Heat transfer analysis of rectangular concrete-filled steel sections

Concrete-filled steel tubular (CFST) columns are widely used in engineering structures, and they have many different cross section types. Among these, normal solid sections and concrete-filled double-skin steel tubular sections are often used. Although many studies have been conducted on CFST columns with these two section types, no studies have been conducted on their damage assessment under blast loading. In this study, experimental analysis and a numerical simulation method were integrated to evaluate the responses and assess the damage of two concrete-filled steel tubular (CFST) columns with different cross sections subjected to near-field blast loading. The results showed that for a scaled distance of 0.14 m/kg1/3, plastic bending deformation occurred on the surfaces of the two CFST columns facing the explosive. The antiexplosion performance of the normal solid-section (NSS) CFST column was better than that of the concrete-filled double-skin steel tubular (CFDST) column. The explosion centre was set at the same height as the middle of column, and the distributions of the peak pressure values of the two columns were similar: the peak pressures at the middle points of the columns were the greatest, and the peak pressures at the bottom were higher than those at the top. With the analysis of the duration of the positive pressure, the damage at the middle was the most severe when subjected to blast loading. Using pressure-impulse damage theory and the validated numerical simulations, two pressure-impulse damage evaluation curves for NSS and CFDST columns were established separately by analysing the experimental and simulation data. Finally, based on the two pressure-impulse damage evaluation curves, the two pressure-impulse damage criteria for these two different fixed-end CFST columns were defined based on the deflection of the surfaces facing the explosives. Furthermore, the mathematical formulae for the two different column types were established to generate pressure-impulse diagrams. With the established formulae, the damage of the CFST columns with these two cross section types can be evaluated. Damage to other similar CFST columns with different cross section types due to near-field blast loading can also be evaluated by this method.

Concrete filled steel tubular (CFST) columns have a high probability to resist high temperatures compared to steel structures, whose evaluation after a fire is limited by the resulting deformation. A better understanding of the behaviour of CFST columns after a fire, affected by the maximum temperature achieved by the concrete infill, is required to properly estimate their residual strength and stiffness in order to adopt a reasonable strategy with minimum post-fire repair. In this paper, a fiber beam model for the simulation of the post-fire response of slender concrete-filled steel tubular (CFST) columns is presented. First, the model is validated against experimental results and subsequently it is employed to analyse the post-fire response of circular CFST columns. The variation of the residual strength with the load level for realistic fire resistance times is numerically studied. Actually, in a building, the columns support load even while a fire is being extinguished, so it is important to take into account this loading condition when predicting the post-fire behaviour. Therefore, in this research, the complete analysis comprises three stages: heating, cooling and post-fire under sustained load conditions. The model considers realistic features typical from the fire response of CFST columns, such as the existence of a gap conductance at the steel-concrete interface or the sliding and separation between the steel tube and the concrete.

The effectiveness of concrete-filled steel tubular (CFST) sections as compression and flexural members has been investigated by many authors. The combined utilization of CFST columns and steel beams has increased in composite building construction in recent years, but combined use of CFST columns and CFST beams is still unexplored. To explore their potential as part of CFST frame structures, two types of connections using extended end plate and seat angle are proposed for exterior joints of CFST beams and CFST columns. The performance of the proposed bolted connections subjected to static loads has been investigated by executing an experimental program involving eight specimens of exterior beam-to-column joints subjected to monotonically increasing load applied at the free end of the beam. The performance is appraised in terms of load deformation behaviour and failure modes of joints with varying test parameters like the beam section type, length and diameter of bolts. Finite element analysis for the applied load has been performed using ATENA-3D software, and the behaviour of the connections is investigated. The FE simulation results are compared with experimental test data to verify the FE model.

Many authors have established the usefulness of concrete filled steel tubular (CFST) sections as compression members while few have proved their utility as flexural members. To explore their prospective as part of CFST frame structures, two types of connections using extended end plate and seat angle are proposed for exterior joints of CFST beams and CFST columns. To investigate the performance and failure modes of the proposed bolted connections subjected to static loads, an experimental program has been executed involving ten specimens of exterior beam-to-column joints subjected to monotonically increasing load applied at the tip of beam, the performance is appraised in terms of load deformation behaviour of joints. The test parameters varied are the beam section type, type and diameter of bolts. To validate the experimental behaviour of the proposed connections in CFST beam-column joints, finite element analysis for the applied load has been performed using software ATENA-3D and the results of the proposed models are compared with experimental results. The experimental results obtained agree that the proposed CFST beam-column connections perform in a semi-rigid and partial strength mode as per specification of EC3.

