Diameter Of Longitudinal Bars Research Articles

Axial compressive behavior of spiral stirrup-reinforced concrete-filled cold-formed steel built-up box stub columns

This paper aims to explore the enhancement effect of spiral stirrup on concrete-filled cold-formed steel built-up box (CF-CFB) columns. Herein, four specimens were designed and fabricated to explore the impact of the spiral stirrup and riveted form on the axial compression performance of CF-CFB columns. Then, the finite element model of spiral stirrup-reinforced concrete-filled cold-formed steel built-up box (RCF-CFB) stub columns was established, and a systemic parametric investigation was conducted to explore the impact of geometric and material parameters on the compression performance of RCF-CFB stub columns. Afterwards, the analysis included an assessment of contact stress and stress distribution was conducted to uncover the confining effect. The analysis results revealed that the failure mode of the RCF-CFB column was tube buckling and concrete crush, accompanied by the spliced tube separating at the buckling area. The built-in reinforcement could significantly improve the performance of the built-up column, while riveted forms of spliced steel tubes had little effect. Furthermore, the axial bearing capacity of the RCF-CFB column would increase with the enhancement in steel yield strength, concrete strength, tube thickness, and longitudinal bar diameter. Finally, a calculating method to predict the axial bearing capacity of the RCF-CFB column was proposed, and the applicability of the method was demonstrated by comparison with experimental and FE data.

Cyclic behavior of UHPC corner beam-column joints under bi-directional bending

Design codes contain special provisions for anchorage lengths of flexural reinforcement and amount of transverse reinforcement in beam-column joints, to safeguard against joint failure and possible structural collapse. However, those stipulations could lead to congested joints with construction problems such as segregation. This study leverages experimental testing of eight corner beam-column joints under constant axial and cyclic bi-directional bending loads. In addition to being one of few on corner joints under a complex and practical loading, the study’s main aim is to evaluate the effectiveness of ultra-high-performance concrete (UHPC) as a joint fill in lieu of normal strength concrete (NSC). Brittle failure, either by pure shear or shear combined with yielding of beam reinforcement occurred in all NSC joints and varied in shape and intensity depending on the column depth-to-beam longitudinal bar diameter (hc ⁄db) ratio and spacing of transverse beam/column reinforcement. Ductile failure characterized by the yielding of beam reinforcement and plastic hinge at the beam-joint interface occurred in all UHPC joints and was independent of the two aforementioned variables. The application of UHPC in the joint region resulted in increasing the joint ultimate strength by 27.6–54.8%, energy dissipation by 24–163%, and stiffness by 20 to 50%, compared to NSC joints. Such results were obtained when no transverse reinforcement is used in the joint, and only half transverse reinforcement is used in the beam/column members, and with hc ⁄db relaxed to 38% of the code recommended ratio, highlighting the advantages of UHPC joints. Predictions of ACI-ASCE 352R code were found to be overly unconservative and in need of future improvement.

Delaying the Occurrence of Bar Buckling in RC Columns Confined with SRG Jacketing

This paper investigates experimentally the structural performance of substandard reinforced concrete (RC) short columns confined with steel-reinforced grout (SRG) jackets under monotonically increasing uniaxial compression. The study comprised 24 square cross section short RC columns having alternative arrangements of shear reinforcement (ratio of stirrup spacing to longitudinal bar diameter ranging from 4.2 to 12.5). The short columns were retrofitted with externally applied SRG jacketing differing by the density of the fabric (4 cords/in and 12 cords/in) and the number of fabric layers (1 and 2). The test results showed that retrofitting significantly changed the behaviour of the specimens compared to the unconfined counterparts. For columns at risk of premature failure due to insufficient support of compression bars provided by the sparse stirrups, the SRG jackets delayed bar buckling, enabling the members to achieve greater strength and deformation capacity. The well-detailed specimens helped establish the maximum effectiveness of SRG confinement.

Analysis of influencing factors on axial compression performance of BFRP bars-coral reef and sand concrete columns

A more systematic analysis of the factors influencing the axial compression performance of basalt-fiber-reinforced polymer (BFRP) bars-coral reef and sand concrete (CRSC) columns was carried out based on axial compression load capacity tests on 15 columns in 5 groups. The test study factors include the concrete strength, longitudinal bars diameter, stirrup spacing, slenderness ratio and bars type, and type of bars material. The results show that the damage process of BFRP bars-CRSC columns can be divided into elastic stage, cracked stage and damage stage. Under different diameters of longitudinal bars, increasing the diameter of longitudinal bars leads to the reduction of the adhesion between longitudinal bars and concrete protective layer, which reduces the load-bearing capacity; decreasing the spacing of hoop bars helps to improve the ductility of the member, but it also reduces the continuity of CRSC and the strength.

