Concrete-filled Double Skin Steel Tubular Columns Research Articles

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.

Circular concrete-filled double-skin steel tubular short columns under axial compression: An improved theoretical model and design

Existing theoretical analysis methods for concrete-filled double-skin steel tubular (CFDST) short columns generally ignore the reduction of concrete strength due to brittleness. As a result, these methods cannot accurately capture full range behavior for CFDST columns containing high-strength concrete. By focusing on the reduction effect of concrete strength in CFDST columns due to concrete brittleness, this paper proposes a reduction factor for use in design, for the first time. Based on this reduction factor, a theoretical model for axially compressed circular CFDST short columns considering non-uniform confinement effect of the sandwiched concrete is established. A substantial experimental database of 168 CFDST columns with carbon circular steel tubes is then compiled. By comparing with this database, the proposed theoretical model is shown to achieve greater accuracy than existing approaches. Utilizing this model, the influence of column parameters on the performance of the studied CFDST members are investigated and the results are discussed. It is shown that concrete strength, hollow ratio, yield strength of the outer tube, and the diameter-to-thickness ratio of the outer and inner tubes are influential to the ultimate strength and ductility of CFDST columns. However, the yield strength of the inner tube only affects the ultimate strength of CFDST columns but has a negligible influence on the ductility. Finally, utilizing the experimental and theoretical data presented, the accuracy of existing design codes for concrete-filled steel tubes for the design of CFDST members with or without considering the reduction effect of concrete strength is assessed. The results indicate that Eurocode 4 provides a more reasonable prediction result.

Experimental and numerical analysis of assembled steel beam to CFDST column joint

A novel assembled joint between the concrete-filled double-skin steel tubular (CFDST) column and steel beams is developed in this paper. The joint is connected by extended endplates and high-strength bolts, and it is designed to facilitate post-disaster repairs. The pseudo-static test was performed on four 1:2 scaled joints, the endplate thickness, column form, and inner tube configuration were considered. The experimental results suggested excellent seismic performance of the joints, including plump hysteretic curves, high displacement ductility coefficients, and significant energy dissipation values. Then, accurate finite element (FE) models were established. Comparative results showed that the FE models better simulated the failure modes of endplate bending, weld tearing, local buckling of the flanges, and bolt fracture. In addition, there was a small deviation between the lateral load-bearing capacity obtained from tests and FE simulations. Next, stress analyses showed that the joint's yielding mode is from the flanges and endplates yielding to the outer tube web yielding. The stress on the inner tube was always lower than the yield stress, indicating that the CFDST column had a significant reserve of lateral load-bearing capacity. The main findings of numerous numerical analyses were that higher-strength endplates and steel beams with wider flanges substantially increased the lateral load-bearing capacity of the joint. Finally, a proposal was made to enhance the installation of ribs at beam ends. It improved the load-bearing capacity and initial stiffness and alleviated the stress concentration.

Predicting axial compression capacity of CFDST columns and design optimization using advanced machine learning techniques

Concrete-filled double-skin steel tubular (CFDST) columns are fundamental in civil engineering, known for their exceptional mechanical properties. Their load-bearing capacity, influenced by factors such as geometry, material properties, and loading conditions, is a critical aspect of CFDST column design. This study introduces an innovative approach that leverages the synergy between gradient-boosting regression (GBR) and three metaheuristic optimization algorithms: sine-cosine-based swarm optimization (SCSO), grey wolf optimization combined with whale optimization (GWO_WOA), and artificial rabbits optimization (ARO). Its primary aim is to accurately predict the axial compression capacity (ACC) of CFDST columns for optimal design. Using a comprehensive database of 153 empirical results from CFDST columns subjected to axial loads, the proposed model is rigorously compared with eight alternative machine learning models, three established design standards, and two empirical equations to validate its robustness. The results highlight the superiority of the finely tuned GBR model, optimized with the ARO algorithm, over peer models and conventional benchmarks. This framework also serves as the foundation for optimizing the CFDST column design by identifying the optimal dimensions that maximize ACC while meeting the design constraints. Clearly, this model can significantly improve CFDST column design, thereby benefiting structural engineers and designers striving to enhance the safety and efficiency of CFDST structures.

