Conventional Concrete Filled Steel Tube Research Articles

Analysis-oriented stress-strain model for concrete in steel tubed geopolymer concrete (STGC) columns

To alleviate the brittleness of geopolymer concrete and further promote its engineering application for environmental benefits, a novel type of column, i.e., steel tubed geopolymer concrete (STGC) column, has been proposed recently. Note that an accurate axial stress-strain model is necessary for a comprehensive understanding of the compressive behavior of STGC columns, but the existing analysis-oriented models are all developed for the conventional concrete-filled steel tube (CFST) columns. Nevertheless, the confinement mechanism of steel tubed concrete columns is significantly different from that of CFST columns since the axial deformation of steel tube and core concrete is not synchronized for steel tubed concrete columns during the loading process due to the discontinuous steel tube. Furthermore, the dilation properties of STGC columns at the initial loading are significantly overestimated by the existing lateral-axial strain model. Besides, steel-confined geopolymer concrete (GPC) experiences a more remarkable softening behavior resulting from the more significant brittleness of GPC than that of ordinary concrete. Thus, the deformation models are proposed in this study and the active-confinement base model is also developed for GPC based on the mechanical characteristics of STGC columns. Then a new analysis-oriented model is proposed for steel tubed geopolymer concrete. Comparisons between the theoretical calculations and the test results indicate that the proposed model can provide reasonable predictions for both steel stress evolution and the stress-strain relationship of STGC.

Behavior of seawater sea sand concrete-filled plastic-lined steel tube stub columns under axial compression

This study presents the experimental results of seawater sea sand concrete (SSC)-filled plastic-lined steel tube (SSC-PLST) stub columns subjected to axial compression. The concrete strength is set as the main test parameter, and the full-range behavior of SSC-PLST specimens is recorded in terms of failure mode, load-deformation response, load-carrying capacity, and law of stress-strain development. The experimental results show that all the SSC-PLST stub columns behave in a ductile manner, which is similar to the conventional concrete-filled steel tube (CFST) members. During testing, obvious axial deformation accompanied with outward local buckling in the plastic-lined steel tube is observed. The composite actions between the infilled SSC and plastic-lined steel tube are demonstrated through the analysis of load-carrying capacity and strain development. As the concrete strength increases, the SSC-PLST columns tend to have an increasing load-carrying capacity, but the corresponding ductility shows a downward trend. Finally, the ultimate load-carrying capacity of SSC-PLST stub columns is predicted using the calculation methods that are already available for conventional CFST members, and the efficiency is evaluated.

Axial Compressive Behavior of Cross-Shaped CFST Stub Columns with Steel Bar Truss Stiffening.

Concrete-filled steel tube (CFST) columns have been widely used in residential buildings due to their high bearing capacity, good ductility, and reliable seismic performance. However, conventional circular, square, or rectangular CFST columns may protrude from the adjacent walls, resulting in inconvenience in terms of the arrangement of furniture in a room. In order to solve the problem, special-shaped CFST columns, such as cross-shaped, L-shaped, and T-shaped columns, have been suggested and adopted in engineering practice. These special-shaped CFST columns have limbs with the same width as the adjacent walls. However, compared with conventional CFST columns, the special-shaped steel tube provides weaker confinement to the infilled concrete under axial compressive load, especially at concave corners. The parting at concave corners is the key factor affecting the bearing capacity and ductility of the members. Therefore, a cross-shaped CFST column with steel bar truss stiffening is suggested. In this paper, 12 cross-shaped CFST stub columns were designed and tested under axial compression loading. The effects of steel bar truss node spacing and column-steel ratio on the failure mode, bearing capacity, and ductility were discussed in detail. The results indicate that the columns with steel bar truss stiffening can change the final deformation mode of the steel plate from single-wave buckling to multiple-wave buckling, and the failure modes of columns also subsequently change from single-section concrete crushing failure to multiple-section concrete crushing failure. The steel bar truss stiffening shows no obvious effect on the axial bearing capacity of the member but significantly improves the ductility. The columns with a steel bar truss node spacing of 140 mm can only increase the bearing capacity by 6.8% while nearly doubling the ductility coefficient from 2.31 to 4.40. The experimental results are compared with those of six design codes worldwide. The results show that the Eurocode 4 (2004) and the Chinese code CECS159-2018 can be safely used to predict the axial bearing capacity of cross-shaped CFST stub columns with steel bar truss stiffening.

