Steel Hoops Research Articles

Damaged mortise–tenon joint strengthened using an assembled spring–steel hoop based on a design of similar rotational stiffness

The excessive enhancement of rotational stiffness leads to an uneven distribution of rotational stiffness across the entire timber frame. This uneven distribution concentrates the load effect on the strengthened mortise–tenon joints (MJs), causing localized failure in advance. To address this issue, a reversible and non-destructive strengthening method for the damaged MJs is devised, involving the utilization of an assembled spring-steel hoop. The rotational stiffness of a strengthened MJ is derived from the springs that link the steel hoop of the beam and column, resembling the rotational stiffness of an intact MJ. A comparison of the measured moment-rotation curves of various types of MJ reveals that the rotational stiffness similarity necessitates the strengthened MJ's yield moment and ultimate moment, along with the associated rotation angles, to closely resemble those of the intact MJ. Secondly, a design methodology is developed for the assembled spring-steel hoop, which involves determining the parameters of the connecting spring. The comparative analysis of the hysteretic test results on multiple strengthened and intact MJs indicates that the strengthened MJs exhibit rotational stiffness deviations of less than 12.2 % during the elasticity stage and 6.6 % during the plasticity stage, in comparison to the intact MJs. A spring exhibits a lower ultimate extension compared to timber, leading to a maximum discrepancy of 37.5 % in the ductility coefficients between the strengthened MJs and intact MJs. The strengthened and intact MJs display similar patterns of stiffness reduction and energy dissipation progression.

Finite Element Analysis of Seismic Performance of RCS Hybrid Joints of Steel Hoop Structure

Reinforced concrete column-steel beam (RCS) composite structures are widely used in super highrise buildings due to their excellent performance. In this paper, the numerical simulation analysis of the RCS hybrid joints of “beam through” and “column through” RCS constructed with steel hoop structure are carried out, and the influence of five different parameters, i. e. , flange width-thickness ratio, steel yield strength, axial compression ratio, concrete strength and end plate thickness, on the seismic performance and failure mode of RCS hybrid joints is investigated. The results show that the ABAQUS finite element model can simulate the performance of RCS mixing under static load through reasonable parameter settings, which is in good agreement with the experimental results. Under the mechanism of “strong column and weak beam”, the combined node firstly undergoes beam-hinge damage, and the change of the axial compression ratio and the concrete strength parameter of the joint have little effect on the ultimate bearing capacity and deformation performance of the joint. Increasing the flange width-thickness ratio reduces the bearing capacity of the joint by up to 11%, and increases the yield strength of the steel, and increases the bearing capacity of the joint by 36. 4%. The thickness of the end plate of the column through specimen is increased, and the bearing capacity and deformation performance of the joint are improved to a certain extent. The changes of each parameter have no significant effect on the final plastic hinge failure mode of the joint. The research in this paper provides theoretical support for the design and research of this type of node in the future.

Shear behavior of circular concrete short columns reinforced with GFRP longitudinal bars and CFRP grid stirrups

Although fiber-reinforced polymer (FRP) reinforcements have become popular, the post-pultrusion bending leads to significant deficiency in bent portions of FRP hoops and spirals. This study adopts flexible carbon FRP (CFRP) grids as stirrups, and investigates the shear behavior of circular short columns reinforced with GFRP bars and CFRP grid stirrups. Six specimens with different shear reinforcement ratios, axial load ratios and types of stirrups (CFRP grids and steel hoops) were tested to shear failures. The failure modes, crack and strain developments, and shear load-horizontal displacement responses were presented for discussion. All specimens experienced shear-compression failures, and the critical diagonal crack widths increased approximately linearly with shear loads. The typical shear load-horizontal displacement curve consisted of three branches: a linear elastic branch, a cracking branch with progressive slope deteriorations, and an almost flat failure branch. Increasing CFRP grid stirrup ratio could not only enhance strength and ductility performance, but also alleviate the slope degradations, while increasing axial load ratios could enhance the initial cracking loads, restrain crack propagations, and reduce the lateral displacements. In terms of strength performance, steel hoops could be replaced by CFRP grid stirrups even with smaller stirrup ratio. Additionally, analytical investigations suggested that the effective stresses and strains of CFRP grid stirrups at failure may be lower than code limits.