A machine learning based interaction model to predict robustness of concrete-filled double skin steel tubular columns under fire condition

Concrete-filled double skin steel tubular (CFDST) column is a hollow composite structure component, which shows better performance than traditional reinforced concrete and steel columns due to the favorable composite action between steel and concrete. In the current study, a machine learning based interaction model combine with the extended Rankine method is developed to predict fire resistance of eccentrically loaded CFDST cylinder columns. The prediction of the reliable shear bond parameter was conducted by back propagation artificial neural network (BP-ANN) and Extreme Gradient Boosting Tree (XGBoost). To perform a reliable production, the architecture and the parametric setting of both models were constructed. Furthermore, the results of the prediction were verified by experimental results and finite element analysis. The results show that the proposed method can predict the behavior of the eccentrically load CFDST columns under fire attack with reasonable accuracy.

Machine learning models for predicting axial compressive capacity of circular CFDST columns

Machine learning (ML), widely recognized for its advanced predictive capabilities, is progressively being employed in structural design. Concrete-filled double skin steel tubular (CFDST) members are extensively utilized due to their exceptional mechanical behavior. However, existing formulas encounter challenges in accurately calculating the axial compressive capacity of circular CFDST members, particularly when high-strength concrete and steel materials are involved, leading to complexity and low accuracy. This paper presents a data-driven framework that leverages the superior predictive performance of machine learning approaches to predict the axial compressive capacity of circular CFDST columns. Finite element models are developed through secondary development of ABAQUS using custom script, which are then validated against experimental data, confirming their effectiveness and applicability. A general database is established, comprising 226 test results and 550 finite element results obtained from the script, to facilitate the prediction of axial compressive capacity for circular CFDST columns. By incorporating the physical mechanisms of CFDST members and conducting correlation analysis, the input parameters for the machine learning models are determined. Decision Tree, Artificial Neural Network (ANN), K-nearest neighbor (KNN) and two ensemble methods including Random Forest, XGBoost, are trained and optimized using the database to provide accurate predictions. The results demonstrate that the KNN model exhibits superior performance and high accuracy, outperforming the four commonly used calculation formulas in the field, particularly for circular CFDST columns using high-strength materials. The proposed data-driven framework shows promising potential for achieving outstanding predictive performance.

Research on the Concrete-Filled Double Skin steel Tubular (CFDST) columns subjected to axial force after fire

This article investigates the structural performance of Concrete-Filled Double Skin Steel Tubular (CFDST) columns after fire under the axial compress. 13 experiments were conducted on the residual bearing capacity of CFDST columns under axial compression after different fire duration time. The variation parameters included the hollow ratio, fire duration time and strength of concrete. The effect of different parameters on the axial load and displacement changes after fire was revealed. The experimental and finite element simulation results were compared and analyzed, and they were in good agreement. The results indicate that CFDST columns have good fire resistance, and the strength of concrete has little effect on the residual bearing capacity of CFDST columns after the fire. With the increase of hollow ratio from 0 to 0.72, the axial residual bearing capacity of CFDST columns increases first and then decreases after the same fire duration time. Compared with the column without initial load, the residual bearing capacity of CFDST columns will increase with an initial load before fire, but different initial loads have no significant impact on the ultimate bearing capacity after fire.

Influence of anchored orbicular rings on the performance of sandwich concrete‐filled double‐skin steel tube columns

AbstractSince recent years, double‐skin tube column sections formed from two parallel, concentrically spaced steel casings, with the sandwiched area between the casings packed with concrete infill, have been employed extensively. The cross‐sectional shapes of the inner and outer casings might be the same or different, such as round, square, octagonal, etc. These concrete‐filled double‐skin steel tubular (CFDST) columns act similarly to concrete‐filled steel tubular (CFST) sections but have less self‐weight and better stiffness. The present study evaluated the response of CFDST column stiffened with orbicular rings under axial compression. It consists of inner and outer steel tubes stiffened by orbicular rings near both ends of columns. The orbicular rings were provided at different locations as L/3, L/4, and L/5, and a series of tests were conducted to obtain the performance of CFDST columns stiffened with orbicular rings with regard to the maximum load‐carrying capacity, load–strain behavior, and load‐deformation behavior. Furthermore, a parametric study was conducted by varying the diameter to thickness ratio, hollowness ratio, the thickness of orbicular rings, yield strength, and location of orbicular rings. The results showed that CFDST column stiffened with orbicular rings have higher ultimate load capacity than unstiffened CFDST. Stiffened CFDST with orbicular rings showed more ductile response than CFDST column and helps to improve the outer steel tube's restraint on sandwiched concrete. In order to estimate the load‐bearing capacity of CFDST columns stiffened with orbicular rings under axial compression, an empirical equation was proposed. The experimental results and findings of the suggested equations were found to be in good agreement.

