Concrete-filled Double-skin Tubular Columns Research Articles

Ultimate strength and design of CFDST columns with intermittent welded plate stiffeners

This study conducts an experimental investigation and formulates a finite element model and design equation to determine the ultimate strength of concrete-filled double-skin tubular (CFDST) short columns. These columns are equipped with intermittent welded plate stiffeners, and the welding is performed on the inner surface of the outer tube through pre-drilled holes at intervals of 10 t, 20 t, 30 t, and 40 t, where “t” represents the thickness of the outer tube. The study investigates the behaviour of the CFDST column influenced by the presence of plate stiffeners, determines the axial strength of eighteen CFDST specimens and discusses the load-displacement curve, deformation and ultimate strength. The findings demonstrated that the ultimate load of the plate-stiffened CFDST columns is between 24% and 42% higher than the unstiffened CFDST columns. The plate-stiffened CFDST columns also exhibited higher ultimate strength at the smallest weld spacing of 10 t. The experimental results showed that the plate stiffeners influenced the distribution of the internal forces, which changed the effective confinement area of the concrete. The investigated force distribution provides a fundamental theoretical basis for computing the confinement area for the finite element model (FEM) and design model. This study developed a verified nonlinear FEM that employed ABAQUS to replicate the behaviour of the plate-stiffened CFDST columns. Finally, this study proposes a new design model that considers the effective cross-sectional area of the concrete confined in CFDST columns with intermittent welded plate stiffeners. The strength predictions of the plate-stiffened CFDST columns were accurate compared to the experimental results.

Machine-learning-based predictive models for concrete-filled double skin tubular columns

This paper aims to develop a unique artificial neural network (ANN)-based equation as well as MATLAB- and Python-based graphical user interfaces (GUIs) using the most comprehensive and up-to-date database for predicting the behaviour of axially loaded concrete-filled double skin tubular (CFDST) short and slender columns with normal- and high-strength materials. Two machine learning (ML) methods, which are ANN and extreme gradient boosting (XGBoost), are trained and tested using 1721 sets of data, with 129 of them collected from experimental studies and 1592 generated by finite element (FE) simulations. The accuracy of the developed ML models is assessed through comparing their predictions with the experimental and FE results. To demonstrate the effect of each parameter on the predicted results, the SHapley Additive exPlanations (SHAP) method is used. The developed ML models are also used to conduct parametric studies to examine the effect of geometric and material parameters on the predicted results. The accuracy of the ML models and the proposed ANN-based equation in predicting the ultimate axial capacity of CFDST columns is compared with that of six design methods including two design code provisions and four design equations proposed by researchers. A numerical example is presented to illustrate the design procedure of the CFDST column using the proposed ANN-based equation. The results indicate that the ANN model performs better on unseen data than the XGBoost model with lower root mean square error for the test set. The results also show that the ML models and the proposed ANN-based equation are superior to the other design models in prediction accuracy.

Study on axial behaviour of concrete filled steel tubular columns

Concrete-filled steel tube (CFST) columns have gained wide acceptance in the construction of high-rise buildings due to their exceptional ability to withstand axial load. They are commonly employed as columns in large-span high-rise buildings, bridges, and piers. This research aims to investigate the performance of concrete-filled steel tube (CFST) columns when subjected to axial load. One key advantage is that the steel tube serves as a permanent confinement for the concrete. The concrete filling inside the tube prevents local buckling and enhances the lateral stability of the member. Moreover, the external steel tube acts as a formwork. Eight composite columns were constructed, comprising four CFST columns and four CFDST (Concrete-filled Double Skin Tubular) columns, and were subjected to axial loading for evaluation. The experimental study was conducted to determine the relationship between load, axial displacement, and lateral displacement, as well as the ultimate load-carrying capacity. Furthermore, finite element analysis using Abaqus was performed to investigate the composite columns analytically. A parametric study was also carried out, considering the diameter-to-thickness ratio and grade of concrete. Confinement significantly enhances the load-carrying capacity, and the presence of concrete infill reduces the lateral deflection of long columns.