Axial behavior of circular seawater sea-sand coral concrete columns reinforced with BFRP bars and spirals

• Axial behavior of BFRP bars reinforced seawater sea-sand coral concrete columns was studied experimentally. • The load bearing capacity of B-SSCC columns declined as the spiral spacing narrowed paradoxically owing to the significant discontinuity of coral concrete internally. • Columns with dense stirrups showed more pronounced ductility in the post-peak stage compared with B-SSCC square columns. • An ultimate bearing capacity prediction model is established. For the sake of solving the problem of steel reinforcement corrosion in concrete structures and reducing the transportation cost of concrete raw materials at sea in the development of South China Sea Islands, a new type of coral reef sand concrete columns reinforced with basalt fiber reinforced polymer (BFRP) bars and spirals was proposed. In this study a total of 18 circular seawater sea-sand coral reef concrete columns reinforced with BFRP bars and spirals (B-SSCC) were tested under concentric axial loads. Parameters of nominal longitudinal bar diameters and spiral spacing were investigated to obtain variations on crack propagation, failure modes, load bearing capacity, strain and ductility properties. The results indicated that no obvious cracks were observed until the curves reached up to 0.95 P max approximately and the columns experienced more brittle and explosive failure modes with the increase of longitudinal bar diameter and spiral spacing. The load bearing capacity of B-SSCC columns increased with longitudinal reinforcement ratio, however, declined as the spiral spacing narrowed paradoxically. It was owing to the significant discontinuity of coral concrete internally. Compared with B-SSCC square columns, the circular columns with dense stirrups showed pronounced ductile behavior in the post-peak stage, remarked by improved average ductile coefficient β of 22.6%. Finally, a numerical prediction of load bearing capacity was suggested.

Low‐cycle fatigue performance of high‐strength steel rebars in concrete bridge columns

AbstractDesign codes often restrict the use of high‐strength steel (HSS) rebars in seismic applications due to their inferior low‐cycle fatigue performance when compared to conventional steel rebars. In this study, previously established low‐cycle fatigue life models of HSS rebars, namely ASTM A706 Grade 550 and ASTM A1035 Grade 690, were incorporated into a new cumulative damage model to identify conditions under which such rebars can achieve seismic performance comparable to that of benchmark ASTM A706 Grade 420 steel bars in concrete bridge columns. An ensemble of well confined flexure‐dominated circular concrete bridge columns located in high seismic regions was considered where the axial load ratio, and longitudinal and spiral reinforcement ratios were varied. The bridge columns were reinforced with ASTM A706 Grade 420, ASTM A706 Grade 550, and ASTM A1035 Grade 690 steel and numerically analyzed under different displacement ductility levels (2, 4, and 6), earthquake types (crustal and subduction), and ratio of hoop spacing to longitudinal bar diameter ratio (4 and 6). Their low‐cycle fatigue performances were compared based on the computed bar fracture and accumulated damage indices. Based on the reported observations, it is concluded that design codes are overly restrictive in not permitting the use of HSS in seismic application on the basis of inadequate low‐cycle fatigue life. As an alternative, this study proposes imposing certain limits on displacement ductility demand and ratio of hoop spacing to longitudinal bar diameter ratio to promote safe and efficient use of HSS rebars in concrete bridge columns.

Designing and detailing transverse reinforcement to control bar buckling in rectangular RC walls

Bar buckling in RC structures is a commonly-observed failure mode that adversely affects their deformation capacity. To restrict bar buckling in ductile RC walls, design codes only emphasises on restricting the spacing of transverse reinforcement and does not recognise the importance of the effective stiffness of the ties (which is a combination of the tie leg axial stiffness and spacing) to restrict bar buckling. Therefore, in this paper the design requirements for anti-buckling transverse reinforcement are summarised, and improvements to the current design methodology for anti-buckling transverse reinforcement are proposed. To ensure that the transverse reinforcement detailing in plastic hinge regions is adequate to restrict bar buckling to single tie spacing and the compressive stress deterioration in bars due to buckling is controlled, refinements to the current detailing procedures are proposed. The buckling restraining ability of transverse reinforcement depends on the axial stiffness of the tie legs, while the compressive stress reduction in reinforcing bars due to buckling depends on their unsupported length (in bare bar tests) or buckling length that can include multiple tie spacing (inside RC members). Therefore, restrictions on both the axial stiffness of the tie legs and spacing of transverse reinforcement along the longitudinal reinforcing bars are proposed. The effective axial stiffness of tie legs is controlled by ensuring that the length of the tie legs in the direction of potential buckling is well below the critical length evaluated using a mechanics-based approach. Additionally, compressive stress degradation in reinforcing bars is controlled by limiting the ratio of the transverse reinforcement spacing and the longitudinal bar diameter such that any reduction of compressive stress carried by the longitudinal bars due to buckling at the limiting curvature recommended by New Zealand Concrete Standard is within an acceptable range. Furthermore, recommendations to avoid buckling of unrestrained reinforcing bars in the boundary zone and the wall web are proposed. Using the proposed design methodology, buckling of longitudinal reinforcing bars in ductile RC walls can be delayed and the detrimental effects of buckling on the lateral response of walls can be controlled until the design drift or curvature demands are met.