Experimental Study on the Flexural Behavior of Connected Precast Concrete Square Piles

Part of deep foundation pit support engineering needs to select connected precast concrete square piles (CPCSPs). Under the premise that the quality of precast concrete square piles (PCSPs) meets engineering requirements, the quality of CPCSPs becomes the key factor to ensure the safety of foundation pit support structures. This paper puts forward a new connection technology of CPCSPs and carries out the flexural behavior experiment of unconnected precast concrete square piles (UPCSPs) and CPCSPs. The distribution of crack and strain on different surfaces of UPCSPs and CPCSPs are measured by carbon fiber composite strain sense optical cables, glass fiber composite strain sense optical cables, and fixed-point polyurethane strain sense optical cables. The anti-crack load, ultimate load, bending moment, and flexural deformation of UPCSPs and CPCSPs are measured. The experimental results of UPCSPs and CPCSPs are compared. The results show that the anti-crack strength of CPCSPs is greatly increased while the flexural deformation of CPCSPs is decreased before the occurrence of crack. With the development of crack (failure stage), the outside areas of hoop steel plate exhibit cracks. At this moment, the strength of CPCSPs is no longer controlled by the strength of middle areas. The ultimate strength of CPCSPs is basically equivalent to that of UPCSPs. The ultimate bending moment of CPCSPs is higher than its design value (about 66%∼76%). The selection of CPCSPs in the design of foundation pit support has good reliability.

Curvature Ductility Factor of Reinforced Concrete Beams with Confinement under Different Strain Rates of Loading

The purpose of this work is to investigate the influence of confinement on the curvature ductility factor of reinforced concrete beams at low and high strain rates of loading. The curvature ductility of beams is affected by the tension reinforcement ratio, the compression-reinforced ratio, the compression strength of concrete <img src=image/14828877_01.gif>, and the yield strength of steel <img src=image/14828877_02.gif>. The degree of transverse reinforcement is another component that determines beam flexural behavior. A model of steel and restricted concrete under varying strain rates of loading was utilized to compute the curvature ductility factor. The reinforced concrete section is studied in this research to determine the confinement of the beam and the different strain rates of loading. The ratio of the volume of rectangular steel hoops to the volume of the concrete core, <img src=image/14828877_03.gif>, represents the confinement. Six values of <img src=image/14828877_03.gif> are investigated to ensure an acceptable degree of ductility capacity. It is concluded that the ACI-Code balanced reinforcement ratio is impacted by confinement, and that it is lower than the ratio achieved when confinement is present. In this work, specific values of the curvature ductility factor for beam sections constructed with the ACI code were reported for <img src=image/14828877_02.gif>=60 ksi (414 MPa), <img src=image/14828877_01.gif>=4 ksi (27.6 MPa). Furthermore, the maximum quantity of tension reinforcement max influences the curvature ductility factor.

Analysis-oriented model for partially FRP-and-steel-confined circular RC columns under compression

Even though analysis-oriented models exist to simulate the axial and dilation behavior of reinforced concrete (RC) columns strengthened with fiber-reinforced-polymer (FRP) full confinement arrangements, a reliable model developed/calibrated for FRP partially imposed confinements is not yet available, identified as a research gap. Therefore, this paper is dedicated to the development of a new analysis-oriented model generalized for fully and partially confined RC columns under compression. In addition to vertical arching action phenomenon, the influence of the concrete expansion distribution along the column height on confining stress is considered in the establishment of the combined confinement from FRP strips and steel transverse reinforcements. A new unified dilation model is proposed, where the substantial effect of additional axial deformations induced by damage evolution in unwrapped zones is formulated by considering available experimental results. This model is coupled with an axial stress-strain formulation that includes a new failure surface function for simulating the dual confinement-induced enhancements, which are strongly dependent on the confinement stiffness. The developed model considers the influence of partially imposed confinement strategy on the axial and dilation behavior of RC columns, whose validation is demonstrated by simulating several experimental tests. Lastly, a parametric study is performed to evidence the dependence of FRP-steel confinement-induced enhancements on steel hoop and FRP spacing, and on the concrete compressive strength.

