Steel Hoops Research Articles

Studies on stress-strain behaviour of concrete mixes confined with BFRP rebars

In the present study, the stress-stain behaviour of confined concrete made with basalt fibre reinforced polymer bars (BFRP) were taken up. The stress-strain behaviour was studied for the concrete mixes confined with steel rebars and BFRP rebars. The confinement was given in the form of steel hoops in the cylinders, 3 hoops (0.8%), 4 hoops (1.1%), 5 hoops (1.3%) and 6 hoops (1.6%). The addition of basalt fibres along with confinement of concrete with steel and BFRP hoops enhanced the compressive strength, indicating further confinement effect in the concrete. It is observed that the addition of fibres is helpful in lower confinements only. Beyond 1.1% confinement, the addition of any type of basalt fibres doesn’t show any effect on compressive strengths. From the stress-strain behaviour of all types of concrete mixes, it is concluded that the ultimate load-carrying capacity and strains at peak stresses are more in concrete with BFRP hoops for mixes up to 1.1% confinement. The addition of basalt fibres to concrete has increased the ductility in both confined and unconfined states

Experimental Study on the Mechanical Performance of Mortise-Tenon Joints Reinforced with Replaceable Flat-Steel Jackets

The mortise-tenon joint is an important hub transmitting and distributing external loads for load-bearing components (beams, columns et al.) in the ancient-timber frame structure system. However, the conventional steel hoop reinforcement methods often insert wood screws into the timber components. When the reinforced joint rotates greatly, the anchoring failure of the screws will cause damage to the timber joint. To solve this problem, this study proposes a detachable and replaceable non-destructive flat-steel jacket reinforcement method in which horizontal flat steel is placed in the center of the joint, and the bolt is extended to the outside of the timber beam. Nine 1:3.52 scaled straight-tenon joint specimens were subjected to monotonic loading of beam ends, including three unreinforced reference joints, three joints with flat-steel jacket and three carbon fiber-reinforced plastic (CFRP) reinforced joints. The mechanical behaviors of the novel joints with flat-steel jacket were experimentally studied by comparing with those of the joints without strengthening and retrofitted with CFRP, based on the failure modes, the initial stiffness, the ultimate bearing capacity, and the moment-rotation relationship curves. Results indicated that the mortise-tenon joints reinforced with flat-steel jackets maintain the original semi-rigid properties of the unreinforced mortise-tenon joints and can effectively prevent the tenon from pulling out. The initial stiffness and ultimate bearing capacity were improved markedly. The column and beam of the reinforced joints remain intact providing the reference for the practical application of joints reinforcement. The mortisetenon joints reinforced with CFRP lose the semi-rigid properties of the unreinforced tenon -mortise joints. The joints reinforced with CFRP have the largest initial stiffness, while the unreinforced joints have the lowest stiffness. The initial stiffness of the proposed joint is in between the joints reinforced with CFRP and unreinforced joints. The ultimate bearing capacity of the joints reinforced with flat-steel jacket is larger than the other two joints, whereas the unreinforced joints have the lowest ultimate bearing capacity.

Evaluating planner hoop steel plates as web reinforcement for deep reinforced concrete beams

An experimental attempt was conducted in this paper, aiming at enhancing the shear and flexural behaviors of deep beams, where steel plates were applied as web reinforcement. Three beams having the same details were prepared using normal concrete strength. Their dimensions were 1250 mm long, 300 mm deep, and 150 mm wide. The specimens were designed to fail in shear, and therefore, they were supplied with two bars with a 20 mm diameter in addition to the top and skin reinforcement. For web reinforcement, the steel plates were cut in dimensions of 4×20 mm and then were applied as closed rectangular stirrups with the longer side (20 mm) aligned in the plane of the beam section. The beams were inspected by applying four-point bending with three shear span to the effective depth ratios (a/d) of 0.75, 1.25 and 1.75. The experimental outcomes revealed that the shear strength, stiffness, and toughness of deep beams enhanced as the a/d ratio dropped.

