Concrete-filled Double-skin Tube Research Articles

CFRP-steel composite beams with seawater sea sand concrete cores subjected to bending

A redesign was attempted for the concrete-filled double-skin tube subjected to bending, and by adjusting the core tube position and applying carbon fiber-reinforced polymer (CFRP) sheets or steel bars in the tensile region, it was hoped to delay the cracking of the concrete in the tensile region. Epoxy mortar was used to modify the interface. Seawater and sea sand has been successfully applied from reinforced concrete beams to concrete-filled steel tube beams, benefiting from the corrosion-resistant properties of CFRP and epoxy mortar. The four-point bending test parameters included the type of core tube, the number of CFRP layers, and the diameter of the steel bars. The beams with FRP sheets exhibited better flexural capacity and residual stiffness, while the brittle fracture of FRP led to major crack in concrete. The beams with steel bars exhibited better initial stiffness and the cracks in concrete were more uniform in width. When the layers of CFRP or the diameter of the steel bars was increased, the flexural capacity of the composite beams increased by 11.0%-13.6% and 7.0%-9.1%, respectively. The enhancements provided by the core tubes were small, and the design of the composite beams should be considered from various perspectives. The moment-curvature relationships, ultimate states, and flexural stiffness were defined by considering concrete cracking, FRP fracturing, and steel buckling. Several formulas for calculating flexural stiffness in the code were evaluated in combination with the test data.

Comprehensive experimental database and analysis of circular concrete-filled double-skin tube stub columns: A review

Comprehensive experimental database and analysis of circular concrete-filled double-skin tube stub columns: A review

Effectiveness of Stiffeners in CFDST Columns: Comparative Study

Concrete-filled double-skin tube (CFDST) columns are considered one of most efficient forms of steel-concrete composite columns, which provide higher axial strength and better ductility compared with their counterpart concrete-filled tube (CFT) columns. This paper aims to numerically investigate and compare the performance of axially loaded circular CFDST short columns provided with stiffeners in inner tubes and outer tubes. Circular steel hollow sections have been adopted for inner as well as outer tubes, and rectangular steel stiffeners were fixed in inner and outer tubes to check the effectiveness of stiffeners in stiffened CFDST columns. These CFDST columns were investigated numerically by using a verified finite-element (FE) model from commercially available software. Behavior of 36 unstiffened, inner-tube stiffened, and outer-tube stiffened CFDST columns (12 each) was studied. The FE results were verified by comparing them with previous test results. The FE analysis study has exhibited an increase in peak load (load-carrying capacity) up to 26% and 15% in outer-tube stiffened CFDST columns compared with unstiffened and inner-tube stiffened CFDST columns, respectively. Also, enhanced ductility has been observed in case of these stiffened CFDST columns.

Estimation of blast-induced peak response of concrete-filled double-skin tube columns by intelligence-based technique

This paper aims to predict the peak response of concrete-filled steel tubes (CFSTs) under blast loads using regression-based machine learning (ML) algorithms considering the variability of input parameters. Twelve regression metrics that are commonly used are utilized to evaluate and compare the efficiency and effectiveness of the proposed ML models. The Monte Carlo approach is used to propagate the variability in the input space to the predicted output. The results showed that the optimal method varies with perspectives. The XGBoost algorithm has the highest final score, and the SVM algorithm exhibits the highest total score for standard deviation. Furthermore, the confidence interval increases with the peak response, and the single model presents a higher accuracy in predicting large displacements. However, ensemble models do the opposite. Sensitivity and uncertainty analyses are performed, indicating that the scaled distance, section size and ultimate moment capacity exhibit the highest contribution to the peak response of the CFST. Finally, a simple formula is developed based on the MLP model, which can cover three input parameters. The findings of this paper can be used for computer-aided dynamic response design of CFST columns subjected to blast loads.

Research on compressive behavior of CFRP-confined CFDST stub columns with square stainless steel outer tube

Carbon fiber-reinforced polymer (CFRP)-confined concrete-filled double skin tube (CFDST) stub columns with a square outer tube made of stainless steel can be characterized by their low weight, high strength, and good corrosion resistance. They have good application prospects in the field of ocean engineering. This study applied the finite element software ABAQUS to comprehensively investigate the compressive behavior of CFRP-confined CFDST stub columns with a square stainless steel outer tube and a circular inner steel tube in terms of their ultimate bearing capacity, based on previous experimental research. Through a parametric analysis and comparison, it was found that the strength and width-to-thickness ratio of the outer tube have a significant effect on improving the ultimate bearing capacity of the specimen, while the strength and diameter-to-thickness ratio of the inner tube and the number of CFRP layers have a limited effect. The development stage of the interaction stresses between the inner and outer tubes and concrete was clarified, and the location of the maximum interaction stress was determined. Finally, based on the limit equilibrium method, formulae for predicting the ultimate bearing capacity of square CFDST stub columns and CFRP-confined square CFDST stub columns were derived.

