Fiber-reinforced Polymer Confinement Research Articles

Axial compressive behavior and failure mechanism of CFRP partially confined ultra-high performance concrete (UHPC)

With comparable mechanical properties, higher cost-effectiveness and ease of column joints rehabilitation, partial fiber-reinforced polymer (FRP) confinement holds promise as an alternative to full FRP confinement in ultra-high performance concrete (UHPC). Nevertheless, the compressive behavior and underlying failure mechanism of FRP partially confined UHPC are not fully understood. To address this issue, axial compression tests on ultra-high strength concrete (UHSC, removing fibers in UHPC) and UHPC cylinders fully and partially confined by FRP are conducted in this study. The stress-strain relationship, strain distribution and crack development of specimens subjected to axial compression are profoundly analyzed using a combination of strain gauges and digital image correlation (DIC). Experimental results indicate that partial FRP confinement has a comparable effect on compressive performance compared to full FRP confinement. FRP confinement and fiber inclusion can effectively overcome the brittle nature of UHSC in compressive failure and consequently extend the transition stage, raising the stress/strain retention ratio from 0.91% to 26.45% and improving the enhancement ratio of ultimate strain from 1.97% to 34.4%. Furthermore, a more reliable and accurate prediction model is proposed in this study for evaluating the stress-strain relationship of partial FRP confinement under ultimate conditions.

Effect of aggregate sizes on dilation properties and stress−strain behavior of FRP-confined recycled aggregate concrete

Aggregate size plays a crucial role in determining the mesoscale uniformity and compactness of concrete, directly influencing its dilation characteristics. This dilation behavior, especially under axial load, has significant implications for the tri-axial stress conditions when the concrete undergoes passive confinement using Fiber-Reinforced Polymer (FRP). This study focuses on comprehensively investigating the stress−strain behavior and dilation properties of FRP-confined recycled aggregate concrete (RAC), with a specific emphasis on the effects of recycled aggregate (RA) sizes. A series of compression tests were conducted to examine the stress−strain response of FRP-confined RAC, shedding light on how the dilation properties are influenced by varying RA sizes. The results revealed notable differences in the physical properties (crushing index and water absorption ratio) of RA compared to natural aggregates, with these differences significantly impacting the unconfined concrete strength. Although, an increase in aggregate size demonstrated an improvement in FRP confinement efficiency, it did not bring about a change in the ultimate strength of FRP-confined concrete due to the interaction between the weakening of unconfined concrete strength and the increase in confinement efficiency. In addtion, the transition stress on the stress−strain curve proved highly dependent on both aggregate size and FRP confinement efficiency to RAC. Consequently, a new modified transition stress model, accounting for the effect of aggregate size and FRP confinement, was introduced. Incorporating this newly proposed transition stress model into the existing stress−strain expression yielded predicted stress−strain curves that closely matched the experimental results for FRP-confined RAC with varying RA sizes.

Failure patterns and mechanical behavior of the cement mortar column partially confined by the FRP jacket

Fiber-reinforced polymer (FRP) composite has been recently adopted to maintain the integrity of infrastructure made of cement mortar, attributed to its high strength-to-weight ratio and easy-to-construct. To obtain an in-depth understanding about the compressive behavior of cement mortar samples with variable FRP center wrapping heights, both the acoustic emission (AE) response and deformation characteristics of which were comprehensively investigated in the present research. The relationship between the confinement provided the exterior FRP jacket and the mechanical characteristic was also explored, followed by the economic analysis on this reinforced technique. Test results indicated that both the deformation ability and the load-bearing capacity gradually increased with the increased center wrapping height of the FRP jacket. When the larger the AE energy generated at the initial compression stage, the more dispersed AE localization. Meanwhile, these main cracks occurred on the surface of reinforced cement mortar generally transformed from the FRP wrapping strip towards other extremities. Correspondingly, the number of tensile cracks of FRP partially wrapped cement mortar decreased with the increased axial deformation. Note that the residual integrity of these reinforced cement mortar with larger FRP-center wrapping height is much superior than its counterparts. The force state of partially reinforced cement mortar with FRP confinement was also discussed in the present research and the results of which were compared with existing theoretical models. Compared to other theoretical models, Teng's model shows the most accuracy in predicting the mechanical behavior of FRP partially reinforced cement mortar.

