Special-shaped Concrete-filled Steel Research Articles

Design of improved multi-cell L-shaped CFST columns under compression and bending

In recent years, there has been a notable surge in proposals for various forms of special-shaped concrete-filled steel tubular (CFST) columns. Despite various methods developed by researchers for determining the bearing capacity of multi-cell special-shaped CFST columns under different loading conditions, these methods remain lack unified. In this study, FE models were developed to simulate performances of improved multi-cell L-shaped CFST (ML-CFST) column under compression and bending. Parametric analyses have been performed to evaluate impact of various factors, including steel yield strength fy, compressive strength of concrete fcu, steel thickness t, loading direction θ, web height c, and axial load ratio n, on performances of ML-CFST columns. The findings of study indicate that the unidirectional flexural capacity of ML-CFST member exhibits a nearly linear relationship with the variables of t, fy, and c, with a minimal impact observed from fcu on flexural capacity. The width of extended section b, fcu, fy, and t have a certain effect on flexural capacity at any angle, particularly fy, t, and b. While fcu, fy, and t exert some influence on N/Nu-M/Mu correlation curves, their influences are marginal compared to the loading direction, which is the predominant influencing factor. Additionally, Mx0/Mπ/4-My0/M3π/4 curves gradually protrude outward with the increase in n. The effects of t, fy, and fcu on the shape of Mx0/Mπ/4-My0/M3π/4 curve were relatively insignificant. Simplified unidirectional flexural capacity calculation models were proposed based on stress analysis of cross-section under ultimate states. Given that flexural capacities estimated by simplified calculation model are slightly conservative, it is recommended to increase the flexural capacities by 10 %. Furthermore, an elliptical equation was formulated to predict the flexural capacity at any angle, with the predictions being slightly conservative. Based on ultimate equilibrium theory, a simplified calculation method was presented to predict unidirectional eccentric bearing capacities of ML-CFST columns. Through regression analysis, a simplified calculation method expressed as polar coordinate form for biaxial eccentric bearing capacity was established, with calculated values aligning well with FE results.

Investigation on special-shaped concrete-filled steel tubular column under combined compression and torsion

This paper discussed the combined compressive and torsional performance of special-shaped concrete-filled steel tubes (SCFST) columns. A pseudo-static test research of eight specimens including two kinds of stiffening forms was carried out under compression and torsion. The investigated factors include section shape, stiffening form, stiffening position and axial compression ratio. From the test, specimens presented the characteristics of compression-torsion failure. The steel tube experienced buckling deformation and separated from the concrete at the middle and bottom of the column. The welds of some specimens cracked from the top to the bottom at the concave corner during the stage of decreasing load capacity. This resulted in localized crushing and the generation of torsional oblique cracks in the concrete. The hysteresis curves of the SCFST columns were relatively full in spindle shape without obvious pinching phenomenon, representing a good energy dissipation capacity. The ductility of the multi-cell CFST (MCFST) column is slightly better than that of the tensile-bar stiffened CFST (BCFST) column. There is a positive correlation between the torsion bearing capacity and parameters, such as the column limb width-thickness ratio, steel tube thickness and yield strength. When the axial compression ratio is less than 0.5, the axial compression can improve the torsion bearing capacity; when the axial compression ratio exceeds 0.5, the existence of axial compression will weaken the torsional capacity. The finite element model was established for parameters analysis, and calculation method for the T/Tu-N/Nu bearing capacity correlation curve was proposed, and the calculation results are more conservative and have certain safety reserve.