Experimental Behavior of Concrete Columns Confined by Transverse Reinforcement with Different Details

Introduction:Most of the existing reinforced concrete buildings often have columns with poor transverse reinforcement details. Models for computing the confined concrete strength were developed using experimental tests performed on specimens with transverse reinforcement typical of seismic design. The paper presents the results of an experimental program performed to investigate the effect of type, amount and pitch of transverse reinforcement on the behavior of confined concrete.Aim:The paper is also aimed at evaluating whether the current code models are suitable for estimating the confined strength of concrete in existing buildings.Methods:A total of 45 reinforced concrete columns with four volume ratios of transverse reinforcement were tested under axial loads. Type and pitch of transverse reinforcement typical of existing r/c buildings not designed according to seismic standards were considered. Therefore, columns reinforced by spiral and hoops with 135° or 90° hooks at the end are investigated for comparing their behavior. The confinement of spirals and hoops to core concrete is discussed as the amount of transverse and longitudinal reinforcement varies. Small increases in strength due to the concrete confinement were measured for hoop pitch of 150 mm (ranging between 2% and 7%), but also for hoops with 90° hook and pitch of 75 mm. Greater increments were obtained by spirals and hoops with 135° hook in the case of 75 mm pitch and when rhomboidal hoops or cross-ties were arranged in addition to the perimeter hoops. A comparison with some similar experimental results is also performed, achieving quite similar results. The mean experimental stress-strain curves are also analyzed.Results:The results show how the increase in concrete strength due to the confinement is more dependent on the transverse reinforcement pitch than the type and detail of transverse reinforcement or even less diameter of longitudinal bars. Finally, the experimental strength of confined concrete is then compared with the values provided by Eurocode 8 and the new Italian Building Code, showing that the higher the volumetric percentage of transverse reinforcement, the greater the overestimation of code models.Conclusion:An overestimation of codes up to 30% is assessed, systematically lower in the case of spirals, and higher in the case of hoops with 90° hooks at the end. The results highlight the need to develop specific equations to determine the strength increase due to the concrete confinement in the case of existing buildings with poor transverse reinforcement.

Overstrength requirements to avoid brittle shear failure in RC slender beams with stirrups

In this paper, an analytical expression was derived for the prediction of shear strength of slender reinforced concrete (RC) beams with stirrups. Failure occurring in B and D regions of the beam was considered. An expression derived for the shear strength of the B region was based on a modified constant angle truss model. The analytical expression derived consisted of three terms. The first one was related to the concrete contribution expressed through the square root of cylindrical compressive strength, geometrical ratio of longitudinal bars, and depth to width ratio. The second one was relating to the shear force induced by yielding of stirrups to the inclination of the main crack occurring at failure; its value depends on the geometrical ratio of longitudinal bars, transverse stirrups, and modulus of elasticity of concrete and steel. Finally, the third term was related to the shear force induced by the flexure of steel bars in the space s, which depends on the number and diameter of longitudinal bars and the yielding stress of longitudinal bars. Failure of the D regions was related to the concrete crushing of loaded joint under the effect of shear force and compressive force, including also the confinement effect due to the presence of stirrups. The original contribution of the paper was the determination of the maximum mechanical ratio of longitudinal and transverse steel to ensure over strength in shear respecting the strength hierarchy.Expressions derived and some of those available in the codes were verified against three different data banks of experimental researches available in the literature. The data banks utilized considered a very wide range of the geometrical ratio of longitudinal bars and transverse stirrups, including, for the latter, very high value producing brittle failure due to concrete crushing.

