In order to reduce the damage caused by the earthquake, the seismic capacity of high-rise buildings must be effectively improved. However, due to the suddenness and uncertainty of the earthquake, the seismic design of high-rise buildings can not only rely on theoretical calculations, more should be combined with the causes of past disasters and the previous summary of the seismic concept. In order to realize the basic concept of small earthquake is not bad, medium earthquake can be repaired, large earthquake does not fall, as far as possible to reduce the casualties and economic losses caused by the earthquake, to the general public a comfortable living environment.
1 Characteristics of earthquakes in China
Based on the activity characteristics of tectonic earthquakes, and special natural conditions and topography, social conditions, and historical factors, China's seismic hazards are mainly characterized by the following: ① The frequency of earthquakes is high. China is located in the world's two major seismic belts - the Pacific Rim seismic belt and the Eurasian seismic belt of the intersection, there is a very obvious seismic activity, the light recorded more than seven earthquakes more than a dozen, five earthquakes occurring a year of thousands; ② disaster is more serious. Earthquakes in mainland China are generally shallow, mostly in the crust inside 10 to 25km, the destructive power can be imagined. More than five magnitude earthquake, in our country will cause a large number of house collapses and casualties; ③ accompanied by very serious secondary disasters. China's topography is high in the west and low in the east, a terraced distribution, the terrain is complex and diverse, hills, mountains, plains, basins, plateaus are distributed. The direct disaster of earthquakes often causes fires, floods, landslides, mudslides, typhoons, tsunamis, due to the post-earthquake resettlement of people is more centralized, the density is larger, coupled with the incomplete medical equipment after the earthquake, the health environment to keep up with the spread of large-scale epidemics can easily result in the dissemination of the disease, to the people's physical and psychological harm caused by the multiplicity of damages, far more than the direct damage caused by earthquakes to be more serious; ④ Disaster area is wide, the earthquake and the scope is large. A large-scale earthquake may affect hundreds or thousands of kilometers of land around the epicenter. Such as China's May 12, 2008 Wenchuan earthquake, in addition to Heilongjiang and Liaoning, the other provinces have obvious tremors, the farthest wave to 3,000km away from Bangkok, you can imagine the power of it, the scope of the wide range of; ⑤ seismic hazards is a certain cycle, China is currently in a period of high seismicity, may be maintained until the beginning of the next century, so we must do a good job in advance to prevent, strengthen the building's seismic capacity.
2 Influence of the regularity of building form and component arrangement on the seismic resistance of high-rise buildings
2.1 Plane irregularity
The types of plane irregularity are mainly divided into torsional irregularity, concave-convex irregularity, and localized discontinuity of the floor slab. The Code provides: the building and its lateral force-resisting structure layout should be regular, symmetrical and have good integrity. The arrangement and division between floors belongs to the arrangement of the building plan. The arrangement of internal wall structure, the area of movable space, the location of the arrangement of stairs and passages, the size of the distance between the bamboo, and the change of the arrangement between floors. If the walls are arranged asymmetrically and the columns and walls are not coordinated and distributed asymmetrically when the building plane is in plan, it will cause the quality and stiffness of the building structure to be uncoordinated and distributed asymmetrically, which will make the building undergo a torsional seismic effect during an earthquake. Some buildings have a large stiffness of the elevator shaft, but it is arranged on the side or corner of the building plan, when an earthquake occurs, so that the elevator shaft by the side of the building damage is serious, this is due to the main part of the seismic effect of the elevator shaft is very large lateral stiffness attracted by the elevator shaft, a typical example of earthquake damage is the 1972 South America, Nagua fifteen-storey high Central Bank Building. Because part of the building in the layout of the plane on one side of the wall less, one side of the wall more, the internal partition wall interrupted or asymmetric, so in the event of an earthquake, the seismic force transfer is impeded and the stiffness of the sudden change, etc., which all show that the layout affects the structure of the seismic is very obvious.
