Concrete-filled Steel Tubular Research Articles

Axial compressive behaviour and design of concrete-filled wire arc additively manufactured steel tubes

The axial compressive behaviour of concrete-filled wire arc additively manufactured (WAAM) steel tubular columns is investigated experimentally in this paper. Firstly, the manufacture of a series of WAAM steel plates and tubes is described. The results of tensile testing performed on coupons cut from the WAAM plates, to obtain the mechanical properties of the printed material, are summarised. 3D laser scanning was employed to generate digital models and to capture the geometric features of the WAAM steel test specimens. Concrete was then cast into the WAAM steel tubes, creating a total of nine concrete-filled steel tubular (CFST) specimens of different diameters, thicknesses and lengths that were subjected to compressive loading. The axial compressive load-deformation responses and ultimate loads of the specimens were obtained and the influence of the as-built surface undulations of the WAAM sections was assessed. Comparisons of the test results against existing structural design provisions highlight the need to consider the influence of the weakening effect of the geometric undulations that are inherent to the WAAM process on the structural response of CFST sections, in order to achieve safe-sided strength predictions.

Comparative Study of Life-Cycle Environmental and Cost Performance of Aluminium Alloy–Concrete Composite Columns

As is widely known, the construction industry is one of the sectors with a large contribution to global carbon emissions. Despite numerous efforts in the construction industry to develop low-carbon materials, there is a limited number of studies quantifying and presenting the overall environmental impact when these materials are applied in a construction project as structural members. To address this gap, this study focuses on assessing the life-cycle performance of novel structural aluminium alloy–concrete composite columns. In this paper, the environmental impacts and economic aspects of a concrete-filled aluminium alloy tubular (CFAT) column and a concrete-filled double-skin aluminium alloy tubular (CFDSAT) column were assessed using life-cycle assessment (LCA) and life-cycle cost analysis (LCCA) approaches, respectively. The cradle-to-grave system boundary is considered for these analyses to cover the entire life-cycle. A concrete-filled steel tubular (CFST) column is also assessed for reference. All columns are designed to have the same load-carrying capacity and, thus, are compared on a level-playing basis. A comparison is also made of the self-weight of these columns. In particular, the self-weight of the CFST column is reduced by around 17% when the steel tube is replaced by an aluminium alloy tube, and decreased by 47% when the double-skin technique is adopted in CFDSAT columns. The LCA results indicate that the CO2 emission of CFST and CFAT is almost the same, which is 21% less than the CFDSAT columns due to the use of high aluminium in the latter. The LCCA results show that the total life-cycle cost of CFAT and CFDSAT columns is around 29% and 14% lower, respectively, than that of the CFST column. Finally, a sensitivity analysis was carried out to evaluate the effects of data and assumptions on the life-cycle performance of the examined columns.

Modified plastic stress distribution method incorporating size effect for strength prediction of CFST members

Large-size concrete-filled steel tubular (CFST) members are widely applied in long-span and heavily-loaded structures due to increasing engineering demands. However, due to the size effect, applying the existing design methods to large-size CFST members may overestimate the load capacity, leading to unsafe designs. The plastic stress distribution (PSD) method is widely used as the primary method in typical design codes for calculating the strength of CFST members that are not susceptible to local buckling, while the PSD method was mainly used previously to evaluate the strength of CFST members with relatively small size. In this work, the experimental database of short CFST members with compact sections was complied, and the applicability of the PSD method was evaluated. The results showed that the test-to-calculated ratios decreased with increasing size of CFST members, and unsafe designs emerged for large-size CFST members. Based on the observations, the mechanics-based modified plastic stress distribution (MPSD) method was proposed, which not only considered the size effect at the material level similar to the way treated in most of the previous calculation methods, but also incorporated the size effect of the interaction action between the steel tube and concrete at the member level. Finally, the MPSD method was evaluated against the experimental database, and good agreement was achieved. These results provide strong justification for changes to the PSD method in design standards.