Axial stress-strain model for concrete in partially FRP wrapped reinforced concrete columns

The application of fiber-reinforced polymer (FRP) composites has become a commonly-used technique for the strengthening/retrofitting of existing reinforced concrete (RC) columns. To date, extensive experimental studies have been conducted in this field, resulting in many stress-strain models of FRP-confined concrete for analysis or design use. Few models, however, are concerned with the concrete in FRP-confined RC columns, especially for the columns under partial FRP confinement. This paper first investigates the mechanical properties of the concrete under FRP strip-transverse steel reinforcement composite confinement via the assessment of existing analysis-oriented models (AOMs). Then, the peak stress of actively confined concrete and the dilation model in AOM are calibrated based on existing research, leading to a new AOM for FRP strip-steel confined concrete. Finally, a test database is assembled to evaluate the reliability of the new model in terms of the predictions of ultimate conditions and stress-strain curves. By comparing the experimental results and theoretical predictions, the proposed model exhibits higher accuracy for the FRP strip-steel confined concrete compared with the other existing models.

Behavior of large-scale FRP-confined square RC columns with UHP-ECC section curvilinearization under eccentric compression

Implementing section curvilinearization (SC) by curving flat sides into curved sides before fiber-reinforced polymer (FRP) jacketing has been proposed to enhance the FRP confinement effectiveness of FRP-confined rectangular concrete columns. In this paper, ultra-high performance engineered cementitious composite (UHP-ECC) is proposed as the added concrete for SC, with an aim to enhance both bending and axial strengths of concrete columns given that the UHP-ECC can contribute to resisting tensile stresses for columns. Thirteen large-scale FRP-confined square reinforced concrete (RC) columns with SC using UHP-ECC were tested under eccentric compression loading to explore their eccentric compressive behavior. The test parameters included the rise-to-span ratio, the FRP thickness, the eccentricity and the surface roughness of the old concrete. The test results show that FRP confinement with SC using UHP-ECC can substantially enhance the peak axial load and the bending moment than those without SC, while the ultimate lateral displacement is marginally increased. An increase in the rise-to-span ratio leads to an increase in the peak axial load.

Design-oriented stress–strain model for RC columns with dual FRP- steel confinement mechanism

Many research studies have been conducted to evaluate confinement-induced enhancements on the mechanical properties of FRP (fiber-reinforced polymers)-confined plain concrete elements subjected to axial compressive loading, leading to the development of extensive predictive models. Nevertheless, experimental stress–strain results for FRP-confined RC columns (FCRC) have demonstrated some behavioural features that cannot be simulated accurately through this kind of model, developed exclusively for FRP-confined concrete columns (FCC). In this paper, a new design-oriented stress–strain model is proposed for the prediction of load-carrying capacity versus axial strain relationship of FCRC. For this purpose, a new parabolic stress–strain expression is developed for calculating the first branch of FCRC’s response up to the transition zone, followed by a linear function. New formulations are proposed to determine the first branch’s stress–strain gradient, transition zone-related information and the second branch’s slope, calibrated using a large test database of FCRC. The proposed design-oriented model is capable of simulating accurately the combined influence of the dual FRP and steel confinement on load-carrying capacity versus axial strain relationship of FCRC. Lastly, the capability of this model is validated by comparison to existing experimental data of FCRC and those obtained from some of existing models in the literature.

Compressive behavior of FRP-confined 3D printed ultra-high performance concrete cylinders

Three-dimensional (3D) concrete printing technology has attracted increasing applications due to its merits such as labor-saving. Due to difficulties in implementing reinforcements in 3D printed concrete (3DPC), 3DPC structures are commonly designed to predominantly resist compressive loadings. This paper proposes to further enhance the compressive performance of 3D printed ultra-high performance concrete (3DPU) elements by fiber-reinforced polymer (FRP) wrapping. Axial compression tests on FRP-confined 3D printed UHPC (FC3DPU) and unconfined 3D printed UHPC (UC3DPU) cylinders were conducted. The key variables include the loading directions (i.e., X, Y, Z directions) of the 3DPU cylinders and the FRP confinement thickness (i.e., one and two layers). Test results show that FRP wrapping can substantially enhance the strength and deformation capacity of 3DPU. Furthermore, the compressive strengths of the UC3DPU and FC3DPU in the X-direction are the highest, while they are the lowest in the Z-direction. The actual confinement ratio threshold for sufficient confinement of FC3DPU is 0.1. Two existing models of FRP-confined concrete were assessed, and results show that models of Liao et al. and Teng et al. have a comparable accuracy in predicting the ultimate axial stresses and strains of FC3DPU. Microscopic analysis reveals that 3DPU has more large defects (i.e., equivalent diameter (Eq) > 2 mm) in interlayers but less small defects (Eq < 2 mm) than the cast counterparts.