Axial compression behavior of CFRP-confined square concrete-filled double skin tube stub columns with stainless steel outer tube

Concrete-filled double skin tube (CFDST) stub columns with a stainless steel outer tube is a new kind of composite columns and potential to be widely utilized in ocean engineering owing to the excellent performance characterized by high corrosion resistance. To improve the ultimate bearing capacity and delay the local buckling of stainless steel tube, confining materials such as carbon fiber reinforced polymer (CFRP) can be adopted in these composite columns. Thus, this study conducted an experimental investigation on the axial compression behavior of CFRP-confined square CFDST stub columns with a stainless steel outer tube. The main variables were the width-to-thickness ratio of stainless tube and the number of CFRP layers. The typical failure modes, load-shortening curves, and load-strain curves of the CFDST and CFRP-confined CFDST columns were obtained. The test results indicated that the local buckling of CFDST columns can be well suppressed with CFRP wrapping, prior to CFRP rupturing. The CFRP can provide an effective confinement on the sandwich concrete and the ultimate bearing capacity enhancement increased with increasing the number of CFRP layers. Based on theoretical analysis a design model for the inner steel tube has been proposed to prevent the premature failure of the inner steel tube which can leads serious ultimate bearing capacity lose. Additionally, new methods for predicting the ultimate bearing capacity of the square CFDST and CFRP-confined CFDST are proposed based on the limit equilibrium method and twin shear unified strength theory. The predicted results are in good agreement with the experimental results and collected test data.

Analytical model for concrete-filled double skin tube columns with different cross-sectional shapes under axial compression

The existing theoretical analysis models of concrete-filled double skin tube (CFDST) columns are limited to the analysis of CFDST columns with circular outer and inner tubes (CC-CFDST). The purpose of this paper is to propose a unified model for CFDST columns with different cross-sectional shapes, including CFDST columns with rectangular outer and inner tubes (RR-CFDST), CFDST columns with square outer and inner tubes (SS-CFDST), CFDST columns with square outer and circular inner tubes (SC-CFDST), CFDST columns with circular outer and square inner tubes (CS-CFDST), CC-CFDST columns. The CFDST columns with different shapes (original columns) are replaced with equivalent CC-CFDST columns by assuming that they have the same cross-sectional areas of the inner and outer steel tubes and concrete. The difference in mechanical properties between the original column and the equivalent column is attributed to the difference in the confining effect of the steel tube on the core concrete caused by the different shape, which is considered by the shape effect coefficient ke. The proposed model is verified by collected experimental data of CFDST columns with different cross-sectional shapes. The mean value and coefficient of variation of the ratio of the calculated bearing capacity to the experimental bearing capacity are 1.006 and 0.119, respectively. The results show that the proposed model can predict the axial compression bearing capacity and load–deflection curves of CFDST columns accurately.

Experimental Study on the Behavior of a Novel Stiffened Hexagonal CFDST Stub Column under Axial Load

A concrete-filled double skin tube (CFDST) consists of an outer steel tube, an inner steel tube, and the space between them filled with concrete. Existing studies have shown that the buckling of steel tubes can be significant in noncircular CFDST for nonuniform confinement or a large width-thickness ratio. Stiffening of the steel tubes is necessary to fully utilize the strength and improve the load-carrying capacity of CFDST. Against this background, the author proposed a novel stiffened hexagonal CFDST column with steel strips welded on both the inner tube and outer tube to form several closed cavities. This paper presents an experimental study on the axial compressive behavior of this new type of CFDST column. The experimental program consisted of five stiffened hexagonal CFDST specimens and two reference specimens for comparison, and the test variables were the hollow ratio and rib width. The test results showed that the ultimate bearing capacity and ductility of the CFDST specimens were greatly enhanced by the steel strips and ribs. Moreover, the finite-element (FE) method was used to develop a three-dimensional model of the CFDST column subjected to axial loading and validated against the experiment. The average error of the FE analysis when predicting the column bearing capacity was 0.029 compared with experimental results. Based on the FE model, a parametric study was conducted to analyze further the effect of each parameter on the axial behavior. Furthermore, a strength design formula was developed to estimate the compressive strength of the novel hexagonal CFDST column.

