Column Bars Research Articles

Seismic performance of earthquake-damaged HSRAC beam-column interior joints rehabilitated by bonding steel plates in the plastic hinge of beams

In order to investigate the seismic performance of earthquake-damaged high-strength recycled aggregate concrete (HSRAC) beam-column interior joints employing ultra-high strength bars (UHSB) in columns, four specimens were designed and manufactured. The research variables included whether the steel tube was encased in the column and whether beams were rehabilitated by bonding steel plates. Among them, the rehabilitated specimens were firstly loaded to 1.0 % drift ratio, and then loaded to failure after rehabilitating the plastic hinge of beams. The test results showed that the beam hinge failure mode occurred in the specimens loaded directly to failure. The damage of columns and joint cores was relatively slight, and the crack width satisfied the repairability limits before 2.0 % drift ratio. The shear failure of the joint core occurred in the rehabilitated specimen without steel tube, as shear deformation and stirrup strain in the joint core increased. The shear bearing capacity of joints increased after steel tubes were encased in columns, and the damage of joint cores was reduced. The bearing capacity and deformation capacity of specimens improved after rehabilitating. Finite element models were established and verified with experiment results. Parameter analysis was conducted for different pre-damage degrees, and the results indicated that as pre-damage degree increased, the bearing capacity of specimens decreased.

Retrofitting performance of precast UHPC jackets and NSM GFRP bars in pre-damaged RC columns with deficient lap splices

Fiber-reinforced polymer (FRP) jackets and the near surface-mounted (NSM) technique have been widely used to improve the seismic performance of intact reinforced concrete (RC) columns with deficient lap splices. For damaged lap-spliced RC columns, use of ultra-high-performance concrete (UHPC) jackets is more powerful in eliminating the bond splitting failure and recovering the stiffness than FRP jackets. This study proposed an L-shape precast steel-reinforced UHPC (PR-UHPC) jacket to improve the retrofit performance of conventional cast-in-place (CIP) and precast UHPC jackets. Further, PR-UHPC jackets were combined with NSM GFRP bars to develop an innovative method. Cyclic loading tests were performed on three three-fourths scaled deficient lap-spliced RC columns. One intact column was directly retrofitted with PR-UHPC jackets, while the other two were first damaged and then retrofitted with PR-UHPC jackets and the combination of PR-UHPC jackets and NSM GFRP bars, respectively. To investigate the effects of UHPC details and NSM bar types, a previous study on lap-spliced RC columns retrofitted with CIP plain UHPC jackets and NSM steel bars was incorporated. The test results indicate that the retrofitted columns failed with rebar fracture, and plastic hinge shifting was prevented. The peak load, effective yield stiffness, and ultimate drift ratio of the retrofitted columns were 29∼50 %, 37∼39 %, and 126∼165 % higher than those of the unretrofitted columns, respectively. NSM GFRP bars outperformed steel bars in terms of residual drift ratio control. The numerical model well predicted the cyclic behavior by addressing the pre-damage effect and the actual confinement of PR-UHPC jackets. The parametric study revealed that the increase of PR-UHPC jacket width effectively improved the strength and stiffness, while the pre-damage degree had a minor effect. The recommended UHPC jacket height for the present columns was 2.5 times the plastic hinge length. Finally, a design method was developed for the UHPC jacket width.

Study on interior beam–column joint sub-assemblage under bi-directional loading using finite element analysis

Purpose The purpose of this paper is to study the shearing performance under bi-directional loading of an interior beam–column joint (BCJ) sub-assemblage using the finite element analysis (FEA) tool (midas fea), validated in this research. Design/methodology/approach The BCJ can be defined as an essential part of the column that transfers the forces at the ends of the members connected to it. The members of the rigid jointed plane frame resist external forces by developing twisting moment, bending moment, axial force and shear force in the frame members. On the type of joints, the response to the action of lateral loads depends on reinforced concrete (RC) framed structures. The joint is considered rigid if the angle between the members remains unchanged during the structural deformation. This work examined the shear deformation, load displacement and strength of a non-seismically detailed internal concentric RC joint using non-linear FEA. The bi-directional loading imposes the oblique compression zone on one joint corner. This joint core’s oblique compression strut mechanism differs significantly from that under unidirectional loading. The numerical results are compared with experimental results in this study, with the data published in the literature. Findings Numerical analysis results show that, in the comparative study of numerical and experimental values, the FEA tool predicts the behaviour of the RC BCJ well. The discrepancy between the experimental and numerical results amounts to 6 to 12% end displacement of the beam, 7% resultant joint shear force, 4.23% column bar strain and 0.70% hoop strain. Originality/value The current code of practice describes the joint sub-assemblage behaviour along the single axis individually. In the non-orthogonal system, the superposition of the two axes for joint space results in overlapping the stresses and, hence, the formation of the oblique strut. This may result in a reduction in the joint capacity under bi-directional loading. The behaviour must be explored in depth, and an attempt is made for further exploration.