Cyclic axial compressive performance of the RC columns reinforced with FRP confined concrete core encased rebar

During an earthquake, the collapse of a structure is generally controlled by the failure or damage of its columns. It is important to enhance the seismic performance of reinforced concrete (RC) columns, including load capacity, ductility, and energy dissipation capacity. Column configurations with several small confined concrete components, namely, the confined concrete cores in this paper, are regarded as an effective way to improve the mechanical performance of square RC columns based on previous studies. The external confining material of a confined concrete core can be steel spirals, steel tubes, or FRP tubes. In this study, a new component, namely, FRP confined concrete core encased rebar (FCCC-R), and a new member, i.e., an RC column reinforced with FCCC-R were investigated. Eight RC columns with/without FCCC-R were cast and tested under cyclic axial compressive loading. The test result indicates that the RC columns reinforced with FCCC-R generally have a much higher axial load capacity. The axial load-displacement curves of all the tested specimens are compared, and the influences of the steel hoops, FCCC-R, and center FCCC are discussed. Moreover, the behaviors of different parts in a hybrid column, including steel hoops, and FRP tubes, are analyzed according to the strain development during the test. Finally, a section superposition method is proposed to predict the axial load-axial deformation behavior, whose results agree well with the experimental results.

Compressive behavior and strength prediction of ultra-high performance concrete confined by normal and high strength steel hoops

Ultra-high performance concrete (UHPC) confined by steel hoops exhibits obvious performance advantages under high temperature or corrosive environment compared with the external steel or fiber-reinforced polymer (FRP) jackets, due to protection to the confinement component by UHPC cover. However, some fundamental knowledge gaps need to be filled to promote the application of this type of composite column, including the compressive behavior of UHPC confined by high-strength steel hoops, influence of UHPC cover on the axial load carrying capacity as well as lack of acceptable strength model for predicting the load carrying capacity of steel hoop-confined UHPC column. Therefore, a series of axial compression tests on 78 UHPC stub columns confined by normal- and high-strength steel hoops were tested, with varying lateral confining levels and thicknesses of UHPC cove. The experimental results show that using high-strength steel hoops as lateral confining component can more effectively improve the post-peak behavior of confined UHPC column than its ascending behavior, compared with the UHPC confined by normal-strength hoops. The UHPC cover can directly carry a considerable share of peak axial load, thus cannot be ignored for evaluating the load carrying capacity of the confined UHPC column. A special strength model is proposed for UHPC columns confined by normal- and high-strength steel hoops to predict the axial load carrying capacity, and it is validated with experimental data derived from this investigation and opened literature.

Experimental Investigation of Special-Shaped Concrete-Filled Square Steel Tube Composite Columns with Steel Hoops under Axial Loads.

Special-shaped concrete-filled steel tube (SS-CFST) columns can be embedded in the wall, thus preventing the columns from protruding. This feature makes it popular in steel residential buildings. This paper proposes a new special-shaped concrete-filled square steel tube (SS-CFSST) composite column composed of multiple square steel tubes connected by steel hoops to form L-, T- or cross-shaped sections. Eight specimens were tested under axial loads with section shape, construction method, slenderness ratio, steel tube thickness, and steel strength as variation parameters. The structural performance, such as failure modes, peak load, load–displacement curves, load–strain curves, and Poisson’s ratio of the steel tubes, were analyzed. The tests illustrated that the failure modes of hoop-type specimens and weld-type stub columns were mainly the local buckling of steel tubes and bending failure, and those of the weld-type slender columns were mainly overall bending failure. The load-carrying capacity of the hoop-type specimen was higher than that of the weld-type specimen with the same cross-sectional dimensions and slenderness ratio. Next, the stress–strain relationship model of core concrete in the SS-CFSST composite column was established by considering the restraint effect of the connection coincidence area of steel tubes and steel hoops on concrete. Additionally, the finite element model (FEM) of the column was established using this constitutive model. By comparing the failure modes, load–strain curves and bearing capacities obtained from the tests and FEM, the established FEM can accurately evaluate the mechanical properties of SS-CFSST composite columns with steel hoops under axial compression.