Seismic Performance of Recycled Aggregate Geopolymer Concrete-Filled Double Skin Tubular Columns with Internal Steel and External FRP Tube

The large production of cement is resulting in a high-carbon footprint, which is essential to minimize for sustainable concrete construction. Moreover, the large quantity of recycled coarse aggregate (RCA) from the demolition of old concrete structures is creating problems for landfill and disposal. The primary goal of this study is to investigate the seismic efficiency of innovative fiber-reinforced polymer (FRP)-recycled aggregate geopolymer concrete (RAGC) steel-tubed columns (FGSTCs) with an internal steel tube (STT), an external glass-FRP tube (GLT), and RAGC located between the two-tubed components to develop a serviceable structural element. To study their seismic functioning under axial load and lateral repeated load, five FGSTC specimens were manufactured and analyzed under quasi-static loads. The influence of three variables on the performance of FGSTC specimens, consisting of STT reinforcing ratio, compression ratio, and recycled coarse aggregate (RCA) replacement ratio, was investigated in this investigation. The produced specimens' ductility, hysteretic loops, strain distribution, skeleton curves, stiffness functioning, energy capacity dissipation, damaging functioning, and strength loss were all assessed and discussed. The results of this investigation revealed that percentage substitution of RCA had a minor impact on the seismic functioning of FGSTCs; however, the compression-load ratio depicted a substantial impact. The energy loss of the FGSTCs was 24.5% higher than that of their natural aggregate equivalents. FGSTCs may have a 16.9% lower cumulative failure rate than their natural aggregate counterparts.

Influences of the strengthening methods on axial and eccentric compressive behaviors of circular concrete-filled double-skin tubular columns

This study presents a comprehensive investigation on the compressive behaviors of concrete-filled double-skin tubular (CFDST) columns with two different types of strengthening methods (installations of reinforcement rings and steel nails). A total of five specimens were tested under axial and eccentric compression, and their damage patterns, load-displacement curves, and strain developments were studied. Results revealed that under axial compression, the ultimate load bearing capacity could be enhanced by around 15% by installing the reinforcement rings, and the occurrence of peak load could be postponed after the yielding of steel tube. This is because the reinforcement rings restrained the steel tube buckling at the loading ends. The steel nails had more contributions in ductility performance, when the column was under eccentric compression. The steel nails helped maintain the integrity of steel tubes which could provide confinement to the annular concrete to delay or alleviate the concrete deterioration and crushing failure, ultimately reducing the lateral deflection of the column. A finite element model has been developed and verified against the test results with less than 10% error in predicting the peak load capacities. The model may be used in future studies in the behavior of CFDST columns with less investigation costs.

Estimating the Axial Compression Capacity of Concrete-Filled Double-Skin Tubular Columns with Metallic and Non-Metallic Composite Materials.

This research focuses on estimating the ACC (axial compression capacity) of concrete-filled double-skin tubular (CFDST) columns. The study utilised algorithms and ‘six’ evaluation methods (XGBoost, AdaBoost, Lasso, Ridge, Random Forest Regressor and artificial neural network (ANN) architecture-based regression) to study the empirical formulae and utilise the parameters as the research’s features, in order to find the best model that has higher and accurate reliability by using the RMSE and R2 scores as performance evaluation metrics. Thus, by identifying the best model in empirical formulae for estimating the ACC of CFDST, the research offers a reliable model for future research. Through findings, it was found that, out of the existing evaluation metrics, the ABR for AFRP, GFRP and Steel; RFR for CFRP; and RR for PETFRP were found to be the best models in the CFDST columns.

