Reinforced Concrete Short Columns Research Articles

Seismic performance of RC short columns with buckling restraint reinforcement

This paper considers the application of the buckling restraining concept in reinforced-concrete (RC) columns subjected to seismic-type loading. The main objective of the experimental study is to investigate the seismic performance of RC short columns with buckling-restrained casing provided to the main reinforcement at the critical region. The debonding of reinforcement by providing a rebar restraining collar is carried out at the plastic hinge region where maximum bending moment occurs. The columns are subjected to quasi-static cyclic lateral loading combined with a constant axial loading to simulate seismic-type loading. The performance of these columns is compared – in terms of ultimate load, ductility, load displacement envelope and energy dissipation capacity – with that of counterpart short columns designed as per Indian Standard code IS:456-2000 and ductile detailing as per IS:13920-2016. The main variables selected in this investigation are the axial load ratio and the length of the buckling-restrained casing of reinforcement. The test results indicate that the columns with buckling-restrained casings have improved performance in terms of lateral load-carrying capacity, energy dissipation characteristics and reduced stiffness degradation relative to columns without casing.

EFFECT OF STRUCTURAL SIZE ON SEISMIC PERFORMANCE OF REINFORCED CONCRETE SHORT COLUMNS—A MESO-SCALE STUDY

Under the action of earthquake, reinforced concrete (RC) short columns often undergo brittle compression-shear failure due to the combined effects of vertical and horizontal loadings. The brittle failure may exhibit obvious size effect of the columns. Considering the meso-scale structural characteristics of concrete, the heterogeneity of concrete and the nonlinear interaction between steel bars and surrounding concrete, a three-dimensional meso-scale numerical model is established to study the mechanical behavior and seismic performance of reinforced concrete short columns with different structural sizes. The meso-scale simulation results are in good agreement with the available experimental results. Moreover, the effect of the axial compression on the seismic performance of RC short columns with different sizes, especially large-sized ones is investigated, including the shear bearing capacity, hysteretic behavior, ductility and energy dissipation capacities. Finally, based on the classical Size Effect Law (SEL) proposed by Bažant, a modified formula is proposed to reflect the size effect on the shear capacity of reinforced concrete short columns.

Shear strengthening of RC short columns with ECC jacket: Cyclic behavior tests

This study aims at developing an easy-to-apply strengthening method for reinforced concrete (RC) short columns by using Engineered cementitious composite (ECC), which is a high-performance composite material with tensile strain-hardening behavior and multiple cracking mechanism in tension. Seven identical RC short columns were prepared, and five of them were strengthened with ECC jackets or ferrocement jacket. The main experimental parameters involve the jacketing schemes and the axial load. The failure modes, hysteresis response, deformation capacity, stiffness degradation and energy dissipation capacity of the specimens were studied by the lateral cyclic loading tests. The tests results show that the ECC jacketed columns failed in ductile modes, and exhibited significant improvement in plastic deformation ability and energy dissipation capacity compared with the control specimens. At the ultimate displacement point, the increase of the drift ratio and cumulative energy dissipation of the ECC jacketed columns were 19–111% and 102–549%, respectively, compared with the corresponding control specimens. Under higher axial loads, the ECC jacketing was still certified to be effectively in enhancing the seismic behavior of the original columns. The shear deformation and the shear strength of the original RC columns were effectively improved by the external ECC jacket, and a prediction method for the shear strength of the ECC jacketed columns was given based on the current Chinese codes.