Rapid repair of RC bridge columns with prestressed stainless-steel hoops and stainless-steel jackets

A fast repair method is developed for earthquake induced-flexure/shear damage at the bottom of the reinforced concrete (RC) columns. The method is a kind of hybrid jacket that employs a thin stainless steel jacket wrapped around the column and the outside stainless prestressed steel hoops. The prestressed steel hoops provide active confinement for the column and prevent premature buckling of the steel jacket. The stainless steel jacket provides confinement to the core concrete, and, as well as, prevent the prestressed hoops from penetrating the concrete to cause stress loss. A large-scale RC column was damaged during pseudo-static cyclic loads, firstly, and then repaired with this method as experimental validation. The pseudo-static cyclic test results from the original and repaired RC columns were compared for failure mode, hysteresis behavior, strength and stiffness degradation, ductility, energy dissipation, and damage status. The repair method was proved to be effective in restoring the structural behavior of the damaged columns to prevent bridge collapse. The ductility of the repaired specimens was 111.4% that of the original specimen; the lateral force capacity was 113.3% that of the original specimen. This repair method can be used to repair columns with repairable damage as an emergency repair, or construction method for new structures.

A new model for reinforced concrete columns strengthened with fibre-reinforced polymer

Much work has been published in recent years on the strength benefits of confining rectangular reinforced concrete columns less than 300 mm across and with an aspect ratio less than 3 with fibre-reinforced polymer wraps and anchors. However, there has been little published research on the effects of polymer confinement on larger, flatter columns. This paper provides a new strength-prediction model with a wider scope than those in the existing literature. Test results provided by two independent researchers for 44 specimens were used as a basis for the model expressions. The test database included columns with aspect ratios ranging from 1·5 to 4·0 and depths of section varying from 100 mm to 600 mm. The key parameters influencing the confined strength, including aspect ratio, number of polymer layers, hoop steel reinforcement and cross-sectional area, and configuration of polymer anchors, were all accounted for. For a sufficient amount of polymer confinement, the threshold values of the lightly and heavily confined columns were proposed to be 0·125 and 0·25, respectively. Finally, the accuracy of the proposed model was verified by comparing its predictions with the database, together with the predictions of other existing models.

Study on fire resistance of prefabricated concrete beam-column joint with end-plates

Purpose The purpose of this study is to put forward a high-strength bolt end-plate connection of prefabricated concrete beam-column joint and carry out the pseudo-static test. Design/methodology/approach ABAQUS finite element software is used to study the fire resistance performance of high-strength bolt end-plate prefabricated joint. This mainly considers the influence of axial compression ratio, screw preload, end-plate thickness and steel hoop thickness. Findings The results show that the thickness of end-plate and steel hoop has a certain effect on the fire resistance. The change of screw preload has little effect on the fire resistance limit. Compared with the cast-in-place concrete beam-column joint, the deformation trend of column-beam end of the fabricated joint is basically the same as that of cast-in-place joint. Originality/value To study the mechanical performance of this kind of joint more comprehensively, the finite element software is used to study the prefabricated concrete beam-column joint with end-plates, and the effects of axial compression ratio, screw preload, end-plate thickness and steel hoop thickness on the fire resistance of joints are mainly considered.

Experimental investigation and confinement model of composite confined concrete using steel jacket and prestressed steel hoop

This paper presents the results of an experimental study of the uniaxial compression behavior of circular section concrete columns confined by composite confinement (CC). The composite confinement includes prestressed steel hoops (PSH) active confinement and steel jackets (SJ) passive confinement. PSH can solve the problem of passive confinement stress lag, whereas SJ retains the prestressing well and prevents the PSH from penetrating the concrete or causing stress loss. The composite confined concrete specimens were tested with the variations of different initial prestress levels and steel hoop centerline spacing. Their behavior was compared with different types of confinement forms, including unconfined concrete (UC), active confined concrete (PSH confined concrete, PC), passive confined concrete (SJ confined concrete, SC). The results of the experiment show the composite confined concrete (CCC) had a higher bearing capacity and deformation ductility than the other groups of specimens. The initial prestress level increased from 0.3 to 0.4, the bearing capacity increased by 5.1%. As the distance between the centerlines of the steel hoop was reduced from 132 mm to 88 mm, the bearing capacity increased by 10.63%. The capacity of CCC is 1.61–1.78 times that of UC, 1.13–1.24 times that of SC, and 1.53–1.69 times that of PC. Based on the transverse reinforcement-confined concrete model of Mander, a composite confined concrete analysis model is proposed. Then the formula of bearing capacity of CCC column was established. The formula calculation results agree well with the experimental results. Compared with the SJ confined concrete, due to the presence of external PSH, the failure mode is changed, and the shear capacity of the composite confined concrete column is improved, thereby limiting the deformation of concrete and overcoming the tendency of diagonal cracks formation. This paper analyzes the shear failure mechanism of CCC based on the force balance model.