Shear mechanism of RCS joints with whole column-section diaphragm

The authors proposed a new precast RC column-steel beam (RCS) joint with whole column-section diaphragm, aiming to address the shortcomings including bearing failure and difficulty in prefabrication of the RCS joint details available in the existing literature. Due to its unique details, the proposed RCS joint may have a dissimilar force-transferring mechanism from the conventional beam-through type RCS joint. Moreover, the effect of various parameters on this novel joint was still unclear by limited test results. The objective of this study was to clarify the shear mechanism of the proposed joint and the impact of critical parameters. Specimens with different axial load ratios and thickness of parallel FBPs were tested under cyclic loading. The results showed that the increase in axial load ratio and thickness of parallel FBPs improved joint strength in this test. A detailed three-dimensional finite element (FE) model was developed and verified using the experimental results. The difference in the force transferring between the proposed joint and conventional beam-through RCS joints was explained in detail based on the numerical results. The dowel action of longitudinal column bars were evident in the proposed joint and the orthogonal FBPs were ineffective in activating the inner concrete strut. A parametric study was further conducted. The impacts of width of parallel FBPs, thickness of orthogonal FBPs, and axial load ratio were investigated. The joint strength linearly increased with the width of parallel FBPs. A parabolic distribution of shear stress in the parallel FBP section was obtained, while the yield zone was concentrated in the central area of the parallel FBP. The ductility could be improved with the thickness of orthogonal FBPs when the thickness was less than 10 mm. Further increasing the thickness of orthogonal FBP beyond 10 mm did not significantly affect the overall joint behavior. The joint may attain the optimal shear strength when the axial load ratio is between 0.3 and 0.4. The modeling approach in this study could be adopted as an effective tool to assess joint behavior. Moreover, the conclusions obtained by numerical study potentially promoted the application of the proposed joint.

Experimental Investigation of GFRP-Reinforced Hollow Square Concrete Column

Due to their great structural efficiency and efficient utilization of materials, steel-reinforced hollow-core concrete columns are often employed in utility poles, ground piles, and piers for bridges. Based on research, these columns' performance is impacted by many design parameters. However, corrosion can be a problem in steel-reinforced concrete structures. This paper examines the differences between using steel and GFRP longitudinal bars in hollow-section square concrete columns and explores the potential benefits of using GFRP bars as an option that is economically viable and non-corrosive. According to the study results, the computational results clearly show how an increased longitudinal GFRP reinforcement ratio improves the columns' bearing capability, but when compared to steel reinforcement, it provides less bearing capability. For the same reinforcement ratio (1.46 %, 3.29 %, and 4.9 %), The findings demonstrated that GFRP columns had a decrease in the axial bearing load by 13.1%, 9.2 %, and 9.4%, respectively.

Effect of beam-column depth ratio on seismic behavior of precast RC column-steel beam joint

To achieve the simultaneous horizontal casting of the joint and column in the factory, the authors proposed a new precast RC column-steel beam (RCS) joint. The overall behavior of the proposed joint has been investigated. However, the effects of various design parameters were unclear. This paper focused on the impact of the beam-column depth ratio on the proposed RCS joints. Three 2/3 scale interior RCS joint specimens were designed and tested under cyclic loading. Experimental results showed that the joint shear strength and ductility of the specimen improved as the beam-column depth ratio decreased. A large beam-column depth ratio could modify the bond condition of column bars. Moreover, a numerical study was conducted to further investigate the influential mechanism of the beam-column depth ratio. According to the distribution of compressive stress, the concrete strut was activated in the joint. The angle between the strut and the horizontal increased with the beam-column depth ratio, leading to less horizontal component to resist the joint shear force. The joint shear strength tended to be overestimated as the beam-column depth ratio increased by using current design methods. Ignoring the effect of the beam-column depth ratio in the design methods may cause significant errors in predicting the joint shear strength.