Axial hysterestic behavior of prestressed CFDST columns for lattice-type wind turbine towers

The escalating power outputs of wind turbines necessitate enhanced load-bearing capabilities in their support structures. A new type of prestressed concrete-filled double skin steel tubular (CFDST) lattice-type wind turbine tower has been proposed to replace the original steel-concrete hybrid tower. The corner columns of the tower are made of prestressed CFDST columns, with the prestressing steel strands situated within the hollow area. While numerous studies have researched the axial characteristics of concrete-filled steel tubular (CFST) columns, investigations into prestressed CFDST columns subjected to axial cyclic loading remain sparse. To address this research gap, this study carried out the experimental and finite element studies of eight prestressed CFDST columns under axial tensile, axial compressive, and tensile-compressive cyclic loads. Detailed analyses of failure modes, hysteresis curves, stiffness degradation, skeleton curves and ductility were conducted. The test results indicate that the prestressing enables the concrete to establish good contact with the steel tubes, thereby preventing the premature cracking. At a cost of approximately 6.8 % reduction in axial compressive load, the axial tensile load of the structure is enhanced by about 47.2 %. Furthermore, an advanced finite element (FE) model, refined based on the test, closely matched the experimental data, thereby validating its accuracy for subsequent mechanism and parameter investigation.

Study on seismic performance and restoring force model of framed CFST column-composite box beam joints

Quasi-static tests are conducted on framed concrete-filled steel tubular (CFST) column-composite box beam joints to study the deformation characteristics and restoring force performance under seismic loads. The joints' seismic performance is evaluated. This study shows that the joints with three different connection forms (single-limbed diagonal brace, cross brace, and diaphragm) in the core area have good bearing and energy dissipation capacities. As a result, the joints exhibit excellent seismic performance. The theoretical and fitting analyses of test results reveal the stiffness degradation law for this type of joint. Additionally, a fixed-point directional degradation trilinear restoring force model is established and compared with the test results. The research results indicate that the proposed restoring force model accurately predicts the nonlinear recovery behavior of the joint under seismic loads. This model is in good agreement with the hysteretic curve of the test, providing a theoretical basis for the seismic design and performance evaluation of the joint and for the seismic design and toughness optimization of a structure.

Prediction of the Strength of the Concrete-Filled Tubular Steel Columns Using the Artificial Intelligence

Introduction. The machine learning algorithms are highly promising for predicting the load-bearing capacity of the building structures. The paper aims at building the predictive models for calculating the strength of the concrete-filled steel tubular (CFST) columns to enable a highly accurate prediction of the ultimate loads for the entire possible range of parameters affecting the load-bearing capacity of the eccentrically compressed columns.Materials and Methods. The article studies the eccentrically compressed short concrete-filled steel tubular (CFST) columns of circular cross-section. Model input parameters: column outer diameter, pipe wall thickness, yield strength of steel, compressive strength of concrete, relative eccentricity. Output parameters: the ultimate loads without taking into account and taking into account the random eccentricities. The models were trained on synthetic data generated based on the theoretical principles of the limit equilibrium method. Two machine learning models were built. When training the first model, the ultimate loads were determined at a given eccentricity of the longitudinal force without taking into account the additional random eccentricity. When training the second model, the additional random eccentricity was taken into account. The effect of the features on the model predictions was assessed using the Feature Importance function. The Optuna method was used to select the hyperparameters. The machine learning models were implemented in the Jupiter Notebook environment using the Gradient Boosting learning method. The total volume of the training sample was 179 025 samples.Results. The importance of the features most affecting the predictive values of the model have been determined. For both models, the outer diameter of the column and the relative eccentricity have proved to be the most important features, which is consistent with the existing experience of designing and calculating such structures. Optimisation of the hyperparameters using the Grid Search method enabled getting the improved results. The high accuracy of prediction has been ascertained by the low values of the regression metrics: MSE = 9.024; MAE = 9.250; MAPE = 0.004 — for the model built without taking into account the additional random eccentricity; MSE = 8.673; MAE = 8.673; MAPE = 0.004 — for the model built taking into account the additional random eccentricity.Discussion and Conclusion. The developed Gradient Boosting models for predicting the ultimate loads of the eccentrically compressed short concrete-filled steel tubular (CFST) columns of circular cross-section, both without taking into account and taking into account the additional random eccentricities, have demonstrated high accuracy and stability of prediction, they can be applied for assessing the strength of the columns during design and construction, which will reduce the time and resources involved in physical testing. In the future, it is planned to expand the data range by including other materials, different cross-section geometries of the columns and a slenderness parameter, which may improve the generalization ability of the model.