Investigation on special-shaped concrete-filled steel tubular column under pure torsion

This paper discussed the torsional performance of special-shaped concrete-filled steel tubes (SCFST) columns. A pseudo-static test research of nine specimens was carried out under pure torsion. The investigated factors include section shape, stiffening form, width-to-thickness ratio of column limbs and steel tube thickness. The failure modes, hysteresis curves, skeleton curves, energy dissipation capacity, and stress distribution were analyzed to investigate the torsional performance of specimens. The overall failure modes of different section forms are ductile damage. There are a number of 45°oblique buckling in the steel tube, and the steel plates at the top and bottom of the column are transversely torn. At the concave corner, the weld of multi-cell and the welded point of the tensile steel bar were cracked from the top to bottom. Besides, the concrete is mainly crossed by torsional oblique cracks. The hysteresis curves of the SCFST columns are plump shape without pinching phenomenon, representing good energy dissipation capacity. The effect of stiffened methods is the most significant for the torsional performance. The ductility and bearing capacity of multi-cell specimens are better than those of tensile-bar stiffened specimens. Based on the same torsion ratio of concrete and steel tube and ignoring the effect on the tensile bars, calculation methods for initial torsional stiffness and torsional bearing capacity were proposed. Comparison results show that the calculation method has good performance in terms of the accuracy and efficiency.

Seismic performance of T-shaped CFST column to U-shaped steel composite beam joints

U-shaped steel-concrete composite beams have the advantages of high bearing capacity, excellent fire resistance and quick construction, when it is used with special-shaped concrete-filled steel tube columns could achieve the architectural effect of no exposed beams and columns in the interior, showing great application potential in residential buildings. In this study, three full-scale T-shaped CFST column to U-shaped steel composite beam joints with different configurations were tested under cyclic loads. The damage modes, and hysteresis performance of the joint were discussed in detail. The test results indicated that the three joints with side plate connection all showed plastic hinge damage at the beam ends of which hysteresis curves show bowed shape, with excellent energy dissipation. The setting of truss reinforcement within the U-shaped steel beam could effectively improve the ductility of the member. Based on the validated finite element model, the effects of side plate configuration, the side plate position, and tube wall thickness on the seismic performance of the joints are investigated. The design methods for the bending capacity of the joints have also been proposed.

Fire research of joint between special-shaped CFST column and U-shaped composite beam

The steel-concrete composite joint plays an important role in the building structure of transmitting load and maintaining structural stability, and the research on fire performance of composite joints is significant. The special-shaped concrete-filled steel tube (SCFST) column and U-shaped steel-concrete composite beam (USCB) have good fire resistance which is proved by authors' previous fire tests. Therefore, the fire performance of the joint between SCFST column and USCB, the temperature distribution, failure mode, fire resistance, and internal force distribution of the joint under fire are investigated based on the fire tests results of column and beam component. The results showed that the temperature of the joint zone is lower than that of the non-joint zone, which ensured that the strength and stiffness of the joint zone are higher than that of the non-joint zone (beam and column). Then the two failure modes including beam failure and column failure occurred in the composite joint in fire conditions. Furthermore, the internal force analysis indicated that internal force redistribution happens during the elevated temperature. The parameter analysis expressed that fire resistance decreases with the increase of load ratio and the beam-column moment ratio and increases with the increase of the beam-column stiffness ratio and the fire protection layer thickness. Finally, the high-temperature calculation formula for the bearing capacity of beam-column joint was proposed.

United composite strength calculation method for special-shaped CFST columns with multiple cavities

The present study investigates the method of calculating the composite strength of special-shaped concrete-filled steel tube (CFST) members with multiple cavities. Based on the research findings of CFST unified theory and some tests, a quantitative method to evaluate the concrete constraint effect is proposed for cross-sectional shapes ranging from regular triangles, squares…regular octagons to circles. The cross-sectional composite strength of regular polygonal CFST members is determined by the way dividing the concrete into effective and ineffective constraint regions, discounting constraint coefficients, and applying unified formulas. Additionally, a regression analysis is conducted to establish the relationship between interior angles and constraint effects. Subsequently, using quadrangle CFSTs as an example, this study investigates the relationship between cross-sectional regularity and constraint effect, and proposes a composite strength calculation method related to it. Finally, the special-shaped multi cavity cross section is separated into several simple polygonal cross sections, taking into account shape efficiency and regularity, to calculate the composite strength of each cavity CFST member and summarize them for determining the total axial compressive bearing capacity. The research findings indicate that a linear relationship exists between the interior angle of the cross section and the initial tangent line angle of the second-degree curve, which serves as the boundary between effective and ineffective constraint regions in regular polygonal steel tube confined concrete. The proposed unified composite strength calculation method, specifically designed for special-shaped CFST columns with multiple cavities, can accurately predict their actual strength.