Concrete-filled steel tubular (CFST) columns have been widely used for constructions in recent decades because of their high axial strength. In CFSTs, however, steel tubes are susceptible to degradation due to corrosion, which results in the decrease of axial strength of CFSTs. To further improve the axial strength of CFST columns, carbon fiber reinforced polymer (CFRP) sheets and basalt fiber reinforced polymer (BFRP) sheets are applied to warp the CFSTs. This paper presents an experimental study on the axial compressive capacity of CFRP-confined CFSTs and BFRP-confined CFSTs, which verified the analytical model with considering the effect of concrete self-stressing. CFSTs wrapped with FRP exhibited a higher ductile behavior. Wrapping with CFRP and BFRP improves the axial compressive capacity of CFSTs by 61.4% and 17.7%, respectively. Compared with the previous composite structural systems of concrete-filled FRP tubes (CFFTs) and double-skin tubular columns (DSTCs), FRP-confined CFSTs were convenient in reinforcing existing structures because of softness of the FRP sheets. Moreover, axial compressive capacity of CFSTs wrapped with CFRP sheets was higher than CFFTs and DSTCs, while the compressive strength of DSTCs was higher than the retrofitted CFSTs.

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.

Numerical investigation and design of concrete-filled double square steel tube columns under axial compression

The effect of ribs and stiffeners on axial load carrying capacity on nineteen concrete-filled steel tubular (CFST) column models and the performance of three unstiffened square CFST columns and two octagonal CFST columns under seismic load by nonlinear static analysis using ANSYS Workbench 16.2 was numerically studied. The models used for this study are unstiffened square CFST columns, diagonal binding rib stiffened square CFST columns, longitudinally stiffened [1] square CFST columns [2], and unstiffened octagonal CFST columns. CFST column was validated against the experimental results under axial load [3]. An increase in thickness of steel tube, stiffener and rib show higher load-carrying capacity. Square CFST columns have more axial load carrying capacity than octagonal CFST columns. Stiffened CFST column has a higher load carrying capacity than unstiffened CFST column. The diagonal rib stiffened CFST columns having ribs with square openings had higher strength than those with circular openings. The ultimate strengths of the diagonal rib stiffened CFST column slightly decreased when the opening diameter in ribs increased and when the opening spacing increased. The unstiffened square CFST columns with thicker steel tubes performed better under seismic load than unstiffened octagonal CFST columns.KeywordsDiagonal binding ribsStiffenersCFST columnSeismicAxialLongitudinalOctagonalSquareCircularUltimate strength

Three-dimensional creep calculation model for reliability analysis of concrete-filled steel tubular (CFST) structure

Concrete filled steel tubular (CFST) columns are becoming popular due to its advantages from both steel and concrete in resisting loads. CFST members are made up of steel hollow section of rectangular or circular sections filled with plain or reinforced concrete. CFST columns can be used effectively in high raised structures and in bridges. The reduced cross sections of columns can be achieved by the composite action of steel and concrete. The steel tube provides confinement to the concrete and helps to improve load carrying capacity. The concrete core offers higher resistance to axial compression. The inward buckling of steel tube will be avoided due to concrete and the out buckling resistance may be improved due to higher stiffness of the column due to higher gross moment inertia with combined effect of steel and concrete. This experimental work made an attempt to understand the effect of thickness of steel tube in behavior of CFST stubs.In the present experimental study, a total of 24 cylindrical concrete filled steel tube specimens were prepared to evaluate the uniaxial compressive behavior and to understand the effect of thickness and aspect ratio (h/D) on the compressive strength. Two different thickness of steel tubes 3 mm and 5 mm and four aspect ratios (h/D) 1, 2, 3 and 4 are considered in the present study. M30 grade of concrete is used. From this study, as the aspect ratio increases the buckling loads decreases. The decrement is up to 37% in aspect ratio 4 when compared to aspect ratio1.

