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Steel Construction Published the List of Highly Influential Articles of 2020
2023 Vol. 38, No. 12
Abstract
PDF (1874KB)Steel-concrete composite structures that meeting the needs of national new-type urbanization construction were widely used in urban and high seismic precautionary intensity areas due to outstanding flexural stiffness, bearing capacity, ductility, and convenient construction. In the background of enhancing seismic precautionary criterion of engineering structures, studying the seismic performance of steel-concrete composite structures, further improving their seismic toughness, and establishing seismic design methods with higher toughness for steel-concrete composite structures are of great significance in promoting the achievement of China’s “dual carbon” target in building structures.
The authors summarized the research progress on the seismic performance of steel-concrete composite structures, including experimental research on the hysteresis performance of steel-concrete composite beams, CFST columns, and composite joints, as well as pseudo-static, pseudo-dynamic, and shaking table tests of steel-concrete composite structural systems. Various seismic analysis models and methods were compared, and some existing problems and directions for further research were proposed in this paper.
Based on the analysis of the current research, the conclusions and prospects are as follows: 1) Steel beams are the main energy dissipation components of composite beams, and increasing the cross section of steel beams can improve the energy dissipation capacity of composite beams. CFST columns mainly rely on steel tube and core concrete for energy dissipation, and the strengthened restraint measure of the column end stirrup-confined can be taken to directly constrain the core concrete, transforming it from brittle to plastic, thereby enhancing the energy dissipation capacity of frame columns. Compared with other types of composite joints, the energy dissipation capacity of rigid connection composite joints is stronger; 2) Beams are the major energy dissipation components of frame structures in rarely occurred earthquakes. However, it will significantly increase engineering costs when beams act as the major energy dissipation components in extremely rare earthquakes. As is well known, the probability of extremely rare earthquakes occurring is extremely low. If some strengthened restraint measures are taken, which enable the columns to have energy dissipation capacity and participate in energy dissipation, the structure can own the ability to resist extremely rare earthquakes while increasing costs appropriately. Therefore, the seismic design concept of composite structures can be advanced from “strong columns and weak beams with beam energy dissipation mainly” in rarely occurred earthquakes to “beams and columns energy dissipation together” in extremely rare earthquakes; 3) The software for fiber model based on the assumption of flat cross-section is usually suitable for the stages of elastic and elastic-plastic in small deformation, while this assumption will fail in the stage of large plastic deformation. Dynamic simulation of composite structural systems under strong earthquakes requires overcoming the assumed conditions of small elastic-plastic deformation stage, and it is an effective method to use the triaxial elastic-plastic constitutive model of concrete and the corresponding solid & shell element during the stage of large plastic deformation; 4) Shear wall structures with advantages of good integrity and high lateral stiffness have weaker seismic resistance under traditional construction. In order to enhance the energy dissipation capacity of shear wall structures, measures such as steel-concrete composite coupling beams, steel reinforced concrete coupling beams, or steel coupling beams, as well as steel reinforced concrete or CFST wall piers can be adopted; 5) Engineering structures face various disasters during their use stage. Traditional design methods based on independent various external loads neglects the coupling mechanism of multiple disasters, making it difficult for the structure to meet the usage requirements under multiple disasters. Therefore, establishing safe and reliable multi-disaster design methods and structural systems is a major task for structural engineers in the field of disaster prevention and reduction.
Based on pseudo-static tests of column end stirrup-confined on CFST columns, CFST column-composite beam joints, and CFST column-composite beam planar frame structures, a three-dimensional solid and shell element model of CFST column-composite beam spatial frame structures was established for dynamic time-history analysis. The influence of the column end stirrup-confined, the main beam heightening, the variable cross-section composite beams with the height of the mid span web reduced, and multidimensional earthquake on the seismic performance of composite structures were discussed. The displacement response, axial compression ratio time history curve, stress-strain curve of composite frame structure under various seismic wave were discussed, as well as the distribution mechanism of plastic energy dissipation, the formation and development law of plastic hinge, the evolution law of structural stiffness damage and other seismic performance indicators.
The finite element analysis results indicate that: 1)the strengthened restraint measures, such as the column end stirrup-confined and the main beam heightening, can effectively improve the seismic performance of composite frame structures and reduce the structural stiffness damage, and the greater of the seismic wave is, the better the effects of the enhanced constraint measures are. Under the same seismic wave, the strengthened restraint measure of column end stirrup-confined has more advantages than the main beam heightening, and the combined effect of two measures can further enhance the ultimate seismic capacity of the composite structure; 2)the strengthened restraint measure of column end stirrup-confined can reduce the interface slip between steel tube and infilled-concrete, enhance the energy dissipation capacity of concrete, reduce the proportion of plastic energy dissipation of CFST columns, increase the proportion of plastic energy dissipation of steel beams, and increase the total plastic energy dissipation of the structure. At the same time, the strengthened restraint measure of column end stirrup-confined can also reduce the number of plastic hinges at column ends and increase plastic hinges at beam ends, delay the emergence of “compression hinges” at the column ends, and extend the transition from “compression hinges” to “tension hinges”; 3)the variable cross-section composite beam with the height of the mid span web reduced has no effect on the seismic performance of composite frames. Using this variable cross-section composite beam not only meets the basic functional requirements of the structure, but also saves structural materials, reduces costs, and further expands the space of the building; 4)compared with the horizontal seismic wave, the ultimate seismic capacity of composite frame structure decreases significantly under the multi-dimensional seismic wave, and the failure mode of composite frame structure changes from shear failure to shear torsional failure. In addition, vertical acceleration in three-dimensional seismic wave has an amplification effect on axial compression ratio, while it has no significant impact on maximum displacement, interlayer displacement angle, plastic energy dissipation and stiffness damage. The strengthened restraint measure of column end stirrup-confined still has an improvement effect on the seismic performance of composite frame structures.
