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In this paper, a novel methodology is developed for the characterization of the capacity of rectangular-shaped concrete-filled steel tubes (CFSTs). In the scientific research field, of particular interest is the behavior of long CFST columns under eccentric compressive load. These conditions promote failure mechanisms involving global member buckling. The developed methodologies are based on machine learning techniques found on artificial neural networks (ANNs). Furthermore, optimization methodologies, employing the grey wolf optimization algorithm and the firefly algorithm, have been attempted. For the training and validation of the models, a database consisting of 1,641 experimental tests collected from literature sources has been prepared, containing long and short specimens as well as specimens with or without load eccentricity. As the vast majority of the available experimental tests involve short specimens, the database has been augmented with 216 3D finite element models (FEMs), featuring increased member slenderness values. The calibration of the FEMs has been performed against experimental tests. The performance of the developed models has been measured through a number of performance indices, and compared with available code procedures. They have been found to provide significant improvements, both for short and long CFST columns, with the ANN model optimized with the firefly algorithm outperforming the others. Furthermore, a graphical user interface (GUI) has been developed which can be readily used to estimate the axial load capacity of CFST columns through the optimal ANN model. The developed GUI is made available as a supplementary material.

Prefabricated concrete-filled steel tube shear walls leverage the advantages of both concrete and steel and have found applications in high-rise buildings and regions with high seismic activity. This paper proposes a type of thin-walled concrete-filled steel tube with superimposed shear walls (CFST-SSWs) that can be conveniently installed on-site to improve construction efficiency and quality. Four full-scale specimens were experimentally tested under constant axial forces and lateral cyclic loads to examine their seismic performance. The two main variables during tests were the end column type and the CFST column width. The X-shaped crack in the CFST-SSWs specimen only extended to the edge of the superimposed wall, while that of the cast-in-place concrete column composite shear wall specimen connected to a crack in the end column. As the widths of the CFST column increased, the ductility increased to 13.47%, and the bearing capacity increased by an average of 14.5%. The average ductility coefficients of the specimens ranged from 2.23 to 3.95, and the CFST-SSWs showed moderate ductility. Increasing the width of the CFST column had little effect on its overall energy dissipation capacity in the early stage, but helped slow the stiffness degradation and intensity degradation of CFST-SSWs. It also significantly increased the ultimate inter-story displacement angle. A calculation model was developed to predict the CFST-SSWs’ cracking load, and experimental data were used to validate the model’s accuracy.

Due to its superior strength and ductility, the utilisation of concrete-filled steel tube (CFST) columns has grown over the last few years. However, due to unequal lateral inflation properties, infill concrete and steel tube slip against one another during early loading phases. This paper presented a novel type of CFST section, a diagonally stiffened CFST column as a solution to this problem. It consists of a circular steel tube with stiffening bars installed inside the outer steel tube from one top end to the diagonally opposite bottom end. The suggested section is presented using various stiffening strategies, including binary, tertiary, and quaternary arrangements. The effectiveness of the suggested column section under an axial load is examined using finite element (FE) modelling, and the validation of the FE model was established using experimental testing carried out by previous researchers on unstiffened and stiffened CFST columns. Analysis was done on specimens of diagonally stiffened CFST columns to evaluate the load-carrying capacity, load vs deformation behaviour, stress distribution, and failure pattern. According to the findings of this study, CFST sections with diagonal stiffeners have higher ultimate load capacity than unstiffened CFST columns. Stiffening bars increase the ductility of brittle infill concrete and eliminate localised steel tube buckling. It was recommended that the number of stiffeners be altered to even numbers since odd numbers of stiffeners can cause asymmetry in the section, which can increase the concentration of stress.

The concept of concrete-encased concrete-filled steel tube (CE-CFST) columns was conceived during the quest to enhance the safety of structures in high seismic regions. Over the past two decades, researchers have extensively investigated the performance of CE-CFST columns under a range of loading conditions using experimental, analytical, and numerical methods. Previous research has shown that integrating outer RC and encasing steel tubes into concrete-encased concrete-filled steel tube columns boosts their structural performance under a wide diversity of loading conditions. This paper aims to provide a comprehensive analysis of various models proposed by researchers in different regions and summarize all results that have been breakthroughs in the last decade. Researchers revealed that this type of structure only uses less than 85% of its flexural capacity. There are still significant research gaps in depicting these structures' behavior under diverse loading conditions, both analytical, numerical and experimental. The authors show the highlights of CE-CFST columns' performance and give a detailed review of the corresponding reinforced concrete (RC) columns and concrete-filled steel tube (CFST) columns. The highlights for further researches on CE-CFST are provided for elaborating other mechanical properties. To sum up, the CE-CFST composite columns provide resilient and sustainability in structure during engineering applications.

In this study, novel cross-shaped concrete-filled steel tube (CFST) and steel tube (ST) columns were developed. CFST columns have a high load-carrying capacity and excellent performance under seismic conditions, and the construction process is fast. In order to investigate the axial load bearings and failure mechanisms, six specimens of CFST and ST columns were tested under the axial load. Three different forms of CFST were employed in this study; one was an ordinary cross-shaped CFST (OC-CFST), while the other two were executed with significant inner changes; namely, stiffeners cross-shaped CFST (SC-CFST), and multi-cell cross-shaped CFST (MC-CFST) filled with concrete. The other group has the same OC-ST, SC-ST, and MC-ST, but these test subjects were without filled concrete. Through discussion of the failure mechanism, load displacement and load strain correlations are determined. The effects of parameters on ultimate resistance, failure pattern, and ductility index were studied. The axial load-carrying performance of the cross-shaped CFST columns was 75–80% better than that of ST columns; and each ST column displayed cooperative behavior. The finite element model (FEM) was simulated, and the outcomes of the experiments were used to validate it. The load–displacement relationships were established using parametric analysis. Existing design standards were used to calculate CFST column loading capacity. Finally, mathematical formulas were improvised to determine the ultimate load of the cross-shaped CFST column.