Influence of Spirals on the Behavior of Short RC Columns Strengthened by External CFRP

An experimental study was carried out to investigate the behavior of normal strength reinforced concrete (RC) circular short columns strengthened by carbon fiber reinforced polymer (CFRP) sheets. Three series comprising total of (15) specimens were loaded to failure under axial concentric compression load. Strengthening was varied by changing CFRP strips number, spacing and wrapping methods. The finding of this research can be summarized as follows; for the columns without CFRP, the influence of the spiral pitch was significant: compared with 100 mm spiral pitch, dropping the spacing to 65 mm and 30 mm increased the load carrying capacity by 25% and 43% respectively. The columns with less internal confinement (lesser amount of spirals) were strengthened more significantly by the CFRP than the ones with greater amount of internal spirals. As an example of the varying effectiveness of the fully wrapped CFRP, the column with spiral pitch at 100 mm was strengthened by 74% with the CFRP. In contrast, the ones with 30 mm spiral pitch only increased in strength with CFRP by 51%. Compared with the control specimen (no CFRP), the same amount of CFRP when used as spiral strips led to more strengthening than using CFRP as hoop strip-the former led to nearly 8% more strengthening than the latter in the case of 100 mm internal spiral pitch. In the case of 65 mm the internal spiral pitch, the difference (between the hoops &spiral CFRP strengthening)is close to 10.5%. The difference between the two methods of strengthening in the heavily spiral columns (30 mm spiral pitch) is more significant.

Seismic Performance Assessment of Corroded Reinforced Concrete Short Columns

AbstractThis paper presents an experimental study carried out on eight full-scale uncorroded and corroded reinforced concrete short columns subjected to the simulated seismic loading to investigate...

Investigation of the performance of externally collared RC short columns via aspect ratio

This paper presents the experimental study of nine pieces of reinforced concrete (RC) short columns. RC short columns were tested with cyclic loading with displacement control under the influence of constant axial load with load index of 0.2. Three columns within the tested nine columns are reference columns which have the details of the reinforcement given in the modern regulations and six of them are 150 mm and 100 mm externally collared columns. In addition to the parameter of the collar spacing, aspect ratio (as=2-1.5-1) is also considered as a parameter. The data obtained from experimental results have shown that externally collar contributes significantly to increasing the shear resistance of RC short columns and limiting the shear dominant behavior. It has been observed that the effectiveness of the externally collar increases with the decrease of the aspect ratio.

Seismic Performance of High‐Ductile Fiber‐Reinforced Concrete Short Columns

This study mainly aims to investigate the effectiveness of high‐ductile fiber‐reinforced concrete (HDC) as a means to enhance the seismic performance of short columns. Six HDC short columns and one reinforced concrete (RC) short column were designed and tested under lateral cyclic loading. The influence of the material type (concrete or HDC), axial load, stirrup ratio, and shear span ratio on crack patterns, hysteresis behavior, shear strength, deformation capacity, energy dissipation, and stiffness degradation was presented and discussed, respectively. The test results show that the RC short column failed in brittle shear with poor energy dissipation, while using HDC to replace concrete can effectively improve the seismic behavior of the short columns. Compared with the RC short column, the shear strength of HDC specimens was improved by 12.6–30.2%, and the drift ratio and the energy dissipation increases were 56.9–88.5% and 237.7–336.7%, respectively, at the ultimate displacement. Additionally, the prediction model of the shear strength for RC columns based on GB50010‐2010 (Chinese code) can be safely adopted to evaluate the shear strength of HDC short columns.

Behavior of Axially Loaded CFRP Confined Large-Scale Capsule-Shaped RC Short Columns with Higher Cross-Sectional Aspect Ratio

This paper aimed to experimentally and analytically investigate the behavior of reinforced concrete (RC) short capsule-shaped columns confined with carbon fiber-reinforced polymer (CFRP) sheets. The efficiency of FRP-confined strengthening system depends mainly on different encountered parameters such as the FRP confinement ratio, shape and size of cross-section, and cross sectional aspect ratio of non-circular columns. The effects of varying the aspect ratio (h/b=2, 2.5, 3 and 4), FRP confinement ratio (number of FRP layers), and FRP-wrapping configuration (full and partial system) are examined in this study. The experiment results showed that the efficiency of FRP confined system was proven to be lower with higher aspect ratio than that with the lower aspect ratio. Confinement by CFRP sheets enhances the performance of capsule-shaped RC columns subjected to axial compressive loads, and it is an efficient technique to improve the strength and ductility of capsule shaped RC columns regardless the cross sectional aspect ratio and the type of confinement. Analytical model was proposed by the authors to predict the axial load carrying capacity of short capsule-shaped reinforced concrete columns. The proposed model is compared with the existing model, showing good agreement with the experimental results and it is improved performance and gives reasonable predictions of load carrying capacity of FRP-confined columns.

