Rectangular Concrete-filled Steel Tubular Research Articles

Prediction of the axial compression capacity of stub CFST columns using machine learning techniques.

Concrete-filled steel tubular (CFST) columns have extensive applications in structural engineering due to their exceptional load-bearing capability and ductility. However, existing design code standards often yield different design capacities for the same column properties, introducing uncertainty for engineering designers. Moreover, conventional regression analysis fails to accurately predict the intricate relationship between column properties and compressive strength. To address these issues, this study proposes the use of two machine learning (ML) models-Gaussian process regression (GPR) and symbolic regression (SR). These models accept a variety of input variables, encompassing geometric and material properties of stub CFST columns, to estimate their strength. An experimental database of 1316 specimens was compiled from various research papers, including circular, rectangular, and double-skin stub CFST columns. In addition, a dimensionless output variable, referred to as the strength index, is introduced to enhance model performance. To validate the efficiency of the introduced models, predictions from these models are compared with those from two established standard codes and various ML algorithms, including support vector regression optimized with particle swarm optimization (PSVR), artificial neural networks, XGBoost (XGB), CatBoost (CATB), Random Forest, and LightGBM models. Through performance metrics, the CATB, GPR, PSVR and XGB models emerge as the most accurate and reliable models from the evaluation results. In addition, simple and practical design equations for the different types of CFST columns have been proposed based on the SR model. The developed ML models and proposed equations can predict the compressive strength of stub CFST columns with reliable and accurate results, making them valuable tools for structural engineering. Furthermore, the Shapley additive interpretation (SHAP) technique is employed for feature analysis. The results of the feature analysis reveal that section slenderness ratio and concrete strength parameters negatively impact the compressive strength index.

Flexural behavior of high-strength RCFST column-to-column connections

This paper investigates the flexural behavior of high-strength rectangular concrete-filled steel tubular (RCFST) column-to-column connections with different configurations. Based on the tests, 35 additional finite element analysis (FEA) models were developed to perform the parametric analysis varying with the cross-sectional size effect of (D), the width-to-thickness ratio of steel tube (D/t), steel tube strength (fs,y), steel bars strength (fr,y), concrete strength (fc,r), length of steel bars (L), arrangement and quantity of steel bars, respectively. The current code underestimated the yield and plastic bending moment compared with the calculated results from GB 50010–2010, ACI 318–19, EN 1992–1-1:2004, and JSCE-SSCS-2007. Finally, the strain coordination method and the full plastic section method were proposed to calculate the yield and plastic bending moment of the connection.

Application of machine learning models in the capacity prediction of RCFST columns.

Rectangular concrete-filled steel tubular (RCFST) columns are widely used in structural engineering due to their excellent load-carrying capacity and ductility. However, existing design equations often yield different design results for the same column properties, leading to uncertainty for engineering designers. Furthermore, basic regression analysis fails to precisely forecast the complicated relation between the column properties and its compressive strength. To overcome these challenges, this study suggests two machine learning (ML) models, including the Gaussian process (GPR) and the extreme gradient boosting model (XGBoost). These models employ a range of input variables, such as the geometric and material properties of RCFST columns, to estimate their strength. The models are trained and evaluated based on two datasets consisting of 958 axially loaded RCFST columns and 405 eccentrically loaded RCFST columns. In addition, a unitless output variable, termed the strength index, is introduced to enhance model performance. From evolution metrics, the GPR model emerged as the most accurate and reliable model, with nearly 99% of specimens with less than 20% error. In addition, the prediction results of ML models were compared with the predictions of two existing standard codes and different ML studies. The results indicated that the developed ML models achieved notable enhancement in prediction accuracy. In addition, the Shapley additive interpretation (SHAP) technique is employed for feature analysis. The feature analysis results reveal that the column length and load end-eccentricity parameters negatively impact compressive strength.