Compressive and Bonding Performance of GFRP-Reinforced Concrete Columns

The use of glass-fiber-reinforced polymer (GFRP) bars as an alternative to steel bars for reinforcing concrete (RC) structures has gained increasing attention in recent years. GFRP bars offer several advantages over steel bars, such as corrosion resistance, lightweight, high tensile strength, and non-magnetic properties. However, there are also some challenges and uncertainties associated with the behavior and performance of GFRP-reinforced concrete (GFRP-RC) structures, especially under compression and bonding behavior. Therefore, there is a need for comprehensive experimental investigations to validate the effectiveness of GFRP bars in concrete columns. This paper presents a study that aims to address these issues by conducting experimental tests on GFRP-RC columns. The experimental tests examine the mechanical properties of GFRP bars and their bond behavior with concrete, as well as the axial compressive behavior of GFRP-RC columns with different reinforcement configurations, tie spacing, and bar sizes. The findings reveal that GFRP bars demonstrate a comparable, if not superior, compressive capacity to traditional steel bars, significantly contributing to the load-bearing capacity of concrete columns. The study concludes with a set of recommendations for further exploration, underscoring the potential of GFRP bars in revolutionizing the construction industry.

Seismic behavior test and plastic hinge theory for HRB500 prefabricated steel reinforcement cage-cast-in-situ concrete columns

An innovative method for constructing concrete columns has been proposed, namely utilizing the hot-forged double-lock mother combined mechanical sleeve connection for the HRB500 prefabricated steel reinforcement cage－cast-in-situ concrete (PRC-CISC) columns. The purpose of this method is to promote the industrialization of steel reinforcement projects and enhance the quality of structural construction. Two traditional reinforced concrete (RC) columns and four PRC-CISC columns were designed for quasi-static tests with design axial compression ratio (ACR) and reinforcement connection methods as parameters. Firstly, the results of the study showed that all specimens exhibited typical flexural failures, with well-rounded hysteresis curves, gradual stiffness degradation in the later stages, good ductility, and energy dissipation. Mechanical sleeve connections had a minor impact on the failure mode of PRC-CISC columns. Increasing the ACR improved the carrying capacity but reduced ductility, narrowed the hinge zone, and increased longitudinal bar stress. The effect of sleeve connection on energy dissipation, stiffness, ductility, and carrying capacity of the specimens was limited, and the overall behavior was stable and controllable. Furthermore, the carrying capacity and stress distribution of core area steel bars in traditional RC columns and PRC-CISC columns were further calculated to assess the differences. Finally, a two-stage plastic hinge calculation theory was formulated, which takes into account the influence of axial pressure, and construction recommendations for two types of PRC-CISC columns were provided.

Seismic fragility analysis of high‐strength concrete frame structures reinforced with high‐strength steel bars

SummaryTo investigate the influence of using high‐strength steel bars in columns on the seismic resistance capacity and seismic resilience of frame structures, seismic fragility evaluation of three 8‐story reinforced concrete (RC) frame structures was conducted based on the incremental dynamic analysis (IDA) using 11 ground motion records. The main parameter is the longitudinal reinforcement configuration in the frame columns, where the first structure is reinforced with HRB 600 grade steel bars in the columns, the second structure is replaced with equal area ultra‐high‐strength (UHS) steel bars (i.e., with a yield strength of approximately 1425 MPa), and the third structure is replaced with equal strength UHS steel bars. A numerical model of the RC frame structure was developed and then validated using previous experimental results. The exceeding probabilities at various performance limit states were calculated based on two typical engineering demand parameters (EDPs) of maximum interstory drift and residual interstory drift. The results showed that using UHS longitudinal steel bars instead of HRB 600 grade steel bars in frame columns could reduce the energy dissipation capacity of the structure, inevitably leading to an increase in the maximum interstory response of the frame. However, much lower exceedance probability was observed in the UHS‐enhanced frame under the repair available limit state based on the residual interstory drift, indicating that the UHS‐enhanced RC frame had higher seismic resilience. In addition, compared to equal area substitution, equal strength substitution is a more ideal design method that can use fewer UHS steel bars to achieve comparable reparability and a smaller increase in maximum interstory drift.