Compressive Behavior and Strength Prediction of Ultra-High Performance Concrete Confined by Normal and High Strength Steel Hoops

Compressive Behavior and Strength Prediction of Ultra-High Performance Concrete Confined by Normal and High Strength Steel Hoops

Study on Behavior of Steel Hoop Connections for Raw Bamboo Members.

Bamboo structures have various types of connections, such as bolting and lashing. One crucial issue in bamboo structures is that the connection with bolts and nails has a lower load-carrying capacity associated with the bamboo failure resulting from the bolt or nail invading them. This paper focuses on the connection for raw bamboo members with steel hoops (BHC), of which the two semi-circular steel hoops are fastened to the raw bamboo with high-strength bolts. The sliding friction is controlled by the interfacial pressure, which can be increased by tightening the bolts. A push-out experiment on thirty-six specimens was conducted considering the following two parameters: the different surface conditions of raw bamboo (with or without the epidermis) and the different interfacial pressure. The test results mainly showed the two failure modes of specimens under certain conditions: continuous longitudinal slip after the vertical load reached the peak; and the steel hoop stuck in the bamboo skin after a period of slip. It is found that the sliding friction was controlled by the interfacial pressure, and the difference in the anti-sliding capacity between the epidermal bamboo specimen and the non-epidermal bamboo specimen was magnified with the increase of interfacial pressure. The contact stress on the surface of bamboo is approximately uniformly distributed based on the finite element analyses. The interfacial pressure can be predicted by the torque value of the digital electronic torque wrench and the equations established by mechanical analysis, respectively. Moreover, the design formulae of bearing capacity for BHC under three guaranteed rates (50%, 95%, and 99.9%) were developed based on probability theory, while the fourth design formula was derived by regression analysis. The reliability indices of the four design formulae were up to 0.07, 1.44, 3.09, and 0.97, respectively, and the resistance partial coefficients were suggested accordingly.

Data-oriented analysis of axial capacity of externally CFRP-confined concrete columns transversely reinforced with steel hoops or spirals

Limited research is available in the literature for exploring the axial compressive behavior of circular concrete columns confined by both external CFRP wraps and internal transverse steel reinforcement (SCC columns). The main objective of the present work is to develop an empirical model and an analytical model for predicting the axial compressive strength of SCC columns by considering the interaction between the CFRP composite and steel reinforcement confinement action. An extensive experimental database of 76 SCC columns has been developed from the literature work. A general regression analysis method and curve-fitting techniques on the experimental test results of SCC columns are used for developing the new empirical model. The confinement mechanics-based analytical model has also been suggested by considering the influence of CFRP and transverse steel confinement. The comparative study solidly substantiated the superiority of the proposed empirical model over the proposed analytical model and the previous models with =0.892, =12.83, and =9.78. A parametric study is also performed using the proposed empirical strength model to examine the effect of different parameters of SCC columns confined by both CFRP and transverse steel.

Analytical Review on Eccentric Axial Compression Behavior of Short and Slender Circular RC Columns Strengthened Using CFRP.

Although reinforced concrete (RC) columns subjected to combined axial compression and flexural loads (i.e., eccentric load) are the most common structural members used in practice, research on FRP-confined circular RC columns subjected to eccentric axial compression has been very limited. More specifically, the available eccentric-loading models were mainly based on existing concentric stress–strain models of FRP-confined unreinforced concrete columns of small scale. The strength and ductility of FRP-strengthened slender circular RC columns predicted using these models showed significant errors. In light of such demand to date, this paper presents a stress–strain model for FRP-confined circular reinforced concrete (RC) columns under eccentric axial compression. The model is mainly based on observations of tests and results reported in the technical literature, in which 207 results of FRP-confined circular unreinforced and reinforced concrete columns were carefully studied and analyzed. A model for the axial-flexural interaction of FRP-confined concrete is also provided. Based on a full parametric analysis, a simple formula of the slenderness limit for FRP-strengthened RC columns is further provided. The proposed model considers the effects of key parameters such as longitudinal and hoop steel reinforcement, level of FRP hoop confinement, slenderness ratio, presence of longitudinal FRP wraps, and varying eccentricity ratio. The accuracy of the proposed model is finally validated through comparisons made between the predictions and the compiled test results.