Experimental and numerical study on seismic performance of L-shaped multi-cellular CFST frames

Steel-concrete composite frames have been widely used in high-rise buildings. To improve the utilization efficiency of the indoor area, irregular-shaped concrete-filled steel tubular (CFST) columns were proposed for an alternative of the traditional rectangular CFST columns. In this paper, the L-shaped multi-cellular CFST (LM-CFST) frame was proposed, consisting of LM-CFST columns and H-shaped steel beams. Firstly, four LM-CFST frames were tested under combined axial compression and cyclic lateral loads. The local buckling was observed at both the top and bottom ends of the column, and fracture of steel occurred when the drift ratio was large. The hysteresis curves were spindle-shaped with the ductility indexes of 2.61–3.24. Then, a refined finite element model was established and validated against the test results. A parametric analysis was conducted to study the effect of axial compression ratio, web cell length, concrete strength and steel tube thickness on the seismic performance of LM-CFST frames. It was found that the axial compression ratio could affect the seismic performance of LM-CFST frames significantly, while the concrete strength had little effect. Finally, the compression-bending capacities obtained from the sectional analysis method specified in EC4 were compared with those obtained from tests. It was found that this method could predict the compression-bending capacity of the LM-CFST frame under combined axial compression and cyclic lateral loads with reasonable discrepancies.

Fiber–Based Model for Simulating Inelastic Interaction Buckling of Rectangular CFST Slender Columns with Unequal Wall Thickness

Rectangular concrete-filled steel tubular (RCFST) columns can be designed to have unequal wall thickness to maximize their resistance to combined axial and uniaxial bending loads. However, there is very little published research on their local–global interaction buckling behavior. This paper presents an efficient fiber-based simulation model for analyzing the inelastic interaction buckling of eccentrically loaded RCFST slender columns with unequal wall thickness. The model considers the progressive local–global interaction buckling, distributed plasticity, concrete cracking and crushing, second-order effects, and geometric imperfections. The mathematical model is programmed to compute not only the axial load–displacement responses but also the interaction curves of axial load and moment of slender RCFST columns. The simulation modeling is validated by experimentally measured data. The validated computer model is used to parametrically study the interaction local–global buckling responses of RCFST columns while the range analysis is conducted to identify the relative significance of key parameters. It is demonstrated that the proposed computer model accurately simulates experimentally measured responses. Hence, it can be used with confidence in practice to analyze and design RCFST slender columns fabricated with steel plates of unequal thickness. The existing design standard AS/NZS 2327:2017 provides acceptable strength predictions of RCFST slender columns and beam-columns while the procedure specified in Eurocode 4 overestimates their capacities.

Parametric study of steel-beam to flat-rectangular CFST column connections with small-size diaphragms

This paper studies the influence of various design parameters on the hysteretic behavior of beam-to-column connections between steel beams and flat rectangular concrete filled steel tubular (CFST) columns with small-size external diaphragms. Finite element models for the corner and exterior connections in a frame structure are developed and validated using test results. The influence of the parameters including the axial compression ratio, the thickness and flatness of the rectangular steel tube, and the size of the diaphragm on the hysteresis, load capacities, ductility, and energy dissipation capacities of the connections are particularly evaluated. Based on the evaluation, recommendations on the optimum design of the parameters are provided to ensure reliable hysteretic behavior of the connections for the seismic design of steel structures. This study provides the knowledge for the use of small-size external diaphragms beyond the limitation of existing design specifications for beam-to-column connections, which is meaningful to best exploit the unique properties of flat rectangular CFST columns and widen their applications in residential buildings.

