circle of columns is supported in the span to support the top five floors, which has the problem of floor transfer and is heavily stressed. Therefore, in the design: add 1) ring beam at the bottom of 39-story column to strengthen the embedding and integrity of the bottom of each column; 2) The floor beam adopts 650x 1400mm steel reinforced concrete beam, considering the vertical earthquake action. The steel bars in the beam are hinged with the inner cylinder to facilitate the construction, but a sufficient number of ordinary steel bars are provided to resist the negative bending moment at the support of the inner cylinder. 3) The floor thickness is increased to 150mm, and double-layer bidirectional reinforcement is adopted.

3. The roof of the revolving restaurant is made of light and long-span plate ring net, with the inner side supported on cylindrical corbel and the outer side supported on eight steel reinforced concrete columns. Due to the lack of external support, large free boundary, complex stress of grid structure and large internal force of members in four corners, it is strengthened in concrete design. At the same time, the influence of vertical earthquake action is considered in the calculation of plate-ring grid structure.

This article was written by Rui Huang and Jin Jianmin. The design of this project has been reviewed and guided by experts inside and outside the province, such as President Xu and Chief Engineer Mo Yong, from the national and provincial seismic fortification evaluation committees for over-code high-rise buildings. Thank you.

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