Axial performance of RC columns with glass-fiber grids as peripheral hoops

ABSTRACTThe present study aimed to investigate the significance and limitation for the practical application of high-flexibility glass-fiber (GF) grids as an alternative of conventional steel hoops in reinforced concrete (RC) columns. The application of GF grids was also tested for the jacket section to retrofit the deficient columns of which configuration is impractical due to the interference of the existing column. Five virgin columns and two RC jacketed columns were tested under concentric axial loads. Main test parameters were the arrangement type and amount of wrapped GF grids with the combination of conventional steel hoops. The effectiveness of GF grids on confining concrete was examined through the assessment of axial ductility ratios of columns and stress–strain curves of confined concrete. All columns exhibited the tensile fracture of GF grids immediately after the peak strength of columns, which accelerated the crushing of core concrete and buckling of longitudinal reinforcement. Thus, the axial ductility ratios measured in the present columns with GF grids were commonly lower than those of the tied columns with conventional steel hoops at the same transverse reinforcement index. Overall, a more elaborate treatment is required for improving the tensile strength and elongation capacity of GF grids to apply for an alternative of steel hoops in tied columns.

Axial stress–strain model developed for rectangular RC columns confined with FRP wraps and anchors

It is widely reported that using fiber-reinforced polymer (FRP) as confinement jackets for concrete can significantly enhance its strength and ductility. The confinement effectiveness for rectangular columns has revealed significant enhancements in their strengths, in particular for well-confined columns with a cross-sectional depth of <300 mm. However, for larger-sized columns with a section depth of larger than 300 mm and an aspect ratio of larger than 2.0, the confinement effectiveness is most limited. One of the key solutions to significantly increase the confinement effectiveness is by using a combination of FRP anchors and wraps. For good analysis and design of FRP confinement for rectangular RC columns, a model for the stress–strain behavior of rectangular reinforced concrete (RC) columns confined with FRP wraps and FRP anchors is provided in this paper. The model is mainly based on results from tests reported in recent studies, in which a series of axial compressive tests on a total of 80 FRP-confined rectangular RC columns of large size were conducted. The database included columns with aspect ratios ranging from 1.5 to 4.0. Furthermore, the depth of the section is up to 600 mm. key parameters such as the aspect ratio and size of cross section, number of layers of FRP wraps, volumetric ratio of internal hoop steel reinforcement, and cross-sectional area and configuration of anchors were all considered. All predicting expressions were validated against the test database used in the model calibration. In terms of average absolute error and mean square error, comparisons with other existing expressions were conducted, showing that the expressions of the present model reveal more accuracy. Finally, comparisons revealed the model can estimate the confinement effectiveness and predict the stress–strain relationships of FRP-confined rectangular RC columns with additional FRP anchorage.

Study on Stress-Strain effect of reinforced Metakaolin based GPC under compression

Abstract The main aim of the present study is to comparatively evaluate Stress-Strain properties of Reinforced alkaline activated Metakaolin based Geopolymer concrete with Conventional concrete of M25 grade prepared with 3 and 4 numbers of hoop steel reinforcement. For studying Stress-Strain behavior under compression totally the 36 cylinders were tested out of which the 24 cylinders was prepared with Metakaolin based GPC and the remaining with conventional concrete M25 Grade. The flexible think was measured in this study is that the volumetric percentage of Reinforcement. The results exposed that confinement with hoop reinforcement significantly enhanced the strength of Metakaolin based GPC but not more than OPC and also it was found that the rate of stress increases when the percentage of the number of hoop reinforcement increased, similarly in case of strain it was decreased when the percentage of the number of hoop reinforcement increased. Hence it was concluded that the Metakaolin based GPC is not profitable that using instead of OPC along with any major form of reinforcement work.

Research on the Influence of the Stiffening Plates on the Stress of Hold Hoop of Bent Cap

In order to improve the mechanical behaviour of bridge steel hoops, the plate shell finite element models of several steel hoops were established by using the general finite element software ABAQUS. Through changing the structural parameters of the stiffening plates, the influence of the stiffening plates on the mechanical properties of the steel hoops was explored. The calculation results show that the stress distribution at both ends of the steel hoop is uneven and there is a phenomenon of stress concentration. The spacing of stiffening plates has great influence on the mechanical properties of steel hoop. Some measures to improve the mechanical properties of steel hoop are given.

Experimental investigation of damaged circular reinforced concrete columns with pre-tensioned steel hoops

External confinement can be applied to improve the ductility and energy dissipation capacity of reinforced concrete columns. In this study, the seismic performances of the damaged and undamaged reinforced concrete columns with pre-tensioned steel hoops are investigated. Considering the effects of the damage rate and axial compression ratio, the pseudo-static test was carried on the eight experimental specimens with the reduced scale of 1:1.5. The experimental results show that the ductility and energy dissipation capacity of damaged and undamaged reinforced concrete columns can be effectively improved using the pre-tensioned steel hoops. Moreover, the size effects of columns and the effects of concrete strength are also examined. Finally, based on the experimental results and discussions, several design considerations of pre-tensioned steel hoops for damaged columns are proposed.