Bond-Slip Performances of Ultra-High Performance Concrete Steel Tube Columns Made of a Large-Diameter Steel Tube with Internally Welded Steel Bars.

Large-diameter concrete-filled steel tube (CFST) members are being increasingly utilised owing to their ability to carry larger loads and resist bending. Upon incorporating ultra-high-performance concrete (UHPC) into steel tubes, the resulting composite structures are lighter in weight and much stronger than conventional CFSTs. The interfacial bond between the steel tube and UHPC is crucial for the two materials to effectively work together. This study aimed to investigate the bond-slip performance of large-diameter UHPC steel tube columns and the effect of internally welded steel bars in steel tubes on the interfacial bond-slip performance between the steel tubes and UHPC. Five large-diameter UHPC-filled steel tube columns (UHPC-FSTCs) were fabricated. The interiors of the steel tubes were welded to steel rings, spiral bars, and other structures and filled with UHPC. The effects of different construction measures on the interfacial bond-slip performance of UHPC-FSTCs were analysed through push-out tests, and a method for calculating the ultimate shear bearing capacities of the interfaces between steel tubes containing welded steel bars and UHPC was proposed. The force damage to UHPC-FSTCs was simulated by establishing a finite element model using ABAQUS. The results indicate that the use of welded steel bars in steel tubes can considerably improve the bond strength and energy dissipation capacity of the UHPC-FSTC interface. R2 exhibited the most effective constructional measures, resulting in a significant increase in ultimate shear bearing capacity by a factor of approximately 50 and energy dissipation capacity by a factor of approximately 30 compared to R0 without any constructional measures. The load-slip curve and ultimate bond strength obtained from finite element analysis and the interface ultimate shear bearing capacities of the UHPC-FSTCs obtained using the calculation method agreed well with the test results. Our results provide a reference for future research on the mechanical properties of UHPC-FSTCs and their engineering applications.

Seismic behavior of composite shear walls with boundary columns using rectangular and U-shaped concrete-filled steel tubes

Steel-concrete composite shear walls with boundary concrete-filled steel tube (CFST) columns have drawn the attention of researchers due to their good seismic performance such as high strength, ductility and energy dissipation capacity. The reported study proposes a novel composite shear wall design that aims to complete the majority of the work in the factory, leaving only concrete pouring, steel welding, and formwork removal to be done on-site, so that the constructional time and cost can be saved. Two types of CFSTs were used for the boundary columns of composite shear walls, one is conventional rectangular CFSTs, another one is U-shaped CFSTs. In order to study the seismic behavior of the novel composite shear walls, cyclic loading tests were conducted on four full-scale specimens. The bearing capacity, energy dissipation, skeleton curves, hysteretic loops, deformation capacity and failure mode are analyzed. The experimental results have demonstrated that the bearing capacity and stiffness of shear walls with U-shaped CFST columns are slightly higher than that of shear walls with rectangular CFST columns, whereas the latter has better deformation capacity. In addition, the reported study investigated the methods of calculating the shear capacity of the novel composite shear walls and provided recommendations on the calculation method to be used for design and structural analysis.

