Since the reform and opening up in China, the construction industry has developed by leaps and bounds. In recent ten years, China has built 1 10,000 high-rise buildings with a building area of 200 million square meters. Among them, the representative buildings, such as Shenzhen Wang Di Building, have 8 1 floor and the height is 325m. Guangzhou Zhongtian Plaza, 80 floors, 322 meters high; Shanghai Jinmao Tower has 88 floors and is 420.5 meters high. In addition, Nanning has also built the tallest building: Wang Di International Chamber of Commerce Center Wang Di Building, which is 54 stories high and 206.3 meters high. With the acceleration of urbanization, high-rise buildings have sprung up all over the country. As civil engineering designers, we must fully understand the structural design characteristics and structural system of high-rise buildings. Only in this way can the design achieve the basic principles of advanced technology, economic rationality, safety and applicability, and quality assurance.
First, the characteristics of high-rise building structure design
Compared with low-rise and multi-storey buildings, the structural design of high-rise buildings occupies a more important position in all disciplines. The choice of different structural systems is directly related to the layout of building plane, facade modeling, floor height, the setting of electromechanical pipes, the requirements of construction technology, the length of construction period and the level of investment cost. Its main features are:
(a) Horizontal force is the main design factor.
In low-rise and multi-storey building structures, the vertical load represented by gravity often controls the structural design. In high-rise buildings, although vertical load still has an important influence on structural design, horizontal load plays a decisive role. Because the values of axial force and bending moment caused by building self-weight and floor load in vertical members are only proportional to the first power of building height; However, the overturning moment caused by horizontal load on the structure and the axial force caused by vertical members are proportional to the square of building height. On the other hand, for buildings with a certain height, the vertical load is usually constant, while the values of wind load and earthquake action as horizontal load vary greatly with the different structural dynamics.
(B) the lateral displacement becomes the control index
Different from low-rise or multi-storey buildings, structural lateral displacement has become a key factor in the design of high-rise structures. With the increase of building height, the lateral deformation of the structure under horizontal load increases rapidly, which is proportional to the fourth power of building height h (△ = qh4/8ei).
In addition, with the increase of height, the application of lightweight and high-strength materials, the emergence of new architectural forms and structural systems, and the rapid increase of lateral displacement, the design of high-rise buildings requires not only sufficient strength, but also sufficient anti-push stiffness to control the lateral displacement of the structure under horizontal load within a certain range, otherwise the following situations will occur:
1. Lateral displacement will produce larger additional internal force, especially for vertical members. When the lateral displacement increases, the eccentricity will increase. When the additional internal force exceeds a certain value, the house will collapse.
2. Make residents feel uncomfortable or panic.
3. The filler wall or building decoration is cracked or damaged, the mechanical and electrical equipment pipeline is damaged, and the elevator track is deformed, resulting in abnormal operation.
4. The main structural members are cracked or even damaged.
(C) seismic design requirements are higher
The structural design of seismic fortification high-rise buildings should not only consider the vertical load and wind load in normal use, but also make the structure have good seismic performance, so that it will not be damaged in small earthquakes and will not collapse in large earthquakes.
(d) It is more important to reduce the weight of high-rise buildings than multi-storey buildings.
It is more meaningful for high-rise buildings to reduce their own weight than multi-storey buildings. Considering the bearing capacity of foundation or pile foundation, under the same foundation or pile foundation, if the weight of buildings is reduced, more floors can be built, which has outstanding economic benefits in soft soil.
The earthquake effect is directly proportional to the building weight, and reducing the building weight is an effective way to improve the seismic capacity of the structure. High-rise buildings are heavy, which not only has a large earthquake shear force on the structure, but also has a large additional axial force on vertical members and a large additional bending moment due to its high center of gravity and large overturning moment.
(e) axial deformation can not be ignored.
In high-rise buildings with frame system and frame-shear wall system, the axial compressive stress of the middle column in the frame is often greater than that of the side column, and the axial compressive deformation of the middle column is greater than that of the side column. When the building is very high, the difference of axial deformation will reach a great value, and the consequence is equivalent to the sinking of the middle support of the continuous beam, thus reducing the negative bending moment at the middle support of the continuous beam and increasing the positive bending moment at the span and the negative bending moment at the end support.
(6) Conceptual design is as important as theoretical calculation.
Seismic design can be divided into two parts: calculation design and conceptual design. The seismic design and calculation of high-rise building structures are carried out under certain assumptions. Although the analysis method is constantly improved and the analysis principle is constantly improved, due to the complexity and uncertainty of earthquake action, the complexity of foundation soil influence and the complexity of the structural system itself, the theoretical analysis and calculation may be several times different from the actual situation, especially when the structure enters the elastic-plastic stage, there will be local cracks and even component damage, so it is difficult to analyze the structure with conventional calculation principles. Practice shows that it is also very important to grasp the conceptual design of high-rise buildings in design.
Second, the structural system of high-rise buildings
(A) structural design principles of high-rise buildings
1. The structural design of reinforced concrete high-rise buildings should be closely coordinated with buildings, equipment and construction, so as to be safe, applicable, technologically advanced, economical and reasonable, and actively adopt new technologies, new processes and new materials.
