Steel-reinforced Concrete-filled Steel Research Articles

Fire test and simulation of circular steel tube confined steel-reinforced concrete columns

Steel tube confined steel-reinforced concrete (STCSRC) columns are novel steel-concrete composite columns where the steel tubes are disconnected at beam-column connections. Few researches have been conducted on the fire behaviour of STCSRC columns by now, though there are some relevant studies on the fire performance of steel-reinforced concrete-filled steel tubular (SR-CFST) columns. However, the mechanical behaviour of STCSRC columns under fire exposure differs from that of SR-CFST columns due to the discontinuous external steel tube. It is essential to carry out a study on the fire performance of STCSRC columns. The fire tests of four axially loaded STCSRC columns were carried out in this paper, and the failure mode, distribution of cross-sectional temperature, fire resistance, and lateral deflection were discussed. A finite element model was developed and validated with test results, and then a parametric analysis of the STCSRC stub and slender columns under fire exposure was performed. The load ratio, steel tube strength, the area ratio of steel tube to concrete, and slenderness ratio have an adverse effect on the fire resistance of STCSRC columns, whereas cross-sectional dimension, area ratio of steel section to concrete, and strength of steel section have a favourable impact. Concrete strength has a different influence on the fire resistance of STCSRC stub and slender columns. Finally, based on the specifications JGJ/T 471-2019 and EN 1994-1-2, simplified calculation methods for assessing the fire resistance of STCSRC columns were proposed.

Fire design of steel-reinforced CFST stub columns with high-strength materials

Innovative composite sections are gaining popularity as they reduce the environmental impact of construction and take advantage of the collaboration of concrete and steel properties at room temperature and in fire. By introducing high-performance materials, these synergies are boosted. One typology is the steel-reinforced concrete-filled steel tubular (SR-CFST) section, in which a steel profile is embedded inside a CFST section. Despite their impressive load-bearing capacity and fire resistance, the available investigations on their thermo-mechanical capabilities are scarce. The present paper develops a numerical model to analyse the thermo-mechanical behaviour of SR-CFST stub columns subjected to fire. The model is validated against previous experiments conducted by the authors by comparing the temperature distribution and the mechanical behaviour with the experimental measurements. Afterwards, an extensive parametric study (1440 cases) is conducted to analyse the influence of several geometrical parameters – cross-sectional shape, outer tube dimensions and thickness, inner steel profile dimensions, section factor – and the use of high-strength materials for the concrete and the inner steel profile. Afterwards, a simplified temperature distribution proposal formerly designed by the authors for SR-CFST sections was tentatively tested. Based on this previous simplified temperature field and in line with the current provisions of EN1994-1-2, a new proposal is developed to evaluate the cross-sectional plastic resistance of SR-CFST stub columns under a fire scenario. This method unifies the initially developed thermal method and provides practical guidance for designers to evaluate the thermo-mechanical behaviour of SR-CFST columns, filling an existing gap in the European design code.

Numerical analysis of circular steel–reinforced concrete-filled steel tubular stub columns

This paper presents a computational model for determining the axial responses of short circular steel–reinforced concrete-filled steel tubular (SRCFST) columns. A novel confinement model is formulated for the concrete core, which is effectively confined by the external circular steel tube and the embedded steel section. The modelling scheme of the confinement is programmed using a mathematical model that utilises fibre-element discretisation of column cross-sections. The numerical predictions are verified by experimental measurements and results obtained from finite-element analysis, demonstrating the accuracy of the modelling technology. In addition, existing concrete confinement models for concrete in circular concrete-filled steel tubular columns are assessed. The new confinement model is shown to be superior in replicating the responses of SRCFST columns. The influences of the design parameters on the column's performance are numerically investigated and ranked in order of importance through a sensitivity analysis. In this study, not only is the validity of current design standards in determining the axial load capacity of SRCFST columns examined but a new design formula is also proposed. The proposed confinement model can be employed in numerical procedures for the inelastic simulation of SRCFST columns; the design formula is suitable for use in practical design.