Axial Compressive Strength Models of Eccentrically-Loaded Rectangular Reinforced Concrete Columns Confined with FRP.

The majority of experimental and analytical studies on fiber-reinforced polymer (FRP) confined concrete has largely concentrated on plain (unreinforced) small-scale concrete columns, on which the efficiency of strengthening is much higher compared with large-scale columns. Although reinforced concrete (RC) columns subjected to combined axial compression and flexural loads (i.e., eccentric compression) are the most common structural elements used in practice, research on eccentrically-loaded FRP-confined rectangular RC columns has been much more limited. More specifically, the limited research has generally been concerned with small-scale RC columns, and hence, the proposed eccentric-loading stress-strain models were mainly based on the existing concentric-loading models of FRP-confined concrete columns of small scale. In the light of such demand to date, this paper is aimed at developing a mathematical model to better predict the strength of FRP-confined rectangular RC columns. The strain distribution of FRP around the circumference of the rectangular sections was investigated to propose equations for the actual rupture strain of FRP wrapped in the horizontal and vertical directions. The model was accomplished using 230 results of 155 tested specimens compiled from 19 studies available in the technical literature. The test database covers an unconfined concrete strength ranging between 9.9 and 73.1 MPa, and section’s dimension ranging from 100–300 mm and 125–435 mm for the short and long sides, respectively. Other test parameters, such as aspect ratio, corner radius, internal hoop steel reinforcement, FRP wrapping layout, and number of FRP wraps were all considered in the model. The performance of the model shows a very good correlation with the test results.

Research on performance of rigid-hoop-reinforced multi-DOF soft actuator

In order to improve the bearing capacity of soft actuators, this article presents the development of the rigid-hoop-reinforced (spring or steel hoop) multi-DOF soft actuator. The actuator is composed of a rotary module with spring reinforcement on the silicone rubber-based body and a bending module with steel hoop reinforcement on the body. Compared with fiber-reinforced actuators, the bearing capacities of rigid-hoop-reinforced actuators made of 65Mn spring steel are improved. The radial and the axial bearing capacity for the bending module and the rotary module is raised by 29.6%, 28.2%, 30.6%, 49.6% respectively; under the same pressure, the spring-reinforced interval increases the maximum rotary angle of the rotary module, the steel hoop-reinforced interval increases the maximum bending angle of the bending module; with the same reinforcement type, the bending module with reinforcement interval of 10 mm has good bending characteristics that the bending angle changes with the pressure gently; the lower the hardness of silicone rubber base body, the better the adaptability and flexibility of the actuator, and the higher the hardness, the greater the bearing capacity of the actuator. Due to the above advantages, the rigid-hoop-reinforced multi-DOF soft actuator can be applied to medical devices which need high load-carrying capacity.

Axial Load Behavior of Ultrahigh Strength Concrete-Filled Steel Tube Columns of Various Geometric and Reinforcement Configurations

This paper presents the behavior of concrete-filled steel tube (CFST) columns infilled with fiber-reinforced self-consolidating ultrahigh strength concrete (UHSC) subjected to axial concentric monotonic loading to failure. UHSC is expected to improve ease of fabrication, strength, and ductility of CFST columns. Seventeen columns having varying geometric properties such as tube wall thickness, cross-sectional shape (circular, rectangular, and square), and slenderness were constructed and tested by applying load through both steel tube and concrete core. Circular columns were further distinguished by the presence or absence of main and hoop steel reinforcing bars in the core concrete. Axial load-displacement response, axial/transverse strain development, and failure modes were recorded during the loading history to analyze the performance. Experimental confined concrete strength and axial strength of UHSC-filled CFST columns were compared with those obtained from three suggested analytical models and three code-based design procedures including Eurocode 4, Canadian CAN/CSA S16, and American AISC. Analytical models were found to over-predict the confined concrete strength and the axial strength of CFST columns. Canadian and American codes were found to be most applicable for predicting axial strength of UHSC-filled CFST columns while remaining conservative.