Optimisation of Beam Member Loading

Introduction. The I-beams are deemed to have the highest load-bearing capacity. However, in practice, due to wide spreading and affordability of pipe-rolling products, the tubular beams are being used quite often. The load-bearing capacity of these beams should be compared under the condition of their equal mass per running meter. An I-beam according to the GOST R 57837-2017 with the mass of running meter equal to 194 kg and a pipe according to the GOST 33228-2015 with the mass of 194 kg/m have been compared. The load-bearing capacity of an I-beam was almost twice as high as that of a tubular beam. The data about the concrete filled steel tubular (CFST) beams, including the ones with the prestressed concrete core at the bottom, is also provided. In such beams, a steel pipe works as an external reinforcement — exo-reinforcement. The load-bearing capacity of the CFST beams is quite considerable taking into account their low cost and good processability. The present research aims at increasing the load-bearing capacity of the tubular beams, which will expand the range of the construction products.Materials and Methods. The geometric optimisation and mental experiment methods have been used. The idea of using the fluid filling material for a tubular beam is based on the well-known property of fluid — its almost complete incompressibility. The maximum volume of a geometric long body with the rectilinear generatrix of lateral surface (for a given lateral surface) is reached if its cross-section has the shape of a circle, which corresponds to a round pipe.Results. A tubular beam with the fluid filling material (a hydraulic beam) is a round pipe blanked off at both ends, completely filled with fluid (without air pockets). When a hydraulic beam is loaded, its lateral surface tends to deform. Consequently, the internal volume of the pipe tends to decrease. However, since fluid is incompressible, its volume doesn’t decrease, which, in turn, prevents the pipe from deformation.Discussion and Conclusion. In a hydraulic beam, due to fluid, the entire load is distributed relatively evenly over the whole internal surface of a beam. The load-bearing capacity of a hydraulic beam has been estimated, which is five times higher than that of an I-beam and ten times higher than that of a tubular beam.

Group resistances of tensile-loaded anchored-bolted connections with CFST columns: Tests, simulations, and theoretical models

The performance of the connections in concrete-filled steel tubular (CFST) columns fabricated with a group of two anchored blind-bolts under tensile forces is evaluated. Based on the bolt group arrangement, two sets of experiments, comprising eight full-scale specimens, were tested. Test parameters include the presence of concrete, bolt anchorage length, pitch distance, and gauge distance. Further, numerical models were developed, which, after validation, were used for parametric studies. Key findings indicate that bolt gauge distance, anchorage length, concrete grade, and column cross-section slenderness significantly influence the connection behaviour, but pitch distance has the least influence. Tube and concrete failure modes, as well as critical gauge and pitch distances, are identified, which govern tube deformation and concrete crushing zones. Finally, a bi-linear theoretical model is developed that can provide a fair prediction of the connection strength and stiffness.

Structural behavior of partially confined circular CFST columns under lateral cyclic loading

The seismic performance of concrete-filled steel tubular (CFST) columns is severely influenced by the failure at the potential plastic hinge zone. This paper proposes a novel confinement technique at the potential plastic hinge region using an inner circular steel tube for the circular CFST columns named partially confined concrete-filled steel tubular (PCCFST) columns. This confinement technique strengthens the plastic hinge region, and hence, the overall structural performance of the columns is enhanced. To verify the effectiveness of the proposed technique, a total of 7 columns: 6 PCCFST and 1 counterpart CFST have been tested under lateral cyclic loadings while subjected to constant axial load. Failure mode, hysteretic behavior, flexural capacity, ductility, and energy dissipation ability of the PCCFST specimens were investigated by varying the diameter and height of the inner circular steel tube. It was seen that the height of the circular inner steel tube has an obvious effect on the cyclic performance of the columns. For the PCCFST columns considered, the maximum lateral strength, ductility, and energy dissipation ability were enhanced by a maximum of ∼ 16 %, 24 %, and 14 %, respectively, when compared with the counterpart CFST column. A new design guideline has been proposed based on the Eurocode and limited experimental data obtained from the present study. A comparison between the experimentally determined maximum lateral strength and the proposed guideline has been made and is found to be in good agreement.