Comparative Study on Behavior of Circular Axially Loaded CFDST Short Columns under Different Loading Arrangements

So far, almost all studies have concentrated on the compressive performance of fully loaded concrete-filled double-skin steel tubular (CFDST) columns, but there have been few studies on partially loaded CFDST columns, and the difference in their behavior remains unclear. Thus, by focusing on the mechanical behavior of circular CFDST columns under different loading arrangements, this paper conducts a comparative study on circular axially loaded CFDST columns based on the author’s previous experimental results. A total of 28 experiments were conducted, in which 14 fully and 14 partially loaded specimens were tested. The influence law of crucial parameters on the compressive behavior of CFDST specimens with two different loading arrangements were compared and the results were discussed. It is shown that when the void ratio is 0 and 0.2, the ultimate strength of partially loaded CFDST columns is greater than that of fully loaded ones, but the results are opposite for the columns with the void ratios of 0.4 and 0.6. Subsequently, based on the experimental data, a finite element analysis (FEA) model was developed and used to ascertain the compressive behavior of CFDST columns with two different loading arrangements. The results suggested that, at the beginning of the loading phase, the outer tube of partially loaded CFDST columns provided a certain lateral confining stress to the concrete, while the lateral confining stress of fully loaded CFDST columns fluctuated around zero. The axial load of the inner tube of fully loaded CFDST columns first increased linearly and then remained almost unchanged. Due to the existence of friction, the inner tube of partially loaded CFDST columns also undertook a certain axial load, which first increased and then decreased to zero.

Experimental study on torsional behavior of tapered high strength thin-walled concrete-filled double-skin steel tubular columns

Comparing with traditional straight concrete-filled double-skin steel tubular (CFDST) members, tapered CFDST members have been increasingly used in the domain of power industry due to esthetics and special spatial experience. In addition, the utilization of high strength materials can reduce cross-sectional area and further lower construction cost. In this paper, the torsional behavior of tapered high-strength thin-walled CFDST (THSTW-CFDST) columns with or without longitudinal stiffeners was investigated and a total of twenty tapered THSTW-CFDST specimens were tested. The failure modes, torque-torsional angle curves and torsional capacity of the THSTW-CFDST columns were investigated. The results demonstrate that the installation of stiffeners can significantly improve the local stability of thin-walled steel tubes, but reduce the torsional capacity of the members. Compared with THSTW-CFDST specimens without stiffeners, the torsional capacities of those with stiffeners have been apparently reduced. Based on the experimental results, the finite element (FE) model was developed and verified, and then utilized to ascertain the force-transfer mechanism and the stress development process of each component of THSTW-CFDST columns. In addition, the effects of the various parameters including hollow ratio, diameter-to-thickness ratio, inner steel tube strength to outer steel tube strength ratio, concrete strength the torsional behavior of THSTW-CFDST members were quantitatively assessed. Finally, the accuracy of the existing methods for predicting the torsional capacity of THSTW-CFDST members was evaluated.