Numerical studies on creep behaviour of SRCFST columns with initial stress of steel tube

The long-term creep behaviour of steel-reinforced concrete-filled steel tubular (SRCFST) column under whole life cycle is a key issue in engineering design. Therefore, a finite element (FE) analysis based on ABAQUS software is conducted to study the creep performance of SRCFST columns considering the initial stress of steel tube. Based on the verified FE model, the whole process load-deformation curves, load distributions of each component and longitudinal strain versus time curves are obtained. The stress distributions of each component are analyzed at the feature points. The effects of main parameters on the creep behaviour are studied, including the sustained load level, initial stress ratio, compressive strength of concrete, steel ratio of steel tube and encased steel section. The studies show that the SRCFST member exhibits an elastoplastic behaviour subjected to the coupling effect of initial stress on steel tube and sustained load. However, the creep ratio undergoes a change from linear to nonlinear by increasing long-term load or initial stress ratio. Finally, a method for calculating the creep in SRCFST columns under combined loads is provided, and the long-term performance can be better predicted.

Lateral impact behaviour of post-fire steel-reinforced concrete-filled steel tubular members: Experiment and evaluation method

Steel-reinforced concrete-filled steel tube (SRCFST), which exhibits superior mechanical behaviour and fire resistance than traditional concrete-filled steel tube (CFST), is widely used as a main load-carrying component in different structures. Structure might experience severe damage even collapse as a result of both impact and fire in extreme events. This study reports an experimental investigation on the responses of post-fire SRCFST members under lateral impact. Impact tests on 12 SRCFST specimens after fire were conducted, and the main parameters considered include the fire exposure time, impact height, axial load ratio, and section steel type. The temperature-time curves, failure modes, impact force, midspan deflection, axial force, plastic deformation, and energy absorption are investigated. The results show that post-fire SRCFST specimens under lateral impact experience global flexural failure. As the fire duration increases, the global deformation and local buckling of the specimen become more visible, and the plateau impact force decreases while the midspan deflection increases. In addition, based on the equivalent temperature of SRCFST members and the membrane force factor method, new simplified evaluation methods are proposed for calculating the dynamic flexural capacity and maximum deflection of SRCFST members subjected to lateral impact.

Experimental investigation on residual capacity of square steel-reinforced concrete-filled steel tubular columns after fire

PurposeSteel-reinforced concrete-filled steel tubular (SRCFST) columns have been increasingly popular in engineering practice for the columns' excellent seismic and fire performance. Significant design progress guidance has been made through continuous numerical and experimental research in recent years. This paper tested and analysed the residual loading capacity of SRCFST columns under axial loading after experiencing non-uniform ISO-834 standard fire.Design/methodology/approachThe experimental research covered the main parameter of heating conditions, 1-side and 2-side fire, through two specimens. Two specimens were heated and loaded simultaneously in the furnace for 240 min. After cooling, the columns were moved to the hydraulic loading system and loaded to failure to determine the columns' residual capacity.FindingsThe experimental results indicated that the non-uniform heating area plays an essential role in the overall performance of SRCFST columns, the increasing heating area of columns results in lower residual loading capacity and stiffness. The SRCFST columns still had a high loading capacity after heating and loading in the fire.Originality/valueThe comparison of experimental data against design results showed that the design method generated a 16% safety margin for S2H4 and a 39% safety margin for S1H4.