Optimization study of strengthened T-shaped concrete-filled steel tubular (CFST) column-steel beam joint

In this paper, a strengthened T-shaped concrete-filled steel tubular (CFST) column-steel beam joint is proposed. The joint is strengthened in the core region of the column wall as well as the beam, to avoid buckling of the column wall and outward displacement of the plastic hinge at the beam end. To study the damage modes and seismic performance of the joint, low-cycle loading experiments on two full-scale specimens of the joint were carried out. The results show that each specimen was damaged by plastic hinging of the steel beam flanges, the column walls did not buckle, and all had a good seismic performance. In addition, Abaqus finite element software was utilized to optimize the joints by analyzing the effects of changing the dimensions of the strengthened column and modifying the structure of the beam-column connection. Simulation results show that increasing the thickness and height of the strengthened column wall enhances the joint's load capacity, stiffness, and energy dissipation. it is recommended that the width-to-thickness ratio of the strengthened columns should be at most 0.08 and the beam-to-column height ratio should be kept at least 1.3. Additionally, two optimization methods, reducing the wall thickness of the steel corbel and weakening the steel beam flange, can effectively reduce weld tearing at the beam-to-column connections, while guaranteeing a good seismic performance. It is suggested that the slope of the steel corbel is not less than 1:6.25, and the weakening ratio of the steel beam flange is not greater than 0.267.

Axial compressive performance of spirally reinforced high-strength CFST column

The concrete-filled steel tubular (CFST) member using high-strength materials could have less ductility than its counterpart using normal-strength materials. In this study, spiral reinforcement is used to enhance the performance of CFST columns using high-strength materials. Experiments were conducted for spirally reinforced high-strength CFST stub columns. The highest yield strengths of spiral reinforcement and steel tube were 1597.6 MPa and 771.7 MPa, respectively, and the highest compressive strength of concrete was 103.1 MPa. Results showed that the presence of high-strength spiral reinforcement effectively improved both the strength and ductility of composite columns, while the fracture of spiral reinforcement was observed in tests. A numerical study revealed that spiral reinforcement provided effective confinement to the high-strength core concrete. The steel tube and spiral reinforcement reached respective yield strength corresponding to peak load. The spirally reinforced CFST column exhibited higher resistance than the equivalent normal CFST column using a thick tube. A design-oriented strength prediction method was proposed and was validated against the test data. The average ratio of formula-predicted to experimental resistances was 0.934, with a coefficient of variation being 0.079.

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.

Experimental study on seismic behaviour of thick-flange steel beam to square CFST column joints with internal diaphragms

Composite structures made of concrete-filled steel tubular (CFST) columns and steel beams have gained widespread usage in high-rise and long-span structures, and beam-to-column joints strengthened by an internal diaphragm (ID) are expected to be efficient joint patterns. However, the thickness ratio (ηbc) limit of beam-to-column flanges is not considered sufficiently in current design codes and existing studies. Moreover, the previous tests rarely included specimens with a beam flange thickness reaching 40 mm. Thus, three joint specimens were designed and tested under cyclic loadings to further investigate the mechanical properties of the ID-type joints and guide engineering applications. The effects of the thickness ratio of the beam-to-column flange were mainly considered in this study. All beam flange thicknesses were 40 mm. Two failure modes were observed, including excessive development of beam plasticity and failure of local connections. Specifically, as the thickness of column walls decreased, the seismic behaviour, such as the resistance, stiffness and ductility of the joints, deteriorated obviously. Two out of three joint specimens satisfied the AISC requirements for composite special moment frames (C-SMF). Based on the test results, the limit value of the thickness ratio of beam-to-column flange (ηbc) has been proposed for design purposes.

The ultimate capacity of geopolymer recycled aggregate concrete filled steel tubular columns: Numerical and theoretical study

Concrete-filled steel tubular (CFST) columns have been extensively used due to their numerous advantages including high load-bearing capacity, stiffness, energy absorption, and ductile failure mode. Human awareness toward environmental problems is increasing and research on geopolymer recycled aggregate concrete (GRAC) has gained much attention as a green and sustainable material. However, there is little experimental and numerical research on the behavior of CFST columns with the GRAC as an infill material with the available design codes not yet modified for addressing the utilization of unconventional concrete mixes, such as the GRAC. This research introduces a critical modification of the ACI code provisions using the nonlinear finite element analysis method (FEA) on the behavior of square GRACFST columns under axial compression where the proposed modification was verified using experimental data from the literature. The influence of the length-to-width ratio, width-to-thickness ratio, geopolymer concrete strength, and the recycled aggregate replacement ratio was investigated. Results show that the outward local buckling was the primary failure mode for both short and slender columns. Considerable improvements were observed in the column's load-bearing capacity, initial stiffness, ductility, and energy absorption by (81.0, 101.9, 46.5, and 87.8)%, respectively, achieved upon increasing the steel tube thickness from 3 mm to 9 mm. However, the increase in length-to-width ratio negatively affects the energy absorption by 39 % while the ductility was reduced up to 20.4 %. The predictability of the AISC360-16, ACI318-19, and BS5400-5 codes on the capacity of GRACFST columns was evaluated where conservative predictions were guaranteed with the closest one recorded for the ACI318-19 code with a 24.5 % overestimation percentage. Finally, a modification was proposed to the ACI code prediction, taking into account the inclusion of recycled aggregate and geopolymer concrete with only a 2 % average error.