Behavior of Axially Loaded Concrete Columns Reinforced with Steel Tubes Infilled with Cementitious Grouting Material

The paper presents a novel method of reinforcing concrete columns using small-diameter steel tubes instead of traditional steel bars. The researchers conducted experimental investigations on twelve mid-scale circular concrete column specimens, which were divided into two groups consisting of six specimens each: short and long columns. Two of the specimens in each group were reinforced with steel bars, while the remaining four were reinforced with steel tubes filled with cementitious grouting material. The study proposed two concepts for cementitious grouted steel-tube reinforcement. The first concept utilized steel tubes with equivalent net areas to the steel bar areas used in the reference column, while the second concept used steel-tube reinforcement with the same diameter as the steel bars in the reference column. Nonlinear Finite Element (FE) analyses were conducted on experimental specimens using ABAQUS software. The results showed that using steel tubes with an area equivalent to that of steel bars instead of conventional columns increased the bearing capacity of reinforced concrete columns by 17%. Moreover, using steel tubes whose area matched 30% of the steel bar area achieved a bearing capacity of about 81% of the conventional concrete columns. The experimental and FE analysis findings indicate that this methodology can increase the bearing capacity of reinforced concrete columns when compared to traditional methods. The axial load-axial displacement curves, axial load-axial strain curves, and failure load of the FE model all demonstrated good convergence with the experimental data. Doi: 10.28991/CEJ-2024-010-02-017 Full Text: PDF

EXPERIMENTAL STUDIES OF GRATE PROFILE VARIANTS ON THE COARSE DEBRIS MODULE EXPERIMENTAL PLANT

The article deals with development of new grid profiles in the equipment for cleaning of seed cotton from major impurities, improvement of the cleaning efficiency of the machine, and calculation processes. Four types of grid profiles were considered for cleaning machines when cleaning seed cotton from small and large impurities. Two versions of the profiles were installed: a radius-shaped and a curved metal grill. Profile fences were developed at the "Pakhta Djin KB" practical plant and joint research was conducted on installing of an experimental full-profile bench. Experiments were carried out at different speeds to determine the performance and cleaning efficiency by studying different profiles of the column bars in the experimental equipment for cleaning cotton from large impurities. The results of the experiment showed that the iron sheet bent colosnik grid is preferable. The recommended iron columnar grating is recommended for industrial implementation.

Behaviour and design of monotonically loaded reinforced concrete external beam-column joints

This paper describes an experimental investigation into the influence of beam reinforcement detailing, column axial load and shear reinforcement on the joint shear strength of reinforced concrete external beam-column connections. A series of twelve external beam-column joint specimens were tested in three groups of four specimens. Series 1 investigated the influence of beam reinforcement anchorage type and bend radius. Series 2 studied the influence of column axial load on joint shear strength while series 3 studied the influence of intermediate column bars and joint shear reinforcement. The paper presents the experimental results and describes the influence of the considered variables on joint shear strength. Three existing empirical design equations for external beam-column are evaluated using data from this program and previously tested beam-column joint specimens. Finally, design recommendations are made.

Prediction of Ultimate Capacity of Concrete Columns Reinforced with FRP Bars.

FRP bars are used in concrete structures as an alternative to steel bars as they have many advantages such as high tensile strength, high strength-to-weight ratio, electromagnetic neutrality, lightweight and no corrosion. There is a perceived lack of standard regulations for the design of concrete columns with FRP reinforcement, e.g., in Eurocode 2. This paper describes a procedure for predicting the bearing capacity of concrete columns with FRP reinforcement based on the interaction of axial force and bending moment, which was developed on the basis of existing design recommendations and standards. It was shown that the bearing capacity of eccentrically loaded RC sections depends on two parameters, which are the mechanical reinforcement ratio ω and the location of the reinforcement in the cross-section expressed by the β factor. The analyses carried out showed the existence of a singularity in the n-m interaction curve indicating the fact that in a certain loaded range, the curve is concave, and more it was shown that the balance failure point for sections with FRP reinforcement takes place for eccentric tension. A simple procedure for calculating the required reinforcement from any FRP bars in concrete columns was also proposed. Nomograms developed from n-m interaction curves provide for the accurate and rational design of FRP reinforcement in columns.