Exploration on unified calculation of axial compressive load-carrying capacity of square and rectangular concrete-filled steel tubular columns

Due to the nonuniform constraints of the steel tubes on concrete, the calculations of the axial compressive load-carrying capacity of square concrete-filled steel tubular (SCFST) columns and rectangular concrete-filled steel tubular (RCFST) columns are complex problems. In this work, the calculation of axial compression load-carrying capacity of RCFST columns was finely divided into five parts creatively by applying superposition principle. It analyzed the constraint mechanism of rectangular steel tubes on the concrete, the stress state of the concrete in the strong and weak constraint zones, and steel tubes on the height and breadth sides. A unified model of axial compression load-carrying capacity of SCFSTs and RCFSTs was established. Based on the established database including 449 published RCFST and SCFST columns, the rationality, accuracy and wide applicability of the proposed model was verified. The parameter sensitivity analysis of the factors affecting the load-carrying capacity of RCFSTs was carried out, the influences of parameters on the improvement of concrete load-carrying capacity and the reduction of the axial load-carrying capacity of the steel tube were studied in detail.

Comparison and design of stiffened rectangular concrete-filled steel tubular members

A stiffened rectangular concrete-filled steel tubular (CFST) member offers improved composite action over the unstiffened counterpart in terms of delaying local buckling of the rectangular steel tube and enhancing confinement to the infilled concrete, and thus various stiffening forms were proposed. Choosing an appropriate stiffener in practical design needs comprehensive understanding and comparison on the efficiency of various stiffeners. However, most studies focused on a single type of stiffener only, no guidance was provided on how to account for the benefits of stiffened rectangular CFST members in mainstream design specifications, and no unified calculation method for load capacity was formed. In this work, finite element models for stiffened rectangular CFST members were established and verified by test results. Then parametric analyses were conducted to compare the axial compressive and flexural behavior of rectangular CFST members with various types of stiffeners. The appropriate arrangement and detailing of stiffeners were proposed, and the diagonal ribs showed the best performance under the same material strength and steel consumption. Finally, mechanics-based models and unified calculation methods were proposed to calculate the axial compression and interaction strength, and were found to well predict the strength of stiffened and unstiffened CFST members. These results will further facilitate the use of stiffened rectangular CFST members in practical engineering projects.

A unified design equation for square and rectangular concrete-filled steel tubular short columns

This paper develops a new unified design equation to estimate the axial compressive strength of square and rectangular concrete-filled steel tubular (CFST) short columns. The following four steps are involved: (i) compiling an experimental database consisting of 348 square and 113 rectangular CFST short columns, (ii) conducting 245 parametric analyses using benchmarked finite element (FE) models to address the gaps in the database, (iii) evaluating existing design equations and identifying their insufficiencies, and (iv) proposing and validating the new design equation. As compared with the existing design equations, the proposed design equation has the following advantages: (i) allowing a fast and convenient estimation of the axial compressive strength of square and rectangular CFST short columns, (ii) getting rid of the calculation of the section slenderness, and (iii) extending the codified material strength limits of the concrete infill and steel tube, i.e., it is suitable to square and rectangular CFST short columns with the compressive strength of the concrete infill up to 150 MPa and yield stress of steel tube up to 1000 MPa.

Evaluation of the load-carrying effect of rectangular concrete-filled steel tubular columns under axial compression based on the multilevel extension method

The load-carrying effect of rectangular concrete-filled steel tubular (RCFST) columns under axial compression has complex influencing parameters. Previous relevant studies have mostly included experimental and modeling approaches, but there are few studies on evaluations based on multiple parameters. This study introduces the extension evaluation method, which is good at solving contradictory problems, and scientifically constructs a multilevel extension evaluation model of the load-carrying effect of RCFST columns. Key parameters such as the cross-sectional height-to-breadth ratio (Sr) and hoop confinement coefficient (ξs) that affect the load-carrying effect of RCFST columns are refined and converted into evaluation indices. The weights of the indices are calculated scientifically via the analytic hierarchy process (AHP). Through the extension evaluation of the tested RCFST columns, the accuracy and rationality of the proposed evaluation model are verified. The conclusion of the multilevel extension evaluation shows that for the fixed section tested in this study, the RCFST columns with moderately thick-walled steel tubes have the best load-carrying effect and economy among thin-walled, moderately thick-walled and very thick-walled steel tubes.