Seismic behavior of low-corroded reinforced concrete short columns in an over 20-year building structure

In reinforced concrete (RC) structures, reinforcement corrosion, concrete carbonization, and chloride ion erosion occur during service life, which deteriorate the seismic structural capacity of the structures. Most researchers have focused on the effect of the reinforcement corrosion on the seismic behavior of RC columns showing flexural behavior. On the other hand, research on the influence of the combined reinforcement corrosion and concrete carbonization on the seismic behavior of in-service RC columns showing shear-flexural behavior has not been performed. The present study evaluates the seismic performance of low-corroded in-service RC columns showing shear failure under cyclic load. Four RC columns with shear-span ratio 2.3 and light transverse reinforcement were obtained from an in-service RC moment frame building constructed in 1987, and quasi-static cyclic tests were conducted to investigate the seismic behavior of these columns. For the test parameters, axial compression force ratios and longitudinal bars arrangements were considered. The test results showed that as the compression force ratio increased, the failure modes of the in-service RC columns shifted from shear-bond failure to shear-compression failure, and the ductility and energy dissipation capacity decreased. The shear strength of the column specimens was evaluated by five existing shear strength models, considering the effect of reinforcement corrosion. Finally, seismic performance of RC short columns with corroded bars was discussed.

Experimental investigation of substandard RC columns confined with SRG jackets under compression

This paper aims to explore the behaviour of substandard reinforced concrete (RC) columns confined with Steel-Reinforced Grout (SRG) jackets under monotonically increasing uniaxial compression. A total of 24 specimens of short RC columns of square cross section were designed to fail due to longitudinal reinforcement buckling. Single-layered SRG jackets were applied to 18 of these specimens, whereas the rest served for control without SRG jackets. Parameters of this investigation were the type and density of the steel fabric as well as the corner radius of the cross section. The employed SRG jacketing managed to increase the strength and strain capacity and postpone the buckling of the longitudinal steel bars to occur at higher compressive strain level. Confinement effectiveness with respect to the lateral confining pressure exerted by the used SRG jacketing is discussed along with the observed mode of failure.

Emergency repair of severely damaged reinforced concrete column elements under axial compression: An experimental study

Reinforced concrete (RC) columns in a framed construction can be severely damaged under earthquake, blast, fatigue and other loads. It is essential to quickly restore the strength and stiffness of columns after its damage for maintaining the serviceability of the building. The objective of this study is to assess the efficiency of different strengthening techniques on emergency repair of the severely damaged RC columns under axial compression. Six short RC columns elements were cast and tested under axial compression until 20% drop in the peak strength. Thereafter, these severely damaged columns were repaired using strengthening schemes, namely (i) repairing the damaged portions using quick setting cement mortar, (ii) Near Surface Mounting (NSM) of CFRP (Carbon Fiber Reinforced Polymer) laminates and (iii) Hybrid strengthening technique. The hybrid strengthening technique uses the combination of CFRP NSM laminates as additional longitudinal reinforcement and external bonding with CFRP fabric for confinement. The hybrid strengthening scheme was found to be the most efficient in restoring the lost strength and initial stiffness when compared to the only NSM strengthening or repair with quick setting cement mortar.