Experimental study of large-scale stiffened thin-walled CFDST columns under aixal compression

Wind turbine towers offer good application potential for concrete-filled double-skin steel tubular columns with large hollow ratios (LHR-CFDST). With the increasing size of this engineering structure, the requirement to reduce construction expenses can be met by enhancing the diameter-to-thickness ratio of steel tubes. Few studies have been conducted to report on thin-walled LHR-CFDST columns. Meanwhile, local buckling and size effect shall be considered for this type of structure. Based on this, the first experimental investigation on large-scale stiffened thin-walled LHR-CFDST columns under axial compression is reported in this paper. Two large-scale thin-walled LHR-CFDST columns with different stud spacings are discussed. Failure modes, local buckling behaviours, load-strain responses, and some common mechanical indexes are investigated. According to the test results, adding studs was an effective method to increase the ultimate capacity of LHR-CFDST columns, as evidenced by the 16.6% increase in peak load for the specimen with smaller stud spacing than the other. Besides, the arrangement of studs leads to an obvious effect of section confinement, thus improving the overall compressive performance of the specimens. Then, four existing specifications all give a conservative prediction for the ultimate capacity of the specimen with smaller stud spacing, indicating that the large-scale thin-walled LHR-CFDST can be evaluated utilizing the present specifications with suitable stiffening measures.

Coupled behaviour and strength prediction of tapered CFDST columns with large hollow ratios for wind turbine towers

Concrete-filled double-skin steel tubular (CFDST) tower has emerged as a promising type of wind turbine support structure with excellent strength and stiffness. As the basic member in CFDST towers, tapered concrete-filled double-skin steel tubular columns with large hollow ratios (TLHR-CFDSTs) are frequently subjected to complex combined compression-bending-torsion loads. This paper presents a comprehensive analytical study on the coupled behaviour of the TLHR-CFDST column under combined loads and the relevant ultimate strength prediction method. A FE model was established to conduct numerical analyses and extensive parameter studies while the data from the previous experimental research was used to provide a verification basis. It is found that the column subjected to combined loads has three typical failure patterns, which are mainly governed by the bending-to-torsion ratio (mt). Columns with moderate mt undergo the bending-torsion failure and show obvious coupled performances. The load-deformation relation, the ultimate characteristics and the effects of important parameters, including axial compressive ratio (n), tapered angle (θ), hollow ratio (χ) and height-to-diameter ratio (λ) were discussed. The ultimate strength correlations of the column were revealed by analysing strength data within the parameter scopes commonly used in wind turbine towers. Lastly, a practical and accurate prediction method for the ultimate strength of the TLHR-CFDST column under combined compression-bending-torsion loads was proposed.

Seismic performance and damage assessment of CFDST frames with SBYC damper

Nowadays, the dual system consisting of moment resisting frame with energy dissipation devices was suitable to be employed for high-rise buildings in the area with high seismic intensities. Current investigations merely concentrated on the story drift as the main index to evaluate the seismic performance of such kind of system, which could not reveal the component damage state in a structure. It is more comprehensive to evaluate the seismic performance through the damage state of components. In this paper, the seismic damage of components in the dual system consisting of concrete filled double skin steel tubular (CFDST) moment resisting frame with shear -bending yielding coupling (SBYC) damper was analyzed. The damage factors of different components including CFDST column, semi-rigid joint and SBYC damper were determined based on existing test results. The nonlinear model of the dual system was established by Opensees and verified by the test results. Afterwards, the 20- and 10-story dual structures were designed with different strength of SBYC damper and moment resistances of joint. Parametric analyses were conducted on these two dual systems through the endurance time method. Further, the uniform damage ratio was defined to represent the uniform level of the damage distribution of the components. It found that increasing the strength of joint was beneficial for achieving a uniform damage distribution of SBYC damper, joint and SBYC damper. The analytical results were beneficial for the application of dual system and provided a reasonable method to evaluate the seismic performance and damage of dual systems.

Axial compression behaviour of circular concrete-filled double-skin steel tubular columns with bolted shear studs: Numerical investigation and design