Compressive Behaviour of Square CFDST Short Columns with Double Inner Steel Tubes

This study focuses on the experimental analysis of concrete-filled double-skin tubular (CFDST) short columns with double circular inner steel tubes under concentric axial loading. This cross-section layout promises to increase the ductile behavior of the compressive element and its energy absorption capacity, not to mention its ultimate axial strength. This research analyses main twofold variables: (i) Hollow ratio and (ii) eccentricity ratio (i.e., distance ratio between the inner tube’s separation and the sandwiched concrete width). As a result, load Vs. Axial deformation, load Vs. Axial strain curves, ductility index, strength index, energy absorption capacity, and ultimate axial capacity formulae for square CFDST columns with double circular inner tubes are reported. Key findings of this study show that (a) ductility is an intrinsic property of these types of sections; (b) the variation effect of hollow ratio influence inversely in the confined concrete strength of the element, (c) Eccentricity ratio has proved to be the least ultimate strength capacity influencer. However, its impact increases when it is jointly analyzed with the hollow ratio values. (d) The formulae proposed for predicting the ultimate capacity of CFDST columns showed good agreement with the experimental results. Thus, these expressions could be extended to the design of composite CFDST elements, provided a resistance factor based on a reliability analysis is incorporated.

Axial behavior of novel CFDST columns with outer welded corrugated steel tubes

A novel type of welded corrugated steel tube-concrete-flat steel tube composite structural column was proposed, and the axial compressive behavior of this column type was evaluated by testing of 14 short column specimens. The test parameters included hollow ratios, inner tube diameter-thickness ratios, and concrete strength and reinforcement ratios. The test results indicated that the outer corrugated steel tube could provide good hoop confinement to the core concrete. The welded corrugated steel tube-concrete-flat steel tube composite structural columns failed with serious extrusion of the wave crests and inward buckling of the inner steel tubes. The effects of the test parameters on the failure modes, strength, and ductility and strain responses were also discussed. Moreover, some existing design models for concrete-filled double skin tubular (CFDST) columns were assessed based on the test results available in literature and this study, and new models for CFDST columns and welded corrugated steel tube-concrete-flat steel tube columns were proposed. The proposed analytical models were found to be in good agreement with both the test results of the specimens with outer flat steel tubes and corrugated steel tubes under axial compression.

STUDY ON THE FIRE PERFORMANCE OF CONCRETE-FILLED DOUBLE-SKIN TUBULAR COLUMNS WITH EXTERNAL STAINLESS STEEL TUBES

To investigate the fire performance of concrete-filled double-skin tubular (CFDST) columns with external stainless steel tubes, the fire resistance of this kind of composite members was analyzed by ABAQUS. The differences in the fire mechanisms between CFDST columns with external stainless steel tubes and those with external carbon steel tubes were studied. The effects of the cross-sectional dimension, hollow ratio, load ratio and material strength on the fire resistance of components were analyzed. The results show that the CFDST columns with external stainless steel tubes present better fire resistance than those with external carbon steel tubes. The fire resistance of members was greatly affected by the cross-sectional dimension and load ratio. The hollow ratio and material strength had a moderate influence on the fire resistance of the members. Based on a parametric analysis, a design method for the fire safety of CFDST columns with external stainless steel tubes was suggested.

Analytical Behaviour of Stiffened and Unstiffened CFDST Short Column

Concrete filled steel tubular (CFST) members comprise of a hollow steel section of circular or rectangular shape filled with plain or reinforced concrete. Concrete-filled double-skin tubular (CFDST) columns are formed by sandwiching concrete between two concentric hollow steel tubes. When compared to a traditional concrete-filled steel tube (consisting of a single hollow steel tube instead of two), the CFDST column is found to have greater axial, flexural, and torsional strengths as well as improved strength-to-weight ratios. They are widely used in high-rise and multi-story buildings as columns and beam-columns. This paper presents the analytical results obtained by using ANSYS software and to develop various kinds of CFDST columns by varying cross-sections, by providing stiffeners and by changing the position of the reinforcements. And also to plot the load-deformation curve and to evaluate the load-carrying capacity of the different models. From the analysis, it is concluded that the square–squaren CFDST with plain stiffeners attached to the outer steel tube has the best axial load carrying capacity.

