Concrete-filled Steel Tubular Specimens Research Articles

Numerical Response of Concrete-Filled Steel Tubular (CFST) Columns Externally Strengthened with FRP Composites Subjected to Cyclic Loading

The ultimate load-carrying capacity of concrete-filled steel tubular (CFST) columns exposed to monotonic loadings can be greatly increased by strengthening those columns, and the occurrence of the steel tube's outward buckling can be postponed. The current research aims to study the possibility of improving the structural characteristics of CFST columns exposed to cyclic loadings in terms of lateral load capacity and absorbed energy by strengthening them with different patterns of fiber-reinforced polymer (FRP) sheets. The ABAQUS software was used to create a three-dimensional (3D) non-linear finite element model (FEM) to simulate the behavior of FRP-strengthened CFST columns exposed to monotonic and cyclic loadings. After ascertaining the accuracy of the proposed model's results in successfully predicting failure patterns and lateral loads compared to the experimental results of tested specimens available in the literature, the model was used to create a parametric study. The parametric study focused on the impacts of the thickness, location, and length of the strengthening sheets on the failure pattern, lateral load-carrying capacity, stiffness, cumulative energy, absorbed energy, and viscous damping factor of the CFST columns exposed to cyclic loadings. The results revealed that the un-strengthened specimen displayed a maximum lateral load of 185 kN and a viscous damping factor of 45.2% at a lateral drift of 5.7%. On the other hand, strengthening the CFST column using five layers of FRP sheets exhibited the highest lateral load of all investigated columns (50% more than the un-strengthened specimen). Additionally, at a lateral drift of 5.7%, the decrease in viscous damping factor of CFST specimens due to strengthening using 1, 2, 3, 4, and 5 layers of FRP sheets with respect to the control specimen was 7.9%, 14.9%, 20.8%, 27.7%, and 30.3%, respectively.

Flexural performance of round-ended concrete-filled steel tubular specimens

Round-ended concrete-filled steel tubular (CFST) columns have numerous benefits, including easy construction, aesthetic appearance, and variable section arrangement. This study experimentally and numerically investigated the flexural behavior of round-ended CFST columns. The six testing results showed that as the aspect ratio increased from 1.38 to 1.86, the flexural resistance and stiffness increased by 82.0% and 159.9%, respectively. Every round-ended CFST specimen had excellent ductility. Concrete can be adequately confined by the round-ended steel tube. Mechanistic and parametric analyses were carried out using a finite element model. Mechanistic analysis demonstrated that the round segment of the steel tube can effectively confine the concrete, resulting in a considerable strength gain near the edge of the compression zone. The parametric analyses showed that as the concrete strength increased from 24 to 110 MPa, the flexural resistance increased by average 20.4%; as the steel strength improved from 235 to 700 MPa, the flexural resistance increased by average 155.4%; as the steel ratio enhanced from 0.05 to 0.2, the flexural resistance increased by average 179.7%; and as the aspect ratio increased from 1 to 3, the flexural resistance of the major (minor) axis increased (decreased) by about 59.9% (36.5%). The analysis showed that the flexural stiffness of this composite specimen can be accurately predicted by AISC 360–2016. Finally, a simplified model was proposed for estimating the flexural resistance of the round-ended CFST specimens.

Effect of imperfection on behaviour of axial loaded square and rectangular concrete-filled steel tubular columns

Axial compressive strength of Concrete Filled Steel Tubular (CFST) columns is significantly improved owing to lateral confinement and the interaction between steel tube and concrete core. Occasionally there exists an initial gap between the concrete core and steel tube which may occur due to improper compaction, shrinkage in concrete, lack of curing of concrete core, etc., consequently, the compressive strength of the column is reduced. Present study shows that the presence of a gap between the concrete core and steel tube reduces the axial strength of CFST column to some extent; however, it results in an enormous fall in the ductility of column. A total of 22 specimens, including 2 hollow steel tube subjected to axial compressive loading, were tested. The main parameters of these tests were the circumferential gap. The influence of gap on the failure mode and ultimate strength of the CFST specimens were investigated. The results show close agreement between calculated and experimental results in terms of load-deformation response and ultimate strength. The behavior of CFST columns with a gap is analyzed, to investigate the impact of different parameters on ultimate strength. A maximum limit of 1.4 to 5.9 gap ratio is proposed, and a simplified formula is suggested to estimate the effect of a gap on ultimate strength.