Contribution of longitudinal GFRP bars in concrete filled FRP tubular (CFFT) cylinders under monotonic or cyclic axial compression

Most of the current design guidelines/codes ignore the compressive contribution of glass fiber reinforced polymer (GFRP) bars in compression members, due to: (1) the compressive strength of GFRP bars is significantly lower than their tensile strength due to fiber micro-buckling or shear failure of GFRP bars under compression; (2) the peak axial load of an unconfined concrete column is achieved at a relatively low axial strain at which the GFRP bars develop small axial stresses due to their low elastic modulus. However, existing studies have demonstrated that, if adequate transverse bars are provided, the contribution of longitudinal FRP bars to the total load resistance of the column is significant partially due to the enhanced axial strain at peak axial load because of the confinement provided by the transverse bars. In concrete filled FRP tubular (CFFT) columns, which is an emerging and attractive form of columns for new construction, the ultimate axial strain of concrete is significantly enhanced by FRP confinement and thus the contribution of internal longitudinal GFRP bars is also expected to be enhanced. However, the compressive behavior of GFRP bars in CFFT columns has never been investigated. This paper therefore presents the results of an experimental program on CFFT cylinders with internal longitudinal GFRP bars. The test results demonstrated that FRP bars contribute significantly to the total load resistance of CFFT cylinders. The FRP bars in CFFT cylinders experienced crushing failure with load resistance close to that derived based on the compression tests on bare FRP bars. A simple analysis method for the axial load at FRP bar crushing for CFFT cylinders was finally proposed.

Behaviour of FRP spiral-confined concrete and contribution of FRP longitudinal bars in FRP-RC columns under axial compression

Fibre-reinforced polymer (FRP) bar-reinforced concrete (RC) columns (FRP-RC columns) have become increasingly popular since they are durable against harsh environmental attacks. For a reliable and cost-effective design of FRP-RC columns, it is essential to gain an in-depth understanding on the compressive behaviour of FRP spiral-confined concrete and the role of FRP longitudinal bars in resisting the axial stresses in the columns. Although extensive studies have been carried out on FRP-RC columns, few studies have been conducted to clarify the axial compressive behaviour of each component (i.e., FRP spiral-confined concrete and FRP longitudinal reinforcing bar). To this end, an experimental study was conducted to explore the compressive behaviour of FRP spiral-confined concrete in FRP-RC columns with and without longitudinal reinforcing FRP bars under axial compression. The contribution of the FRP bars in the FRP-RC columns is also investigated. The key variables of the experimental study include the spiral pitch and the diameter of FRP longitudinal bars. Test results show that the FRP spiral-confined concrete in FRP-RC columns exhibits a bilinear axial stress–strain behaviour. The threshold of sufficient confinement ratio of FRP spiral-confined concrete is 0.12, which is larger than that of the FRP jacket-confined concrete documented in the literature. The axial load carried by the FRP bars with a diameter of 16 mm (corresponding to a reinforcement ratio of 4.6%) accounts for 25% ∼ 35% of the total axial load in the column at the ultimate state, while the contribution of the FRP bars is significantly reduced with the decrease in the bar diameter. The value of the tensile elastic modulus of the longitudinal FPR bars should be reduced when estimating their axial load contribution to the FRP-RC columns. The axial load–strain behaviour of FRP-RC columns can be well predicted by the existing model with some modifications.

Controlled synthesis of Bi2O3–YSZ composite powders and their sintering behavior for high‐performance electrolytes

AbstractBismuth oxide (Bi2O3) is a promising additive to decrease the sintering temperature of yttria‐stabilized zirconia (YSZ)‐based electrolyte for solid oxide fuel cell application. However, Bi2O3 tends to grow into large column bars (>50 µm) in a chemical coprecipitation method, which dramatically limits the mixing uniformity of Bi2O3 and YSZ, even much worse than that of mechanical mixing. In this study, the reaction temperature was increased from room temperature to 90°C to increase the number of nucleation during the violate reaction between Bi3+ solution and YSZ suspension in NaOH. On this basis, the violence of the reaction was further moderated by adding half of NaOH first, then YSZ powders and the other half of an NaOH solution. The size of Bi2O3 was further decreased to sub‐micrometer and Bi2O3 was homogeneously mixed with YSZ particles, even when its addition amount was as large as 20 mol%. These composite powders effectively promoted the sintering behavior of YSZ. The sintering temperature of YSZ was decreased to 900 and 1000°C with 10 and 5 mol% Bi2O3 doping, respectively. Increasing the doping ratio induced severe volatilization of Bi2O3 and pore formation. Raising the sintering temperature (no more than 1200°C) enhanced the doping effect of Bi2O3 into the YSZ lattice but induced instability in the YSZ crystal structure.