Cyclic Stress–Strain Behavior and Model for FRP-Confined Engineered Cementitious Composite

A fiber-reinforced polymer (FRP) confined engineered cementitious composite (ECC) column as a new high-performance composite member achieves significant ductility. Understanding its cyclic behavior is necessary to guide seismic design. This paper presents an experimental study on the compressive behavior of large rupture strain (LRS) FRP and traditional glass FRP (GFRP) jacketed ECC cylinders subjected to full and partial cyclic loadings. Effects of FRP confinement level, FRP type, and loading scheme on the failure pattern, stress–strain relationship, ultimate condition, plastic strain, and stress deterioration ratio were examined. Existing equations for plastic strain and stress deterioration ratio of the envelope and internal cycles were evaluated. Based on the experimental results, new calculation formulas were derived for the plastic strain and stress deterioration ratio. A threshold involving partial unloading/reloading factors was determined to define the effective cycles. In addition, a cyclic model composed of a monotonic envelope model and an unloading/reloading model was developed for estimating the stress–strain hysteresis loops of both GFRP- and LRS FRP-confined ECC cylinders.

Uniaxial compressive behaviour of a FRP standing support made of mine wastes

Twenty-seven specimens were tested to investigate the uniaxial compression behaviour of an innovative standing support for underground space applications. The innovative standing support consisted of an external fibre reinforced polymer (FRP) jacket and the infill column made of cementitious grout, tailings and coal wash rejects. Effects of the FRP layers number and water to the cementitious grout (w/g) ratio were evaluated. Test results indicated that lower w/g ratios produced stronger infill columns. With FRP confinement, the standing support demonstrated strain-hardening loading characteristics with a significant improvement in both strength and ductility. The highest strength and strain of the specimens achieved was 58.4 MPa and 11.8% respectively. Compared with the unconfined specimens, the confinement with four FRP layers increased the specimen strength and associated strain up to 3.6 and 27.0 times respectively. A correlation between the compressive strength of the infill material and the ultrasonic pulse velocity was also investigated. Furthermore, a simple design-oriented model was proposed to predict the peak strength and the corresponding strain of the innovative standing support.

Analytical model to predict axial stress-strain behavior of heat-damaged unreinforced concrete columns wrapped by FRP jacket

Although there are several confinement models to obtain analytically the axial stress-strain response (fc-εc) of concrete columns wrapped with fiber-reinforced-polymer (FRP) jacket at ambient conditions, a reliable design-oriented model to determine the fc-εc of heat-damaged concrete columns post-confined with FRP is still lacking in the literature. This study aims to address this research gap, by proposing a formulation that predicts the favourable effects of FRP confinement on concrete elements previously exposed to high temperatures. This model proposes a closed-form formulation to derive a fc-εc expression, including a set of strength and strain sub-models to calculate the stress/strain information at transition and ultimate points defining the stress-strain response. To develop the model and calibrate its key components by data analysis of statistical treatment techniques, a large test database of FRP confined unheated/heat-damaged concrete of circular/square cross-section consisting of 1914 specimens was collected. The proposed design-oriented model is able to demonstrate the influence of pre-existing thermal damage on the axial fc-εc relationship, whose reliability is revealed comprehensively through predicting data from several experimental heat-damaged concrete specimens confined with FRP systems.