3D numerical study of axial compression behavior of concrete-filled steel tubular columns from mesoscopic perspective

This study investigates the axial compression behavior of Concrete-Filled Steel Tubular (CFST) short columns from a mesoscopic perspective. Concrete is regarded as a three-phase composite material consisting of aggregates, mortar, and interfacial transition zones (ITZs). The Concrete Damaged Plasticity (CDP) model was employed to simulate the nonlinear mechanical response and fracture characteristics of concrete. Verified against experimental results, the numerical model excels in realistically simulating the uniaxial compression behavior of CFST short columns. Subsequently, factors influencing the macroscopic behavior of CFST short columns were investigated. Results indicate that the random aggregate model adopted for concrete enables a more realistic prediction of the behavior of CFST short columns. Additionally, circular section steel tubes demonstrate a stronger confinement effect compared to square ones. With increasing lateral confinement, the properties of CFST short columns, such as bearing capacity and ductility, are significantly enhanced.

Interaction and compatibility between steel and concrete of circular CFST stub columns with high-strength materials

To promote the application of high-strength-materials in concrete-filled steel tubular (CFST) columns, finite element modeling, and parametric analysis were conducted on circular CFST stub columns filled with normal-strength concrete (NSC) and ultra-high strength concrete (UHSC). For simulating properties of concrete under passive confinement, a three-dimensional constitutive model for NSC and UHSC was established, of which the stress-strain relation and evolution of dilation angle are both related to confining pressure and axial plastic strain. The model was verified by tests of CFHSST (concrete-filled high-strength steel tube) and UHSCFST (ultra-high-strength concrete-filled steel tube), demonstrating that the interaction between concrete and steel tube can be revealed correctly. Parameter analysis of CFST stub columns filled with NSC and UHSC was carried out, with the grade of steel tube covering from Q235 to Q890 and the D/t ratio ranging from 8.9 to 75. With the decrease of D/t ratio, the deformation capacity and post-peak performance of columns are improved. A recommended range of steel contribution ratio of UHSCFST was proposed for the consideration of cost and safety, which is 25% ∼ 63%. Based on the analysis of interaction between concrete and steel tubes, the compatibility of the two materials is recommended, of which the yield of steel tube occurs after contact with concrete and before the concrete reaches its peak strength. Besides, the formula in EC4 overestimates the load-bearing capacity of CFST stub columns with high-strength materials. Therefore, a modified formula was proposed that reduces the enhancement factor of concrete strength and enables a safer prediction of load-bearing capacity.

Experimental investigations on the partially concrete-filled steel tubular slender beams

This paper focuses on the flexural behaviour of partially concrete-filled steel tubular (PCFST) beams having slender cross-sections (D/t > 150). PCFST beam is developed by optimising the tension zone concrete in a concrete-filled steel tubular (CFST) beam. Twelve specimens were investigated under a four-point load to understand the flexural behaviour and load transfer mechanism of PCFST slender beams. Parameters considered for the tests are concrete compressive strength, shear-span-to-depth ratio, and concrete wall thickness. The failure mode, load-deflection, strain distribution and moment-rotation data of the test specimens are reported and compared. In general, PCFST beams exhibit the same flexure behaviour as CFST beams in terms of flexural resistance and stiffness, and the weight reduction is around 40 %. However, composite action is lost in PCFST without tension zone concrete, and the beam behaves like an empty steel tube. Therefore, a minimum concrete wall thickness (tc,min) in the tension zone is required in PCFST and an equation for tc,min is proposed based on the test results. Concrete compressive strength is found to have lesser significance in the PCFST beam, whereas a decrease in the shear span-to-depth ratio from 2 to 1 can alter the mode of failure from flexure to shear. A numerical model is developed using ABAQUS to support the test results. This study finds the PCFST beam an optimised replacement for CFST beams in flexure-predominant structural systems. Additionally, a flexure capacity equation is proposed using the plastic stress distribution method, and made good predictions with the experimental and finite element results.

