Load-deflection Curves Research Articles

Axial-flexural behaviours of concrete-filled square GFRP tubular columns under low temperatures

This study reported experimental investigations on the axial-flexural behaviours of concrete-filled square glass fiber-reinforced polymer (GFRP) tubular (CFSFT) columns at low temperatures. Twenty CFSFT columns under eccentric loads and two-point loading were conducted to evaluate the influences of low temperatures (T = 20 and −60 °C) and eccentricity ratios (e/r = 0.00, 0.25, 0.50, and 0.75). Failure modes, load–displacement behaviours, and load–strain behaviours at low temperatures have been reported. Test results indicated the load-vertical displacement curves of CFSFT columns under eccentric loads exhibited three different stages, i.e., linear, transition, and strain hardening or softening stage. Meanwhile, the bilinear load–deflection curves were observed for CFSFT columns under four-point bending. Based on these test results, the experimental axial load-moment (NT-MT) curves of CFSFT columns were obtained. These curves indicated that decreasing the temperature from 20 to −60 °C observably increased both the ultimate axial load (NuT) and ultimate bending moment (MuT) of CFSFT columns. However, the increment of both ultimate axial load and ultimate bending moment gradually decreased with the increasing eccentricity ratios.

Flexural performance and patterns identification of UHPC subjected to alternating elevated and cryogenic attacks with acoustic emission characterization

This paper investigates the flexural performance of ultra-high-performance concrete (UHPC) subjected to a simulated lunar ground temperature environment between 200 °C and −170 °C termed as alternating elevated and cryogenic attack (AECA). Additionally, the cracking behavior during the failure process was characterized by acoustic emission (AE) technique. Compared with unexposed UHPC specimens, those exposed to AECA up to five cycles still exhibit superior flexural performance. The AE results imply that tensile cracks are mainly generated by the failure of cement matrix, and shear cracks are mainly attributed to the steel fiber–matrix debonding and steel fiber pull-out behaviour. AECA exhibits the dual effect of enhancement and deterioration on the flexural performance of UHPCs, and there is even a competitive relationship between the two effects. Due to the dual effect, cement matrix strength and bond strength between cement and steel fiber show different responses to AECA. Finally, four prediction equations that matched excellently with the experimental results are proposed to estimate the residual flexural load and compute the flexural toughness directly according to Newton-Leibniz law instead of integrating the area under the load–deflection curve.

Flexural Performance of Steel Bar Reinforced Sea Sand Concrete Beams Exposed to Tidal Environment

The advancement of marine engineering has brought close attention to the durability of concrete structures. In order to investigate the time-varying performance of reinforced concrete beams in a marine environment and to better apply sea sand directly in marine engineering, this paper describes tests and analysis on the flexural performance of reinforced sea sand concrete beams after being exposed to a tidal environment. Eight beams were tested using four-point static loading equipment. The variation parameters included the type of mixing water, longitudinal reinforcement rate, sea sand replacement rate and duration of service. The force damage process and damage pattern were observed. The load–maximum width crack curve and load–deflection curve were obtained. The effects of each variation parameter on the mechanical properties such as ultimate bearing capacity, initial rigidity, energy dissipation coefficient and ductility coefficient were analyzed. The test results show that compared with the specimens exposed to the tidal environment for 90 days, the peak load of the specimens decreased by 5.6%, the initial rigidity decreased by 60.9% and the ductility coefficient decreased by 41% after 270 days of exposure, while the peak deflection and energy dissipation indexes first increased and then decreased. The seawater mixing can enhance the peak load and cracking load of the specimens, but the initial rigidity, peak deflection, energy dissipation coefficient and ductility coefficient of the specimens are reduced to some extent. The initial rigidity of the specimens tended to increase with the increase in the sea sand replacement, but the peak load decreased. Under the same reinforcement rate, reducing the diameter of the reinforcement is beneficial to improve the initial rigidity of the specimen, while using the reinforcement with higher elongation can effectively enhance the peak deflection of the specimen. Based on the Chinese code, the calculation method of flexural bearing capacity with modified concrete strength is proposed, and the calculation results are in good agreement with the test results.