A new numerical model for polymer-confined rectangular concrete columns

A new numerical model to estimate the strength of unreinforced and reinforced concrete columns of rectangular cross-section confined with fibre-reinforced polymer wraps is introduced. The model is based on data from 456 tests compiled from 30 studies available in the technical literature. The test database covers unconfined concrete strengths varying from approximately 10 to 80 MPa and the width and depth of the cross-sections ranged between 90 and 500 mm. The effects of aspect ratio and size of the cross-section, effective rupture strain of the polymer, corner radius and internal hoop steel reinforcement are all accounted for in the model. Based on the test database, thresholds of lightly and heavily confined concrete are proposed to ensure that columns have a sufficient level of polymer confinement. Model predictions were found to be in good agreement with the test results.

A Stress-Strain Model Considering Bilinear and PostPeak Softening Behaviors for FRP-Confined RC Columns

Objectives: A model with more generalized scope than those in the literature has been proposed to predict the stressstrain response of Fiber-Reinforced Polymer (FRP) confined rectangular RC columns under axial compression. Methods/ Statistical Analysis: 255 columns with their experimental data were used as a basis for the model expressions. Their database included columns with aspect ratios ranging from 1.0 to 2.0. Furthermore, the cross sectional depth varied from 100 to 500 mm and the unconfined concrete strength varied from 18.3 to 51.5 MPa. Findings: Based on interpretation of the test results, bilinear and post-peak softening stress-strain responses are proposed as referencing models, respectively, for columns having a sectional depth of smaller or larger than 300 mm. The softening model is composed of a parabolic expression for describing the first stage of the response and two linear expressions for the second and third stages. The influences of aspect ratio and size of cross sections, number of FRP layers and internal hoop steel bars on the key features of the envelope stress-strain curve model (i.e. ultimate axial stress and strain) are all considered in the model. Application/ Improvements: Reasonable agreement was revealed between the analytical responses of the proposed model with the selected tested responses, confirming its ability to simulate the stress-strain behavior of FRP-confined rectangular RC columns of small or large sizes. Keywords: FRP-confined Concrete, Rectangular RC Columns, Stress-strain Model

Axial capacity of steel tube-reinforced concrete stub columns

The steel tube-reinforced concrete (ST-RC) column is an innovative composite structure that has been increasingly applied in high-rise buildings and bridge piers in China. This study aims to investigate the axial capacity of the ST-RC stub column by means of a modified superposition method. Primarily, the sectional confining force equilibrium was elaborated to explain the confinement mechanism of the ST-RC column under concentric compression, which involves the confining stress induced from the inner steel tube and secondary confining stress induced from the peripheral steel hoops. Thereafter, a modified method for predicting the axial capacity of the ST-RC column, considering the secondary confinement, was developed according to a stress and strain analysis. Thirdly, 41 published specimens were collected to verify the accuracy of the proposed method. It was demonstrated that the current method fitted strongly with the tests results and exhibited satisfactory adaptability. Finally, four other methods according to ACI 318, AIJ, EC 4, and CECS 188-2005 were verified in the same scenario. It was found that the current method exhibited superior correlation with the experimental results compared with conventional approaches. It will be favorable to obtain the viable axial capacity of the ST-RC column by using the presented solution.

Study on mechanical properties of a new type of prefabricated GRC mold shell

The GRC formwork structural column adopts the factory-based vertical prefabrication production process, which can reduce the floor space, reduce the formwork loss, speed up the construction progress, promote the full decoration of the prefabricated building, and improve the efficiency of the assembly construction. major. In order to optimize the production process of prefabricated GRC formwork column, the overall stress system of GRC formwork structure is analyzed in the concrete pouring process, and the thickness of GRC formwork, the number of steel hoops and the GRC mode are considered. The influence of the shell cross-section size on the mechanical properties. The research results can provide reference for the optimization and design of prefabricated GRC formwork column production process.