Conceptual design of long span steel-UHPC composite network arch bridge

An innovative half-through network arch bridge with a main span of 470 m was proposed and designed using coarse aggregate ultra-high-performance concrete (CA-UHPC) and steel. To effectively resist the tremendous axial forces, we utilized circular hollow section arch ribs with a CA-UHPC outer layer and an inner steel tube in this proposal. To minimize the bending moment of the arch ribs, we arranged the inclined crossed hangers above the deck and similarly the columns below the deck, connecting the arch ribs with the main girder, and adopting a structural system of double-hinged arches. By incorporating the recently developed multi-material bi-directional evolutionary structural optimization method and through various parametric studies, we determined the best configuration for the bridge. We also used finite element analysis to investigate the corresponding structural responses. The numerical analysis demonstrates that the proposed bridge has excellent mechanical performance in terms of stability, overall structural stiffness, stress, and fatigue strength of each component. A comprehensive comparison between the proposed bridge and the more conventional concrete-filled steel tube (CFST) arch bridge under the same conditions reveals that: the construction-related CO2 emissions of the proposed bridge are significantly lower than those of the CFST bridge, while the economic indicators are slightly better. The proposed bridge is simpler, slenderer, and more elegant. Due to its innovative design, the proposed bridge can overcome a series of problems associated with conventional bridges such as the excessive self-weight of long-span concrete arch bridges, the challenges of welding thick steel plates on all-steel arch bridges and the high cost of these structures, as well as the disadvantages of CFST arch bridges.

New shear transfer system for concrete‐filled steel tube (CFST) columns

AbstractConcrete‐filled steel tube (CFST) columns are frequently used in construction due to their high ductility and improved loadbearing capacities. At beam‐to‐CFST column joints, gusset plates are welded to the outer face of the steel tube, and specific elements should ensure the shear transfer between the inner face of the steel tube and the concrete core. The current shear transfer systems are shear studs, internal rings, angles, steel plates, steel bars, tab stiffeners, etc. Although good, these systems also have disadvantages, such as slowing down the construction process, complexity of execution, greater material consumption and restricting the flow when pumping the concrete into the tube. UPCCFST is a new connection system that enhances the shear bond strength of CFST columns and consists of punching the steel tube inwards to produce crown‐shaped protrusions that act as shear connectors. This system also speeds up the construction process, simplifies connections, does not restrict the flow when casting the concrete and has economic and environmental advantages. Push‐out tests were performed to evaluate the shear bond strength. The shear strength improvement using UPCCFST has been compared with the reference conventional CFST columns. The new system increased the shear bond strength by up to 6.67 times for the SHS 200x4 specimens and 2.89 times for the SHS 110x2 specimens. This paper explains the UPCCFST system, the prototyping process, the testing and the most important results and observations.

Behavior of circular-steel-tube-confined square CFST short columns under axial compression

This paper proposes circular-steel-tube-confined square concrete-filled steel tube (CFST) to improve the mechanical property of the newly built and existing deficient square CFST. The external adding circular steel tube does not enter the beam-column connection area and mainly functions as lateral confinement to the inner square CFST. Twelve circular-steel-tube-confined square CFST short column specimens and twelve conventional square CFST short column specimens were designed and tested under axial compressive load. The main variables of the test include column type, steel ratio, outer circular steel tube size, and concrete strength. The overall failure modes, axial load-displacement curves, macroscopic deformation characteristics, and the inner and outer circular steel tube confinement were analyzed and compared systematically. The study revealed that the outer circular steel tube yielded horizontally in tensile strength and provided effective confinement to the inner square CFST. Therefore, the bearing capacity of the square stub column was improved significantly, and the brittle failure in the confined area was prevented. Due to the effective confinement by the outer circular steel tube, the stress state in the square steel tube was much more uniform. The column configuration led to an effective but uneven confinement in the outer circular steel tube, concentrating near the square steel tube corner. Based on the superposition method, a suitable design method is proposed to predict the compressive strength of circular-steel-tube-confined square CFST, and the theoretical predictions are in good agreement with the test results of this paper.

Seismic behavior of high-strength concrete-filled square steel tube columns reinforced with ultrahigh-strength reinforcing bars

This paper investigated the seismic behavior of high-strength concrete-filled square steel tube columns reinforced with ultrahigh-strength reinforcing bars. Three full-scale specimens with different amounts of ultrahigh-strength longitudinal reinforcing bars under a high axial load ratio and cyclic lateral load were tested. The test results demonstrate that the existence of ultrahigh-strength longitudinal reinforcing bars had a significant influence on the seismic behavior of high-strength concrete-filled steel tube columns. Compared with conventional concrete-filled steel tube (CFT) columns, the deformation capacity, bearing capacity and cumulative energy dissipation of these innovative CFT columns are greatly improved, and the cumulative damage and plastic deformation of the specimens decreased with the introduction of steel bars. A calculation model based on AISC360-16 using the superposition method was proposed and verified to evaluate the compressive-flexural strength of the specimen.