Comparison of the Efficiencies of Web Cutouts for Steel Plate Girders with Corrugated Webs Versus Flat Webs Under Pure Shear

Abstract Corrugated steel-plate girders are used as structural elements in many applications because of their properties, cutouts are always provided in these plate elements to enable inspection and servicing. This study presents an experimental and analytical study to investigate the efficiency of a steel-plate girder with corrugated webs (GCWs) and flat webs (GFWs) having cutouts under shear loading. In this study, we investigated the influence of some important parameters on the girders’ load-bearing capacity. The experimental program was conducted on six full-scale plate girders, which have been tested under central load at mid-span. The analysis was conducted using ANSYS V20 to perform a nonlinear technique for the determination of the ultimate load of the tested girders. Finally, experimental and finite element model were used to define the ratio of increasing carrying capacity in plate girder with flat and corrugated web, having cutout under shear loading. The ultimate strength, failure mechanism, and load-deflection curves from the experimental and analytical study show that the shear capacity of the girder with corrugated web girders extends loads by 22 % compared with the flat web.

Flexural Behaviors of Modified Recycled Aggregate Concrete Reinforced with Pultruded GFRP

Corrosion of steel reinforcement in underwater concrete structures continues to be a problem in the modern era. The possibility of employing pultruded glass fibre reinforced polymer (GRFP) I beams to replace traditional steel reinforcement in an encased beam is still being explored. The purpose of this research is to further investigate the effects of incorporating treated recycled concrete aggregate (RCA) and discrete synthetic fibres, specifically Polyolefin (PO) and Fibrillated Polypropylene (PP) fibres, in the production of concrete beam reinforced with GFRP I beam for underwater construction. Five beams were constructed and tested in this investigation using a four-point loading arrangement. The load–deflection curve, stress–strain curve, and initial and ultimate fracture loads are all included in the test parameter set. The results indicate that treating RCA and fibres greatly increases the flexural capacity of modified recycle aggregate concrete (RAC) beams compared to unmodified RAC beams.

Effect of Fiber Content on Mechanical Properties and Microstructural Characteristics of Alkali Resistant Glass Fiber Reinforced Concrete

Owing to its enhanced strength, ductility, and resistance to harsh environments, increasing research attention has been paid to alkali-resistant glass fiber reinforced concrete (ARGFRC). This paper presents experimental studies concerning the effects of fiber content on the mechanical properties and microstructural characteristics of ARGFRC. The amount of glass fiber was considered at levels of 0.0, 0.3, 0.5, 0.8, 1.0, 1.3, and 1.5% of the concrete volume. The compression, flexural, impact resistance, scanning electron microscopy, and energy dispersive spectroscopy tests were conducted. The flexural load-deflection curve, flexural strength, flexural toughness index, flexural fracture energy, postcracking stiffness, postpeak stiffness, and impact resistance energy absorption were obtained. Then the changing law affected by fiber content on these mechanical properties was further analyzed, and the corresponding equation was fitted. When fiber content was 1.5%, the flexural toughness index I5, I10, and I20 values were 4.0, 5.9, and 8.9, respectively, and increased by 3.0∼7.9 times. Glass fiber incorporation could increase the ductility and delay the brittle failure when the fiber content reached 0.8%. The largest postcracking stiffness was calculated at 36.174 kN/mm with a fiber content of 0.8%. The higher the fiber content, the larger the postpeak stiffness of the tested beams. Impact resistance test results demonstrated that the optimum fiber content was 1.3%. As the fiber content increased, the effect of the concrete grout on the fiber packaging decreased, according to the scanning electron microscopy analysis. The energy dispersive spectroscopy observation proved that adding a certain fiber content did not affect the concrete hydration reaction.