Axial Compression Behaviors of the Steel Tube Confined Reinforced Concrete Columns with Binding Bars

An experimental study on the steel tube confined reinforced concrete (STCRC) column with binding bars under axial compression is conducted. The bearing capacity and failure modes are obtained. It can be known that the axial deformation of concrete occurred under compression. The core concrete is wrapped and constrained by the steel tube wall, and the steel tube wall is constrained by binding bars locally, so the local buckling shape of the wall between the binding bars is like wave shape. The 3D finite element model is also developed to analyze the behavior of this type of column under axial compression. Good agreement is shown between the test and predicted results in terms of the load‐deformation curves and ultimate strength. The parametric studies indicate that the spacing of binding bars, diameter of longitudinal bars, concrete strength, thickness of the steel tube wall, and section dimension of the column generate different influence on the mechanical properties and bearing capacity. The diameter of longitudinal bars, concrete strength, and section dimension of the column have a great effect on the ultimate bearing capacity. The numerical results also show that the spacing of binding bars has little effect on the ultimate bearing capacity. The larger thickness of the steel tube wall leads to adverse effect on the specimen performance. Finally, the theoretical calculation is carried out, and the result is good.

Deformation Capacity Limits for Reinforced Concrete Walls

The use of deformation capacity limits is becoming increasingly common in seismic design and assessment of reinforced concrete (RC) walls. Deformation capacity limits for RC walls in existing design and assessment documents are reviewed using a comprehensive database. It is found that the existing models are inconsistent and do not account for variation in deformation capacity with changes in the ratio of neutral axis depth to wall length ( c/ L w) and ratio of transverse reinforcement spacing to longitudinal bar diameter ( s/ d b) at the wall end region. A new mechanics-based model considering strain limits on the concrete and reinforcement is recommended. Concrete compressive strain limits for different levels of wall end region detailing are selected based on curvature ductilities for the walls in the database. Reinforcement tensile strain is limited based on a model for bar buckling. The proposed model, which accounts for c/ L w and s/ d b, is shown to have less dispersion and more accuracy than existing models when compared against experimental data and provides consistency between assessment and design provisions.

Flexural ductility of reinforced concrete beams with lap-spliced bars

In this paper, the flexural ductility of lap-spliced reinforced concrete (RC) beams is experimentally investigated. Twenty-four specimens were designed and manufactured for laboratory experiments. Concrete compressive strength, amount of transverse reinforcement over the splice length, and the diameter of longitudinal bars were selected as the main variables. The ductility of tested specimens is evaluated based on a previously defined ductility ratio. Results show that concrete strength and amount of transverse reinforcement over the splice have major effects on ductility. With an appropriate amount of transverse reinforcement, a satisfactory ductility response for different concrete strengths can be obtained. The CSA-A23.3-04 Standard provisions on bond strength and ductility of lap-spliced RC beams are evaluated and discussed. This study shows that the provisions in predicting the bond strength of lap-spliced concrete beams are adequate but may not achieve a satisfactory performance for ductility. An equation is proposed to achieve the appropriate ductility.

The Effect of Lap Splice Length on the Cyclic Lateral Load Behavior of RC Members with Low-Strength Concrete and Plain Bars

Many existing reinforced concrete buildings in developing countries located in seismic areas do not possess sufficient strength and ductility characteristics to resist the effects of severe earthquakes. Among other deficiencies, improper detailing of reinforcement and poor quality of materials are major causes of poor seismic performance. More specifically, inadequate lap splice lengths provided at floor levels on plain column longitudinal bars, particularly when concrete strength is low, is a widespread deficiency that has not been investigated in detail to date. Information on the behavior of such columns under earthquake actions is extremely important for reliable assessment of the seismic safety of many existing structures with detailing deficiencies and poor construction quality. In this study, the effect of lap splice length on the cyclic lateral load behavior of low-strength RC columns with plain longitudinal bars (14 mm diameter) was investigated experimentally. The specimens were designed to represent columns with low axial loads. The geometric ratio of the longitudinal reinforcement and the volumetric ratio of the lateral reinforcement of the columns are 1% and 0.8%, respectively. The test program included five RC columns with lap splice lengths of 25, 35, 44 and 55 times longitudinal bar diameter, as well as a reference specimen with continuous longitudinal bars. Test results clearly demonstrated that presence of 180-degree hooks at the ends of the lap splice reduces the negative influence of the inadequate lap splice length on RC member performance, even in the case of low-strength concrete. All specimens reached their flexural strengths and did not experience considerable strength degradation until large drift levels. Test observations were supported with findings of a fiber-based analytical model, which also demonstrated the influence of hooks on improving the bond slip behavior along inadequate lap splices.