Finite element analysis on compressive behaviour of tin slag polymer concrete with fibre reinforced polymer composites confinement

Finite Element Analysis (FEA) forecasts how tin slag polymer concrete with confinement would react to real-world stresses, minimizing the need for a physical prototype. Currently, there is no research on FEA designated for tin slag polymer concrete with fibre reinforced polymer (FRP) confinement. The purpose of this study is to investigate the compressive behaviour of tin slag polymer concrete (TSPC) with FRP confinement through finite element method (FEM). Carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) were used to enhance the compressive properties of TSPC column. A comparison was made between the finite element result and the experimental result by analysing the stress–strain response. According to the comparison, the finite element model and experimental sample for unconfined and confined TSPC share a similar bilinear stress–strain curve. It was also discovered that TSPC with GFRP confinement has substantially more accurate stress–strain curve with 4.46% compared to 13.47% for CFRP, a percentage difference of 9.01%. TSPC with CFRP confinement has higher compressive strength than GFRP. The highest strength increment from one layer to the next occurs between one and two confinement layers. According to the simulation, the failure mode of finite element model is similar to the experimental sample.

Eccentric compression behavior of concrete-filled steel tube columns strengthened by CFRP/steel strip

Concrete-filled steel tube (CFST) column structures have been widely applied in structural engineering because of their superior bearing capacity and ductility. During the service lives of CFST columns, due to the change in the service function, the increase in the load, and the corrosion of the steel tubes, some existing CFST columns face the strengthening demand of increasing bearing capacity and durability. In this study, two efficient strengthening methods for CFST columns were proposed, namely, winding carbon fiber-reinforced polymer (CFRP) or welding steel strips around the columns. A total of 11 specimens were tested to investigate the behavior of the strengthened CFST columns under eccentric compression according to parameters such as the height-diameter ratio, fiber-reinforced polymer (FRP) layer, eccentric distance, and steel strip thickness. The results indicated that the ultimate strength and initial stiffness decreased with increasing eccentric distance. The arrangement of the FRP and steel strips effectively improved the eccentric carrying capacity and ductility. The improvement in the ultimate load due to FRP confinement exceeded 34.31%, and that due to the steel strips exceeded 13.47%. Finally, considering the effect of the eccentric distance and height-diameter ratio on the behavior of the structure under compression, a model for predicting the bearing capacity under eccentric load was proposed and verified by a database that included various design parameters.

Evaluation of Hybrid Fiber Multiscale Polymer Composites for Structural Confinement under Cyclic Axial Compressive Loading

Fiber reinforced polymer (FRP) confinement is recognized as the most promising technique for the strengthening and retrofitting of concrete structures. In order to enhance the performance of conventional epoxy-based FRP composites, nano filler modification of the epoxy matrix was implemented in the current study. In particular, the cyclic loading response of standard concrete specimens externally confined by epoxy-based natural and hybrid fiber reinforced polymer systems was investigated. The confinements were realized with sisal fiber reinforced polymer (SFRP) and hybrid sisal basalt fiber reinforced polymer (HSBFRP). Moreover, the effects of multiwalled carbon nanotubes (MWCNT) were also investigated. Three different specimen sets were considered for study: (i) unconfined specimens, (ii) epoxy-based FRP confined specimens and (iii) MWCNT incorporated epoxy-based FRP confined specimens. The specimens were tested in repeated compressive mode in loading-unloading cycles at increasing displacement levels. The test results revealed that FRP wrapping could enhance the mechanical behavior of unconfined columns in terms of strength and ductility. Moreover, it was evident that the mechanical properties of the epoxy matrix were enhanced by MWCNT incorporation. The developed epoxy-based FRP confinement containing MWCNT ensures improvement in axial strength by 71% when compared with unconfined specimens. The epoxy-based FRP confinement, with and without MWCNT, exhibited a high strain redistribution behavior around the concrete core. In comparison to the unconfined specimens, the confinement could increase the sustained axial strain from 0.6 to 1.4% using epoxy-based FRP confinement and to 1.6% with MWCNT incorporated epoxy-based FRP confinement. Further, an empirical model was developed to predict the ultimate axial stress of concrete columns confined externally with FRP jackets. The ultimate compressive strength obtained from the experimental study was compared with the proposed model, and the observed deviation was lower than 1%.