Nonlinear Numerical Analysis on Seismic Performance of New-type Outer Hoop Box Column Joints

The connection between the upper and lower structure is the key to seismic design of RC frame-lightweight steel mixed structure. This paper proposes a new type of outer hoop box column joint for this kind of mixed structure, which aims to strengthen the connection between exiting and new building, reduce the stiffness mutation and facilitate construct. In order to study the seismic performance of the new type of joint, the ABAQUS finite element software was used to carry out the numerical analysis of the new type of joint under the action of low-cycle cyclic loading with a 1/2 scale model, and the factors that affect the seismic performance were discussed in-depth. The results show that the hoop effect of the outer hoop steel plate strengthens the restraint on the core area of the joint and its surrounding members, which causes the plastic hinge position of the concrete beam to move outward, effectively protects the core area of the joint, and is beneficial to the bearing capacity of the new joint. Appropriately increasing the axial compression ratio can increase the bearing capacity, initial stiffness and yield displacement of the new joints to a certain extent.

Seismic behaviour of a new type of external hoop assembled joint for RC Frame- added layer hybrid structure

The connection between upper and lower structures is the key to the seismic design of reinforced concrete frame-added layer hybrid structure. A new type of external hoop assemble joint for this hybrid structure was proposed in this study. This joint was used to strengthen the connection between existing and new structures, reduce the stiffness mutation, and enhance the overall cooperative seismic capacity of a structure. The experimental and finite element analysis results of four specimens with different upper column section types and joint types under reversed cyclic loading were discussed in terms of failure phenomena, failure modes, process of angle change of beam and column, strength degradation, stiffness degradation, hysteretic curves, skeleton curves and energy dissipation capabilities to study the seismic performance of this new type of joint. Results indicate that the plastic hinges of the new type of external hoop assemble joints are moved outwards to the edge of the external hoop steel plate of column foot. The damage position is moved away from the interface between the column and the beam, thereby protecting the core area of joint effectively. The new type external hoop joint has a fuller hysteretic curve, higher load-bearing capacity and slower strength and stiffness degradation than the traditional exposed rigid joint. Specimens with box-shaped cross section upper steel column have better energy consuming ability than specimens with I-shaped cross section upper steel column. The section form and stiffness of the upper steel column has a greater influence on the seismic performance of the hybrid joint. Reasonable suggestions are provided. This study can provide scientific basis and technical support for light steel hybrid structures.

Stress - strain behaviour of confined nano silica-based concrete

In the present study, the stress-stain behaviour of confined concrete made with nano-silica (nano-SiO2) were taken up. The stress-strain behaviour was studied for the M30 and M50 grades nano-silica (nano-SiO2) concrete mixes confined with steel 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 nano-silica (nano-SiO2) along with confinement of concrete with steel hoops enhanced the compressive strength, indicating further confinement effect in the concrete. It is observed that the addition of nano-silica (nano-SiO2) is helpful in lower confinements only. Beyond 1.1% confinement, 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 nano-silica (nano-SiO2) concrete with steel hoops for mixes up to 1.1% confinement. The addition of nano-silica (nano-SiO2) to concrete has increased the ductility in both confined and unconfined states

Studies on stress- strain behaviour of fibre reinforced self-compacting concrete in confined state

In the present study, the stress-stain behaviour of self-compacting concrete (SCC) and fibre reinforced self-compacting concrete (FRSCC) were taken up. The stress-strain behaviour was studied for the SCC and FRSCC mixes in unconfined and confined states. 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 fibres along with confinement of FRSCC with steel hoops enhanced the compressive strength, indicating further confinement effect in the FRSCC. It is observed that the addition of fibres is helpful in lower confinements only. Beyond 1.1% confinement, the addition of any type of fibres doesn’t show any effect on compressive strengths. From the stress-strain behaviour of all types of FRSCC, it is concluded that the ultimate load-carrying capacity and strains at peak stresses are more in SFRSCC and HFRSCC for mixes up to 1.1% confinement. The addition of fibres to SCC has increased the ductility in both confined and unconfined states