Investigation of cyclic behavior of partially concrete-filled steel tubular columns

A partially concrete-filled steel tubular (CFST) member is formed by partially filling concrete into a steel hollow section (SHS). Partially CFST members are mainly used as bridge piers and frame columns due to the improved structural performance, since the potential plastic hinge is expected to occur at the composite section when subjected to an earthquake. In practice, the shear transfer between the upper SHS and the lower CFST part of a partially CFST column needs careful consideration, sometimes leading to complex details. In this study, two types of shear connectors, namely bolted shear connectors and diagonal-rib shear connectors were proposed and arranged below the concrete-filling interface. Five square partially CFST specimens and a reference steel tubular column were designed and tested under combined constant axial compression and cyclic lateral loading. The test variables included the concrete-filling ratio, types of shear connectors, and strength of shear connectors. The results demonstrated the effectiveness of the two types of shear connectors, and the specimens with diagonal-rib shear connectors showed excellent performance. The method and corresponding formulas about the full shear connection were proposed to calculate the resistance of square partially CFST columns under combined compression and flexure.

Performance of concrete filled steel tubular (CFST) members with circumferential gap under lateral impact

To investigate the influence of circumferential gap on the impact-resistance capacity of CFST columns, 12 CFST specimens were tested under constant axial load and lateral impact loading at mid-span. The test parameters included gap ratio, impact velocity, and slenderness ratio. The failure mode, impact force versus time curves, and deflection versus time curves of CFST specimens with circumferential gap were experimentally investigated and compared with those of the reference samples without any gap. Based on the test results, the failure mode, impact force history, deflection response, and energy dissipation of the specimens were experimentally investigated, and the influence of the above parameters on the impact-resistance of CFST specimens was analyzed. To further explore the force transfer mechanism of CFST with gap under lateral impact, a finite element analysis (FEA) model was established, and the influence of the gap on the contact stress, internal force transfer mechanism, and dynamic flexural capacity of CFST specimens was analyzed. Lastly, a simplified formula for predicting the dynamic flexural capacity of CFST considering the influence of circumferential gap was proposed, which could provide a reference for practical engineering applications.

Experimental and numerical investigation on the lateral impact responses of CFST members after exposure to fire

The impact load acting on structure can cause a completely different structural response from the static load, and structure may be subjected to both impact and fire in extreme events, leading to severe structural damage or collapse. This paper reports the lateral impact responses of concrete-filled steel tubular (CFST) members after fire. Nine CFST specimens with different parameters, i.e., the fire duration time, impact heights, and boundary conditions, were tested using an electric furnace and a drop hammer impact system. The failure modes, impact force, midspan deflection, and energy absorption were investigated and compared. Results reveal that post-fire CFST members exhibit global bending failure under lateral impact, and increasing the fire duration time reduces the impact resistance of CFST members, with lower peak and plateau impact forces but larger deflection. The finite element (FE) model of post-fire CFST members under lateral impact was developed and calibrated against the test results to further analyze the impact responses. In addition, simplified methods were proposed for the evaluation of dynamic flexural capacity, plateau impact force and maximum deflection of CFST members subjected to lateral impact.

Dynamic response of concrete-filled square steel tubular columns under lateral impact load at flat or corner zone

The dynamic response of columns under lateral impact is of great significance for guiding structural design under accidental loads. The present study aims to investigate the effect of different impact directions on the dynamic response of square concrete-filled steel tubular (CFST) columns by conducting drop hammer tests at flat or corner zones. The main experimental parameters included hammer mass, drop height, indenter radius, built-in steel bars, shear span ratio, width-thickness ratio, and column sizes. The experimental results showed that the impact zone has a significant effect on the dynamic response of CFST specimens. Under higher impact energies, specimens impacted at corner zones were more likely to fracture on the tensile side of the steel tube than specimens impacted at flat zones. And the specimens impacted at the corner zone were more sensitive to the change of indenter radius. Smaller shear span ratios, built-in steel bar, and larger column sizes improved the impact resistance of tested specimens. The 3D finite element models were developed and analyzed by ABAQUS/Explicit to explore dynamic behavior further.