Experimental Study and Finite Element Analysis on the Flexural Behavior of Steel Fiber Reinforced Recycled Aggregate Concrete Beams.

This paper reports on the flexural behavior of nine steel fiber reinforced-recycled aggregate concrete (SFRAC) beams through combined experimental and finite element analysis. The test parameters in this study include the steel fiber volume fraction, recycled aggregate replacement ratio, and concrete strength. The failure modes, crack development, load-deflection curves, and flexural bearing capacity of SFRAC beams are investigated in detail. The test results indicated that cracks and concrete crushing are formed in the compression zone of all specimens. The flexural bearing capacity of SFRAC beams increases with the increase of steel fiber volume fraction and concrete strength and the decrease of recycled aggregate replacement ratio. In addition, the results are compared with those of the finite element analysis. Based on the uniaxial compressive constitutive model of SFRAC, a new model for calculating the flexural bearing capacity of SFRAC beams is proposed. The prediction and test results are compared to evaluate the accuracy of the developed formula. The studies may provide a considerable reference for designing this type of structure in engineering practice.

Torsional buckling and postbuckling behavior of stiffened FG-GRCL toroidal shell segments surrounded by elastic foundation

In this study, a new algorithm for the nonlinear buckling behavior of the stiffened functionally graded graphene-reinforced composite laminated (FG-GRCL) toroidal shell segments in the thermal environment surrounded by the elastic foundation and subjected to torsional load is introduced. The FG-GRCL shell segment is stiffened by the FG-GRCL ring and/or stringer stiffener system. Based on the Donnell shell theory with the Stein and McElman approximation, considering the von Karman geometrical nonlinearity and the anisotropic smeared stiffener technique for FG-GRCL stiffeners, the formulations to investigate the critical buckling load and postbuckling torsional load-deflection curves are determined. A new algorithm is fully developed by adding the effect of circumferential stress into the stress function. The significant effects of temperature, stiffener system, geometrical parameters, and elastic foundation are considered and studied in detail.

Experimental and Finite Element Study on Bending Performance of Glulam-Concrete Composite Beam Reinforced with Timber Board.

In this research, experimental research and finite element modelling of glulam-concrete composite (GCC) beams were undertaken to study the flexural properties of composite beams containing timber board interlayers. The experimental results demonstrated that the failure mechanism of the GCC beam was the combination of bend and tensile failure of the glulam beam. The three-dimensional non linear finite element model was confirmed by comparing the load-deflection curve and load-interface slip curve with the experimental results. Parametric analyses were completed to explore the impacts of the glulam beam height, shear connector spacing, timber board interlayer thickness and concrete slab thickness on the flexural properties of composite beams. The numerical outcomes revealed that with an increase of glulam beam height, the bending bearing capacity and flexural stiffness of the composite beams were significantly improved. The timber boards were placed on top of the glulam members and used as the formwork for concrete slab casting. In addition, the flexural properties of composite beams were improved with the increase of the timber board thickness. With the elevation of the shear connector spacing, the ultimate bearing capacity and bending stiffness of composite beams were decreased. The bending bearing capacity and flexural rigidity of the GCC beams were ameliorated with the increase of concrete slab thickness.

Experimental analysis of corroded RC continuous beams rehabilitated by ICCP-SS under unsymmetrical loading