Experimental residual capacity of steel-reinforced concrete-filled steel tubular stub columns after fire exposure

Technological advances in the development of steel–concrete composite structures have led to the introduction of novel types of sections in the sought of higher load-bearing capacities. Particularly for composite columns, the axial capacity of concrete-filled steel tubular (CFST) sections may be enhanced by the introduction of an open steel profile embedded within the concrete infill, forming the so-called steel-reinforced concrete-filled steel tubular (SR-CFST) columns. An important aspect that should be revised, in order to safely use these sections in design, is their performance after a fire. In this experimental program, six SR-CFST stub columns are tested in the post-fire situation. Two series of columns comprising three circular and three square sections with the same steel usage are tested for comparison purposes. The size of the inner steel profile is varied in order to investigate its effect over the post-fire capacity of the columns. The specimens were initially exposed to elevated temperatures inside an electric furnace, and then cooled to ambient temperature; afterwards compressive axial load was applied gradually until failure, to ascertain their residual capacity. The experimental results show the high ductility of the SR-CFST stub columns after heating, with the circular specimens reaching higher post-fire peak loads than their square counterparts. This load increases with the size of the embedded steel profile. The post-fire capacities of the columns are evaluated through the Residual Strength Index, showing similar values for both circular and square SR-CFST columns. Finally, a design equation is proposed to facilitate the evaluation of the post-fire compression resistance of SR-CFST columns, as an extension of the existing room temperature design equation in Eurocode 4 Part 1–2 for CFST columns, accounting for the degradation of the steel and concrete after fire exposure through the corresponding residual factors.

Interaction diagrams of concrete-filled steel tube columns exposed to fire considering the slenderness effect

A procedure for obtaining the interaction diagrams of slender concrete-filled steel tube (CFT) columns exposed to fire was presented. The maximum structural capacity represented by the interaction diagram of the short column was determined via mathematical optimization. The interaction diagram of the slender column with the slenderness effect was obtained using a moment magnifier, which is defined by an effective flexural rigidity. After investigating fire resistances from the developed procedure and experiments of 78 slender CFT columns, the participating factors of 0.2 and 0.8 for concrete and steel, respectively, were proposed for the effective flexural rigidity. The applicability and versatility of the developed analysis procedure were demonstrated using the interaction diagrams of CFT and steel-reinforced concrete-filled steel tube columns exposed to fire.

Performance of SRCFST member under long-term loading and preload on steel tube

To further improve the structural performance, steel-reinforced concrete-filled steel tubular (SRCFST) column is widely used in different building structures. Due to the fact that during the whole life cycle, the columns may be subjected to preload on the steel tube and long-term load, the mechanical behaviour of SRCFST column becomes more complex under the combined load. Therefore, 12 specimens are prepared and tested to study the compressive behaviour of SRCFST column under the combined load. Through the experiment, the longitudinal strain versus time curves of circular SRCFST members in the creep phase, the failure modes and load-deformation curves in the load-bearing capacity failure phase are obtained. The test results show that considering the combined load has an impact on the mechanical behaviour of the specimen. The testing is followed by the numerical methodologies, a finite element model is established and validated using test results. The influence law of the main factors on the strength index of load-bearing capacity is evaluated. Based on the parameter study, the calculation formula of the ultimate load-bearing capacity for circular SRCFST members under combined preload on steel tube and sustained load is proposed, which provides reference for engineering design or assessment.

Cyclic and monotonic behaviour of steel-reinforced concrete-filled steel tubular columns

Steel-reinforced concrete-filled steel tubular (SRCFST) column, exhibits high strength and superior fire resistance, is a type of composite members developed by configuring the steel section in conventional concrete-filled steel tubular (CFST) column. In this paper, the cyclic behaviour of SRCFST columns was experimentally investigated. Twenty columns with cross and I-shaped steel sections under cyclic loading were tested. The effects of the axial load ratio and steel section type on the failure modes, lateral load–displacement curves, and seismic performance were investigated. The test results demonstrate that columns with circular cross-sections, which experience severe local buckling and fracture of the steel tube, exhibit superior cyclic performance compared with those with square cross-sections. The steel section type has a negligible influence on the seismic behaviour of the columns. As the axial load ratio increases, the ductility factor and ultimate load of the columns decrease moderately. A finite element (FE) model was developed to investigate the full-range behaviour of the columns under monotonic loading. Finally, design methods were proposed for calculating the lateral capacity of SRCFST columns with circular and square cross-sections.