Practical temperature calculation and fire-resistant design of special-shaped concrete-filled steel tubular columns

The fire performance of special-shaped concrete-filled steel tubular (SCFST) columns under axial load was investigated in this paper. The ABAQUS based finite element models were conducted for heat transfer and structural analysis. Twelve fire tests on SCFST columns under the ISO-834 standard fire curve by the authors' research were used to verify the model. In addition, heat transfer and fire-resistant mechanism of full-scale SCFST columns were studied; and extensive parametric studies were investigated to examine the influence of fire protection layer thickness, column limb thickness, slenderness ratio and other factors on temperature distribution and fire behavior of SCFST columns. Through the calculation and analysis of temperature field and fire resistance, a simplified temperature calculation method for steel tube, tension reinforcement, and inner steel plate are proposed along with the modularization temperature calculation method for special-shaped concrete sections. Moreover, the ultimate bearing capacity calculation method for SCFST columns under the standard temperature rise curve is proposed allowing for the influence of high temperatures on material properties, which can also be applied to determine fire resistance of SCFST column. Finally, the calculation method for the thickness of fire protection layer and the corresponding fire design is proposed based on the delaying temperature mechanism.

Design of multi-cell T-shaped concrete-filled steel tubular columns under compression and bending

To date, a number of studies have been done on the calculation methods for predicting bearing capacities of multi-cell special-shaped concrete-filled steel tubular (MS-CFST) columns under axial, pure bending, and eccentric load. However, most of the existing methods only apply to a specific combination form of cross-section and have not been established in a unified formulation. In this study, finite element (FE) models were established to predict the performances of the four common combination forms of multi-cell T-shaped concrete-filled steel tubular (MT-CFST) sections subjected to axial, eccentric, and pure bending loads, and verified with the available test data. The parametric analyses were conducted to investigate the effects of steel thickness t, concrete strength fck, steel yield strength fy, aspect ratio of the web c/b, and slenderness ratio λ on the flexural and eccentric compressive performance based on the FE models. The study results demonstrated that the four common combination forms of MT-CFST sections showed the same changing trends of flexural capacity with different parameters, and the flexural capacity rose almost linearly with the increase of the parameters. The parameters (t, fy, fck, c/b, and λ) had a definite effect on the N/Nu-M/Mu correlation curve, especially c/b and λ. Furthermore, by analyzing the stress distribution of mid-span cross-section at ultimate state, the simplified calculation model for asymmetric flexural capacity applicable to different combination forms of MT-CFST section was put forward. The reduction factor of 0.9 for positive flexural capacity (WC) and the restricted width-to-thickness ratio 60235/fy are recommended. Based on the study of the constraint stress of steel tube to core concrete and the concrete constraint region distribution for the columns subjected to compression, the calculation method for predicting the axial bearing capacity applicable to different combination forms of MT-CFST section was proposed. According to the parametric analysis and shape of N/Nu-M/Mu correlation curve, the simplified method for N/Nu-M/Mu correlation curves applicable to different combination forms of MT-CFST sections was developed, which simultaneously took the influence of slenderness ratio into consideration. Besides, the calculation methods for MT-CFST members proposed in this paper have been verified with available test results and FE ones.

Elastoplastic analysis of shear resistance of special-shaped concrete-filled steel tubular connection

The experimental and analytical methods were used in this research to comprehend the elastoplastic shear behavior of special-shaped Concrete-Filled Steel Tubular (CFST) joint and develop damage models for its joint under low-reversed cyclic and axial loading. It is identified that the shear failure occurs in the panel zone and the joint's shear force-lateral displacement hysteretic curves and stress distribution are obtained. The joint is divided into individual components, including webs, steel frame, and core concrete, so as to investigate the elastoplastic shear behavior of the joint at the material level. In order to accurately calculate the elastoplastic shear resistances of joint components while considering the loading and boundary conditions and interactions between web and flange limbs, the shear resistances models of the webs, steel frame, and core concrete are presented based on the analytic function of shear stress, plastic hinge damage mechanism, and concrete compression strut mechanism. Furthermore, the above models are used to develop a design calculation formula for the shear resistances, in which the plastic development of the webs, the stress state of the plastic hinges, and the shear resistance increasing factor of the concrete compression strut can be considered. Finally, a design formulation to calculate the shear capacity of the special-shaped CFST joint is put forward，and the calculated results are compared with experimental data.