Experimental and statistical investigation on synergistic effect of nano based epoxy hybrid FRP on strength and durability of circular concrete columns

Strengthening and retrofitting of concrete structures using fiber reinforced polymer (FRP) wrapping is a promising technique in construction sector. Epoxy is the commonly used matrix which possess a higher rate of degradation under exposure to harsh environment. The evaluation of strength and durability of concrete cylinders confined by multi walled carbon nanotube (MWCNT) incorporated epoxy with hybrid sisal and basalt fiber composite systems exposed to various environmental conditions forms focus of this paper. Specimens are subjected to various environmental exposure such as elevated temperature, acidic, alkaline and sea water conditions. Two varieties of epoxy viz. neat epoxy and MWCNT modified epoxy systems are considered. Mechanical and durability properties are analyzed based on axial compressive behavior, stress strain response, visual inspection and modes of failure. The MWCNT incorporated epoxy based hybrid FRP confined specimens exhibited a strength reduction less than 10% when compared with unexposed confined specimens under aggressive environmental conditions, while unconfined specimens showed strength reduction by 40% when compared to unexposed unconfined specimens. The MWCNT modified epoxy based hybrid confinement showed an energy absorption of 6.24 times that of unconfined specimens upon chemical exposure. Ultrasonic Pulse Velocity test revealed efficacy of confinement system in protecting concrete core from a sudden failure, which in turn increases the compressive strength of system. A statistical analysis using ANOVA was employed to find significance of these factors and confirmed with experimental results. The effect of MWCNT incorporation is significant in FRP confinement and shows the possibility of the FRP system to be adopted as a major retrofitting material in alkaline and sea water environments.

Predictive models for the peak stress and ultimate strain of FRP confined concrete cylinders with inclined fiber orientations

It is proved that fiber orientation significantly affects the confinement of fiber-reinforced polymer (FRP) wrapped concrete members. The inclination of fiber relative to the hoop direction reduces confinement effectivity. In addition, most available models for peak stress and ultimate strain of confined concrete fail to provide accurate estimations for FRP-wrapped concrete in such cases. Accordingly, the main objective of the present study is to suggest modification factors that can be applied to the existing predictive models in the case of inclined fiber orientation and improve their prediction accuracy. For this purpose, a database of experimental results on FRP-confined concrete columns with inclined fiber orientation from previous studies have been collected. Then, by using an evolutionary algorithm, modification factors were proposed to incorporate the effect of fiber orientation. Applying these modification factors to the existing stress and strain models for FRP-confined concrete cylinders leads to a more accurate estimation of the behavior of FRP-confined columns with inclined fibers.

Experimental and Numerical Investigation of Axial Compression Behaviour of FRP-Confined Concrete-Core-Encased Rebar.

The axial compression behaviour of fibre-reinforced polymer (FRP)-confined concrete-core-encased rebar (FCCC-R) was investigated by performing monotonic axial compression tests on seven groups of FCCC-R specimens and three groups of pure rebar specimens. The research parameters considered were the FRP winding angle (0°, ±45°, and 90°), number of layers (2, 4, and 6 layers), and slenderness ratio of specimens (15.45, 20, and 22.73). The test results showed that FCCC-R's axial compression behaviour improved significantly compared with pure rebar. The axial load-displacement curves of the FCCC-R specimens had a second ascending branch, and their carrying capacity and ductility were enhanced substantially. The best buckling behaviour was observed for the FRP winding angle of 90°. The capacity and ductility of the specimens were positively related to the number of FRP-wrapped layers and inversely related to the slenderness ratio of the specimens. A finite element model of FCCC-R was constructed and agreed well with the test results. The finite element model was used for parametric analysis to reveal the effect of the area ratio, FRP confinement length, internal bar eccentricity, and mortar strength on the axial compression behaviour of FCCC-R. The numerical results showed that the area ratio had the most significant impact on the axial compression behaviour of FCCC-R. The confinement length of the FRP pipe and internal bar eccentricity had similar effects on the axial compression behaviour of FCCC-R. Both of them had a significant impact on the second ascending branch, with the post-peak behaviour exhibiting minimal differences. The influence of mortar strength on the axial compression behaviour of FCCC-R was observed to be minimal.

Strengthening RC square columns with UHP-ECC section curvilinearization and FRP confinement: Concept and axial compression tests

In this paper, a novel column strengthening technique incorporating section curvilinearization (SC) using ultra-high performance engineered cementitious composites (UHP-ECC) and fiber-reinforced polymer (FRP) confinement is proposed and validated. Eleven large-scaled FRP-confined square reinforced concrete (RC) columns with SC using UHP-ECC as the additive material were prepared and tested. The main parameters investigated in this paper included the FRP thickness, the rise-to-span ratio, and the strength of the additive concrete. Test results show that the SC technique with FRP jacketing and UHP-ECC as the additive material enhances the load-bearing capacity of the RC columns by 52% with a 17% section increment. By assessing the existing design-oriented stress–strain models of confined concrete in curvilinearized square columns (CSCs) using the test results obtained in this study, it reveals that the current models could underestimate the ultimate strength of the confined concrete in CSCs with a two-layer FRP wrap, and therefore further research on the improvement on the stress–strain model of confined concrete in CSCs becomes necessary.