Investigation on fire resistance of steel-reinforced concrete-filled steel tubular columns subjected to non-uniform fire

The fire performance of steel-reinforced concrete-filled steel tubular (SRCFST) columns under non-uniform fire exposure is analytically and experimentally examined. Four SRCFST columns are methodically tested under 3-, 2-opposite-, 2-adjacent-, and 1-face heating to achieve the failure modes, temperature evolution in time, axial and lateral deformations, and fire resistance. Finite element (FE) analysis models were used to verify and assess the test results. The obtained results reveal that profiled steel benefits the integrity and fire resistance of SRCFST columns. The temperature distribution of the cross-section is substantially affected by uneven temperature distribution, resulting in shifting the stiffness center toward the unexposed side at the elevated temperature. The parametric studies are also conducted over various influential factors to analyze their effects on the fire resistance of SRCFST columns subjected to non-uniform fire. A modified calculation methodology based on the extended Rankine approach is developed to predict the fire resistance of SRCFST columns under various fire conditions, covering uniform fire, 3-, 2-opposite-, 2- adjacent-, and 1-face heating. Based on the quantitative comparisons between the experimentally observed data and the numerically predicted results, the simplified method generally yields accurate predictions on the ultimate load capacity of SRCFST columns under non-uniform fire.

Utilizations of Steel-Reinforced Concrete-Filled Steel Tubular (SRCFST) Constructions

In recent times, there has been a significant amount of research dedicated to traditional concrete-filled steel tubular (CFST) structures. This research has been broad and includes both practical testing and theoretical analysis. As of late, scholars have redirected their attention toward introducing innovations in the domain of steel-concrete composite systems. This piece delves into investigations involving experiments and simulations, as well as the underlying mechanisms and interplays of constraints. The focus is on rectangular columns made of Steel-Reinforced Concrete Filled Square Tubular (SRCFST), subjected to combined compression and torsion forces. By building upon previous investigations, the article not only provides a summary but also evaluates the pathways that research on SRCFST columns has taken. Additionally, it offers a glimpse into the potential directions for future studies. This comprehensive strategy not only serves as a point of reference for upcoming research endeavors but also furnishes practical insights for their real-world implementation.

Dynamic response of cross steel reinforced concrete filled steel tubular columns under impact under fire

To explore the impact response of cross steel-reinforced concrete-filled steel tubular (CSRCFST) columns under fire, a finite element model using ABAQUS software was generated. After validation against test results, parametric studies were carried out to investigate the effects of impact load as well as time of fire exposure on the impact resistance of CSRCFST columns. The numerical results show that the impact behavior of post-fire CSRCFST columns can be divided into three stages: peak stage, plateau stage, and softening stage. For CSRCFST columns, the peak and plateau stages are important, which absorbed 32.4% and 67.6% of the impact kinetic energy, respectively. After two hours of fire exposure, the stiffness and peak impact load of the column decreased by 93.7% and 71.7%, respectively. However, the peak mid-span deflection and residual deflection increased by 6.5% and 20.1%, respectively. When the drop weight tripled, the maximum deflection and residual deformation of the midspan increased by 2.8 and 3.2 times, respectively. However, the peak impact load increases only by 14.5%. When the impact energy is the similar, the maximum midspan deflection of the specimen is almost identical, whereas a larger impact momentum reduces the peak impact force but increases the impact force at the plateau stage. By increasing fire duration, the behaviors of the column deteriorate seriously, with 85% reductions in peak impact force and 39% increase in maximum midspan deflection. In addition, the effects of impact velocity are also significant regarding each stage.