The concrete-filled double-skin steel tubular (CFDST) columns have recently attracted significant attention for practical engineering constructions. The axial resistance behaviour of CFSDTs can be greatly enhanced by installing bolted shear studs as pin supports for the outer. This is a companion paper that expands on the authors' work [1]. The previous paper concentrated on full-scale experimental testing to ascertain the influence of bolted shear studs on the axial resistance of CFSDT columns, while this study focused on finite element (FE) analysis and a simplified design approach for such composite structural members. A three-dimensional FE model for predicting the axial behaviour of circular CFDST columns without and with shear studs was developed and validated using the test results of the specimens' typical failure mechanisms and load-displacement responses. The model was then employed to perform structural analysis by comparing the axial load-displacement curve, internal force distribution, longitudinal stress distribution over the sandwiched concrete, and confinement mechanism of a circular column with bolted shear studs with that of its counterpart without bolted shear studs. Parametric studies were carried out to examine the effects of bolt longitudinal spacing, diameter, length, and yield strength. It was found that the spacing of the bolts-to-width (bs/Do) ratio had significant effects on the axial resistance and ductile performance of columns. Also, a bolt spacing corresponding to bs/Do<0.44 was suggested to attain an acceptable ductile performance in practical application. Verification of the design model revealed that it led to satisfactory predictions of the ultimate strengths of circular CFDST columns without and with shear studs.

Axial compression performance of concrete filled double skin (SHS outer and CHS inner) steel tubular columns strengthened by CFRP

This paper reports the compressive behavior of concrete-filled double skin steel tubular (CFDST) columns strengthened by CFRP under axial loading. The outer skin was made of square hollow sections (SHS), while the inner skin was made of circular hollow sections (CHS). Twelve CFDST columns strengthened by CFRP and three CFDST columns were tested. The failure mode, load-displacement relationship, axial load-carrying capacity and initial stiffness are given and analyzed. The results showed that the CFDST columns strengthened by CFRP have better mechanical properties than the column without CFRP reinforcement (i.e. the CFDST columns). The CFDST columns strengthened by CFRP eventually failed due to CFRP rupture or buckling of the external steel tube near the end and cracked concrete damage from the outer steel tube. The stiffness and axial load bearing capacity of the column can be improved as the number of CFRP layers increases. A simplified design formula of CFDST columns strengthened by CFRP is proposed, which is in good agreement with the experimental results.

Torsional behaviour of tapered concrete-filled double-skin steel tubular columns with large hollow ratios

Tapered concrete-filled double-skin steel tubular columns with large hollow ratios (LHR-CFDSTs) possess light self-weight and good lateral rigidity. They exhibit tremendous potential in lateral load-critical structures, including large-span buildings, wind turbine towers and offshore infrastructures. In these structures, in addition to axial force and bending moment, columns are also commonly subjected to considerable torque. Although the compressive/flexural behaviour of tapered LHR-CFDST columns has attracted attention from researchers, studies on the torsional performance and mechanism are limited. This paper thus presents a comprehensive analytical study on the torsional behaviour of tapered LHR-CFDST columns on the basis of previous experimental research. Numerical analyses were conducted to investigate the influence of the key parameters. Full-range torsional characteristics of the column were revealed with regard to the typical torque–torsional angle relation, the stress development of the concrete and the contact between components. Load distributions as well as the effective twist length were discussed to understand the mechanism of the column under torsion. It is found that significant torsional deformation, oblique concrete cracks and struts, the compressive stress of concrete and the contact pressure all develop concentratedly within the effective twist length, which reflects the segments of the tapered column with a relatively weak torsion-resistant ability. Lastly, a torsional capacity prediction method was proposed by refining the previous theoretical model and it exhibits favourable accuracy for tapered LHR-CFDST columns with tapered angles higher than 1.50°.

Experimental and numerical investigations into the compressive behaviour of circular concrete-filled double-skin steel tubular columns with bolted shear studs

Eight medium-length circular concrete-filled double-skin steel tubular (CFDST) columns were examined under axial compression to evaluate the influence of bolted shear studs on the axial performance of CFDST columns. The specimens comprised six columns with bolted shear studs with longitudinal spacings of 100 mm, 75 mm, and 50 mm, and two control columns without bolted shear studs for comparison. The investigated parameters were the failure mechanisms, load-displacement curves, initial axial stiffness, compressive strength, toughness, ductility, residual strength, as well as load-strain responses. Besides, a finite element (FE) model was developed and validated against experimental results to further investigate the performance of columns. The test results demonstrated that the bolted shear studs avoided shear failure and significantly increased the ductility of the specimens. Reducing the bolted shear stud spacing reduced the intensity of local buckling in the outer tube and improved the confinement effect. Additionally, the confinement effect provided by the outer tube was remarkably improved by introducing the bolted shear studs.