Stone prism encased concrete-filled steel tube columns subjected to axial compression

A new type of concrete-filled steel tube (CFST) structural member is proposed by replacing the central portion of the infilled concrete with a stone prism. The mechanical behavior of the stone prism encased concrete-filled steel tube (SCFST) members under axial compression was studied through experimental testing. A total of twenty-four stub columns, including eighteen SCFST specimens and six CFST specimens, were subjected to axial compression until failure occurred. The primary parameters considered were the size of the stone prism (s = 75 mm, 100 mm or 125 mm), wall-thickness of the steel tube (t = 6.0 mm, 8.0 mm or 12.0 mm), and concrete strength (fcu = 62.3 MPa or 107.0 MPa). Test results demonstrated the composite actions between the three parts of the member, which significantly enhanced the load-carrying capacity of the SCFST column. The effect of the parameters on the failure mechanism, load-carrying capacity, load versus deformation response, and ductility of the SCFST specimens was researched. Meanwhile, the experimentally obtained axial compressive strength of each SCFST specimen was compared with predicted strengths obtained by adopting the formulae from currently available design codes for conventional CFST columns. These design codes could not precisely predict the load-carrying capacity of SCFST stub columns, and an improved model was proposed.

Performance of the novel C-purlin tubular beams filled with recycled-lightweight concrete strengthened with CFRP sheet

This research investigates the performance of double C-purlin sections used in the concrete-filled steel tube (CFST) beams system. A recycled-lightweight concrete was used as infill material to reduce the newly suggested composite beam's self-weight and cost. Up to 70% of the raw coarse aggregate was replaced with the expanded polystyrene beads and recycled aggregate concrete. Also, the strengthening performance of the suggested CFST beam was investigated using carbon fiber reinforced polymer (CFRP) sheets. A total of seven specimens of concrete-filled C-purlin tubular sections were tested under four-point loads, and additional 12 models were numerically analyzed using finite element software. The conducted failure modes, flexural strength, flexural stiffness, and energy absorption index are presented and discussed. The results confirmed that the recycled-lightweight infill concrete material has a slightly lower influence on the investigated CFST beams' flexural performance than those with normal concrete (less than 10%). Like the conventional CFST beams, the double C-purlin tubular beams filled with recycled-lightweight concrete showed a normal response to the CFRP's strengthening quality. Moreover, the flexural strength capacity and stiffness values obtained from the current analyses are analogous with those theoretically predicted using the existing methods, which achieved mean values of about 0.759 and 1.126, respectively, as average prediction mean values of these methods.

Structural performance of nano-silica based blended CFST stub circular column

The objective of this study is to find the optimum percentage replacement of cement with the nano-silica in a concrete mix to improve the structural performance Concrete Filled Steel Tube (CFST) stub circular column under the axial compressive load. The CFST stub columns with circular cross-sectional area of conventional concrete mix (without nS) and nano-silica blended concrete mix (with nS-CFST) had been experimentally and numerically studied. The amorphous silica is a major component of the cementitious materials, and it reacts with the calcium hydroxide, which is comprised of the calcium silicate hydration. The addition of this nano-silica affects the different properties of the concrete and increases its performance. On the other hand, CFST is known for the composite behavior generated between the hollow tube of steel and a core constructed of concrete, which improves the collected performance of the member. In present study, the concrete mix of M25 had been modified with 4%, 6%, 8% and 10% of nano silica. The structural behavior of stub columns (with / without nS) are assessed under axial compressive load. Also, the CFST stub columns were then modelled into ABAQUS for finite element analysis. The nano-silica blended concrete in CFST stub columns showed an improved the structural performance than the conventional cement concrete stub circular CFST column.