2.2 Vertical Irregularity
The type of vertical irregularity is divided into lateral stiffness irregularity, vertical lateral force-resisting construction discontinuity, and sudden change of floor bearing capacity. The Code provides: the vertical section of the building should be regular, vertical lateral resistance to build the strength of the material and section size should be gradually increased from top to bottom, to prevent lateral resistance to lateral force structure of the lateral stiffness and bearing capacity of the sudden change. According to the requirements of the Code of the mutation limits and height of the building to avoid the production of vertical irregularity type, so that the vertical distribution of stiffness and mass is more uniform, to prevent the production of mutation, to avoid the production of weak and torsion.
3 frame shear structure in the shear wall force characteristics
frame shear structure can affect the structural stiffness of a number of factors, such as the interface size of the shear wall, the number, location, and its own shape, etc., which affects the structural stiffness of the deepest is the stiffness theory. In frame shear structure, the structural stiffness and stiffness distribution are determined by the position and number of shear wall arrangement. The High-Rise Code divides high-rise buildings into two levels, i.e., high-rise buildings (Level A), and super high-rise buildings (Level B), which correspond to different seismic design levels and different applicable heights for different structures. In frame shear structures, it is the shear wall that plays the greatest role. Shear walls are lateral force-resisting members, and when this structure is used, shear walls should be arranged on the outside of the two main axes, thus forming a two-way lateral force system. Frame structural system is a structural system formed by using beams and columns to form a frame in both vertical and horizontal directions. It bears vertical and horizontal loads at the same time. Its main advantage is that the building layout is flexible, can form a larger building space, and the building elevation is more convenient to deal with. The main disadvantage is that the lateral stiffness is small, and when the number of floors is large, it will produce excessive lateral displacement, which is easy to cause the destruction of non-structural components (such as partition walls, decoration, etc.) and thus affect the use. In non-seismic areas, frame structures generally do not exceed 15 stories. The internal force analysis of frame structures is usually performed accurately by computer. Frame shear wall structure, also known as box-shear structure, is a structure in which a certain number of shear walls are arranged in the frame structure to form a flexible and free space to meet the requirements of different building functions, and at the same time, there are enough shear walls with considerable lateral stiffness (large lateral stiffness of shear walls means strong resistance to deformation under the action of horizontal loads (wind loads and horizontal seismic forces)). The disadvantage of the frame structure is the small lateral stiffness, when more levels, all produce excessive lateral displacement, easy to cause damage to non-structural components and affect the use, but it has a flexible layout, can form a larger building space advantages, in order to retain this advantage, while improving its lateral stiffness, it produced the frame shear wall structure. The main feature of this structure is to retain the advantages of the frame structure on the basis of additional resistance to the shear force of the shear wall, thus increasing its lateral stiffness, in this structural system, the shear wall bears more than 80% of the horizontal load, while the frame only bears about 20% of the frame, which is different from the frame structure regardless of what direction of the load is borne by the frame of the whole situation. When the building height, defense intensity, building importance category, foundation category are the same, the requirements for the frame in frame structure are much higher than the requirements for the frame in frame shear structure. Characteristics of frame structure: flexible building plan layout, more flexible facade design, its deformation characteristics for shear deformation and bending deformation combined with shear-type deformation, less types of frame structure components, easy to standardize, stereotypes, in the material deformation performance is good, can be constructed up to 30 floors, in general 15 to 20 floors is good. The characteristics of shear wall structure: good overall performance, high stiffness, small lateral deformation under horizontal action, bearing capacity requirements are easier to meet, its lateral deformation for the bending type, but due to the spacing of the shear wall can not be too large, so its plane layout is not flexible enough. In order to overcome its shortcomings, it will use the frame-supported shear wall structure, skipping floor shear wall structure.