Effectiveness of Combined Confinement with Metal Meshes and Ties for Preloaded and Post-Heated RC Short Columns

Recently, innovative materials are used as lateral confinement reinforcement to increase concrete core contribution to load carrying capacity of reinforced concrete (RC) columns. This paper presents experimental assessment for the effectiveness of practical lateral confinement that utilizes metal meshes combined with typical steel ties for RC columns. The experimental plan comprises thirty square short column specimens and investigates two metal meshes types: expanded metal mesh (EMM) and welded wire mesh (WWM). Preloading and post-heating were considered in the experimental study to provide realistic assessment for proposed confinement in real and fire conditions. Prior to testing under monotonic axial compression, all column specimens were subjected to preloading with 65% (of reference column capacity) in addition to post-heating for only fourteen specimens. The lateral reinforcement consisted of metal mesh layers (either EMM or WWM) wrapped above steel ties of various volumetric ratios (\(\rho =0.333\), 0.167% and zero). Results showed that column load capacity was increased (49.15–2.72%) for all specimens enclosing proposed lateral reinforcement with respect to reference specimens whose lateral reinforcement comprising only ties (\(\rho =0.333\)%). Also, the ductility was noticeably improved. Results revealed that metal mesh strength had the upper hand in magnifying ultimate load and ductility. Moreover, optimum lateral reinforcement (that develops significant improvement in load capacity and ductility without degradation of stiffness) was determined. Metal meshes enhanced fire resistance by dropping concrete core temperature and mitigating ultimate load reduction of post-heated RC columns compared to reference specimens.

FRP for seismic strengthening of shear controlled RC columns: Experience from earthquakes and experimental analysis

Abstract The high vulnerability of existing Reinforced Concrete (RC) structures, even to moderate seismic events, has been confirmed from recent post-earthquake surveys. Short and wall-like RC columns are particularly prone to brittle failures, governed by concrete crushing. To reduce the vulnerability of existing RC columns, the use of externally bonded Fibre Reinforced Polymer (FRP) reinforcement has been recognized as an effective method for preventing the aforementioned brittle failure and, hence, increasing members' lateral capacity and ductility. In the first part of this study, the results of an observational analysis on columns shear failures in RC buildings severely damaged after the L'Aquila earthquake are presented. The second part of the study presents and discusses the results of an experimental program carried out on seven short RC columns governed by shear failure under load reversal and compressive axial load. Both columns in “as built” configuration and strengthened in shear with discontinuous carbon FRP (CFRP) strips have been tested. Two classes of concrete have been used, in order to simulate structures with medium or poor material quality, and different external reinforcement ratios have been investigated. The specimens' responses have been analysed in terms of failure modes, strength/deformation capacity and strain distribution in CFRP strips.

Estimating Shear Strength of Short Rectangular Reinforced Concrete Columns Using Nonlinear Regression and Gene Expression Programming

Shear collapse may be considered as the most important reason of the failure of Short Reinforced Concrete Columns (SR-RCCs) subjected to strong earthquake or other extreme loads. Although the current design provisions are mainly based on experimental data, they are too simple to consider all contributing factors. In this study, 30 test results of SR-RCCs from available laboratory tests have been collected. Next, an accurate Finite Element (FE) model is calibrated using 10 out of 30 laboratory tests. It is shown that the calibrated model successfully represents the behavior of SR-RCCs. Subsequently, 53 simulations are performed using the calibrated model. The results are used to establish a Gene Expression Programming (GEP) model, which considers most effective parameters such as axial force, transverse reinforcement ratio, cross-sectional aspect ratio, compressive strength, and ductility. Besides, a Non-linear Regression (NR) is also presented using the same simulation results. Finally, the remaining validation laboratory tests are used to compare the accuracy of the GEP-based and NR models with ACI and EC2 design codes. The validation results in an average error of 15% for GEP, whereas for NR, ACI, and EC2 the average errors are 50%, 45%, and 43%, respectively. It is concluded that the proposed model, developed by GEP, provides more accurate results in the prediction of shear strength of SR-RCCs in comparison with the others at least within available test results.