Analytical behavior of dodecagonal concrete-filled double skin tubular (CFDST) columns under axial compression

ABSTRACT Dodecagonal CFDST columns have been used in power transmission towers in China due to their lighter weight and lower manufacturing cost. In this paper, the axial compressive behavior of dodecagonal concrete-filled double skin tubular (CFDST) columns was investigated by finite element analysis. The different behaviors between dodecagonal CFDST columns and circular CFDST columns were compared through the load-deformation curves, contact pressure, longitudinal stress distributions in concrete and ultimate capacity. The developed finite element model was validated against experimental results in terms of load-displacement curves, failure modes, stress distributions and the ultimate strength of the tested specimens. Then a parametric analysis was performed by the verified finite element model on parameters including slenderness ratio, hollow ratio and diameter of the outer steel tubes. Research results showed that the confinement effects and the ultimate strength of dodecagonal CFDST columns were relatively weaker than those of the corresponding circular CFDST columns. At last, a design equation for the compressive capacity of dodecagonal CFDST columns was derived, and accurate predictions were obtained compared with the experimental results.

Behaviour of Concrete-filled Double-skin Short Columns Under Compression Through Finite Element Modelling: SHS Outer and SHS Inner Tubes

Concrete-filled double-skin tubular (CFDST) columns are formed by sandwiching concrete between two concentric hollow steel tubes. The result is a composite column with the benefits of both steel and concrete properties. When compared to a traditional concrete-filled steel tube (consisting of a single hollow steel tube instead of two), CFDST column is found to have greater axial, flexural and torsional strengths as well as improved strength-to-weight ratios. However, developments in CFDST column configurations can be made by altering the cross-section shape of the steel tubes, which is generally formed from square and circular ones. This paper considers the square CFDST short columns with inner square hollow sections (i.e. SHS outer and SHS inner tubes). This is because; in additional to their advantages shown above, they were seldom considered in literature especially by virtual testing. Accordingly, this paper is devoted for the finite element (FE) modelling of this type of composite columns by using ABAQUS program. Innovatively, this paper, rather than different investigations in literature, uses the most accurate constitutive models of both the cold-formed steel (i.e. Ramberg-Osgood model) and the confined concrete infill in double-skin tubes. Prior to parametric studies, the program has been validated using previous experimental results to ensure that the chosen combination of the materials could provide accurate and reliable results for the proposed models. The paper is then extends through parametric studies to investigate the effects of the key parameters affecting the general behaviour of this column under axial compressive loading, which exceeds the range of materials currently available in the available experiments. The paper additionally explores the effect of the eccentric loads on the behaviour of such columns, rarely done in literature, and comparisons with available theoretical moment-interaction curves have been considered. Overall, this paper provides new results that might aid in widening the practical usage of such column.

Seismic behavior of stiffened concrete-filled double-skin tubular columns

The imperfect steel-concrete interface bonding is an important deficiency of the concrete-filled double skin tubular (CFDST) columns that led to separating concrete and steel surfaces under lateral loads and triggering buckling failure of the columns. To improve this issue, it is proposed in this study to use longitudinal and transverse steel stiffeners in CFDST columns. CFDST columns with different patterns of stiffeners embedded in the interior or exterior surfaces of the inner or outer tubes were analyzed under constant axial force and reversed cyclic loading. In the finite element modeling, the confinement effects of both inner and outer tubes on the compressive strength of concrete as well as the effect of discrete crack for concrete fracture were incorporated which give a realistic prediction of the seismic behavior of CFDST columns. Lateral strength, stiffness, ductility and energy absorption are evaluated based on the hysteresis loops. The results indicated that the stiffeners had determinant role on improving pinching behavior resulting from the outer tube's local buckling and opening/closing of the major tensile crack of concrete. The lateral strength, initial stiffness and energy absorption capacity of longitudinally stiffened columns with fixed-free end condition were increased by as much as 17%, 20% and 70%, respectively. The energy dissipation was accentuated up to 107% for fixed-guided end condition. The use of transverse stiffeners at the base of columns increased energy dissipation up to 35%. Axial load ratio, hollow ratio and concrete strength affecting the initial stiffness and lateral strength, had negligible effect of the energy dissipation of the columns. It was also found that the longitudinal stiffeners and transverse stiffeners have, respectively, negative and positive effects on ductility of CFDST columns. The conclusions, drawn from this study, can in turn, lead to the suggestion of some guidelines for the design of CFDST columns.