Experimental study on reduced I-beam to CFRST column joints under cyclic loading

A joint for I-beam to concrete-filled rectangular steel tubular (CFRST) column connections is proposed and investigated in this study. Five specimens are firstly designed and tested. The hysteresis curves from tests indicate the good energy dissipation capacities of the joint specimens subjected to cyclic loading. It is also found that most of the joint specimens can be classified into the rigid and full-strength joints according to Eurocode 3. Subsequently, the finite element (FE) model is also developed and verified using the test results. A series of FE analyses show that the thickness of the endplates and the beam length are the two main parameters for the behavior of the proposed joint, and the insufficient stiffness of endplates may significantly increase the risk of rupture at the butt welding between the beam flange and endplates. The results of this study show that the proposed joint is suitable for use in the severe seismic condition according to GB 50011-2010, while its application is limited to C-IMF according to ANSI/AISC 341-10.

Ultimate flexural strength of rectangular concrete‐filled steel tubular beam‐columns using high‐strength materials

AbstractThe ultimate flexural strength of concrete‐filled steel tubular (CFST) beam‐columns is calculated using the full‐plastic strength as per the “AIJ Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures”. However, the full‐plastic strength might overestimate the ultimate strength of rectangular CFST beam‐columns, when the width‐to‐thickness ratio is significant and/or high‐strength materials are used. The calculation of the ultimate flexural strength when it exceeds the applicable range of AIJ CFT recommendations is based on a method that considers the stress reduction on the compression side of the steel tube or outermost edge strain of the concrete. However, both stress reduction and outermost edge strain should be considered when high‐strength materials are used in CFST for accurate ultimate strength formulas. In this study, a simple evaluation method for the ultimate flexural strength of rectangular CFST beam‐columns was proposed using the maximum strain at the extreme concrete compression fiber and stress reduction on the compression side of the steel tube. In addition, the appropriate strain at the extreme concrete fiber needed to accurately calculate the ultimate strength was discussed.

Explainable machine learning models for predicting the axial compression capacity of concrete filled steel tubular columns

Concrete-filled steel tubular (CFST) columns have been popular in the construction industry due to enhanced mechanical properties such as higher strength and ductility, higher seismic resistance, and aesthetics. Extensive experimental, numerical and analytical studies have been conducted in the past few decades to assess the structural response of CFST columns under various loading conditions. However, there is still uncertainty in predicting the capacity of CFST columns, and most of the current codes are conservative. In this paper, data-driven machine learning (ML) models have been developed to predict the axial compression capacity of rectangular CFST columns. An extensive database of 719 experiments was collected from literature and is randomly used to train, test, and validate the ML models. Seven ML models, namely lasso regression, random forest, Adaptive Boosting (AdaBoost), Gradient Boosting Machine (GBM), Light Gradient Boosting Machine (LightGBM), Extreme Gradient Boosting (XGBoost), and Categorical Gradient Boosting (CatBoost), are evaluated to predict the compression capacity of CFST stub columns under axial load. The performance of the different ML models in predicting the compressive strength of CFST columns is compared by different code equations prevalent in different parts of the world. It is found that LightGBM and CatBoost models performed better with an accuracy of 97.9% and 98.3%, respectively, compared to the existing design codes in predicting the capacity of CFST columns. Feature importance analyses and SHapley Additive exPlanations (SHAP) explain the ML model performances and make the developed models interpretable. Resistance factor is determined using the best performing ML model for compressive strength prediction of CFST stub columns following AISC 360–16 code provision.