In this study, a total of 14 corroded reinforced concrete (RC) continuous beams rehabilitated by a dual function system—Impressed Current Cathodic Protection-Structural Strengthening (ICCP-SS) with carbon-fabric reinforced cementitious matrix (C-FRCM), were tested under in-plane bending (resulted from unsymmetrical loading) to investigate the influences of corrosion rate, layer of carbon fabric mesh, complete wrapping as the end anchorage and loading scheme. The flexural behaviour including failure modes, load-carrying capacities, load-deflection curves and ductilities of the specimens were carefully studied. The design flexural strengths were calculated based on the current design guidelines, which were compared with the experimental results. It’s shown from the experimental data of five-point bending tests that beams strengthened with C-FRCM had higher yielding loads and ultimate loads than corroded beams without rehabilitation. However, the ductility of RC continuous beams was weakened by the C-FRCM strengthening method and its anodic polarization process, in particular for the specimens with more layers of carbon fabric meshes in prefabricated C-FRCM plates and complete wrapping as the end anchorage. Under unsymmetrical loading scheme, the delamination failure of concrete cover was observed and the application of complete wrappings was found to improve the ductility of the specimen. Comparison of the measured and predicted moment capacities showed that the current design guidelines generally underestimate the moment capacities of C-FRCM strengthened RC continuous beams, especially the specimens further strengthened by complete wrapping.

Single piles under monotonic lateral loads-Full scale tests and numerical modelling

This paper aims to present the results of the analysis of full-scale lateral loading tests of a single pile driven into a bi-layered soil located in Plancoet (France). The pile was subjected to a monotonic sequence of loads, and the P-Y curves along the pile were derived and compared to those recommended by the current design methods. Displacement analysis was then undertaken by inputting these P-Y curves, as well as those currently recommended in the literature, into the software SPULL to predict the load-deflection curve. The calibration process was conducted by a 3D finite element model using Abaqus software based on the elastic-perfectly plastic Mohr–Coulomb constitutive model, and the surface-to-surface contact method was used to take into account the nonlinear response at the pile/soil interface. The determined soil elastic modulus was used to predict the pile deflections by using some usual elasticity-based methods, which led to good prediction of the small pile deflections.

Shear failure mechanism of local buckling-dominated large-scale corrugated steel web

The plate thickness and corrugation wavelength of the corrugated steel web (CSW) used in bridge girders are significantly enlarged compared to the corrugated steel plate in buildings, resulting in a higher elastic critical shear buckling stress than the shear yield stress, and hence the failure may occur in an elastoplastic state. This paper investigates the shear failure mechanism of local buckling-dominated large-scale CSWs used in bridge girders. Shear tests of three beams with standard large-scale CSWs and corresponding nonlinear finite element analyses are carried out. The failure mode, typical load–deflection curve, out-of-plane deformation, and strain distribution are obtained. Besides, the effects of initial imperfections on shear behaviors are analyzed in detail. The results indicate that the shear failure of local buckling-dominated large-scale CSW presents elastoplastic buckling of two adjacent sub-panels across the fold interactions, which shows typical characteristics of snap-through instability. The shear loading process can be divided into four stages: elastic stage, elastoplastic stage, buckling stage, and post-buckling stage. The large-scale CSW belongs to a type of imperfection-sensitive member. The initial imperfections make the local sub-panel enters a plastic state before the nominal shear yield load is reached and the shear stress no longer uniformly distributes over the web height. The ultimate shear capacity and initial yield load decrease significantly with the increase of the imperfection amplitude, and the out-of-plane deformation increases with the imperfection amplitude. The elastoplastic shear buckling mechanism of local buckling-dominated large-scale CSWs is explained based on a rigid-bar frame model.

Reliability of toe-nail connections in a gable roof houses under uplift wind loads

The latest devastating hurricanes have caused considerable damage to light frame wooden houses. The detachment of the roofs because of the high uplift wind loads is one of the typical damages. This detachment often occurs at the contact connections in between the roof trusses and the walls of the houses. For several decades, toe-nail connections, which are poor in uplift resistance, have been used in those contacts. The purpose of this paper is to develop an efficient numerical model for performing reliability assessment of wooden roofs under uplift wind loads taking into account the statistical variability in both the load–deflection curve of toe-nails and the uplift wind load. The model utilizes a three-dimensional semi-analytical solution previously developed to predict the behaviour and failure of wood roofs under spatially variable uplift loads. The random characteristics of the toe-nails were calculated based on experimental results found in the literature and were statistically simulated using a beta distribution. The randomness of the wind speed was evaluated using a normal distribution. A full-scale gable roof was considered as a case study to determine the probability of roof failure at different wind speeds, Monte Carlo simulations along with the semi-analytical model were used to obtain the fragility curve based on two different criteria. It was found that beyond a mean wind velocity of 32 m/s, the probability of roof failure increases rapidly.