Behaviour of Beam-Column Subjected to Reversed Lateral Loading

This paper presents an experimental investigation on the behaviour of Concrete Filled Double Skin Tubular (CFDST) column connected to Concrete Filled Steel Tubular (CFST) beam under reversed lateral loading. The CFDST column consists of two concentric cold-formed steel tubes with concrete sandwiched between them and the CFST member consist of cold-formed steel hollow section completely filled with concrete. The specimen in this study consists of a hollow square tube as the outer skin for both CFDST and CFST members. The inner skin of the CFDST column is a circular steel tube and M30 grade concrete having 40% cement replaced with fly ash was filled in between the tubes. The connection between the beams and the columns were provided using through bolts and angle plates. A pair of specimens considered as the control specimens having CFST members for both beam and column were also tested for comparison. All the four specimens were subjected to a constant axial load and reversed lateral loading which simulates the earthquake forces. The seismic characteristics like hysteretic behavior, peak load displacement, ductility, stiffness degradation and energy dissipation of CFDST beam-column were compared with that of the CFST beam-column. The beam-column specimens with CFDST columns exhibited favorable seismic behaviour with reduced self-weight and hence a better choice than CFST columns in regions of seismic risk.

08.35: Compressive behavior of concrete filled double skin sections consisting of corrugated plates and ultra‐high strength steel corner tubes

ABSTRACTConcrete filled tubular (CFT) sections and the most recent configuration of these sections, concrete filled double skin tubular (CFDST) sections, are being increasingly utilized in structural constructions due to their high strength and great performance under fire. CFDST sections generally consist of two steel hollow sections where the gap between the steel skins is filled with concrete. In this study, the mechanical performance of a new type of CFDSTs consisting of corrugated steel skins and ultra‐high strength steel (UHSS) or mild‐steel (MS) corner tubes is investigated. The efficiency of corrugated skins and UHSS or MS corner tubes is experimentally examined through conducting monotonic compressive tests. It is shown that employing steel skins with corrugated geometry may significantly increase the post‐peak compressive strength of CFDST sections. In addition, the experimental results are revealed that the non‐ductile compressive behaviour of ordinary CFDST sections can be effectively improved by employing UHSS corner tubes in fabrication of these sections. A finite element modelling framework is also developed to simulate the behaviour of the proposed high strength corrugated CFDST columns. The numerical modelling is first validated against the experimental results. The outcome of the paper indicates the potential of corrugated CFDSTs with UHSS or MS corner tubes to be used in various civil engineering construction.

Effect of ultra-high strength steel on mitigation of non-ductile yielding of concrete-filled double-skin columns

Concrete-filled tubular (CFT) sections, and the most recent configuration of these sections, concrete-filled double-skin tubular (CFDST) sections, can provide high strength in structural construction. In this study, an innovative fabrication strategy is proposed to prohibit the non-ductile yielding of CFDST sections by applying ultra-high strength steel corner steel tubes. Hence, the static compressive behaviour of CFDST columns containing ultra-high strength steel tubes is experimentally investigated. The results are then compared with those for the same experiment conducted on a CFDST specimen with corner mild steel tubes. In addition, a numerical investigation is conducted to examine the effect of different parameters on promoting the ductility and compressive strength. In order to pave the way for utilizing CFDSTs with corner tubes in engineering practice, a simple analytical formulation is also proposed to predict the load-displacement relationship of these sections based on the experimental and numerical results.