Evolution Law of Reinforcement Corrosion and Mechanical Properties under the Conditions of Basic Magnesium Sulfate Cement Concrete

Basic magnesium sulfate cement (BMS) is a new kind of cement, which has many advantages. In order to study the corrosion of steel in basic magnesium sulfate cement concrete (BMSC), nine steel bars were tested by dry and wet cycling. The simulated solution of BMS was used in this test. In addition, 24 corroded steel bars in BMSC beams and columns that exposed for two years in the natural environment were intercepted for the tensile test. It can be found that the maximum section loss rate, the yield strength, the ultimate strength, the modulus of elasticity and the elongation at break of the corroded steel bars are linearly related to the corrosion rate. Through the research, the calculation formula of the relationship between the yield strength, ultimate strength, elongation at break and elastic modulus of the steel bar and the maximum corrosion rate of section of the steel bar were established. It provides a reference and basis for the durability study of BMSC beams and columns in the future.

Critical Analysis of Honeycomb to Actualize Building Design Appearance

This research was carried out to analyze honeycomb critically to actualize building design appearances. Such problems are: reduction in the load bearing capacity or the strength of the building structures, increase in the penetration of water inside the building structures, losing of grip between rods and concrete and reduction in the durability of the buildings are problems associated with honeycomb. However, these problems can be solved by addition of water within the allowable limit, using a cohesive concrete to improve on the grip on the rod, proper compaction or vibration of concrete, reduction of the pouring height, ensuring a water tight form work and placing of bars in beams and columns properly

Structural Performance of GFRP Bars Based High-Strength RC Columns: An Application of Advanced Decision-Making Mechanism for Experimental Profile Data

Several past studies have shown the use of glass fibre-reinforced polymer (GFRP) bars to alleviate the reinforced steel rusting issue in different concrete structures. However, the practise of GFRP bars in concrete columns has not yet achieved a sufficient confidence level due to the lack of a theoretical model found in the literature. The objective of the current study is to introduce a novel prediction model for the axial capability of concrete columns made with bars of GFRP. For this purpose, two different approaches, such as data envelopment analysis (DEA) and artificial neural networks (ANNs) modelling, are used on a collected dataset of 266 concrete column specimens made with GFRP bars from previous literature works. Eight parameters were used to predict the axial performance of GFRP-based RC columns. The proposed DEA and ANNs predictions demonstrated a good correlation with the testing dataset, having R2 values of 0.811 and 0.836, respectively. A comparative analysis of the DEA and ANNs models is undertaken, and it was found that the suggested models are capable of accurately forecasting the structural response of GFRP-made RC column structures. Then, a comprehensive parametric analysis of 266 GFRP-based columns was performed to study the effect of different materials and their geometrical shape.

Effect of Tie Bars on Axial Compressive Behavior of Round-Ended Rectangular CFST Stub Columns.

Round-ended rectangular concrete-filled steel tube (RRCFST) columns are prone to local buckling that are close to straight steel plates when used as piers of a bridge and affect its long-term use. In order to solve this problem, tie bars were used in this research to stiffen RRCFST columns. Eleven specimens with tie bars and three specimens without tie bars were tested to analyze influences of cross-sectional aspect ratio, longitudinal spacing, limb numbers and diameter of the tie bar on failure model, confined effect, bearing capacity and ductility of RRCFST stub columns. Finite element models (FEM) with different concrete constitutive models for rectangular and circle parts were established and validated to reveal the mechanism of the constrained effect of tie bars. Experimental and FEM results show that the local buckling scope was decreased and gradually moved to middle height with decreased longitudinal spacings tie bars. The addition of tie bars in RRCFST columns with large aspect ratios slightly enhanced the ultimate bearing capacity, the diameter of tie bars changing from 8 mm to 12 mm greatly enhanced displacement and energy ductility by 58.4% and 85.1%, respectively. However, more tie bars (e.g., two or three limbs) utilization could not further improve the bearing capacity and failure mode. While, the tie bars had very limited contribution to bearing capacity and ductility for RRCFST columns with small aspect ratios, because the outer steel tubes already individually provided for enough confinement on inner concrete. By considering different concrete confined models for rectangular and round-ended parts, an analytical model was proposed and validated to predict the ultimate bearing load for RRCFST stub columns with tie bars.