Experimental Study on Compressive Behavior of Concrete-filled Double-skin Circular Tubes with Active Confinement

A convenient family member of composite columns is concrete filled double skin tube (CFDST); it contains two tubes of concentric steel and also shell concrete that there is between them. The superior or equal potential offering characteristics of CFDST columns are more than counterparts of filled steel tubes of classical concrete (CFST). The purposes of the present study are to provide experimental investigation results into the prestressed load-carrying capacity of CFDST columns. Here, an innovative technique is used to confined concrete prestressing, in which the fresh concrete is compressed for a short-run duration. Sixty-four total specimens were tested with various outer thickness and diameter, inner thickness and diameter, and also CFDST columns of concrete strength that resist axial compression. The experimental results support that the present technique prestressed confined concrete, and it demonstrates that CFDST specimens' load-carrying capacity enhanced significantly.

Behaviour of Concrete Filled Double Skin (CFDS) Steel Tube by Partial Replacement of Coarse Aggregate with Brickbats

Objectives: To find the optimum percentage replacement of brickbats in concrete by performing fresh and hardened concrete tests and is incorporated into CFDS steel tube column to study its behaviour analytically. Methods: The behaviour of a double skin CFST column with brickbats partially substituting the coarse material with Brickbats. Part of the coarse aggregate was replaced with grouted class I brick bats with percentages of 5 to 25%. A study of the compressive and split tensile strength of concrete was conducted fresh and hardened tests on concrete specimens (cubes and cylinders). Performance of CFDS steel tube column partially replacing coarse aggregate with brickbats is examined analytically using ANSYS. Findings: The compressive and split tensile strength of 10 percent replacement brickbats improved by 8.5 and 13 percent, respectively, while the compressive and split tensile strength of 5 percent replacement brickbats increased by 1.67 and 6 percent, respectively. When compared to nominal cube strength, partially replace brickbats compressive strength increases gradually up to 10% brickbat replacement and then tends to decrease from 15percent brickbat replacement. When compared to different mixtures, it was discovered that adding 10% brickbats increases strength, and this amount was deemed the best mix proportion. The highest load taken by CFDS with 10% brickbats is 649.36kN, whereas standard CFDS is 705.16kN. The CFDS tube was replaced by CFDS tube, which took 1.15 percent more load than brick bats. Novelty/Applications: This research yields good result and can be adopted for the construction of tall composite buildings. This technique will be reduced the self — weight of the structure to some extent. Keywords CFDS, brickbats, concrete, confinement effect, ANSYS

Fire resistance of stiffened CFDST columns after earthquake-induced damages

Abstract Despite the awareness about the hazard gained through the past evidence, fire following an earthquake remains a predominant agent of damage to buildings and life-line structures. Often the damage caused by the subsequent fire can be more severe than the damage caused by the ground motion itself. Concrete filled double skin tube (CFDST) columns as a special composite structure are particularly prone to the inadequate steel-concrete interface bonding and subsequent local buckling during separate fire conditions and seismic action. To establish a favorable interface interaction between steel and concrete as well as to postpone overall buckling under post-earthquake fire, the effectiveness of stiffening CFDST columns with longitudinal steel stiffeners are investigated. The analyses on CFDST columns with different patterns of stiffeners embedded in the interior or exterior surfaces of the inner or outer tubes are conducted with a sequentially coupled approach, starting with a heat transfer analysis and followed by a stress/deformation solution. In the finite element modeling, the confinement provided by both the inner and outer tubes on the compressive strength of concrete is accounted for. The effect of concrete fracture is incorporated by inserting a horizontal discrete crack model at the location of the major tensile crack. The results showed that stiffeners on the exterior surface of the inner tubes are among the most effective of various patterns studied in enhancing the fire endurance of the columns up to 64%. For stiffened CFDST columns contrary to the unstiffened ones, the fire-resistance reduction to seismic damage is usually less than 5% of the initial value. The fire-resistance rating reduction obtained for hollow ratios of 0.5 is 58%, with respect to the hollow ratio of 0.25.

Behavior of octagonal concrete-filled double-skin steel tube stub columns under axial compression

This paper presents an experimental investigation on the behavior of stiffened octagonal concrete-filled double-skin tube (CFDST) stub columns subjected to axial compression. A total of eight specimens were prepared and tested under axial compression aiming to study the effects of longitudinal stiffeners and hollow ratio on the elastic-plastic local buckling and ultimate load-carrying capacity of the test columns. The specimens consist of two control specimens without stiffeners (unstiffened columns) and two specimens stiffened with outer-inner tubes welded stiffeners (OI stiffeners) in four sides of the octagonal steel section. In addition, four specimens were stiffened with two types of stiffeners: OI stiffeners and outer tube welded stiffeners (O stiffeners) in eight sides of the outer steel tube section. Furthermore, two hollow ratios were employed. Based on the test results, the ultimate load-carrying capacity, load-displacement curves, load-strain response, ductility, and failure modes were discussed and clarified. The test results indicated that the strength and ductility of the specimens were improved greatly by the presence of the stiffeners. The stiffeners significantly enhanced the confinement and prevented the occurrence of elastic-plastic local buckling of the outer tube. Besides, current design equations were used to predict the ultimate load-carrying capacity of octagonal CFDST stub columns. The test results were then compared with the predicted ultimate capacities of existing design equations. These predictions showed good agreement with the test results.

