This paper briefly introduces the development history, research progress and application of concrete filled steel tube in practical engineering, expounds the characteristics of concrete filled steel tube and its engineering application in steel structure residence, and looks forward to the problems needing further research in the future.
Concrete-filled steel tubular (CFST) is a kind of member filled with concrete on the basis of steel reinforced concrete and spiral reinforced concrete. Concrete-filled steel tube (CFST) makes concrete in a complex stress state through the interaction between steel tube and concrete, thus improving the strength, plasticity and toughness of concrete. At the same time, the existence of concrete can avoid or delay the local buckling of steel tubes, thus ensuring the full play of material properties. It can be seen that they promote each other, cooperate with each other and complement each other, and jointly improve the integrity of concrete-filled steel tubular members, making them have the advantages of high bearing capacity, good plasticity and toughness, good seismic performance, convenient construction, good fire resistance, good post-disaster repairability and advanced economic indicators, so they have been widely used.
Research progress of 1 concrete filled steel tube
According to the section form, concrete filled steel tube can be divided into circular concrete filled steel tube (commonly known as concrete filled steel tube), square concrete filled steel tube, rectangular concrete filled steel tube and polygonal concrete filled steel tube. One of the earliest projects that adopted concrete filled steel tube structure was 1879 pier of Severn Railway Bridge in England. At that time, the steel pipe was filled with concrete to prevent the steel pipe from rusting while bearing pressure. The early research did not consider the interaction between steel tube and its core concrete to improve the bearing capacity of members, but simply superimposed them. With the in-depth study, it is found that concrete is in a complex stress state due to the constraint of steel pipes on concrete, which improves the strength, plasticity and toughness of concrete. At the same time, the existence of concrete can avoid or delay the local buckling of steel tubes, and the interaction between them is complementary, which improves the integrity of concrete-filled steel tubes and makes them have a series of superior mechanical properties and advanced economic indicators.
There are many research methods to study the mechanical properties of concrete filled steel tubes, such as experimental research, regression of experimental coefficients, limit state analysis, and numerical solutions such as fiber model method and finite element method [3-4]. Their difference lies in how to estimate the mutual constraint between steel tube and core concrete, which leads to the complexity of its mechanical properties. Because researchers study the above problems from different angles and have different understandings of the hoop effect between steel pipe and concrete, the calculation methods and results will be different. Researchers from all over the world have systematically studied the mechanical properties of concrete-filled steel tubular members and frame structures composed of concrete-filled steel tubular members and steel beams or reinforced concrete beams under static, dynamic and fire [1-4]. On the basis of relevant research results, countries all over the world have formulated the design and construction codes for concrete-filled steel tubular structures, such as EC4( 1996), DIN 18800( 1997) in Europe and ACI-3 19-89 and SSLC (/KLC) in the United States. China began to study concrete filled steel tubes from 1960s, mainly focusing on pouring plain concrete into steel tubes. Although it started late, especially in recent ten years, it has made remarkable achievements, and issued many design specifications, such as JCJ0 1-89, CECS28:90, DL/T5085- 1999, GJB 65438+. The formulation of these regulations has opened the prelude to the wide application of concrete filled steel tubes in China's construction industry.
2. The application of concrete filled steel tube in residence
From 1960s to 1980s, concrete-filled steel tubular (CFST) began to be used in industrial and civil buildings. With the deepening of theoretical research, the promulgation of design specifications and its own advantages, concrete-filled steel tubular (CFST) has been more and more widely used in single-storey and multi-storey industrial plant columns, equipment frame columns, various frames, supports, trestle columns, subway platform columns, power transmission and transformation towers, truss struts, piles and space structures. In recent years, especially in recent 2-3 years, it has been applied to bridge structures, high-rise buildings and super-high-rise buildings.