4 Effect of aspect ratio on the overall seismic resistance of the structure and economic indicators
4.1 Overall seismic resistance
The so-called dynamic action of the building structure is the seismic action, which is not only related to the intensity of the earthquake, but also a great deal of the relationship lies in the dynamic characteristics of the structure itself, shear-to-weight ratio and the base shear are the two commonly used indicators of the seismic response of the structure. The height-to-width ratio recommended by the high code is taken as the limit of 6, and the height-to-width ratio of 6 base shear is taken as the reference value of the two structures, which yields the relative value of the structural base shear at the change of the height-to-width ratio relative to the height-to-width ratio of 6 (see Fig. 1). The structural base shear increases approximately linearly as the height-to-width ratio increases, the seismic force on the structure increases proportionally with height, and the shear end of the structure grows relatively fast. The ratio of the structural gravity load to the horizontal seismic action is gradually increasing, and at first, the sliding chengdu is relatively fast, and then the change is more stable, showing the advantages of a more flexible structure and reduced seismic response when the structural period is lengthened (see Figure 2).
4.2 Economic Indicators
If only from the point of view of structural safety, the current high code in the actual design work in the height and width ratio restrictions can be relaxed. According to the past engineering experience, when the building height increases there will be two changes. On the one hand, when the aspect ratio is larger, the lateral movement of the structure will change and increase in a positive direction, while at the same time, the overturning resistance and overall stability will be negatively affected, showing a downward trend; on the other hand, the horizontal load will change in a short period of time and increase rapidly. In the horizontal load, the displacement of the top of the building is positively related to the overturning moment. Specifically, the greater the displacement of the top of the building, the greater the overturning moment, and vice versa, which is proportional to the fourth power of the height and inversely proportional to the width. In order to make the structural stiffness, overturning resistance and stability sufficient, the size of the structural elements needs to be increased, and the amount of material used will also increase, and the structural economy is worse.
5 Specific measures to improve the seismic performance of high-rise buildings
In the construction of high-rise buildings, not only to meet the requirements of the strength and hardness of the material, but also to make it have a better ductility, and have a yield region in the specified parts. Specifically divided into the following aspects: ① Improve the stability of the foundation of the high-rise building and the compressive capacity of its short columns. Improve the shear-to-span ratio, thus reducing the cross-sectional area of the short columns and improving the seismic performance. The main method is to use a higher grade of concrete materials and other methods that can improve the ductility of the material combined, used in actual construction to improve seismic performance; ② the use of better performance of concrete - steel pipe concrete columns to do the overall structure of the building, there is concrete inside the steel pipe has been in a state of compression, and its corresponding compressive strain and compressive strength have been greatly improved, to solve the problem of poor ductility of concrete; ① to improve the stability of the foundation of high-rise buildings and its short columns to reduce the cross-sectional area, improve the seismic performance. The problem of poor ductility of concrete; ③ The beginning of the design should be comprehensively analyzed. The model of the high-rise building structure can be appropriately simplified before construction, but its overall stress and the stress of the key parts are absolutely necessary to be reflected in the model, the parameters of the components and the selection of the restoring force model should be in line with the main performance of the structural components and structural characteristics of our country, and can not be copied from the existing parameters of the foreign industry. According to the different seismic levels to select the corresponding elastic-plastic state, to be from the structure of the displacement, bearing capacity, compressive force, yield, damping ratio and other data for a comprehensive analysis and discussion, correctly applied to the actual construction.
6 Conclusion
With the continuous development of the economy, urban construction is becoming increasingly prosperous, more and more skyscrapers rise up, giving people the convenience and the enjoyment of beauty at the same time, the construction industry should pay more attention to is the high-rise building seismic performance. Natural disasters always happen so cruelly and without warning. In order to better build the country, we must do to prevent in advance, in the building itself to do articles, improve its seismic performance and the ability to cope with a variety of natural disasters. Only in this way can people have enough time to react in the face of powerful natural disasters and truly enjoy the convenience of the city.
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