Experimental study on flexural behavior of hollow steel-UHPC composite bridge deck

The application of UHPC might reduce the weight of the bridge deck and improve its crack resistance and durability. This study proposes a hollow steel-UHPC composite deck composed of the hollow steel tube and UHPC without shear connectors. The flexural test was performed on the hollow steel-UHPC composite deck to evaluate the effects of the embedded steel tubes, steel bars, UHPC material, and flange thickness. In addition, the mechanical properties of hollow steel-UHPC composite decks were compared with waffle decks and solid concrete decks. Results indicated that the hollow steel-UHPC composite deck had excellent bending capacity and material utilization. The load-deflection curve of the composite decks could be divided into four stages: elastic stage, working stage with cracks, nonlinear stage, and fully plastic stage. Without any additional shear connection, the steel-UHPC interface slip did not occur until the load reached 0.89Pu. In the ultimate limit state, the strains at the mid-span cross-section of steel and UHPC were almost the same, suggesting that there was a good composite effect between the embedded steel tube and the outer UHPC. The embedded steel tube had a significant influence on improving the original stiffness (K0), the stiffness at the working stage with cracks (K1), and the flexural capacity (Pu) of composite decks. The flexural failure of the hollow steel-UHPC composite deck was controlled by the tensile zone. Finally, a theoretical formula was proposed to predict the flexural capacity of the hollow steel-UHPC composite deck.

Thermal Buckling and Postbuckling Behaviors of Couple Stress and Surface Energy-Enriched FG-CNTR Nanobeams

Small-sized structural elements such as beams, plates, and shells are usually used as nanomechanical resonators, nanoscale mass sensors, nanoelectromechanical actuators, and nanoenergy harvesters. At the nanoscale, the structures usually possess a high surface area-to-bulk volume ratio, leading to the free energy related to surface atoms becoming considerable compared to that of the bulk part. Earlier reports indicated several physical reasons for size-dependent phenomena, e.g., nonlocal stress, surface energy, and couple stress. To provide an in-depth insight into the mechanical behavior of small-scale structures, size-dependent continuum models including two or more physical factors have attracted the attention of the academic community. This research analyzes the thermal buckling and postbuckling characteristics of functionally graded carbon nanotube-reinforced (FG-CNTR) nanobeams with a tri-parameter, nonlinear elastic foundation and subjected to a uniform temperature rise. Chen-Yao’s surface energy theory and Yang’s symmetrical couple stress theory are combined to capture two types of size effects in nanobeams. The postbuckling model is formulated based on the Euler–Bernoulli deformation hypothesis and Euler–Lagrange equation. Using a two-step perturbation technique, the related postbuckling equilibrium path is determined. In numerical analysis, the impacts of surface energy, couple stress, elastic foundation, boundary conditions, geometric factor, layout type, and volume fraction of CNTs on the thermal buckling and postbuckling behaviors of nanobeams are revealed. It is indicated that considering couple stress or surface energy can lead to a significant increase in the postbuckling stability of nanobeams compared to the case in which it is not considered. In addition, there is a reverse competition between couple stress or surface energy effects on the thermal buckling responses of nanobeams. As the temperature rise will cause the material elastic moduli softening, the thermal buckling load–deflection curves of nanobeams with the temperature-independent case are much higher than those with the temperature-dependent cases.