Axial compression performance of rubberized concrete-filled steel tubular stub columns after fire exposure: Experimental investigation and calculation models

Rubberized concrete (RuC) has emerged as a promising eco-friendly solution for structural elements, offering improved energy dissipation, damping characteristics, and ductility. Incorporating rubber particles into concrete-filled steel tubular (CFST) columns presents an environmentally conscious alternative for various structural applications. However, understanding its performance post-fire is crucial for safe design application, an area currently lacking comprehensive research. This study investigates the structural behavior of sixteen axially loaded circular RuCFST columns after exposure to fire. The columns, with diameters of 219 mm and 377 mm, are filled with normal concrete and RuC, containing 5 %, 15 %, and 30 % replacement of natural fine and coarse aggregate with rubber particles. Following ISO-834 fire standards, the specimens undergo elevated temperatures and subsequent application of compressive axial loads to determine their post-fire behavior. Analysis includes examination of failure modes, temperature-time relationships, axial load-deformation curves, and strain and stress development in the columns. Microstructural analysis using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) is conducted to study internal morphology before and after exposure to high temperatures. Results indicate non-uniform and nonlinear distributions of maximum temperatures across cross-sections, with historical temperature affecting residual capacity more significantly with increased rubber content and decreased column size. Load-bearing capacities decrease by up to 30 % and 22 % for small and large RuCFST columns, respectively, after exposure to fire. Residual axial stiffness also declines, with reductions of nearly 55 % and 42 % for small and large columns, respectively. In contrast, ductility improves in fire-exposed specimens, with columns having a 30 % rubber replacement ratio showing the highest enhancement. A novel method for calculating post-fire residual load capacity based on the average historical maximum temperature is introduced, demonstrating high accuracy and offering a valuable tool for assessing and reinforcing RuCFST columns after fire exposure.

Mechanical behavior of eccentrically loaded UHPC-encased CFST medium-long columns

To study the eccentric compression performance and failure mechanism of ultra-high performance concrete (UHPC) encased concrete-filled steel tubular (CFST) medium-long columns (UC-CFST medium-long columns), a total of 27 UC-CFST column specimens were tested under eccentric compression with eccentricity and slenderness ratio as the main parameters. After verified by experimental results, the finite element (FE) analysis of UC-CFST medium-long columns with actual structural cross-sectional size under eccentric compression was further carried out. It is found that as the slenderness ratio and eccentricity increased, the bending stiffness and the ultimate bearing capacity of the UC-CFST columns decreased gradually, and the failure mode changed from strength failure to instability failure, and the smaller the proportion of encasing UHPC area, the greater the upper-bound slenderness ratio of strength failure. Moreover, the influence of eccentricity was nonlinearly coupled with slenderness ratio on the eccentric compressive bearing capacity of UC-CFST medium-long columns. Finally, a calculation method of ultimate load-bearing capacity of UC-CFST medium-long columns considering the influence of eccentricity and slenderness ratio was proposed, which had good calculation accuracy with experimental and numerical results.

Plastic and punching shear strengths of concrete-filled steel tubular gapped K-joints without noding eccentricity under brace axial loading

This paper presents a comprehensive investigation, numerically and theoretically, into the structural behaviour and design aspects of concrete-filled steel tubular (CFST) gapped K-joints under brace axial loading. Finite element (FE) models were developed and validated against test results from existing experimental programmes. Based on the validated FE models, influences of geometric parameters and steel material properties on the load-deformation curve, plastic strength, ultimate strength and punching shear stress distribution of CFST K-joints were systematically assessed. In general, the chord diameter-to-thickness ratio 2γ, brace-to-chord diameter ratio β and steel material strength are crucial to the structural behaviour of CFST K-joints, whilst the brace-to-chord thickness ratio τ was found to have minimal influence. A shear stress distribution model was proposed as well to predict the shear stress distribution at ultimate load. To accurately account for the radial support of inner concrete, the classical ring model was modified accordingly. Based on the proposed shear stress distribution model and modified ring model, analysis-orientated and design-orientated equations were formulated to estimate both the plastic and ultimate strengths of CFST K-joints. The calculated plastic and ultimate strengths were compared against the experimental and numerical results, and the comparison demonstrates the ability of the proposed design methods to accurately and reliably determine the plastic and ultimate strengths of CFST K joints.