Seismic performance and design method of T-shaped CFST column to U-shaped steel-concrete composite beam joint with vertical stiffeners

Special-shaped concrete-filled steel tubular (CFST) columns and cold-formed U-shaped steel–concrete composite beams (CUCB) have recently been developed for efficient steel–concrete composite construction. In this study, five half-scaled T-shaped multi-cell CFST columns to CUCB joint specimens with different vertical stiffener dimensions were designed for pseudo-static test. The seismic performance of the proposed joints was analyzed based on the failure modes, hysteresis curves, ductility, and stress distribution. The experimental results show that joints with vertical stiffeners exhibit favorable seismic performance and can be considered rigid joints in a braced frame according to Eurocode 3. A finite element (FE) model for the joint was developed using ABAQUS and verified with the available experimental results. Furthermore, a parametric study was conducted to explore the effects of the vertical stiffener dimensions on joint strength and stiffness. The numerical results indicate that the vertical stiffener dimensions can significantly enhance joint strength and stiffness. A design method of such joints was proposed for practical engineering applications, including calculation formula for joint flexural capacity as well detailing for joint stiffness.

Seismic performance of T-shaped CFST column to U-shaped steel-concrete composite beam joint

Both novel composite structures, special-shaped concrete-filled steel tube (CFST) column and cold-formed U-shaped steel-concrete composite beam with stiffened rebar truss (CUCB), have advantages of good mechanical behavior, appealing architecture, and convenient construction. In this study, a series of five half-scaled specimens of CUCB to T-shaped CFST column frame joints with vertical ribs were tested under cyclic loading. Different connection detail types were designed considering both mechanical and constructional aspects. The bearing capacity, stress distribution, stiffness degradation, energy dissipation capacity, ductility, and joint stiffness were evaluated to understand the seismic performance of these new joints. Experimental results show that joints with vertical ribs have favorable seismic performance and could be considered as rigid joint in a braced frame according to Eurocode 3. Based on the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method, a multi-criteria analysis model for evaluating the joint performance was performed to determine the most effective connection detail considering the mechanical behavior and construction cost. As a result, the proposed novel joints are seismically feasible.

Experimental Investigation of Special-Shaped Concrete-Filled Square Steel Tube Composite Columns with Steel Hoops under Axial Loads.

Special-shaped concrete-filled steel tube (SS-CFST) columns can be embedded in the wall, thus preventing the columns from protruding. This feature makes it popular in steel residential buildings. This paper proposes a new special-shaped concrete-filled square steel tube (SS-CFSST) composite column composed of multiple square steel tubes connected by steel hoops to form L-, T- or cross-shaped sections. Eight specimens were tested under axial loads with section shape, construction method, slenderness ratio, steel tube thickness, and steel strength as variation parameters. The structural performance, such as failure modes, peak load, load–displacement curves, load–strain curves, and Poisson’s ratio of the steel tubes, were analyzed. The tests illustrated that the failure modes of hoop-type specimens and weld-type stub columns were mainly the local buckling of steel tubes and bending failure, and those of the weld-type slender columns were mainly overall bending failure. The load-carrying capacity of the hoop-type specimen was higher than that of the weld-type specimen with the same cross-sectional dimensions and slenderness ratio. Next, the stress–strain relationship model of core concrete in the SS-CFSST composite column was established by considering the restraint effect of the connection coincidence area of steel tubes and steel hoops on concrete. Additionally, the finite element model (FEM) of the column was established using this constitutive model. By comparing the failure modes, load–strain curves and bearing capacities obtained from the tests and FEM, the established FEM can accurately evaluate the mechanical properties of SS-CFSST composite columns with steel hoops under axial compression.