Flexural–Shear Performance of Lightweight Concrete Panels with High Insulation Capacity

With the increased interest in the inherent fire resistance of organic insulators, various precast concrete insulation panels have been developed. However, precast concrete insulation panels still have structural and fire resistance problems resulting from a low composite action and unclosed cross-sectional details. To improve composite action and fire resistance, this study proposes the closed cross-sectional details of insulator panels with lightweight aggregate concrete, insulation material, and wire mesh. The objective of this study is to examine the flexural–shear performance of precast lightweight concrete panels with closed cross-sectional details developed for exterior cladding with high insulation capacity. Six full-sized insulation panels were tested under two-point top loadings. The main investigated test parameters to vary the moment–shear ratio of the insulation panels were the amount of the shear reinforcement and shear span–effective depth ratio. Test results indicate that the insulation panels with moment–shear ratios of 2.60 or higher were governed by shear, indicating that the longitudinal bars remained in an elastic state until the peak load of the insulation panels was reached. Thus, an increase in the moment–shear ratio of the insulation panels led to more brittle failure characteristics. Meanwhile, the insulation panels governed by flexure exhibited plastic flow performance in the applied load–deflection curve and well-distributed cracks. In particular, the maximal flexural moments of insulation panels with moment–shear ratios of 0.75 or less were higher than those calculated from the equations specified in ACI 318-19, indicating that the composite action was fully exerted. Overall, the developed insulation panels with cross-sectional details must be designed to a have moment–shear ratio of 0.75 or less to fulfil the ductile response under extreme lateral loads and exert full composite action.

Valorization of macro fibers recycled from decommissioned turbine blades as discrete reinforcement in concrete

The extensive use of glass fiber-reinforced polymer (GFRP) composites has inevitably resulted in a large amount of FRP waste, posing a significant environmental threat. A recent study performed by the authors’ group of the present study pioneered a new mechanical method of recycling GFRP wind turbine blades into macro fibers, in which the macro fibers characterized by a fixed-length have been produced using a manual process of low efficiency and high cost, making it impossible for use in a practical application. In the present study, a shredding machine has been therefore used to efficiently process waste GFRP wind turbine blades into macro fibers of hybrid lengths lesser than 100 mm for being incorporated into concrete. A series of tests were carried out to investigate the properties of the resulting concrete, and the test results of beam specimens were then analyzed using a twice inverse analysis approach. The results of compression tests and four-point bending tests showed that the incorporation of recycled macro fibers led to a slump loss of 54%, a compressive strength reduction of 14.07%, a flexural strength improvement of 37.85% and a significant flexural toughness enhancement of 36.8 times at a fiber volume ratio of 2.5%, as compared to those of plain concrete. The direct-tensile strength and the corresponding tensile strain obtained by a twice inverse analysis approach were about 2.26 MPa and 134 με, respectively, as predicted by the inverse analysis based on flexural load-deflection curves. The macro fibers processed using a shredding machine are feasible for enhancing the performance of the resulting concrete, and can be economic-efficiently used for industrial scale applications.

Experimental, Analytical, and Numerical Assessments for the Controversial Elastic Stiffness Enhancement of CFRP-Strengthened Timber Beams.

The strengthening of timber beams with carbon-fibre-reinforced polymers (CFRP) has been widely used in the last decades to enhance the behaviour of historical or new timber structures, usually for bending. While considerable improvement in capacity and ductility is typically achieved, the increase in stiffness was, in many cases, well short of analytical expectations, which tend to overestimate stiffness. This study addresses the problem by investigating the underlying mechanical behaviour using experimental, analytical, and numerical tools on a sample of Norway spruce (Picea abies) beams reinforced with carbon-fibre fabric. In the experimental program, each beam is tested for bending with and without CFRP reinforcement in order to determine specimen-specific stiffness increase on an individual basis. The reinforcement yielded an increase of in capacity, in ultimate displacement, and in compliance, verifying its efficiency. Axial compression tests on an independent sample are also performed to verify modulus of elasticity in compression. Numerical computations based on a beam model and a three-dimensional finite element model are performed with the introduction of separate moduli of elasticity for tension and compression in timber. Inverse computation using the experimental load–deflection curves yielded the moduli and the compression yield stress of timber to provide the best match between tests and simulations. The mean difference of only in stiffness between FEM and the tests is obtained. The dominance of normal stresses in the longitudinal direction is found, in correspondence with the experimentally observed tensile failure of timber (apart from a few defected specimens). Compression yield stresses are within (beam model) and (FEM) error compared with the control axial tests. The differences between FE simulations and tests in ultimate load and compliance are within . This study concludes that the application of CFRP in the composite beams enables the determination of timber material properties opposed to pure timber beams without reinforcement, and the adoption of separate moduli of elasticity for tension and compression leads to adequate modelling of reinforced timber beams.