Seismic Evaluation of the ACI Code Provisions for Lap Splicing of Longitudinal Bars in R/C Rectangular Bridge Columns

In this study, the adequacy of the ACI code provisions for detailing of lap-spliced reinforcement in R/C rectangular bridge columns was numerically investigated. Pushover analysis was conducted on a total of 324 R/C rectangular bridge columns with lap-spliced reinforcement, using a modified version of previously developed computer program. The computer modeling is based on moment–curvature analysis of the column section with the inclusion of a bond/slip mechanism and concrete confinement model. In this study, the program was revised to integrate the effectiveness of lateral hoop reinforcement on both concrete confinement and lap-splice clamping. Constant axial load was applied in the analysis. The parameters involved in the study were lap-splice length, volumetric ratio of column hoops, and axial load ratio. The results indicate that the top lateral load–displacement characteristics of the column are enhanced when the lap-splice length at the column base increased. To ensure a minimum displacement ductility of 4.0 for the column, lap splices as short as 30φb (φb = longitudinal bar-diameter) should be avoided at expected plastic hinge locations. The best performance for a wide range of axial load ratios was exhibited by columns having the ACI’s classes A and B tension splices and laterally reinforced with hoops required by the ACI code for seismic design.

Cyclic Behavior of Smooth Steel Reinforcing Bars: Experimental Analysis and Modeling Issues

This article presents the results of an experimental campaign on the cyclic behavior of smooth steel bars, used as internal reinforcement of existing Reinforced Concrete (RC) structures; the stress-strain relationship is analyzed for different values of L/D ratio, where L is the stirrup spacing and D is the longitudinal bar diameter. Most of the experimental tests concerned smooth bars with L/D ratios ranging between 5 and 100. The analysis of the test results is complemented by some comparisons of the curves obtained by experimental tests on smooth steel bars with those obtained by experimental tests on ribbed steel bars. The theoretical predictions obtained by existing models available in the literature on the cyclic behavior of ribbed steel bars are then discussed. Our review of these formulations underlines their range of applicability as well as their potential to predict the damage and buckling that occur for different L/D ratios of the smooth steel bars.

Comportamento de emenda por traspasse de barras comprimidas em contato e separadas

Neste trabalho é resumido um estudo experimental sobre o comportamento de emendas por traspasse de barras comprimidas em pilares de concreto armado, com o objetivo de investigar a influência da distância entre as barras emendadas sobre o comportamento da emenda. O estudo englobou os ensaios à compressão centrada de dois pilares: um com emenda com barras em contato e outro com emenda com barras separadas a uma distância igual a duas vezes o diâmetro das barras da armadura longitudinal, φl. Nos pilares ensaiados, constatou-se que a tensão média de aderência na emenda praticamente não depende da distância entre as barras. Para ambos os pilares, próximo à carga última, a tensão normal nas pontas das barras apresentou valores muito superiores à resistência uniaxial à compressão do concreto.

Assessment of the seismic behaviour of RC columns

Current codes for earthquake resistant design of RC structures include several provisions that ensure satisfactory behaviour of columns. In order to check the efficiency of relevant code provisions, the behaviour exhibited by tested RC columns is assessed. For this purpose, experimental results on more than 450 RC columns were collected. Their behaviour has been evaluated on the basis of several parameters, namely: the in-section arrangement of hoops, the effective confinement ratio, the spacing of hoops normalized to the longitudinal bar diameter, etc. The main findings of this comparison between experimental results and code provisions are presented in this paper. The present work confirms the beneficial role of multiple hoops, low axial load, high confinement ratios and low values of the spacing of hoops normalized to the diameter of longitudinal bars on limiting the force response degradation due to cycling, as well as on enhancing the ductility of RC columns. In general, code provisions seem to ensure the targeted behaviour.

Practical Performance Model for Bar Buckling

A practical model has been developed to predict, for a given level of lateral deformation, the likelihood that longitudinal bars in a reinforced concrete column will have begun to buckle. Three relationships linking plastic rotation, drift ratio, and displacement ductility with the onset of bar buckling were derived based on the results of plastic-hinge analysis, moment-curvature analysis, and the expected influence of the confinement reinforcement. These relationships, which account for the effective confinement ratio, axial-load ratio, aspect ratio, and longitudinal bar diameter, were calibrated using observations of bar buckling from cyclic tests of 62 rectangular-reinforced and 42 spiral-reinforced concrete columns. A version of the drift ratio relationship is proposed for earthquake engineering applications. The ratios of the measured displacements at bar buckling to the displacements calculated with the proposed model had a mean of 1.01 and a coefficient of variation of 25% for rectangular-reinforced concrete columns. The corresponding mean and coefficient of variation for spiral-reinforced columns were 0.97 and 24%, respectively.