Strength and ductility of innovative hybrid NSM reinforced and FRP confined short RC columns under axial compression

Strengthening of concrete structures using Fiber Reinforced Polymer (FRP) composites has gained prominence in the recent years due to its various advantages. Reinforced Concrete (RC) columns are predominantly subjected to compression loading and often need strengthening to increase their strength and ductility. This paper explores an innovative hybrid strengthening technique where short RC square column elements are strengthened using both Near Surface Mounted (NSM) CFRP laminates and Externally Bonded (EB) CFRP fabrics for confinement. Ten square column elements of cross sectional dimension 230mm and height of 450mm were cast, strengthened and tested under pure axial compression to investigate the efficiency of different combinations of strengthening techniques. Existing constitutive models available from past literature were used to predict the behavior of FRP strengthened RC column elements. A close correlation was observed between the peak strength obtained from experimental and analytical values. Hybrid strengthening technique was found to be more efficient leading to higher increase in strength, stiffness and ductility as compared to only NSM strengthened or only CFRP confined RC columns.

English

English

Axial Compression Performance of Reinforced Concrete Short Columns with Supplementary V-Shaped Ties

Axial Compression Performance of Reinforced Concrete Short Columns with Supplementary V-Shaped Ties

Prediction of Lateral Load Displacement Curves for Reinforced Concrete Short Columns Failed in Shear

Short columns in buildings are generally the members with high stiffness and governed by brittle shear failure. Therefore, lateral load displacement curves for short columns have significant influence on the seismic assessment. In this study, a trilinear relationship is proposed to simulate the seismic behavior of short columns failed in shear with the height-to-depth ratio of less than 2. The turning points of the proposed curve are defined as shear cracking, shear strength and collapse point, and straight lines connecting these points. Shear cracking is an apparent turning point in stiffness from shear cracking of concrete to reaching the shear strength of the short column. As cracks propagate, concrete struts are formed in the cracks and the crushing strength of the struts is subsequently reached. The proposed trilinear relationship of short columns is based on these observations from experimental results and can greatly simplify the analytical model. At last, a comparison with experimental results to verify and to check the applicability of the proposed model is reported.

Studies on the behavior of Reinforced Concrete Short Column subjected to fire

The paper investigates the axial capacity of reinforced columns exposed to fire. The simplified method namely 500°C isotherm method explained in Eurocode 2 is used to assess the capacity of the column. Finite element software ANSYS is used to perform the thermal analysis. A set of numerical studies were carried out to quantify the effect of various parameters on short columns subjected to fire. The study is performed on columns of different cross-sections to investigate the effect of eight parameters, namely the thermal boundary conditions, grades of concrete, grades of steel, types of aggregate, distribution of reinforcement on column faces, concrete cover, load eccentricity and support conditions. The fire ratings based on various failure criteria are determined, and minimum rating is accepted as design fire rating. A simplified interaction curve to predict the failure of columns under fire subjected to axial load and uniaxial moment for square cross-sections was also developed.

Improved confinement of reinforced concrete columns

Abstract Traditional steel ties reinforcement cannot provide superior confinement for reinforced concrete (RC) columns due to the constraints on tie spacing and disturbance of concrete continuity. This paper presents a practical confinement configuration consisting of single Expanded Metal Mesh (EMM) layer in additional to regular tie reinforcement. The EMM layer is warped above ties. The proposed transverse reinforcement, with various volumetric ratios of ties, was investigated in sixteen square short RC column specimens categorized in two groups according to their slenderness ratios. The specimens were cast in vertical position simulating the construction field and they were tested under concentric compression till failure. The results indicated that the columns, confined with proposed lateral reinforcement, revealed significant improvement in the strength and ductility. Also, high reduction in ties volumetric ratio with no loss in ultimate load could be achieved by installing the EMM layer.