Temperature field of intumescent coating protected concrete-filled steel tubular columns under fire

In the fire protection design for concrete-filled steel tubular (CFST) structures, the analysis of temperature field usually acts as a preceding step to further study the mechanical performance in fire. However, there is still a lack of systematic research on the temperature field of CFST sections with intumescent fire coating (IFC). In this study, fire tests of four hollow steel tubular specimens and twenty CFST specimens lasted for 180 min ∼240 min were carried out under ISO-834 standard fire curve and all the specimens were protected with IFC. A technical process to determine the equivalent thermal conductivity of IFC applied to CFST columns was proposed. A verified finite element analysis (FEA) model to predict temperature field of IFC protected CFST section was established and the size effects of CFST sections on the temperature field were studied. It was found that the intumescent coating can reliably work together with CFST specimens without overall crumbling during the long-time fire exposure up to 240 min. The temperature rise of the steel tube of CFST column can be significantly delayed with the effective insulation of IFC together with the heat absorption of the core concrete. The main factors affecting temperature rise of the IFC protected CFST section under fire are the core concrete, dry film thickness (DFT) of IFC and sectional diameter.

Eccentric compression behaviour of diagonal rib-stiffened thin-walled square concrete filled steel tubes

Thin-walled square concrete filled steel tubular (CFST) members have attracted more attention in practice because of less steel consumption, high construction efficiency, and convenient arrangement for furniture or curtain wall. Compared with circular members, the local buckling of thin-walled square CFST members is a big concern since tube buckling often results rapid strength and ductility degradation. So in this investigation, diagonal ribs were welded on the internal tube corners to improve the performance of square CFST members. Nine diagonal rib-stiffened square CFST specimens, with large width-to-thickness ratio B/t=100, were experimentally investigated under eccentric compression. The main test parameters included column slenderness ratios (λ=13.9, 34.6, 55.4) and loading eccentricities (2e/B=0, 0.25, 0.50, 1.00). Detailed finite element models were established and verified. The composite effect of the stiffened column was elaborated through combining test results and the finite element analysis, followed by feasibility of column–column connection details and parametric studies. Finally, design methods for calculating the member resistances subjected to axial and eccentric compression were proposed.

Creep Prediction Model of Concrete-Filled Steel Tube under Different Core Concrete Conditions

To reasonably predict the creep law of concrete-filled steel tubular (CFST) members under different working conditions, four groups of creep tests were carried out, and the creep curves of CFST specimens with defects were obtained. Based on the creep prediction formula given in the code for the design of CFST highway arch bridges issued in 2015, the creep prediction models of CFST under different core concrete conditions are studied. The results show that the concrete microcracks around the cavity defects are continuously generated and expanded during the compression process, resulting in the dislocation and sliding of the concrete. It increases the additional deformation of CFST; debonding defects hinder the redistribution of stress between the steel tube and the core concrete, weaken the interaction effect between the steel tube and the concrete, and increase the creep deformation of the core concrete. According to Different core concrete states, appropriate reference factors are selected to modify the creep prediction formula of CFST, which greatly improves the creep prediction accuracy of CFST members and defective members.

Flexural behaviour of stiffened and multi-cell L-shaped CFSTs considering different loading angles

Stiffened and multi-cell L-shaped concrete-filled steel tubular (CFST) members are often subjected to the loads from different loading angles as corner columns. Moreover, the flexural behaviour is the basis for the research of beam-columns and the determination of axial force-bending moment interaction curves. However, little work has been done on the flexural behaviour of these kinds of members at different loading angles. In this study, a total of 11 L-shaped CFST specimens with the variables of sectional type and loading angle were tested under pure bending. The failure mode, bending moment-deflection curves, moments-strain curves, deflection curves, flexural capacity and stiffness were investigated. It was found that the influence of loading angles is significant. The flexural capacity and stiffness of the specimens are highest at 135°, middle at 0° and 180°, and lowest at 45° and 225°. Setting longitudinal stiffeners or using multi-cell cross-section can improve the flexural capacity by about 20%. Then a finite element model was developed incorporating residual stress and cold-formed effect. Considering the effect of loading angles on the basis of the design model at 0°, the simplified formulas for flexural capacity of the stiffened and multi-cell L-shaped CFST members were proposed.

Collapse Behaviour of a Concrete‐Filled Steel Tubular Column Steel Beam Frame under Impact Loading

The progressive collapse of a concrete‐filled steel tubular (CFST) frame structure is studied subjected to impact loading of vehicle by the finite‐element software ABAQUS, in the direct simulation method (DS) and alternate path method (AP), respectively. Firstly, a total of 14 reference specimens including 8 hollow steel tubes and 6 CFST specimens were numerically simulated under transverse impact loading for verification of finite‐element models, which were compared with the existing test results, confirming the overall similarity between them. Secondly, a finite‐element analysis (FEA) model is established to predict the impact behaviour of a five‐storey and three‐span composite frame which was composed of CFST columns and steel beams under impact vehicle loading. The failure mode, internal force‐time curve, displacement‐time curve, and mechanical performance of the CFST frame were obtained through analyzing. Finally, it is concluded that the result by the DS method is closer to the actual condition and the collapse process of the structure under impact load can be relatively accurately described; however, the AP method is not.