Evaluation of ultimate bending moment of circular concrete–filled double skin steel tubes using finite element analysis

In this study, the ultimate bending moment of circular concrete-filled double skin steel tubes (CFDSTs) was investigated. A CFDSTs made of two concentric circular steel tubes with concrete infill and M16 shear connector system was fabricated. The four-point bending test of the 10 m long CFDST consisting of outer and inner steel tubes with 914.4 mm and 514.4 mm in diameter, respectively, was carried out and the ultimate bending moment of the CFDST was investigated. A finite element (FE) simulation of the CFDSTs subjected to bending was developed using the commercial software ABAQUS and the accuracy of the developed FE model was verified by comparing to the experimental result. The ultimate bending moment of CFDSTs was then evaluated with respect to different concrete infill compressive strengths and yield strengths of the steel tubes. The corresponding design ultimate bending moments of the CFDST with regard to the design codes AISC and EC4 were also computed. The results revealed that EC4 and AISC can accurately predict the ultimate moment capacities of the CFDST with shear connector.
 Keywords: ultimate bending moment; concrete-filled double skin tube; shear connector system; finite element analysis.
 Received 19 November 2018, Revised 04 January 2019, Accepted 04 January 2019

Technical Note: Post-Fire Axial Load Resistance of Concrete-Filled, Double-Skin Tube (CFDST) Stub Columns

This technical note reports findings on a series of squash tests to investigate the effects of a significant fire loading history on the axial load strength of concrete-filled, double-skin tube (CFDST) stub columns. Axial loading tests were conducted on two stub columns that were previously subjected to the first 60 minutes of the standard ASTM E119 (ASTM, 2012) fire. Results were compared to the resistance of an identical virgin stub column. Comparisons indicated an average reduction of 28% in the axial load strength of stub columns when subjected to the mentioned fire loading history.

On the fire performance of unbraced composite frames

PurposeThe purpose of this study is to gain a deeper understanding of the structural behaviour of fire-exposed unbraced composite frames. Designers to date paid little attention to unbraced one-bay composite frames as structural system. There are two main reasons for this. First, codes lack simplified methods for the fire design of these frames due to their sway and the linked P-Δ effects when subjected to fire, which complicates the design. Second, it is demanding to construct external composite joints for the regarded one-bay frames. Thus, external joints in composite constructions are mostly constructed as steel joints. Nevertheless, these frames offer advantages. These include increased usable space and flexibility in the building’s use, large spans, fast construction times and inherent fire resistance.Design/methodology/approachTo profit from these benefits, two different external semi-rigid composite joint were developed for the considered one-bay composite frames. The first solution based on concrete-filled steel tube columns and the second on concrete-filled double skin tube columns. Furthermore, a numerical model was established to study the fire performance of unbraced composite frames. The model was validated against four fire tests on isolated composite joints and two large-scale fire tests on unbraced composite frames.FindingsOverall, the predictions of the numerical model were in good agreement with the test results. Thus, the numerical model is appropriate for further investigations on the fire performance of unbraced composite frames.Originality/valueThe sequence of construction results in significant stresses in the steel section, which creates difficulties in numerical modelling and may account for the relatively few studies carried out at room temperature. For the fire design, there was, to the best knowledge of the author, to date no numerical model available that was capable of considering the sequence of construction.

Experimental Investigation on Lateral Impact Response of Concrete-Filled Double-Skin Tube Columns Using Horizontal-Impact-Testing System

This paper presents an experimental investigation on the lateral impact performance of axially loaded concrete-filled double-skin tube (CFDST) columns. These columns have desirable structural and constructional properties and have been used as columns in building, legs of off shore platforms and as bridge piers. Since they could be vulnerable to impact from passing vessels or vehicles, it is necessary to understand their behaviour under lateral impact loads. With this in mind, an experimental method employing an innovative instrumented horizontal impact testing system (HITS) was developed to apply lateral impact loads whilst the column maintained a static axial pre-loading to examine the failure mechanism and key response parameters of the column. These included the time histories of impact force, reaction forces, global lateral deflection and permanent local buckling profile. Eight full scale columns were tested for key parameters including the axial load level and impact location. Based on the test data, the failure mode, peak impact force, impact duration, peak reaction forces, reaction force duration, column maximum and residual global deflections and column local buckling length, depth and width under varying conditions are analysed and discussed. It is evident that the innovative HITS can successfully test structural columns under the combination of axial pre-loading and impact loading. The findings on the lateral impact response of the CFDST columns can serve as a benchmark reference for their future analysis and design.