Simplified proposal for the temperature field of steel-reinforced CFST columns exposed to fire

Concrete-filled steel tubular (CFST) columns are composite sections that may contribute to reduce the environmental impact in construction through a more efficient use of resources. By embedding a steel profile inside a CFST section, a new typology is generated, the so-called steel-reinforced concrete-filled steel tubular (SR-CFST) columns, which may enhance not only the structural capacity of these composite columns at room temperature but also their fire performance. This paper focuses on studying the thermal behaviour of SR-CFST columns under fire conditions, for which purpose a two-dimensional finite element model was developed by the authors and validated by comparing the temperature distribution results with experimental tests available in the literature. Subsequently, parametric studies were carried out to analyse the influence of the relevant parameters – cross-section shape, outer tube thickness, inner steel profile dimensions, section factor – over the cross-sectional capacity of SR-CFST columns exposed to a standard ISO 834 fire. Using the data obtained from the parametric studies, a simplified temperature distribution proposal was derived. In the presented proposal, the composite section was divided onto four components (hollow steel tube, concrete infill, inner steel profile web and flanges) and simplified equations and tables were developed through statistical data processing in order to find the representative equivalent temperature of each part of the section. By doing so, the reduced cross-sectional capacity of a SR-CFST column at a given fire resistance period can be easily evaluated by using a single strength and stiffness value for each component of the section corresponding to its assigned equivalent temperature. This simplified approach may be helpful for practitioners in the fire design process of SR-CFST columns, for which the current provisions in Eurocode 4 Part 1.2 do not provide guidance in predicting the temperature field.

Fire performance on steel-reinforced concrete-filled steel tubular columns with fire protection

Steel-reinforced concrete-filled steel tube (SRCFST) is widely used as an innovative composite structure to provide better performance in construction. This study focuses on comprehensive experimental and numerical investigations of the fire performance of SRCFST columns with fire protection under the ISO-834 standard fire test procedures. The fire resistance tests were conducted on eight SRCFST columns with various thicknesses of fire resistive coating. The fire test results, including the thermal conductivity of protection material, failure modes, temperature evolution in time, axial and lateral deformation curves, and fire resistance were reported in detail. The results demonstrated that fire protection delayed the degradation of bending stiffness and significantly increased the fire resistance. Further comparisons of fire resistance between SRCFST columns with and without fire protection indicated that the fire resistance of SRCFST column with protection thickness of 12 mm is 3.32 times higher than that of that without fire protection under an equal load level. A sequentially coupled thermal-stress finite element (FE) analysis model was develped and validated against the tested results. The measured thicknesses of fire resistive coating were compared with the predicted results to evaluate the applicatibility of the existing design method of concrete-filled steel tubular (CFST) columns to SRCFST columns with thick fire-resistive coating. The results indicated that the current design method should be adjusted or modified further to provide safe and accurate predictions of the thickness of fire protection for SRCFST columns.

Impact and post-impact behaviours of steel-reinforced concrete-filled steel tubular columns after exposure to fire

This paper presents a numerical investigation on the lateral impact and post-impact behaviours of steel-reinforced concrete-filled steel tubular (SRCFST) columns after exposure to fire. The finite element analysis (FEA) model of post-fire SRCFST columns under lateral impact is developed by the ABAQUS software and calibrated. The impact responses of post-fire columns are analyzed, namely the impact process, the development of internal force, the energy change and plastic damage, etc. The impact resistance of SRCFST and CFST columns after fire is further compared, and the residual capacity of post-fire columns after lateral impact is studied. The results show that the SRCFST column after fire exhibits excellent impact performance, and the influence of fire degree on the impact resistance degradation of SRCFST columns is significantly smaller than that of CFST columns. A parametric study is also carried out to analyze the influence of various parameters on impact responses of post-fire SRCFST columns, and simplified methods are finally proposed to evaluate the impact resistance and residual capacity of post-fire SRCFST columns under lateral impact.