2. 1 application and characteristics of concrete filled steel tube in non-residential buildings
In the 1960s, concrete-filled steel tubes began to be used in industrial workshops. According to statistics, by 1994, there were hundreds of single-storey and multi-storey factories or frame columns in China, and the cost of each component was equivalent to or lower than that of reinforced concrete columns, but the cross section was obviously reduced and the construction period was shortened. Concrete-filled steel tube factories are all over the country, such as Shanghai No.31 Cotton Mill, Dalian Shipyard Hull Assembly Workshop, Wuchang Shipyard and Zhonghua Shipyard Hull Structure Workshop, and Taiyuan Iron and Steel Company No.3 Steelmaking and Continuous Casting Workshop [2]. The application of concrete filled steel tube in the factory building has taken the first step in its application in practical engineering.
In recent ten years, concrete filled steel tube has been widely used in arch bridges and space truss beam bridges. According to incomplete statistics, by the end of 1999, more than 100 CFST arch bridges and 10 CFST rigid skeleton arch bridges had been built in China. At the same time, the application of concrete filled steel tube in high-rise and super-high-rise buildings in China has developed rapidly, and it has gone through the process from local columns to most columns and finally to all columns. Among them, some high-rise buildings with concrete-filled steel tubular columns are: Quanzhou Post Office Building (87.5m high), Nan 'an Post Office Building, Fuzhou Global Plaza, Guangzhou Good World Plaza (1 16.3m), Beijing Sichuan Building, and the second district of Pingshan Comprehensive Building of Fujian Provincial Government; Most of the columns are: Xiamen Fukang Building (86.5m), Beijing World Financial Center Building (120m), Guangzhou New China Building (20 1.8m), Shenzhen Post and Telecommunications Information Center Building and Tianjin Industrial and Commercial Bank Office Building; All the columns are: Xiamen Jinyuan Building (96. 1m), Tianjin Evening News Building (137m), Shenzhen Seg Plaza Building (29 1.6m), Shanghai Land and Sea Project (84.7m) and so on. Among them, Shenzhen Seg Plaza Building won the second prize of National Science and Technology Progress Award in 2000, which is the first high-rise building with combined structure designed, manufactured and constructed by itself in China, all made of domestic steel and invested by itself. The application of concrete filled steel tube in bridges and high-rise buildings has taken the second step of practical application.
In recent ten years, concrete filled steel tube has been applied to composite columns. With the deepening of theoretical research on concrete filled steel tube and the increasing application of engineering, it is urgent to improve the fire resistance of concrete filled steel tube and the brittleness of core high-strength concrete. At present, there are two main ways to solve this problem: one is to solve the brittleness problem of concrete filled steel tube by filling reinforced concrete or steel fiber concrete in the steel tube, and to solve the fire resistance problem of concrete filled steel tube by spraying fire retardant coating on the outer surface of the steel tube; The second is to use composite columns. Composite column is a kind of composite column [5], which consists of steel pipe, plain concrete inside the pipe and reinforced concrete outside the pipe. The concrete filled with steel pipe is placed at the core of the column and surrounded by a circle of reinforced concrete. Because the composite column is wrapped with concrete, it has good fire resistance and corrosion resistance. At the same time, a considerable part of the total axial force is borne by the concrete filled steel tube in the core, and the axial pressure shared by the peripheral concrete (distributed according to the vertical stiffness ratio) is small, so the axial pressure ratio is also small and has good ductility. At present, the projects adopting composite column structure in Liaoning include: Shenyang Daily Building (105m), Liaoning Postal Hub (96.6m), Liaoning Property Building (100m), Shenyang Hetai Building (75.6m), Shenyang Heping District Local Taxation Bureau (76.8m) and Shenyang Electric Power Garden Twin Towers (65438). The application of concrete filled steel tube in composite columns has taken the third step in practical engineering.
2.2 characteristics of concrete filled steel tube and its application in residence
On the one hand, the steel structure itself has the advantages of high-tech content, environmental protection and renewable utilization. On the other hand, due to the substantial increase in China's steel output, the world's steel output is becoming increasingly saturated and the steel price is falling, in recent years, China has begun to vigorously promote the steel structure and encourage its adoption. The Ministry of Construction and other departments have also set the goal of accelerating the promotion of the development and application of building steel structures, and decided to focus on promoting residential steel structures during the Tenth Five-Year Plan period, striving to make the steel consumption of building steel structures in China reach 3% of the total national steel output during the Tenth Five-Year Plan period and 6% by 20 15 years [6]. Residential buildings have always ranked first in the construction industry, so it is imperative to promote steel structure in residential buildings.