Behaviour of tapered CFDST columns with large hollow ratio under combined loads

Tapered concrete-filled double-skin steel tubular columns with large hollow ratio (LHR-CFDST) have a good application potential in wind turbine towers, where they are commonly subjected to the combined action of compression, bending, shearing and torsion. Up to now, research work on the behaviour of tapered LHR-CFDST columns under combined loads is seldom reported and relevant experimental data are limited. This paper presents an experimental and numerical study on the tapered LHR-CFDST column under combined compression-bending-shear-torsion loads. Seven specimens with different parameters including the tapered angle, the hollow ratio, the torsion-to-bending ratio, the axial compression ratio and the height-to-diameter ratio were tested. The experimental results in terms of failure modes, load versus deformation relations and strain responses were discussed. Reliable finite element models were established to conduct numerical analysis in an extended parameter range. The mechanism of columns and the effects of torsion-to-bending ratios, axial compression ratios, height-to-diameter ratios, tapered angles, hollow ratios, diameter-to-thickness ratios of steel tubes and the material strength were revealed. The suitability of the existing ultimate capacity prediction method was evaluated for tapered LHR-CFDST columns. It is found that the failure mode of columns under compression-bending-shear-torsion loads is related to the torsion-to-bending ratio. The columns behave in a ductile manner under moderate axial compression. Tapered angle has a slight effect on the bending capacity of the column, whereas the capacity declines with the increase of hollow ratio and torsion-to-bending ratio. The existing method gives a conservative prediction for the ultimate capacity of tapered LHR-CFDST columns.

Behaviour of tapered concrete-filled double-skin steel tubular columns with large hollow ratio under combined compression-bending-torsion loads: Experiments

Recently, there has been an increasing interest in utilizing circular Concrete-Filled Double-Skin Steel Tubular (CFDST) columns in modern buildings. As a typical member in large-span building structures, the Concrete-Filled Double-Skin Steel Tubular column with Large Hollow Ratio (LHR-CFDST) is characterized by a tapered shape and a hollow ratio larger than 0.75. In engineering practice, this type of column is commonly subjected to a combination of compression, bending and torsion. This paper presents an experimental study on the tapered LHR-CFDST column under combined compression-bending-torsion loads. Specimens with different tapered angles, hollow ratios, bending-to-torsion ratios and axial compression ratios were tested. The failure mode, load versus displacement relation, strain response and effects of important parameters were discussed. The ultimate strength and mechanism of tapered LHR-CFDST columns under combined loads were analysed. The feasibility of predicting the ultimate strength of tapered LHR-CFDST columns using the existing methods was evaluated based on the experimental data.

Seismic behavior and analytical model for a fully bolted joint between CFDST columns and steel beams

A steel beam to concrete-filled double-skin steel tubular (CFDST) column joint was developed to improve the widespread use of CFDST columns in buildings. The comparative studies of six specimens under pseudo-static cyclic tests were designed and performed to examine the influence of endplate thickness, axial compression ratio, bolt diameter, concrete fill degree, and inner tube shape on the mechanical behavior of the joint. The failure modes were the bending of the endplate, warpage and fracture of bolts, fracture of the weld seam, and buckling of beam flanges. The displacement ductility coefficients exceeded 4.89, indicating that the connection had excellent ductility. Increasing the endplate thickness greatly improved the seismic indices of the joint. Although the higher axial compression ratio increased the bearing capacity and initial stiffness of the joint, the ductility and energy dissipation were unaffected. Improving the bolt diameter did not affect the bearing capacity, ductility, and energy dissipation capacity of the joint; however, it increased the initial stiffness and delayed the strength. The bearing capacity and initial stiffness of the joint were improved by increasing the concrete fill degree and using a square tube instead of the inner circular tube; however, it decreased the ductility and energy dissipation and accelerated the strength degradation of the joint. Finally, a calculation model of the initial rotational stiffness was proposed. The mean and standard deviation of the ratios of the predicted results to the experimental results were 0.95 and 0.14, respectively.