Shear resistance of multi-cell special-shaped CFST column to H-section steel beam joint

This paper studies the shear behavior of frame joint between multi-cell special-shaped concrete-filled steel tubular (CFST) column and H-section steel beam based on experimental research. The shear failure mode and hysteretic behavior of the joint were investigated from the experiment. A finite element (FE) model for multi-cell cruciform CFST column to H-section steel beam joint was conducted to study the influences of the following parameters on joint shear resistance: the axial compression ratio of column, aspect ratio of the joint, thickness of steel tube in joint panel zone, and the concrete compressive strength. Stress analysis on the steel tube and the concrete of each cell in joint panel zone was conducted by using the FE model to clarify the load transfer path and shear mechanism. Furthermore, a mechanical model for the joint shear resistance was developed, in which the contributions from the steel tube and concrete in different cells were considered. Based on the shear mechanical model, a calculation method for joint shear resistance is also proposed to facilitate the practical applications and verified by the experimental and numerical results.

Seismic Behavior of Frames with Special-Shaped Concrete-Filled Steel Tubular Columns and H-Shaped Steel Beams

First Name is required invalid characters Last Name is required invalid characters Email Address is required Invalid Email Address Invalid Email Address

The analysis on the seismic behavior of L-shaped prefabricated special-shaped CFST with rectangular multi-cell columns

In this paper, a prefabricated special-shaped concrete-filled steel tube (CFST) with rectangular multi-cell column has been subjected to low cyclic reversed loading experiment and finite element (FE) parameter analysis. A total of three full-scale L-shaped specimens were tested under constant axial compression load and transverse cyclic load. The main variable in the experiment was the direction of loading. The results show that the failure mode of the special-shaped column specimen is compression-bending failure. Within 300 mm of the bottom of the column, the steel tube bulges or even cracks, the concrete powder fells and fails to exit the work. Among them, the L-135° specimen has the highest peak bearing capacity while the L-0° specimen has better ductility. The main variables of FE parametric analysis are axial compression ratio, concrete grade, steel tube thickness and loading angle. The analysis results show that the lower axial compression ratio and higher concrete grade are conducive to the improvement of bearing capacity. As the thickness of the steel tube increases, the peak bearing capacity and displacement of the specimen increases but the ductility decreases. The special-shaped column model has better seismic behavior when loaded along various loading angles. The peak bearing capacity of the special-shaped column specimen loaded at 135° is consistent with the test results and compared with the L-45° specimen, its bearing capacity is increased by up to 62.04%, which has better comprehensive seismic behavior.

Behavior of stiffened and multi-cell L-shaped CFST columns under eccentric compression

The mechanical behavior of the traditional special-shaped concrete-filled steel tubular (CFST) columns can be improved by employing longitudinal stiffeners or using multi-cell cross-section. In practical application, the stiffened and multi-cell L-shaped CFSTs are often subjected to bi-directional loading. However, there are only few research studies on their performance as loaded eccentrically with different orientations of the loading and there is no relevant design guideline. This paper reports tests on 16 L-shaped CFST columns (nine stiffened, six multi-cell specimens and one reference specimen) loaded in eccentric compression. The main variables were the angle indicating the orientation of the load action line (0°, 45°, 135°, 180° and 225°), load eccentricity ratio (0, 0.25 and 0.5) and slenderness ratio (31.5 and 41.9). Finite element models for the eccentrically loaded stiffened and multi-cell L-shaped CFST columns were developed. Based on the parametric analysis, practical design methods were proposed for predicting the bearing capacity based on the stability loss and the axial force vs. bending moment interaction diagrams for the members considering the influence of the load orientation at the end face of the column.

Simulation of Mechanical Properties of Special-Shaped Concrete-Filled Steel Tubular Column and Steel Beam Joints after Fire

To study fire after the mechanical performance of steel girder node special-shaped concrete-filled steel tube column, based on standard ISO - 834 litres of cooling curve, the node temperature field model was established based on finite element software ABAQUS, the compute nodes in the overall uniform temperature field under fire as a result, the reasonable choice of fire after the steel and concrete constitutive model, the temperature field results into the node stress model, considering the factors that influence the whole effect of fire loading in low cycle, the nodes of the finite element model, and contrast analysis of the temperature after the fire of the node and hysteretic performance and ultimate bearing capacity. The results show that the failure modes of special-shaped CFST column-steel beam joints at room temperature and after fire are the same, and the ultimate bearing capacity of the joints after fire decreases significantly by 14.88% compared with that at room temperature.