Residential steel structure can be divided into low-rise, multi-storey and high-rise buildings. Below the third floor is the low floor, below the ninth floor is the multi-storey, and above the ninth floor is the high floor. The 10 ~ 12 floor is also called the small high floor. Considering the seismic requirements, residential steel structures should generally not exceed 12 stories. At the same time, due to China's large population, relatively insufficient land resources, and the rapid growth of urban housing demand, it is appropriate to develop multi-storey and small high-rise steel residential buildings, but in cities with high population density, high-rise buildings are still the mainstay. Residential steel structure has the advantages of less columns, large indoor effective space, flexible residential space layout and good structural performance. The structural system it chooses is generally: 5 ~ 6 floors below, frame system or frame support system; More than 6 stories, frame-supporting system or frame-concrete shear wall (core tube) system; Multi-layer, mostly using dual systems. The frame columns used in steel structure residence include H-shaped steel column, concrete filled steel tube column and steel reinforced concrete column, and the latter two are composite columns. In small high-rise buildings, composite columns save more steel than H-shaped steel columns, which can reduce the project cost. However, the structure of steel reinforced concrete columns is more complicated than that of concrete filled steel tubular columns. Therefore, it is imperative to popularize concrete filled steel tube in residential steel structures.
In Shanghai, Tianjin, Liaoning, Xinjiang and other places, China has built some pilot projects of steel structure residence with concrete filled steel tube as frame column, which has promoted the development of steel structure residence in China. As we all know, Shanghai has a high population density and valuable land resources. Therefore, high-rise buildings are mostly used in Shanghai pilot steel residential buildings. However, the population density in Tianjin, Liaoning, Xinjiang and other places is relatively small, so small high-rise and multi-storey buildings are chosen. Table 1 lists some steel structure projects that use concrete filled steel tubes as frame columns in high-rise residential buildings in these areas [7- 10]. The application of concrete filled steel tube in residential buildings has taken the fourth step of its practical engineering application, which is of great significance, because it has realized the development of concrete filled steel tube structure from high-rise to small high-rise and multi-storey, and opened up a new situation for its extensive application.
3 Conclusion and prospect
The development of concrete filled steel tube in China is only a few decades' history, but the development speed is very fast, and now it has been widely used in various engineering fields. Especially in residential buildings, the application has made it transition from high-rise buildings to small high-rise and multi-storey buildings, creating a broad space for its development. It is estimated that in the first 20 years of 2 1 century, China will need 486-549 million square meters of new housing every year, and 576 million square meters of new housing every year in the next 30 years. In the next 50 years, housing construction will become a new growth point and consumption hotspot of national economic construction. Therefore, the development space of steel structure residence is broad, and the application prospect of concrete filled steel tube in residence is broad. Therefore, it is necessary to do further research on concrete filled steel tube. The following aspects should be done well in the future:
(1) In terms of fire protection design, it is necessary to simplify the design method of fire resistance limit of concrete filled steel tube and formulate the Code for Fire Protection Design of Residential Buildings with Steel Structure (Concrete Filled Steel Tube Structure). Only in this way can it help to popularize the application of concrete filled steel tube in residential buildings.
(2) Strengthen the evaluation of residual mechanical properties of steel structure (concrete filled steel tube) houses after fire and the research of post-disaster maintenance and reinforcement measures.
(3) The key of steel structure residence lies in price, quality, waterproofness, durability and joint treatment of external wall panels. At present, although some wallboard has good performance in all aspects, the price is higher, which is unacceptable to developers and households, so it cannot be vigorously promoted for the time being. Therefore, we should vigorously develop cheap, light, high-strength and durable external wall panels and corresponding supporting components.
(4) Combining with the engineering practice, further improve the design theory, different types of structural design specifications and construction procedures of steel structure (concrete filled steel tube) residential buildings, and compile the supporting atlas of each component as soon as possible.
In addition, the steel structure residence requires higher construction technology of the construction team, while most areas in China mainly carry out the construction of concrete structures. Therefore, the training of steel structure construction team should be strengthened to further promote the development and application of concrete filled steel tube in residence.
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