Chen Libin, Zhu Zhangfeng, Liu Deyuan, Du Yong
(School of civil engineering, Nanjing University of technology, 211816)
Abstract: This paper comprehensively analyzes the structural characteristics and experimental research results of the traditional column through type and beam through type RC column steel beam composite frame structure (RCS) joints and several improved RCS joints, and summarizes their failure modes and overall performance. On this basis, combining the RCS concept with the prefabricated building technology, an assembled composite frame structure joint based on steel joint is proposed, and the performance of the joint is compared with the RCS joint by finite element analysis. The analysis results show that the joint has high bearing capacity and ductility deformation capacity, good lateral resistance, and the steel joint can effectively ensure the integrity of the joint, which has further research and application value.
Key words: reinforced concrete column steel beam; fabricated structure; steel joint; seismic performance; frame joint; composite structure
0 Preface
RCS (reinforced concrete columns and steel beams) composite frame structure is a kind of high-rise building structure which develops rapidly in recent years. It is composed of reinforced concrete columns and steel beams. It has the advantages of large axial bearing capacity, good durability and fire resistance, small self weight, strong bending capacity and larger span under the same load. With the vigorous implementation of China's construction industrialization, there are more and more prefabricated composite frame structures, that is, beams, columns and joints can be prefabricated in the factory and assembled on site. Compared with the traditional cast-in-place structure, the construction efficiency and site working environment are greatly improved. In the assembled composite frame structure, the stress at the beam column joint is complex, which is an important part of the structure. The structure of the joint itself, and the integrity of the assembled beam and column will have a direct impact on the seismic performance of the structure. In RCS structure, the design of joints is broad and various, and there are many effective connection methods between joints, steel beams and concrete columns. Therefore, it is of great significance to study the joint of RCS combined frame and design a more efficient and reliable assembled frame joint.
1 research status of RCS nodes
1.1 column through type RCS node
From the point of view of the joint structure, the column through joint should keep the integrity of the column, that is to say, the steel beam should be disconnected at the joint and should not pass through the concrete column completely. Generally, the steel beam section is welded on the cylindrical steel plate around the joint area. Figure 1 shows a typical column through type RCS joint. It is designed by SAKAGUCHI, a Japanese scholar, and the repeated load test is carried out on it. The strength calculation method of the joint is given.
Since the 1980s, dozens of RCS joints have been developed by major Japanese construction companies, which set off a boom in joint research and development, most of which are column through joints. In the mid-1990s, Japan building society classified the above-mentioned joints into 12 standard types in the "RCS composite joint design criteria", so as to systematically classify the RCS joints.
In 1992, Kuramoto et al. Analyzed the relevant factors influencing the shear performance of the joint through the seismic performance test of the column RCS joint. The research shows that the shear capacity of the joint can be significantly increased by increasing the thickness of the steel beam web in the joint area and setting the end bearing plate.
In China, Dai Shaobin et al. Carried out quasi-static loading on three column through type RCS joints (see Figure 2), and obtained that: column through type RCS joints conform to the seismic design principle and are a reliable RCS joint form. In addition, the ANSYS finite element software is used to simulate the test components, and the simulation results are in good agreement with the test parameters such as the failure mode and skeleton curve.
1.2 beam through RCS node
The beam through type RCS joint comes from the United States. This kind of joint is that the steel beam continuously passes through the reinforced concrete column, at the same time, a hole is opened on the profile steel flange for the internal longitudinal reinforcement to pass through, and a hole is opened on the web for the internal stirrup to pass through. Figure 3 shows a typical beam through joint.
In 1989, sheekh et al. Carried out reciprocating and monotonic loading tests on 15 2 / 3-scale beam through type RCS joints, in which the structural details of the core area of the joint were mainly taken as variables, such as the connection mode of steel beam and joint, face bearing plate, vertical stiffening plate, shear bolt and steel column (see Figure 4). The test results show that the beam through joint can provide enough stiffness and ductility under the ultimate load; the strength of the joint mainly depends on the shear condition of the concrete in the core area of the joint; the increase of the thickness of the internal face plate of the joint will not affect the stiffness of the joint, but can greatly improve the strength of the joint. The author also points out that shear failure and compression failure are two important forms of RCS joint failure, and puts forward his own calculation formula of the bearing capacity of RCS composite joint.
In 2000, Parra-Montesinos produced 9 beam through type RCS joints are tested for different joint internal structures (such as whether there is cover plate, stiffener, u-stirrup passing through the web) and the change of material types in the joint area. Through analysis and research, the beam through type RCS joints have greater rigidity and good seismic performance, which can be applied to the high intensity seismic fortification area. If steel cover plate, face bearing plate are arranged in the joint, or When ECC material is used, the diameter of longitudinal bars in columns and the number of stirrups in joints can be reduced appropriately.
In 2001, Yang Jianjiang et al carried out repeated load tests on one beam through type joint and three columns through type joints, and compared the differences between the failure mechanisms of the beam through type joint and the column through type joint. The test results show that the beam through type RCS joint has better energy consumption capacity, seismic performance and fuller hysteretic curve compared with the column through type. At the same time, it is proved that the core area of the joint is basically in pure shear state, and the steel beam passing through the concrete column as a whole can provide better shear bearing capacity for the core area.