Reflections on Mechanisms Affecting the Behavior of FRP-Concrete-Steel Double-Skin Tubular Columns

This paper presents the results of 20 hollow and concrete-filled double-skin tubular columns (DSTCs), which were tested as part of a comprehensive experimental program that was undertaken at The University of Adelaide on FRP-concrete steel DSTCs. The paper is aimed at providing important insights into the influence of two key parameters, namely the diameter of inner steel tube and presence/absence of a concrete-filling inside the inner steel tube, which play major roles in the column behavior through their influences on a series of interacting mechanisms that govern the complex system behavior. A detailed examination of the results yielded a number of important insights into the mechanisms that influence the compressive behavior of DSTCs.

Compressive behaviour of concrete-filled double-skin sections consisting of corrugated plates

This paper deals with the static compression behaviour of concrete-filled double-skin tubular (CFDST) columns with corrugated plates. These sections take advantage of the high strength of their infill concrete as well as the ductility of their steel skins. Static compression tests were performed on CFDST column specimens with inner or outer skins built up from corrugated plates. The steel hollow sections fabricated from corrugated plates exhibited a more stable response under axial compression loading. Hence, implementing them in CFDST columns is expected to enhance the overall behaviour of these columns. These tests reveal how the overall behaviour of CFDST columns is affected by replacing their inner/outer skin with one fabricated from corrugated plates. The corrugated plates have an insignificant effect on the strength of column specimens. However, the ductility and energy absorption capacity of CFDST columns are improved dramatically. A finite element modelling framework was also developed to simulate the response of CFDST columns with corrugated plates and validated against the results of the experiments.

Theoretical and numerical analysis to concrete filled double skin steel tubular columns under fire conditions

As one of the recently developed steel–concrete composite structural members, concrete filled double skin tubular (CFDST) columns have been used in current construction industry due to its good structural performance. However, CFDST columns could be vulnerable under fire attack. In the current work, the fire resistance of CFDST columns has been studied. Finite element analysis (FEA) models were developed to investigate the fire resistance of unprotected CFDST columns. The FEA models were verified with experimental data from literature. Based on the developed numerical model, the failure mechanism of CFDST columns under fire conditions were studied and a parameter study was performed on the fire resistance design for CFDST columns. Based on the computational analysis, the conventional Rankine approach was extended to predict fire resistance of CFDST columns. Experimental data were used to validate the accuracy of the proposed method; it shows that the proposed method can predict the fire resistance of CFDST columns with reasonable accuracy compared with experimental data.

Influence of Inner Steel Tube Diameter on Compressive Behavior of Square FRP-HSC-Steel Double-Skin Tubular Columns

FRP-concrete-steel double-skin tubular columns (DSTCs) are a new form composite column system that effectively combines the advantages of the constituent materials. The performance of this column system has been experimentally investigated in a number of recent studies. However, apart from a single study reported on square DSTCs, all of the existing studies have been concerned with DSTCs with circular external tubes. This paper reports on part of an ongoing experimental program at the University of Adelaide on FRP-concrete-steel composite columns. The results from 12 square hollow and concrete-filled DSTCs and six companion hollow concrete-filled FRP tubes (H-CFFTs) that were tested under axial compression are presented. Results of the experimental study indicate that hollow DSTCs with larger inner steel tube diameters develop similar ultimate axial stresses to but significantly larger axial strains than companion DSTCs with smaller inner steel tubes. The results also show that, in concrete-filled DSTCs with similar Ds/ts ratios, an increase in the steel tube diameter leads to an increase in both axial stress and strain of concrete. It was observed that H-CFFTs perform significantly worse than both hollow and filled DSTCs under axial compression, and their behavior further degrades with an increase in the diameter of their inner voids.