2. In the structural design of high-rise buildings, attention should be paid to the structural selection and construction, the economic and reasonable structural system with good seismic and wind-resistant performance and the layout scheme of plane and facade, and attention should be paid to strengthening structural connection. In the seismic design, the overall seismic performance of the structure should be ensured, so that the whole structure has sufficient bearing capacity, stiffness and ductility.
(2) the structural system and application scope of high-rise buildings
At present, domestic high-rise buildings basically adopt reinforced concrete structures. Its structural system includes: frame structure, shear wall structure, frame-shear wall structure, tube structure and so on.
Frame structure system. The frame structure system consists of four load-bearing components: floor, beam, column and foundation. Plane frames are composed of beams, columns and foundations, which are the main load-bearing structures. The plane frames are connected by connecting beams to form a spatial structure system, which is one of the commonly used structural forms in high-rise buildings.
The advantages of frame structure system are: flexible building layout, large space, convenient building facade treatment, light structure weight, mature calculation theory and low cost within a certain height range.
The disadvantages of frame structure are: the frame structure itself is flexible, the lateral force is poor, and it will produce large horizontal displacement under wind load, and the non-structural members will be seriously damaged under earthquake load.
Scope of application of frame structure: the reasonable number of floors of frame structure is generally 6 to 15, and the most economical one is about 10. Frame structure has been widely used in offices, houses, shops, hospitals, hotels, schools and multi-storey industrial workshops and warehouses because it can provide large building space and flexible layout, and can meet the requirements of various technologies and uses.
2. Shear wall structure system. In high-rise buildings, in order to improve the lateral stiffness of building structures, reinforced concrete walls are called "shear walls". The main function of shear wall is to improve the shear strength and stiffness of the whole building, and the wall is also used as a component for maintenance and room partition. In the shear wall structure, the reinforced concrete wall bears all horizontal and vertical loads, and the shear wall is arranged orthogonally along the horizontal and vertical directions or obliquely along multiple axes. High rigidity, good spatial integrity and low steel consumption. In the historical earthquake, the shear wall structure showed good seismic performance, and the earthquake damage occurred less and to a lesser extent. Using shear wall structure in residential and hotel rooms can better adapt to the characteristics of many walls and small room area, and make the room clean and beautiful without exposing beams and columns.
There are many walls in shear wall structure, so it is difficult to arrange a large room. In order to meet the requirements of large-scale public rooms such as halls, restaurants and conference rooms in hotels, as well as the requirements of shops and public facilities on the ground floor of residential buildings, frames can be used to replace some shear walls on the ground floor or floors to form a frame-supported shear wall structure.
In the frame-supported shear wall, the stiffness of the bottom column is small, which leads to the sudden change of the upper and lower stiffness. Under the action of earthquake, the bottom column will produce great internal force and plastic deformation. Therefore, this kind of frame-supported shear wall structure is not allowed in the earthquake area.
3. Frame-shear wall structure system. A frame-shear wall structure can be formed by arranging a certain number of shear walls in the frame structure. This kind of structure not only has the characteristics of flexible layout and convenient use of frame structure, but also has high stiffness and strong seismic capacity, so it is widely used in office buildings and hotels in high-rise buildings.
4. Cylindrical structure system. With the increase of the number and height of buildings and the improvement of seismic fortification requirements, the structural system of high-rise buildings with frame-shear wall under plane working conditions often cannot meet the requirements. At this time, the shear wall can be used to form a space thin-walled tube, which can be turned into a vertical cantilever box girder, and the columns can be densified to enhance the stiffness of the beam. It can also form a frame tube that is stressed in space as a whole. A structure in which one or more pipes mainly bear horizontal force is called a tube structure. Generally, the cylinder structure is as follows:
(1) frame-tube structure. Thin-walled tube with shear wall is arranged in the middle, which bears most horizontal forces, and ordinary frame with large column spacing is arranged around it. This kind of structure is similar to the frame-shear wall structure, and it is also applied in Nanning Wang Di Building.
(2) Pipe-in-pipe structure. The pipe-in-pipe structure consists of an inner pipe and an outer pipe. The inner tube is a shear wall thin-walled tube, and the outer tube is a frame tube composed of dense columns (usually the column spacing is not more than 3 meters). Because of the dense peripheral columns, high beam stiffness and small hole area (generally less than 50% of the wall area), the frame-tube work is different from the ordinary plane frame, but it has good overall spatial function, similar to the perforated vertical box girder, and has good wind-resistant and seismic performance. At present, the tallest reinforced concrete structures in China, such as Shanghai Jinmao Building (88 floors, 420.5 meters) and Guangzhou Zhongtian Plaza Building (80 floors, 320 meters), all adopt tube-in-tube structure.
(3) Beam tube structure. A number of thin-walled shear wall tubes are arranged in the plane, each of which is relatively small. This structure is mostly used in buildings with complex plane shapes.
(4) Mega-structure system. Mega-structure is a first-class mega-frame composed of several mega-columns (usually composed of elevator shafts or large-area solid columns) and mega-beams (set every few or more floors, and the beam section generally accounts for one to two floors), which bear the main horizontal force and vertical load, and the beams and columns of other floors form a second-class structure, and only transfer the floor load to the first-class frame structure. The beam-column section of the secondary structure of this structure is smaller, which makes the building layout more flexible and planar.
In addition to the above structural systems, other structural forms can also be applied, such as thin shell, suspended cable, membrane structure, grid structure and so on. But at present, the four most widely used structures are frame, shear wall, frame-shear wall and tube.
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