A novel concrete-filled steel tubular section with diagonal strengthening bars was introduced in the current study. It is constructed from a circular steel tube with strengthening bars placed diagonally opposite from one top end to the bottom end. Analysis was done on specimens of diagonally stiffened concrete-filled steel tubular columns to determine the load-bearing capacity, load versus deformation behaviour, stress distribution, and failure pattern. A parametric study was also carried out to ascertain the impact of the steel casing's outer diameter and thickness, concrete strength, and stiffening scheme on the suggested concrete-filled steel tubular's behaviour. The current evaluation's findings indicate that the column sections'geometrical and material properties significantly impacted the uniaxial performance of concrete-filled steel tubular sections with diagonal stiffeners. Increasing the diameter and grade of diagonal stiffeners improved the peak loading capacity and load versus strain graph of concrete-filled steel tubular columns with diagonal strengthening bars. In addition, the design relations given by the current design standards were altered, and new design equations were used to predict the load capacity of concrete-filled steel tubular columns that were diagonally stiffened. The results showed that every updated design equation from the design codes was determined to be in good agreement with the outcomes of the analysis. With the recommended strengthening strategy, the amended Eurocode design equation results demonstrate a more accurate and consistent prediction of the load capacity of concrete-filled steel tubular columns.

The complication linked with the prediction of the ultimate capacity of concrete-filled steel tubes (CFST) short circular columns reveals a need for conducting an in-depth structural behavioral analyses of this member subjected to axial-load only. The distinguishing feature of gene expression programming (GEP) has been utilized for establishing a prediction model for the axial behavior of long CFST. The proposed equation correlates the ultimate axial capacity of long circular CFST with depth, thickness, yield strength of steel, the compressive strength of concrete and the length of the CFST, without need for conducting any expensive and laborious experiments. A comprehensive CFST short circular column under an axial load was obtained from extensive literature to build the proposed models, and subsequently implemented for verification purposes. This model consists of extensive database literature and is comprised of 227 data samples. External validations were carried out using several statistical criteria recommended by researchers. The developed GEP model demonstrated superior performance to the available design methods for AS5100.6, EC4, AISC, BS, DBJ and AIJ design codes. The proposed design equations can be reliably used for pre-design purposes—or may be used as a fast check for deterministic solutions.

The combination of concrete-filled steel tube (CFST) columns and reinforced concrete flat slabs provides a suitable structural solution that can be used to replace traditional structures in high-rise buildings. The connection between the CFST column and the RC flat slab is an important factor for this construction system to operate efficiently. In this study, a new form for the connection between the CFST column and the RC flat slab has been proposed. This connection has more constructability, where the steel tube is cut at the level of the slab to prevent the separation between the slab and the concrete inside the steel tube as the slab reinforcement is allowed to continue in the connection zone. The joint zone is strengthened with rebar rings to compensate for the loss in confinement caused by cutting the steel tube at the slab level. In addition to using longitudinal rebar in the joint zone to bear part of the vertical load and to ensure bending rigidity. The behavior of the proposed connection was investigated by testing three sets of small-sized samples. In addition, an equation based on the outcomes of the second set of tests has been carried out to determine the joint bearing capacity for the vertical load.

Concrete filled steel tubes (CFSTs) show advantageous applications in the field of construction, especially for a high axial load capacity. The challenge in using such structure lies in the selection of many parameters constituting CFST, which necessitates defining complex relationships between the components and the corresponding properties. The axial capacity (Pu) of CFST is among the most important mechanical properties. In this study, the possibility of using a feedforward neural network (FNN) to predict Pu was investigated. Furthermore, an evolutionary optimization algorithm, namely invasive weed optimization (IWO), was used for tuning and optimizing the FNN weights and biases to construct a hybrid FNN–IWO model and improve its prediction performance. The results showed that the FNN–IWO algorithm is an excellent predictor of Pu, with a value of R2 of up to 0.979. The advantage of FNN–IWO was also pointed out with the gains in accuracy of 47.9%, 49.2%, and 6.5% for root mean square error (RMSE), mean absolute error (MAE), and R2, respectively, compared with simulation using the single FNN. Finally, the performance in predicting the Pu in the function of structural parameters such as depth/width ratio, thickness of steel tube, yield stress of steel, concrete compressive strength, and slenderness ratio was investigated and discussed.

This study proposed a newly built-up CFST column reinforced by diagonal props called a diagonal prop CFST column. Experiments were performed to investigate the response of the proposed column section subjected to axial compression. The CFST column behavior was presented using typical curves of load-deformation, load–strain, and load-carrying capacity. Further, the parametric analysis was performed to explore how changing the prop’s geometry and materials can affect its ultimate strength. The diagonal props inside the CFST columns controlled the steel tube from buckling throughout the height of the columns, which also made the concrete inside the tube more flexible. Under axial compression, it was found that a prop bar with a diameter of 8–16 mm makes the proposed CFST columns more conservative compared to the CFST columns without props. Also, an equation was developed to estimate the CFST column's ultimate strength under axial loading. The experimental results and the proposed equation were observed to be in good agreement with each other.