Experimental behavior of steel-beam to flat rectangular CFST column connections with small-sized external diaphragms

This paper experimentally investigates the hysteretic behavior of beam-to-column connections between steel beams and flat rectangular concrete filled steel tubular (CFST) columns with small-sized external diaphragms to enable the implementation of the connections in steel structure residential buildings. The flat rectangular CFST columns are characterized by the width-to-depth ratio of the cross-section greater than 2.0. Five test specimens, including three corner connections and two interior connections in a frame structure, with 400×200 mm rectangular CFST column cross-section and various sizes of external diaphragms are designed for the investigation. During the tests, the specimens are subjected to constant axial loads on the columns and cyclic vertical loads at the beam ends that impose bending moments about the strong and weak axis of the columns. Failure modes including buckling in beam flanges, cracking at the welded connections between beam flanges and external diaphragms, and shear failure in panel zones are observed. The load-displacement hysteresis and strain distribution are obtained to evaluate the elastoplastic behavior of the connections. The moment resistance capacity, ductility, energy dissipation capacity, as well as stiffness and strength degeneration of the connections are respectively evaluated. The test results demonstrate that the connections have adequate hysteretic performance for earthquake resilient design of steel structure residential buildings. It is applicable to use small-sized external diaphragms beyond the limitation of existing design specifications for beam-to-column connections with flat rectangular CFST columns.

Load-transfer mechanism of distribution beams in giant rectangular concrete-filled steel tubular columns

This study investigates the load-transfer mechanism of a distribution beam and the load-bearing capacity of a huge rectangular concrete-filled steel tubular (RCFST) column through experiments and numerical analyses. Axial compression tests on six giant RCFST columns with a scale factor of 1/5 were conducted. Test results indicated the following: 1) All specimens exhibited excellent ductility and failed by local buckling of the steel tube. The distribution beam experienced shear failure because of the large plastic deformation at both beam ends. 2) The distribution beam effectively improved the ultimate load capacity of the specimen and the concrete core. 3) The yield load of the concrete core was approximately twice the shear capacity of the distribution beam, and its ultimate load capacity was slightly higher than twice the tensile capacity of the distribution beam. Subsequently, corresponding scaled finite element models were established and verified by comparison with the experimental results regarding failure modes, load-bearing capacity, and load–displacement curves. Based on the verified finite element models, numerous parametric analyses were conducted on full-scale RCFST columns, considering different section sizes of steel tubes and distribution beams, as well as layouts of the distribution beam. Based on the experimental and numerical analyses of the load-transfer mechanism of the distribution beam, a simplified design formula for the distribution beam was proposed.

Research on Confinement Effect of the Outer Steel Tube in Notched Square CFST Columns.

The outer steel tube in a concrete-filled steel tubular (CFST) column confines the core concrete and improves the compressive strength of the core concrete. When there is a notch damage in the tube, the confinement effect may be affected. The confinement effects of the notched steel tube in rectangular CFST columns were systematically investigated by using numerical approaches. Refined three-dimensional finite element models with advanced concrete constitutive relations were established. With the verified finite element modeling method, full-sized square CFST columns with horizontal, vertical, or diagonal notches at different locations of the steel tube were simulated. Stress distributions and deformation modes of the steel tube and core concrete were analyzed. Columns with a horizontal notch at the plate center location displayed a higher axial strength reduction than those with vertical notches. A parametric study was performed to investigate the influences of concrete strengths, steel strengths, steel ratios, notch length to column width ratios, and notch angles on the compressive strengths of the rectangular CFST columns. A practical design formula was proposed based on the obtained results. The proposed formula could effectively predict the influences of different notches on the confinement effect in the notched CFST columns.