Simplified analytical solution for axial load capacity of concrete-filled double-skin tube (CFDST) columns subjected to fire

Concrete-filled steel tube (CFST) columns and even more so concrete-filled double-skin tube (CFDST) columns have demonstrated a desirable behavior when subjected to fire loading conditions. Current methods to estimate the axial load capacity for these columns exposed to fire are either limited to simple cases using tables or require complex finite element analysis. In an effort to develop a more general and practical approach to this problem, a simplified step by step analytical procedure is proposed for the calculation of the axial load capacity of CFDST columns subjected to any given time–temperature (fire) curve. The procedure was defined by combining an analytical solution to the heat transfer problem with calculation of axial load capacity using the fire-modified material properties adopted from the Eurocode 4 general rules for structural fire design. The proposed method was verified with existing experimental and advanced finite element simulation results. A number of design recommendations, based on the knowledge gained from using the procedure for different case studies, are proposed for CFDST columns subjected to fire.

Finite Element Simulation of Concrete-Filled Double-Skin Tube Columns Subjected to Postearthquake Fires

AbstractDetailed finite-element (FE) analyses were conducted using commercial software to simulate the behavior of concrete-filled double-skin tube (CFDST) columns subjected to postearthquake fires. The main goal of these simulations was to replicate results from a series of experiments that first subjected CFDST columns to different levels of seismic damage by applying cyclic lateral loading, and then fire testing of the same specimens in a furnace. The numerical simulations paralleled the experiments in the loading sequence. Different material models and modeling techniques were assessed in terms of reproducing experimental observations including local failure modes such as steel tube buckling. Results from the cyclic loading simulations including residual deformations were maintained as initial conditions for the subsequent thermal-stress analysis to simulate the fire testing phase of the experiments. A sequentially coupled nonlinear thermal-stress analysis was conducted on the models of CFDST columns ...

Experimental Study on Post-Earthquake Fire Resistance of Ductile Concrete-Filled Double-Skin Tube Columns

Experimental studies were conducted to examine the behavior of concrete-filled double-skin tube (CFDST) columns exposed to fire after being subjected to simulated seismic loads. The experiments were conducted in two separate phases, consisting of (1) the quasi-static cyclic tests, followed by (2) fire tests. Three nominally identical column specimens were constructed for these studies. One of the specimens was directly tested under fire to quantify its resistance in an undamaged condition. The other two specimens were first subjected to quasi-static cyclic lateral loads, imposing varying degrees of lateral drift to simulate two different seismic events with moderate and high damage levels before being exposed to fire. Both of the specimens were pushed to the maximum drift of 6–6.5% with different residual drifts of 1.4 and 3.9% for moderate and high damage levels, respectively. The undamaged and damaged columns were then subjected to the same fire tests in accordance with a standard temperature-time curve while sustaining an axial load until the column failed due to global buckling. Local buckling of the tubes was also observed in the specimens due to the thermal expansion and separation from the concrete. Overall the results showed marginal differences in the fire resistance of the three specimens, providing evidence for the resilient performance of these columns under post-earthquake fire scenarios. An additional quasi-static cyclic loading test was conducted on the specimen that had been exposed to fire without any prior damage to investigate the behavior of the column subjected to seismic loads after the fire test. Differences in behavior were modest, except for a 5.7% drop in strength attributed to permanent degradation in material properties due to the fire test.

Nonlinear analysis of short concrete-filled double skin tube columns subjected to axial compressive forces

Proper material constitutive models for concrete-filled double skin tube (CFDST) columns with circular cross-section and subjected to axial compressive forces are proposed and verified against experiment data using the nonlinear finite element program Abaqus. It is shown that CFDST columns can provide a good confining effect for concrete core, especially when the diameter-to-thickness ratios of both the outer and the inner tubes are small. In addition, empirical equations are proposed to predict the lateral confining pressure of the concrete core for CFDST columns. Finally, the axial strengths of the CFDST columns calculated by the proposed formulations are compared with AISC formulations against the experimental data. It has been shown that the proposed formulations are better than the AISC formulations.