Experimental and numerical investigation on the flexural behavior of a large-scale prestressed UHPC T-Shaped girder

A large scale (10 m long) prestressed ultra-high-performance-concrete (UHPC) T-shaped girder was experimentally investigated in this paper to study its flexural behavior, cracking moment, and ultimate flexural capacity through the bending test. By considering contribution of the tensile strength of UHPC, this paper suggested a modified tensile constitutive model of UHPC to achieve a reliable cracking moment and ultimate flexural design scheme for the prestressed UHPC girders. Effectiveness of this design scheme was demonstrated through the comparative studies on the developed load–deflection curves and ultimate flexural capacity of the girder obtained from the experimental results and that from the suggested design scheme. In addition, this study developed a nonlinear numerical analysis program to investigate the nonlinear flexural performance of the girder, and its flexural capacity calculated by using this program agreed well with the experimental results. Moreover, parametric studies regarding the influences of reinforcement ratios, prestresses of the prestressing tendons, and girder depths on the girder’s flexural resistance were also conducted. Finally, it is concluded that (1) UHPC girder exhibited good ductility and excellent flexural deformation capacity and cracking performance; (2) the tensile strength of UHPC has limited contribution to the ultimate flexural capacity of UHPC girders; and (3) appropriately increasing reinforcement ratio of the prestressing tendons, ultrahigh compressive strength of UHPC could be fully utilized, and thus the cracking moment and ultimate flexural capacity of UHPC girders could be effectively improved.

Flexural behavior of concrete beams reinforced by partially unbonded steel-FRP composite bars

Fiber-reinforced polymers (FRPs) are linear elastic materials that are widely used to reinforce newly built bridges and strengthen existing structures due to its high strength and good durability. To avoid brittle failure, the strength utilization of FRPs in practical applications is relatively low. A steel-FRP composite bar (SFCB) is a composite bar compound of an inner steel bar and outer longitudinal FRPs; its advantages include a good initial modulus, high durability and controllable postyield stiffness. When a concrete structure is reinforced by SFCBs, due to the limited rupture strain of the SFCB’s outer FRP, the deformability of the corresponding member is often controlled by the rupture of the FRP. To improve the deformability of an SFCB beam, an experimental study on concrete beams with partially unbonded SFCBs was conducted, and the failure modes, load–deflection curves and strain distribution were investigated. The test results showed that with partial unbonding treatment of the SFCB, the failure mode involving rupture of the SFCB’s outer FRP during SFCB reinforced concrete failure can be delayed or transferred into failure due to concrete crushing after the SFCB’s inner steel bar yielded, and the corresponding deformability and energy ductility can be improved. Then, a numerical 3D model was built and validated with the test results, and a parametric study of partially unbonded SFCB beams with different parameters was carried out. The simulation results show that the partial unbonding treatment can increase the deformability of the SFCB beam (with rupture of FRP) by two times while maintaining the stability of the load carrying capacity. When the unbonded length lub is between 0.5D and 1.0D (D is the height of the concrete beam), balanced failure of the SFCB beam is more likely to occur (the postyield stiffness is between 0.20 and 0.25). An excessive unbonded length lub will prematurely lower the depth of the compression zone, and the corresponding bearing capacity will decrease. As a result, when designing an SFCB reinforced concrete beam, the postyield stiffness ratio, reinforcement ratio and unbonded length should be comprehensively considered.