In 2012, Yu Qiong and others carried out comparative tests on a group of beam through type joints and column through type joints respectively. The results showed that the failure location of beam through type joints occurred in the flange and web at the root of the steel beam, and the core area had large deformation. But the whole ductility of the column through joint is not enough, because the weld between the steel beam flange and the connecting anchor plate is broken, the structure fails early. At the same time, it shows that under the same conditions, the beam through joint has better energy dissipation capacity and seismic performance.
1.3 comparison between beam through type and column through type joints
The failure position of the joint is different when it is damaged, the beam through joint is basically the beam flange and web reach to yield, and the component is damaged, and the concrete in the core area is seriously cracked and deformed. In the failure of the column through joint, the web and flange of the steel beam did not yield, but the weld between the flange of the steel beam and the anchor plate of the joint was first pulled off, leading to the failure.
Under the same conditions, the performance of the beam through type RCS joint is better than that of the column through type RCS joint, because in the beam through type joint, the steel beam passes through the concrete column as a whole, and the beam web passing through the core area of the joint can significantly improve the ultimate strength and energy dissipation capacity of the joint, At the same time, in the design of beam through type RCS joints, stirrups are often arranged inside the joints, so as to better restrict the concrete in the core area and delay the failure of the joints. The column through type RCS joints often need to be welded at the beam column connection, but the field welding, especially the upper and lower flange welding, is very difficult, which is easy to produce weld defects, leading to stress concentration phenomenon. Many tests show that before the joint can fully play its role, the weld has been pulled off, which greatly affects the performance of column through type joints.
In terms of construction difficulty, the construction of beam through joint is more difficult, because it needs to consider how the column longitudinal reinforcement and stirrup in the joint pass through the flange and web of the steel beam, which often requires more complex structure and increases the difficulty of concrete vibration in the joint area. The structure of column through joint is simple, the integrity is good, the construction technology is relatively simple, and many of them can be prefabricated by the factory, which shortens the construction period, and is very helpful to save the cost.
1.4 improved RCS nodes
With the continuous development of assembly technology, in recent years, the design of improved RCS joints emerge in endlessly in China. Most of these improved joints are prefabricated in the factory. During the design, the shear resistance and the binding force on the concrete in the core area are increased by setting stiffeners, face bearing plates, stirrups, anchor bolts, high-strength bolts and other structures inside the joints. Compared with the traditional beam and column through type joints, the improved RCS joints improve many adverse effects caused by complex design, inconvenient construction and other factors. At the same time, the advantages of the beam and column through type joints are absorbed, which has good seismic performance.
In 2005, Xiao Yan proposed a new type of RCS composite joint based on bolt end plate connection (see Figure 5). In the manufacturing of joints, the end plate is connected to the column by high-strength bolts, and then the steel beam is connected to the end plate by full penetration welding. This connection method has wide applicability, especially for the connection of steel beam and prefabricated column. Then, the author carried out repeated loading tests on two groups of specimens, and the results show that the improved RCS joint has excellent seismic performance, and its energy consumption capacity, bearing capacity and ductility are very superior.
In 2012, Guo Zixiong carried out an experimental study on four reduced scale fabricated RCS joints under the action of low cycle reciprocating load. This new type of joint consists of a steel plate barrel composed of a cylindrical wall plate, a stiffening web and a horizontal stiffening rib. The concrete is poured inside without stirrups. It is connected to the steel beam through embedded high-strength bolts (see Figure 6). Through the test, the failure mode of all members is beam hinge mechanism failure, which is basically the same as the traditional beam through joint failure mode. The shear capacity of the joint increases with the thickness of the stiffened web. Through analysis, this new type of joint can fully consume energy and has excellent seismic performance.
In 2014, menjinjie et al. Proposed a joint construction form of RCS with steel beam web through and flange partially cut. The joint of this structure is slightly different from the beam through joint in the traditional sense. The joint web through but flange not through (see Figure 7), which realizes the continuity of beam and column to a large extent, so as to fully guarantee the effectiveness of joint force transmission. Based on the different failure conditions of the specimens, two failure modes, local pressure failure and shear failure, are concluded. The hysteretic behavior of members under different failure modes is different. The test not only shows that this type of joint has good seismic performance, but also proves that the joint can enhance the joint performance of internal and external concrete through reasonable construction, so that the joint has better bearing capacity.
2 fabricated composite frame joint based on steel joint
2.1 node construction
In the fabricated frame structure, the connection between many members is weak, and the lack of integrity after assembly leads to poor seismic performance. In order to better adapt to the development requirements of prefabricated buildings and expand the research of prefabricated composite frame structure, a prefabricated reinforced concrete column steel reinforced concrete beam joint based on steel joint is proposed in combination with the design concept of RCS joint (see Fig. 8 and Fig. 9). The joint is welded by two sections of H-shaped steel. The size of joint section steel is consistent with that of beam internal section steel, and there is a belt welded at the upper and lower ends The square steel plate frame of the stiffened angle plate is welded with 8 rectangular stiffeners on the web plate