Thermal Effect on Bond Strength of Rubberized Concrete Filled Steel Tubular Sections

This study aims to assess the effect of high temperature on the bond strength of Concrete Filled Steel Tubular (CFST) sections incorporating crumb rubber particles as partial replacement of fine aggregates in concrete core. High daily temperatures; as in hot countries; was considered as well. Push out tests were conducted on 72 CFST specimens with five different concrete mixes. Prior to these tests, 210 concrete specimens were tested to obtain the mechanical properties of rubberized concrete under high temperature gradients. Some of the tests; on CFST sections or concrete specimens; were tested at the desired high temperature, and some others were tested after cooling down. The main variables considered were the crumb rubber replacement ratio, type of CFST section (circular or square) and temperature gradient. The test results indicated that high temperature reduces the bond strength between steel tube and rubberized concrete core with small percentage of rubber replacement. Recovery in bond strength was observed when the specimens were cooled down. Circular sections showed higher bond strength and ductility on its square counterpart.

Degradation of Axial Ultimate Load-Bearing Capacity of Circular Thin-Walled Concrete-Filled Steel Tubular Stub Columns after Corrosion.

This work aimed to investigate the effects of steel tube corrosion on the axial ultimate load-bearing capacity (AULC) of circular thin-walled concrete-filled steel tubular (CFST) members. Circular thin-walled CFST stub column specimens were made of steel tubes with various wall-thicknesses. These CFST column specimens were subjected to an accelerated corrosion test, where the steel tubes were corroded to different degrees of corrosion. Then, these CFST specimens with corroded steel tubes experienced an axial static loading test. Results show that the failure patterns of circular thin-walled CFST stub columns with corroded steel tubes are different from those of the counterpart CFST columns with ordinary wall-thickness steel tubes, which is a typical failure mode of shear bulging with slight local outward buckling. The ultimate strength and plastic deformation capacity of the CFST specimens decreased with the increasing degree of steel corrosion. The failure modes of the specimens still belonged to ductile failure because of the confinement of outer steel tube. The degree of steel tube corrosion, diameter-to-thickness ratio, and confinement coefficient had substantial influences on the AULC and the ultimate compressive strength of circular thin-walled CFST stub columns. A simple AULC prediction model for corroded circular thin-walled CFST stub columns was presented through the regression of the experimental data and parameter analysis.

Early-age creep behavior of concrete-filled steel tubular members subjected to axial compression

In concrete-filled steel tubular (CFST) structures, the loading age of the concrete core is usually earlier than that of other concrete structures, resulting in early-age creep. Early-age creep has significant influences on stress redistribution within the cross-section, and also on the deformation, of the CFST structures. Experiments were conducted to investigate the early-age behaviors of compressive CFST members. The CFST specimens were loaded at ages of 1, 3, 7, and 28d to acquire the creep growth at different loading ages. Meanwhile, the elastic moduli of concrete at corresponding ages were tested. Moreover, stress transfer between the concrete core and outer steel tube during the creep was analyzed using a modified fib Model Code 2010 (MC 2010) creep model. The findings illustrate that a dynamic process exists in stress redistribution within cross-sections of the CFSTs at early ages, and increase of the stress on the concrete core due to elastic modulus development will counteract decrease of the stress on the concrete core due to the creep.

Compression Behavior of CFST Stub Columns with Holes

Holes are always opened in the steel tubes during the inspection and revision of initial concrete imperfections in concrete filled steel tubular (CFST) columns. The structural performance of such composite columns with holes may have obvious differences in comparison with normal CFST members. This paper intends to investigate the influences of sectional type, holes location, holes size, and holes depth on CFST stub columns. The typical failure modes, load‐deformation responses, the ultimate strength, and ductility were discussed in detail. A total of twenty‐eight specimens, twenty CFST columns with holes, four intact CFST specimens, and four reference hollow steel tubes subjected to axial compressive loading, were tested. The experimental results were compared with predictions of Eurocode 4 and finite element analysis. An empirical equation for predicting the ultimate strength of CFST stub columns with holes was proposed.