In order to study the seismic behavior of square concrete-filled steel tubular (CFT) columns with binding bars under cyclic lateral load, numerical simulation is carried out based on the finite element software OpenSEES. The influence factors of seismic performance on square CFT columns with binding bars were studied. The results indicate that the analysis results exhibit good agreement with the experiment data. The axial load level is the main factor that influences the envelope curve. The stiffness decreases with the incresing of axial load level, while the ultimate lateral load increases with the axial load level less than 0.2, and decreases with the axial load level more than 0.3. The thickness of steel tube has a significant influence on the ultimate lateral load, while has little impact on the shape of envelope curve.

Due to creep and shrinkage of the concrete core, concrete-filled steel tubular (CFST) arches continue to deform in the long-term under sustained loads. This paper presents analytical investigations of the effects of geometric nonlinearity on the long-term in-plane structural performance and stability of three-pinned CFST circular arches under a sustained uniform radial load. Non-linear long-term analysis is conducted and compared with its linear counterpart. It is found that the linear analysis predicts long-term increases of deformations of the CFST arches, but does not predict any long-term changes of the internal actions. However, non-linear analysis predicts not only more significant long-term increases of deformations, but also significant long-term increases of internal actions under the same sustained load. As a result, a three-pinned CFST arch satisfying the serviceability limit state predicted by the linear analysis may violate the serviceability requirement when its geometric nonlinearity is considered. It is also shown that the geometric nonlinearity greatly reduces the long-term in-plane stability of three-pinned CFST arches under the sustained load. A three-pinned CFST arch satisfying the stability limit state predicted by linear analysis in the long-term may lose its stability because of its geometric nonlinearity. Hence, non-linear analysis is needed for correctly predicting the long-term structural behaviour and stability of three-pinned CFST arches under the sustained load. The non-linear long-term behaviour and stability of three-pinned CFST arches are compared with those of two-pinned counterparts. The linear and non-linear analyses for the long-term behaviour and stability are validated by the finite element method.

Concrete filled steel tubular (CFST) columns have been used widely because of the confinement provided by the steel tube to the concrete core and the concrete restraint against local buckling of the steel tube. In engineering practice, end plate connections are often used to connect steel beams and CFST columns, and conventional connection design methods for steel structures are often used. This paper deals with the effects of binding bars in potentially enhancing the integrity of end plate connections to CFST columns. Binding bars and blind bolts are considered to connect the steel beam to the CFST column in composite beam-column connections; with the aim to avoid the separation of the steel tube from the concrete core under tensile loading. A finite element (FE) model has been developed to simulate the end plate connections using binding bars and blind bolts. The various effects of the tube thickness, endplate thickness and binding bars on the connection behaviour have been investigated using the developed FE model. The results show that the binding bars are a promising approach into the development of end plate connections which enhance the initial stiffness and their ultimate load capacity. The tensile load from the steel beam can be transferred into the steel tube and concrete core of the CFST column through the combined action of the blind bolts and binding bars. Thus, the integrity of the steel beam to CFST column connections can be enhanced by applying the binding bars in bolted end plate connections.

Seismic behavior of fire-exposed concrete-filled steel tubular (CFST) columns

As China's infrastructure continues to grow, concrete filled steel tubular (CFST) structures are attracting increasing interest for use in engineering applications in earthquake prone regions owing to their high section modulus, high strength, and good seismic performance. However, in a corrosive environment, the seismic resistance of the CFST columns may be affected to a certain extent. This study attempts to investigate the mechanical behaviours of square CFST members under both a cyclic load and an acid rain attack. First, the tensile mechanical properties of steel plates with various corrosion rates were tested. Second, a total of 12 columns with different corrosion rates were subjected to a reversed cyclic load and tested. Third, comparisons between the test results and the predicted ultimate strength by using four existing codes were carried out. It was found that the corrosion leads to an evident decrease in yield strength, elastic modulus, and tensile strain capacity of steel plates, and also to a noticeable deterioration in the ultimate strength, ductility, and energy dissipation of the CFST members. A larger axial force ratio leads to a more significant resulting deterioration of the seismic behaviour of the columns. In addition, the losses of both thickness and yield strength of an outer steel tube caused by corrosion should be taken into account when predicting the ultimate strength of corroded CFST columns.