Development and validation of a FRP-wrapped spiral corrugated tube for seismic performance of circular concrete columns

This paper presents the cyclic behavior of circular concrete columns confined with a proposed FRP-wrapped spiral corrugated tube (FWSCT) that serves as a permanent ribbed surface between FRP and concrete. Five FWSCT column specimens with GFRP layers and axial load ratios as design parameters were tested for the flexural and shear performances. The FWSCT specimens that did not use transverse steel hoops in the entire column used longitudinal reinforcement for the flexural capacity and FWSCT for the shear capacity. Specimen FWSCT-0 was confined with only a spiral corrugated steel tube without GFRP wraps; Specimens FWSCT-1, FWSCT-3, FWSCT-5 and FWSCT-8 were confined with a spiral corrugated steel tube and additional 1, 3, 5 and 8 GFRP layers, respectively. Test results showed that Specimen FWSCT-0 experienced shear failure at about 1% drift; Specimen FWSCT-1 experienced flexural shear failure at about 4% drift, while Specimens FWSCT-3, FWSCT-5 and FWSCT-8 exhibited rupture of longitudinal reinforcement at drift ratios of 7% to 8%, without shear failure. A plastic hinge was developed in un-wrapped column ends, further extending into the footing. An analytical method that used the residual shear model and a proposed plastic hinge length reasonably predicts the flexural and shear capacities of FWSCT columns.

Modeling the axial compressive stress-strain behavior of CFRP-confined rectangular RC columns under monotonic and cyclic loading

This paper presents a stress-strain model for rectangular reinforced concrete (RC) columns confined with carbon fiber-reinforced polymer (CFRP) composites and subjected to cyclic axial compression. The model is mainly based on experimental observations and results from a series of tests reported in the companion study, in which 24 large-size CFRP-confined rectangular unreinforced and reinforced concrete columns under monotonic and cyclic axial compression were tested. Relative experimental results compiled from the available literature are also used. The test database covers unconfined concrete compressive strengths ranging from 17.6 MPa to 51.5 MPa and cross-sectional side lengths ranging from 90 mm to 400 mm. The proposed stress-strain model is composed of three main components: (1) a monotonic stress-strain model for the envelope curve of cyclic response; (2) a parabolic expression for the unloading path; and (3) a straight line for the reloading path. The model also considers the effects of key parameters including cross-sectional size, aspect ratio, effective rupture strain of the CFRP, and internal longitudinal and hoop steel reinforcement. The performance of the model is finally verified through comparison of predictions made by this model with selected test results, as well as the predictions of more than 20 models of other researchers.

Strut-and-tie model for interior RC beam-column joints with substandard details retrofitted with CFRP jackets

A softened strut-and-tie model (STM) is developed for interior reinforced concrete (RC) beam-column joints without any steel hoops in the joint or intermediate longitudinal column steel reinforcement, and discontinuous bottom beam flexural steel reinforcement. The STM model is extended to identical joints retrofitted with Carbon Fiber-Reinforced Polymer (CFRP) composites. The STM model is compared to experiments of two full-scale RC beam-column interior joints, one of which was retrofitted with CFRP composites. Failure modes for the original joint included anchorage failure of bottom steel beam bars, crushing of concrete nodal zones, diagonal joint shear failure, and for the retrofitted joint CFRP laminate delamination and crushing of joint core concrete. The STM model is based on the joint reinforcement details and experimental performance, including column axial load effects and the contribution of CFRP horizontal and diagonal laminates as tension ties. STM model assumptions were verified with strain gauge measurements. The STM model was successful in estimating the ultimate shear strength of the original and retrofitted joints. Recommendations are presented for evaluating the strut width for both original and retrofitted joints that include the quality of the bond of beam steel reinforcement to concrete in the beam-column joint.

Rehabilitation of earthquake damaged external RC beam‐column joints by joint enlargement using prestressed steel angles

SummaryThe effectiveness of a rehabilitation method based on joint enlargement using prestressed steel angles to enhance the seismic behavior of damaged external reinforced concrete beam‐column joints was experimentally investigated. Three half‐scale joints having either non‐seismic or seismic reinforcement details were tested both before and after rehabilitation by applying lateral cyclic loading of increasing amplitudes. Two defects were considered for the two non‐seismic units, being the absence of transverse steel hoops and insufficient bond capacity of beam bottom steel reinforcing bars in the joint panel zone. The damaged specimens were rehabilitated by injecting epoxy grout into existing cracks and installing stiffened steel angles at the re‐entrant corners of the beam‐column joint, both above and below the beam, that were mounted and held in place using prestressed high‐tensile strength bars. The test results indicated that the seismic performance of the rehabilitated specimens in terms of strength, stiffness, and ductility was fully recovered and comparable with the performance of the seismically detailed specimen. Copyright © 2016 John Wiley &amp; Sons, Ltd.