Reactive powder concrete-filled steel tube (RPCFT) members subjected to axial tension: Experimental study and design

Reactive powder concrete-filled steel tube (RPCFT) consists of rectangular or circular steel tube filled with reactive powder concrete (RPC). Currently, there is a general lack of knowledge regarding the behavior of RPCFT members subjected to axial tension. This paper addresses the knowledge gap by conducting experimental tests on 57 specimens, including 33 RPCFT specimens, 21 RPC specimens, and three hollow steel tube (HSS) specimens. The test parameters are the diameter-to-thickness ratio of the steel tube, fiber reinforcement ratio, and curing temperature. The test results indicate that the tensile strength of RPCFT members increases with increasing fiber reinforcement ratios and curing temperatures or with decreasing diameter-to-thickness ratios of the steel tube. As compared to conventional concrete-filled steel tube (CFT) members, RPCFT members have: (i) a similar tensile behavior, i.e., the steel tube has an increased tensile strength and does not have any pinching due to the interaction between the steel tube and concrete infill; and (ii) a higher tensile strength because RPC with fibers has a higher tensile strength than conventional concrete and can maintain its tensile capacity at large strains. This paper also evaluates the applicability of current design equations for estimating the axial tensile strength of RPCFT members. The evaluation indicates that the current design equations are over-conservative for RPCFT members. This paper then proposes a new design equation for estimating the axial tensile strength of RPCFT members. The accuracy of the proposed equation is confirmed by using it to predict the strength of the 33 RPCFT specimens tested in this research.

Behavior of novel CFST circular column-to-foundation connections under cyclic loading

Despite several advantages of Concrete Filled Steel Tube (CFST) columns, these are still not very popular in construction due to the non-availability of well-established, efficient, and ductile connections of these columns with the reinforced concrete (RC) foundations. The present study proposes two novel CFST column-to-foundation connections that are efficient and have adequate ductility to withstand inelastic deformation cycles under moderate to extreme seismic loading. The first proposed connection was developed by shape modification of the cavity used in the first-stage concrete of the RC foundation, whereas the second proposed connection was developed with a concept of mechanical interlocking. The connections were tested under cyclic lateral loading for simulating the seismic excitations. Two more connections representing controls were also tested. The first was a conventional CFST column-to-foundation connection, and another was a regular RC column-to-foundation connection. The test results establish the better performance of the proposed connections. Finite element (FE) models were developed to predict the behavior of experimentally investigated RC and CFST column-to-foundation connections. The prediction was in good agreement with the experimental results.

Flexural behaviour of steel hollow sections filled with concrete that contains OPBC as coarse aggregate

Oil-palm-boiler clinker (OPBC) is an agricultural waste from the palm oil industry and is considered a serious threat to the environment. Moreover, the high consumption of concrete as a construction material results in a continuous demand for natural aggregates, thereby negatively affecting the environment. Thus, channeling OPBC waste materials into the concrete industry aids in promoting the use of a sustainable and lightweight member. This research presents a novel sustainable composite beam that uses an OPBC as a replacement of the natural coarse aggregate. Flexural behaviour of steel tubes infilled with conventional and OPBC concretes were investigated. The results showed that the ductility, flexural stiffness and structural efficiency were higher in the OPBC concrete filled steel tube (CFST) than conventional CFST by 15%, 12% and 20%, respectively. Furthermore, in comparison to conventional CFST, the 10% less self-weight in OPBC CFST will significantly reduce the construction cost of the material. Conclusively, the utilisation of OPBC as infill material for CFSTs will solve disposal problem, preserve natural resources, reduce environmental pollution and will make the structural system sustainable.

Experimental performance of concrete-encased CFST columns subjected to full-range fire including heating and cooling