Debonding detection for rectangular CFST using surface wave measurement: Test and multi-physical fields numerical simulation

Abstract The interfacial debonding detection approach for concrete-filled steel tubular (CFST) members using surface-mounted piezoelectric-lead-zirconate-titanate (PZT) actuators and sensors is experimentally and numerically investigated. In this study, an interfacial debonding detection approach using surface wave measurement with PZT patches is propoed firstly and experimental investigation on the feasibility of the proposed approach with two CFST specimens with different levels of interface debonding defects is carried out. For comparison, a PZT sensor is also embedded in concrete core to measure the stress wave traveling from the surface-mounted PZT actuator. Experiment results indicate that the voltage amplitude of the embedded PZT sensor representing the wave propagation from the steel plate into the concrete core decreases due to the existence of interfacial debonding defect, but the output signal of the surface-mounted PZT sensor increases accordingly when interface debonding occurs. Moreover, the increment of surface-mounted PZT sensor measurement is related to the dimension of the interface debonding defects. The experimental finding implies that the interface debonding is detectable using surface wave measurement. In order to investigate the mechanism of the proposed approach, numerical study on the wave propagation process in CFSTs is further discussed by using three-dimensional (3D) solid element models and two-dimensional (2D) plane strain models, respectively. Finally, in order to consider the effect of nonhomogeneous meso-scale structure of concrete on the wave propagation, a random aggregate method (RAM) is employed to model the concrete core and multi-physical fields coupling numerical analysis on stress wave propagation of the CFST member with different interface debonding defects is performed. The numerical findings meet well with the experimental observations and the results indicate that the interface debonding of CFST can be detected efficiently using surface wave measurement.

Experimental Study and Numerical Assessment of the Flexural Behaviour of Square and Rectangular CFST Members under Monotonic and Cyclic Loading

This paper focuses on the characterization of the behaviour of Concrete Filled Steel Tubular (CFST) columns made with Rubberized Concrete (RuC), and on the development of an accurate numerical model for the simulation of CFST columns under monotonic and cyclic bending. The test campaign involves 18 CFST specimens with different configurations, namely the cross-section slenderness, the concrete strength, the axial load level and the lateral loading type. All CFST members tested exhibited good ductility under monotonic loading. The Eurocode 4 design provisions was verified against the test results and the design capacities of the CFST members were validated to be conservative. A comprehensive 3D Finite Element (FE) model was developed and calibrated based on test results. The FE model proved to be reliable in predicting the bending behaviour of CFST member, in terms of local buckling deformation modes, ultimate capacity and ductility of the CFST columns.

Performance and dynamic behaviour of FRP strengthened CFST members subjected to lateral impact

Due to the increasing popularity of concrete-filled steel tubular (CFST) members, there will be more chances of vehicles/vessels or terrorist attacks on these structures in near future. Fibre-reinforced polymer (FRP) strengthening can be an effective option to reduce impact damage or failure of CFST members. However, existing knowledge is very limited in understanding the behaviour of FRP strengthened CFST structures under lateral impact loading. This paper outlines drop hammer impact test results of a series of experimental programs of bare and FRP strengthened CFST specimens. A total of sixteen CFST specimens were prepared and tested under lateral impact at their mid-span. The results indicate that permanent lateral displacement of CFST members can be reduced up to 18.2% by externally bonded FRP sheets. The effects of FRP type, FRP wrapping direction, carbon fibre-reinforced polymer (CFRP) wrapping layers, wrapping length, and impact velocity were investigated to understand the influences of these parameters on the behaviour of strengthened CFST specimens. CFRP laminates were found to be weak under impact loading when wrapped in only longitudinal direction. However, a combination of longitudinal and hoop layers of CFRP laminates, or only GFRP wrapping, can remarkably minimise the severity of damage and failure of FRP in CFST specimens under lateral impact. A comparison of current test results with recent works has been presented to understand the effect of impact energy on the lateral displacement control ability of FRP strengthened CFST members.

Experimental behaviour of tapered CFST columns under combined compression and bending

Tapered concrete filled steel tubular (CFST) columns have been used in some spatial structures, however, research on the structural performance of such tapered composite columns under combined compression and bending is relatively seldom, and this paper thus intends to fill this knowledge gap through experimental investigations. A total of twenty-four tapered CFST specimens were tested under combined compression and bending, including twelve square columns and twelve circular ones. The test parameters included sectional type, tapered angle, slenderness ratio and load eccentricity. Typical failure modes, the load versus deformation relations, the strain development of tapered composite members and the influence of significant parameters were evaluated based on the testing results. Finally, simplified design method for predicting the load carrying capacity of tapered CFST column under combined compression and bending was discussed.