Behavior of T-shaped CFST column to steel beam connection with U-shaped diaphragm

The connection between special-shaped concrete-filled steel tubular (CFST) column and steel beam is a particular concern to engineers. A novel U-shaped diaphragm connection is introduced in this paper to transfer the moment at beam ends in the frame with special-shaped CFST columns and steel beams. Five T-shaped CFST column to H-section steel beam connections were tested under low-cycle horizontal loading. U-shaped diaphragm was used as the connection to transfer loads from beam to joint panel zone. The experimental parameters include the size of U-shaped diaphragm and the axial load ratio of column. Based on the experimental results, the strength, ductility and strain distribution of the connections were calculated. Test results show that the connection can develop plastic bending capacity in the beam, satisfying the design code requirements of America, Great Britain, and China. Furthermore, a parametric analysis was conducted using the finite element method . The influences of U-shaped diaphragm size, tube thickness, and axial load ratio of column were analyzed. Based on the parametric analysis results, a mechanical model is proposed to calculate the yield strength and ultimate strength of U-shaped diaphragm connections. In conclusion, with a proper design, the T-shaped CFST column to H-section steel beam connection using the U-shaped diaphragm can be effectively used in practical applications. • Seismic loading tests are conducted on five large-scale connection specimens. • Finite element model (FEM) is developed and verified with the experimental results. • Parametric study using the FEM is carried out and mechanical model is proposed.

Research on seismic behavior of special-shaped CFST column to H-section steel beam joint

To investigate the seismic behavior of joint between special-shaped concrete-filled steel tubular (CFST) column and H-section steel beam, a pseudo-static test was carried out on five specimens with scale ratio of 1:2. The investigated factors include stiffening types of steel tube (multi-cell and tensile bar) and connection types (exterior diaphragm and vertical rib). The failure modes, hysteresis curves, skeleton curves, stress distribution, and joint shear deformation of specimens were analyzed to investigate the seismic behaviors of joints. The test results showed the connections of exterior diaphragm and vertical rib have good seismic behavior and can be identified as rigid joint in the frames with bracing system according to Eurocode 3. The joint of special-shaped column with tensile bars have better seismic performance by using through vertical rib connection. Furthermore, a finite element model was established and a parametric analysis with the finite element model was conducted to investigate the influences of following parameters on the joint stiffness: width-to-thickness ratio of column steel tube, beam-to-column linear stiffness ratio, vertical rib dimensions, and axial load ratio of column. Lastly, preliminary design suggestions were proposed.

Seismic behavior of plane frame with special-shaped CFST columns, H-shaped steel beams and vertical rib connections

The seismic behavior of special-shaped concrete-filled steel tubular (CFST) column to H-shaped steel beam plane frame (SCFSTF) with vertical rib connections was investigated experimentally and numerically in this study. Double-bay two-story SCFSTFs with the scale ratio of 1/2 were tested under constant vertical compressive load and cyclic horizontal loads (or displacements), which is a pseudo static experiment considering the axial load ratio. Based on the experimental results, failure modes, bearing capacity, hysteretic curve, skeleton curve, energy dissipation, ductility, rigidity degradation and strength degradation of SCFSTFs were investigated. As a result, a typical strong column-weak beam failure mode was determined. The degradation in strength and rigidity were minimal and energy dissipation capacity and ductility of frames were adequate, demonstrating excellent seismic performance. Besides, energy dissipation capacity and horizontal resistance were improved with axial load ratio increasing within the parametric range of this test. After the test, a finite element method based on software ABAQUS was carried out to further investigate the seismic performance. The numerical results of skeleton curve, rigidity, yield load, peak load and stress distribution agreed well with the test results. The plastic hinge formation process and ultimate failure mode were further investigated using the finite element model.