Predicting axial load capacity of CFST columns using machine learning

AbstractOwing to their economic and structural advantages, concrete‐filled steel tubular (CFST) columns have been implemented in diverse structural applications, especially in high‐rise buildings, suspension bridges, and subway stations. However, there is no agreement between international standards regarding the ultimate compressive strength of CFST columns subjected to concentric axial force or combination of bending moment and axial force, especially for slender sections and high‐strength materials. Considering such limitations, a comprehensive dataset on rectangular and circular (built up and hot rolled) CFST sections with different slenderness ratios was collected. The compiled experimental results were compared with corresponding values calculated by pertinent design code provisions, including AISC 360‐16 and Eurocode 4. The accuracy of design codes in predicting the axial capacity of CFST sections under concentric and eccentric loading was assessed accordingly. The distribution of the experimental‐to‐predicted data confirmed that the prediction error was dependent on the slenderness ratio of columns. However, the effects of other parameters, including the mechanical properties of materials and eccentricity level on the model error, were negligible. In the second phase of the study, a surrogate Machine‐Learning (ML) model was developed to estimate the axial capacity of circular and rectangular CFST columns under centric or eccentric loading condition. The accuracy of the proposed ML predictive model was appraised using several statistical metrics. The novel informational model attained superior accuracy and could be used to simplify generative design in future computational intelligence structural design platforms.

Optimal allocation of material and slenderness limits for the rectangular concrete-filled columns

The current American Specification AISC 360–16 for concrete-filled steel tubular (CFT) columns exhibits particular limits in terms of section slenderness and material strengths. This paper proposes a decision tree algorithm, together with a comprehensive compiled experimental database of rectangular CFT columns under concentric loading and reliability analysis, to establish new optimal limits for section slenderness and material strength. The decision tree analysis yielded “data-driven classification rules” indicating that a relative section slenderness of 5, concrete compressive strength of 175 MPa, and steel yield strength of 850 MPa are considered optimal limits for the design equations in the AISC 360–16 in terms of achieving the minimum reliability requirements. The reliability analysis results are used to run a sensitivity analysis on the CFT columns to identify the dominant parameters that affect their axial loading capacity. Based on the sensitivity results, special attention should be given to the variability of concrete strength. The last part of this study demonstrates that the design equation of the high-strength CFT member in the new AISC Specification (AISC 360–22; obtained from a public review draft) can be extended to the conventional materials using the same proposed optimal limits.

Research on a Buckling Detection Technique for Rectangular Concrete-filled Steel Tubular Columns based on Grating Projection

<p>The surface buckling of concrete-filled steel tubular (CFST) columns under axial compression was observed in real time by using fringe projection, and pre-set fringes with phase information were projected onto the surface of the member. The buckling information (the size and shape of drum buckling) and its evolution were obtained by analysing the buckling images during the loading process. Nine images of key moments during the experiment were obtained by a technique of obtaining the actual measurements synchronously, and the position of the bulge on the whole side of the steel pipe was obtained by unwrapping. The shape and evolution of the drum buckling of the steel pipe developed symmetrically before point C (the ultimate bearing capacity) and evolved from 2 to 3 peaks. After passing through point C, the buckling deformation was asymmetrical. The deformation near the loading end increased gradually, and the buckling deformation increased with loading. The fastest growth stage was between points C and F, and the load displacement curve showed a rapid reduction in the load. These experimental results show that this noncontact measurement method based on fringe projection is effective in detecting the buckling of rectangular CFST column surfaces.</p> <p> </p>

Optimization of axial load carrying capacity of CFST stub columns

Concrete filled steel tubular (CFST) columns are widely used due to their enhanced mechanical properties. The interaction between the concrete core and the steel casing increases structural stability and magnifies the compressive strength of concrete. Besides the structural performance, in alignment with the commitment of the concrete industry to reduce its environmental impact, lowering the carbon emissions caused by the production of concrete structures is gaining importance in recent years. The current paper gives an overview of the equations available in the literature that predict the axial load carrying capacity of rectangular CFST columns. A modified version of the Jaya metaheuristic algorithm is being proposed and the outcome of this algorithm is being presented. The algorithm is used in order to maximize the axial load-carrying capacity of a stub column. As an optimization constraint the CO2 emission associated with the production of the CFST column is being kept below a predefined level throughout the optimization process. The optimization process as well as the cross-sectional dimensions associated with the optimum solution are presented.