Concrete-encased CFST (concrete-filled steel tube) columns are gaining popularity in constructions especially in high-rise buildings and large-span structures. This type of composite column features an inner CFST component and an outer reinforced concrete component. Extensive literature is available on the structural performance of concrete-encased CFST columns at ambient temperature, however, the fire performance of this type of composite columns has seldom been addressed. To fill in this research gap, this paper thus experimentally studies the performance of concrete-encased CFST columns subjected to full-range fire including heating and cooling. A set of tests, including six fire resistance tests and four postfire tests, respectively, were conducted under the combined effect of load and fire. Test observations and results are presented, including fire resistance, postfire residual strength, failure modes, temperature versus time relationships and deformation versus time relationships. The fire performance of concrete-encased CFST columns is analysed and discussed based on these observations. It is found that due to the insulating effect of the outer reinforced concrete, concrete-encased CFST columns exhibits higher fire resistance in fire and higher postfire residual strength as compared with conventional CFST columns without concrete encasement. The simple calculation model of analysing the fire resistance of composite column in Eurocode 4 was extended and then employed to calculate the fire resistance as well as the postfire residual strength of concrete-encased CFST columns. The predictions are found to be conservative as compared with test data, and this simplified method are applicable to assess the fire resistance and postfire residual strength of concrete-encased CFST columns.

Analytical behavior of concrete-encased CFST columns under cyclic lateral loading

The analytical behavior of concrete-encased CFST (concrete-filled steel tube) columns under cyclic lateral loading is reported in this paper. A finite element analysis (FEA) model is developed to investigate the cyclic behavior of such composite column. Comparisons are made between measured and predicted results on failure modes, load versus displacement relationships and ultimate strength. It is found that the proposed FEA model can reproduce the experimental results with good accuracy. The full-range load–displacement relationships, the contact stress between steel tube and concrete and the axial load distribution among different components are analyzed using the verified FEA model. The cyclic behavior of concrete-encased CFST is compared with that of conventional CFST and reinforced concrete (RC) columns. The parametric analysis is conducted to investigate influences of various parameters on the moment–curvature (Μ−ϕ) envelope curves. The parameters include the material strength, the steel ratio of inner CFST, the longitudinal reinforcement ratio, the stirrup characteristic value, the tube diameter to cross-sectional width ratio (D/B) and the axial load level. Finally, a simplified hysteretic model for the Μ−ϕ relationship of concrete-encased CFST column is proposed.

Confined Expansion and Bond Property of Micro-Expansive Concrete- Filled Steel Tube Columns

The shrinkage/expansion behavior and bond carrying capacities were investigated through 4 micro-expensive concrete-filled steel tube(MCFST) and 3 conventional concrete-filled steel tube(CFST) short columns. The results show that the temperature field in MCFST is similar to that of ordinary concrete members. Concrete core has obvious effect on shrinkage-compensating with the addition of swelling agent. Pre-stress is produced in the core concrete when it is confined by the steel tube. Both water cement ratio and expansive agent have obvious influence on expansive behaviors of MCFST. The tests also indicate that the pre-stress in core concrete can improve bond strength of core concrete and steel tube of MCFST columns and proposed a new method to improve the interface bond strength of composite structures.

Push-Out Test on Bond Property of Micro-Expansive Concrete-Filled Steel Tube Columns

The shrinkage/expansion behavior and bond carrying capacities of 4 short, micro-expansive concrete-filled steel tube (MCFST) and 3 short, conventional concrete-filled steel tube (CFST) columns were experimentally investigated. The results indicate that pre-stress is produced in the core concrete under the confined by the steel tube. Both expansive agent and water cement ratio have important influence on expansive behaviors of MCFST. The current work also indicates that the MCFST columns have higher bond strength than conventional CFST columns and this recommends a new method to improve the bond strength of composite structures.

Mechanical performance of eccentrically loaded pre-stressing concrete filled circular steel tube columns by means of expansive cement

This paper presents experimental and numerical investigations of pre-stressed concrete filled circular steel tube columns by means of expansive (PCFT) under eccentric loading. Conventional concrete filled steel tube (CFT) specimens are also tested in this paper just for comparison purposes. Based on the steel follow-up strengthening and concrete bounding surface constitutive relationships, this paper writes a nonlinear finite element program for calculating complete load mid-height deflection curves of eccentrically loaded PCFT/CFT columns. A good agreement is obtained between the calculated and test results. The results indicate that the eccentrically loaded PCFT columns generally have higher load carrying capacities than the CFT ones when other parameters are kept the same. But by increasing the load eccentricity ratio, the difference of load carrying capacity between them decreases. Other factors such as pre-stress, load eccentricity ratio, and steel ratio, etc. are also investigated.