Performance of steel-reinforced concrete-filled square steel tubular members under sustained axial compression loading

Steel-reinforced concrete-filled steel tubular (SRCFST) members have been used in the construction owing to the superior structural performances, and are usually subjected to sustained loads during their service period. The study aims to evaluate the structural behaviour of SRCFST columns under long-term sustained load. Twenty square SRCFST columns were designed and tested to investigate the influence of sustained load on the compression behaviour. Various parameters, namely, long-term load ratio, steel ratio of profiled steel and slenderness ratio are considered to evaluate their influences on the creep in sustained load, failure mode, load-deformation curve and load–strain relation of the SRCFST columns respectively. Furthermore, the finite element (FE) model was established via software ABAQUS and its reliability was verified. The results indicate that the long-term load ratio has slight effect on the ultimate strength of SRCFST specimens. When considering the long-term load, the ultimate strength of SRCFST stub columns under axial compression nearly remains unchanged, while the load-carrying capacity for long columns actually decreases. Finally, a simplified calculation method considering long-term loading effect for the load-carrying capacity index of SRCFST columns under axial loading is proposed through parametric regression.

Analysis on Seismic Performance of Steel-Reinforced Concrete-Filled Circular Steel Tubular (SRCFST) Members Subjected to Post-Fire.

Steel-reinforced concrete-filled steel tubular (SRCFST) columns have a great development prospect in engineering practice due to their high load-bearing capacity, good ductility, and energy consumption capacity. This paper established the post-fire seismic analysis model of SRCFST with a circular-cased H section using the sequential coupled thermal-stress method by ABAQUS. The P-Δ curve, stiffness, ductility, and energy dissipation were calculated. Then, the post-fire seismic performance of CFST members was compared while keeping the total steel ratio constant, and it revealed that the SRCFST had superior ductility to CFST. Finally, the ductility coefficient and skeleton curve were parametrically evaluated. The results of the study showed that the effects of heating time (th), axial compression ratio (n), slenderness ratio (λ), and steel tube ratio (αt) on the skeleton line of SRCFST columns are more significant; the axial compression ratio (n), slenderness ratio (λ), and steel tube ratio (αt) have a negative influence on the ductility subjected to post-fire.

Performance of steel-reinforced square CFST stub columns loaded in axial compression: Experiments

A steel-reinforced concrete-filled steel tubular column is a type of concrete-filled steel tubular (CFST) column reinforced by embedded steel profile. The inner carbon steel tube reinforced square concrete-filled stainless steel tubular column (SR-CFSST) has not been systematically investigated. In this project, stub composite columns with various experimental parameters were tested as loaded in axial monotonic compression, considering the effects of the concrete strength, column size and wall thickness of the steel tubes. The mechanical behaviour of stub columns was analysed in terms of ultimate load-bearing capacity, strength index, ductility, and failure modes. The calculation methods in five codes, including GB 50936, DBJ/T 13–51, Eurocode 4, ANSI/AISC 360 and AIJ, which are widely used for ordinary CFST stub columns, were compared. Finally, a simplified calculation method for the SR-CFSST stub columns was proposed for predicting the ultimate bearing capacity. The predicted results agree well with test results and the simplified calculation method can be used to design SR-CFSST stub columns loaded monotonically in axial compression.

Evaluation of the fire resistance of steel-reinforced concretefilled steel tubular columns with a circular cross-section

In the present investigation, an analysis of the fire resistance of the steel-reinforced concrete-filled steel tubular columns with circular cross-sections was carried out by means of numerical simulation. The development of the study was carried out by means of numerical simulation to predict the behavior of the column against fire. The results of the numerical model are validated by comparing the temperature levels obtained through experimental tests. From the results obtained, it is shown that the increase in the contact area between the steel and the concrete reduces the average temperature of the column, which implies a greater resistance to fire. The fire resistance of the columns with the steel profile designs are between 3.4 - 3.6 times higher compared to the column only made of concrete, which is an indication of the excellent performance of the steel-reinforced concrete-filled steel tubular columns with circular cross- sections columns. In general, the methodology proposed in this research allows the analysis of the thermal physical phenomena of the different columns used for the construction of buildings.