In order to fully utilize the load-bearing capacity and seismic resistance potential of the CFST frame-core tube structure and enhance the safety of important engineering structures under strong earthquakes, the enhanced constraint measures including stirrup-confining concrete at column end and “strengthening coupling beam and wall pier ” were used in the CFST frame-core tube structure system. The influence of these measures on displacement response, plastic energy dissipation, stiffness damage, second line of defense and failure mode were discussed. The analysis results showed that:
1)The core tube as the first line of defense for the structure, when the plastic deformation of the structure is small, the measure of “coupling beam and wall pier strengthened” has a greater impact on structural deformation. However, the measure of column end stirrup-confined has a significant impact on the interlayer deformation of the bottom reinforcement area when in the stage of large plastic deformation.
2)The measure of column end stirrup-confined has a significant impact on the seismic performance of the external frame as a second line of defense. It can enhance the loading-bearing capacity of CFST columns and ensure that the columns in external frame do not suffer serious damage during super strong earthquakes. In this case, more torque and shear force are borne by the external frame, coordinating with the core tube to resist seismic force, reducing the damage and plastic deformation of the core tube, effectively exerting the seismic performance of the second line of defense, and ultimately achieving the goal of not collapsing.
3)The measure of column end stirrup-confined has changed the failure mode of “strong core tube and weak frame”, and improved the energy dissipation capacity of concrete-filled steel tube column， significantly improving the toughness of CFST columns and limiting the displacement of the core tube in large plastic deformation, further expanding the energy dissipation range of the wall pier. At the same time, this measure makes the plastic energy dissipation distribution of wall pier in core tube and CFST columns in external frame more reasonable, forming a reasonable double line of defense failure mode, slowing down the degradation of structural stiffness, improving the ductility and seismic toughness of the structure.
Fundamentals based on periodic structure in Solid-State Physics are applied to seismic isolation and propagation in periodic structure.
Firstly, a compound periodic steel structure parameterized by its phase differences between cells is constructed: SupposeIt is supposed that a steel structure contains no less than 2 orientable periodic cells, and these cells are glued together one by one. Beginning from the fixed first cell, by isometric mapping composed of gluing and orientation maps, it aligned with the 2nd cell forms a 2-cell periodic structure with positive phase difference between their orientations, then the 2-cell structure extends at its both ends into an 1-dimensional periodic structure containing at least 2 orientable periodic substructures. In this 1-dimensional compound periodic structure, each cell further extends on plane, or into space, according to isometric mapping composed of gluing and orientation maps, and shapes a 2-dimensioanal, and those compound periodic structures of dimensions bigger than 2. Composition of gluing maps with orientations plays the mapping role at those common parts of periodic structures glued together one by one, propagates seismic wave from the beginning cell to the next cell, in turn seismic wave ranges through the integrated compound periodic structure.
Secondly, seismic response parameterized by wave number, number of gluing mappings, scale of the structure, and in these parametersphase differences in the structures is computed in compound periodic structure. By linear response assumption, response with each of parameters is computed respectively at first, then total response of compound periodic structure is obtain from their summation.
Seismic wave is stochastic, and obeys a given stochastic differential equation, and then structural response is also set of values with probability. Seismic responses in 2-cell and 1-dimensional periodic structure are discussed at first. In 2-cell compound periodic steel structure, the given stochastic differential equation is input at its first cell, under certain starting condition and boundary condition, and with the help of set-valued stochastic process and martingale knowledge, seismic response in the first cell is calculated; further by phase difference and trigonometric function operations, seismic responses in the second cell of 2-cell structure and each cell in 1-dimensional structure are calculated.
Finally, Markov controlling to structural seismic response is performed to select the optimal phase angle, and realize a sort of polynomial response goal: according to the set-valued stochastic process property of the response in periodic cell, it is represented by a diffusion process, and its infinitesimal generator is used to construct the corresponding Dirichlet-Poisson equation, further the corresponding Hamilton-Jacobi-Bellman（HJB）equation is constructed to solve the optimal phase angle, substituted back into the Dirichlet-Poisson equation to solve the controlling result. The result decreases as propagating time and initial amplitude of seismic wave increase. Simulating the given stochastic seismic wave by Monte Carlo method, and generating a stationary distribution, then sampling the given stochastic seismic wave by this distribution, calculation of variances of the sampling series and the Markov controlling result and comparison of expectations of indicator function of the given stochastic seismic wave and the Markov controlling result show that variance of the Markov controlling result is smaller than that of sampling series, what is more, the Markov controlling deceases expectation of the given stochastic seismic wave.
Realization of both representation to seismic response in compound periodic steel structure by stochastic equation and its Markov controlling is in servitude to reference for its stochastic prediction.
The static behavior of chevron brace when its beam is designed by a smaller unbalanced force is reviewed and it is revealed that the post-buckling behavior is fully dependent on the capacity of beam carrying unbalanced force. Lateral force-drift angle curves of dual systems composed of frame and chevron brace with their lateral capacity ratios of 1∶1 and 1∶3 are introduced, the capacity of the beam in chevron brace can carry 1.0 and 0.3 times the codified unbalanced force, respectively, totally 4 series of examples. Because the drift angles at brace buckling, tension yielding and frame lateral mechanism increase in order, brace buckling occurs first and then its capacity deteriorates, but the capacity of the frame develops and compensates the decrease of the compressive brace force, the total lateral capacity of the system after brace buckling depends on both the frame/brace capacity ratio and the capacity of the beam in chevron brace carrying unbalance force in chevron brace. Under the demand of assuring the total deteriorating percentage less than 20%, a relation between the frame/brace capacity ratio and the unbalance force ratio ratio is proposed.