FRP-confined rubber concrete with effect of strain rate: Tests and analysis-oriented stress–strain model

Rubber concrete is attracting more and more investigations and applications in recent years, due to its environmental advantages and lightweight performance. As the replacement by rubber particles can reduce the concrete strength, an efficient and lightweight solution for strengthening and repairing rubber concrete is to apply fiber-reinforced polymer (FRP) confinement. This paper is the first to consider the influence of strain rate on the stress–strain relationship of rubber concrete with FRP confinement. To develop a stress–strain model of FRP-confined rubber concrete under different strain rates, an experimental program with 36 FRP-confined concrete specimens was conducted in this work. In the experiments, rubber particles were used to replace 0–30 % fine aggregates by volume. Strain rates of 3.3 × 10-5/s, 3.3 × 10-4/s, and 3.3 × 10-3/s were used to simulate three different strain rate levels of quasi-static loading, fast loading, and seismic loading, respectively. The experimental results showed that the compressive strength of FRP confined rubber concrete obviously increased with the strain rate increases, and decreased with an increasing rubber replacement ratio. Based on the experimental results, this paper proposed analytical models to predict the lateral strain-axial strain relationship, peak strength, and peak strain for FRP confined rubber concrete considering the strain rate influence. Finally, an analysis-oriented stress–strain model of FRP confined rubber concrete under different strain rates was established. The predicted results of the analysis-oriented stress–strain model showed good agreement with the experimental data.

Strength enhancement due to FRP confinement for coarse aggregate-free concretes

Concrete strength can be effectively enhanced by external fiber reinforced polymer (FRP) confinement. However, it is found from the literature that the strength enhancement coefficient (k1) varies significantly, especially for different types of concrete. This paper presents an in-depth investigation of the variety of strength enhancement coefficients with concrete types. First, two state-of-the-art databases for FRP-confined UHPC and FRP-confined ECC specimens are constructed in this paper. From analyzing the databases, although the mechanical performances and components are different for UHPC and ECC, nonetheless, their strength enhancement coefficients are quite similar (k1 = 2.27 and 2.01, respectively). With the validation by an experimental program on FRP-confined mortar specimens, it can be concluded that the similarity of strength enhancement coefficients for FRP-confined UHPC, ECC, and mortar, which are much lower than that for normal concrete, is caused by the lack of coarse aggregates. The reasons are discussed and well explained by a simplified experiment with steel balls and shear-friction mechanism. Based on the research, this paper proposed a unified strength enhancement coefficient, i.e., 2.13, for all coarse aggregate-free concretes. As the existence of coarse aggregates is found to be the main governing factor for the strength enhancement coefficient, this paper provides a fundamental basis for future research on confinement for next-generation coarse aggregate-free cementitious materials, such as lunarcrete and 3D-printed concrete.

Hybrid FRP-concrete-steel double-skin tubular columns of varying slenderness ratios under eccentric compression

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) are an emerging form of hybrid column with excellent mechanical performance and durability. Such columns consist of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes. While extensive experimental and theoretical studies have been conducted on such columns, these studies have been focused on short specimens under concentric compression. This paper presents the results of a series of tests on hybrid DSTCs of varying slenderness ratios subjected to eccentric compression. The effects of FRP confinement stiffness, load eccentricity, and slenderness ratio on the behavior of hybrid DSTCs were examined. The test results showed that the test hybrid DSTCs exhibited excellent performance with great ductility. A theoretical model with incorporations of existing stress-strain models for FRP-confined concrete was then adapted to simulate the behavior of the test hybrid DSTCs. The comparisons showed that the direct use of a concentric loading stress-strain model led to underestimation of the deformation capacity of the test hybrid DSTCs. Compared with the other two models, the eccentric-loading stress-strain model previously developed by the authors' group provided more accurate predictions for the ultimate deformation of the test hybrid DSTCs.