Concrete-filled Double-skin Tube Columns Research Articles

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.

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 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.

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.

On the fire performance of unbraced composite frames

Purpose The 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/approach To 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. Findings Overall, 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/value The 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 ...

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.

Testing of self-consolidating concrete-filled double skin tubular stub columns exposed to fire

A concrete-filled double skin tube (CFDST) is an innovative steel and concrete composite construction with the potential to be used as columns in high-rise buildings. Self-consolidating concrete (SCC) can offer convenience for construction and ensure the construction quality of CFDST columns. There is very limited knowledge about the fire performance of CFDST columns, which is a key issue in the application to high-rise buildings. This paper reports an experimental investigation into the fire behaviour of CFDST stub columns filled with SCC and fibre reinforced SCC. The study aims to obtain thermal and structural responses of the stub columns through standard fire testing. Details are given in terms of the failure mode, temperature distribution, critical or limiting temperature and fire resistance. It was found that the added fibre does not affect the temperature distribution whereas the increased concrete thickness and outer tube perimeter reduces the temperature in CFDST specimens. The added steel fibre increases the fire resistance of CFDST stub columns especially for load levels less than 0.6. The critical or limiting temperature for CFDST sections was found to be higher than for CFST (concrete-filled steel tube) or unfilled tubes.