High-strength Reinforced Concrete Columns Research Articles

Combined nomograms for calculating the bearing capacity and reinforcement of high-strength and normal concrete RC columns

PurposeThis paper aims to obtain a calculation method by hand without iteration.Design/methodology/approachThis paper adopts strains as known quantities to solve the internal forces and deformations of the section, simplifies the deflection curve of the column and obtains nomograms that can calculate the bearing capacity and reinforcement of circular reinforced concrete (RC) columns by hand.FindingsNomograms include five variables: mechanical reinforcement ratio, relative normal force, dimensionless bending moment, slenderness ratio and ultimate dimensionless curvature. Nomograms corresponding to all classes of concrete have been drawn, and their dimensionless form makes them widely applicable. The calculation results of nomograms are compared and analysed with numerical calculation results, and the difference is within 5%, meeting the engineering requirements.Originality/valueCalculating the bearing capacity of compression bending components requires considering second-order effects. Therefore, the calculation of the bearing capacity of circular RC columns requires iterative calculation, as it includes dual nonlinearity of material and geometry, and the two are coupled with each other. To calculate the bearing capacity of the section adopting ordinary concrete, it is necessary to solve the transcendental equation iteratively. For high-strength concrete, it can only be solved by numerical integration. A fast calculation method by hand is proposed in this paper.

Crack-controlled allowable forces for high-strength reinforced concrete columns under cyclic loading using finite element analysis

High-strength reinforced concrete (HSRC) columns have been gradually promoted in use. However, the crack-controlled design method for HSRC columns is still insufficient to ensure serviceability and reparability. Therefore, the main purpose of this work is to provide a simplified method for assessing crack damage to HSRC columns. For this purpose, Finite Element Analysis (FEA) is developed to simulate crack development in selected specimens. Additionally, the FEA results are used to identify the reduction factors that are related to the stiffness and the unloading stiffness and the ratio of shear deformation to total deformation, which are required in the force-crack quantification model. Finally, for practical use in the design, a simplified formula for building the relationship between the shear force and maximum residual crack width for an HSRC column is also proposed. For practical use in the design, this work conducts a case study to build the relationship between the shear force and damage level for an HSRC column. Restated, an engineer can use this relationship to determine the allowable shear forces for ensuring serviceability and reparability.

Evaluation of Mechanical Characteristics of High-Strength Reinforced Concrete Columns with Hexagonal Chicken Wire Mesh Under Cyclic Loading

Recently, new materials and methods have been implemented to improve the performance of reinforced concrete columns. This paper studies experimentally the behavior of reinforced high-strength columns, with additional internal confinement using hexagonal chicken wire mesh (HCWM) to enhance ductility, energy absorption, and ultimate strength and reduce the brittleness of traditional columns. Eight double-ended one-third-scale columns with different volumetric ratios of transverse rebar have been constructed and loaded under the uniform axial compression and lateral cyclic loads. Different volumetric ratios of transverse rebar were provided based on special, intermediate, and ordinary moment frames specifications of ACI 318–14 (named SD, ID, and OD, respectively). The specimens were confined by transverse rebars and HCWM and the results were compared with reference specimens with traditional transverse ties. Experimental findings demonstrated that HCWM specimens exhibited better performance compared to their control specimens concerning crack control efficiency, load capacity, ductility, energy dissipation, and failure modes. Implementation of HCWM increased the ultimate deformation, ductility, and energy absorption of OD specimens up to 21%, 50%, and 175%, respectively, compared to their control specimen. Confinement of the concrete columns by HCWM increased the ultimate strength of OD and ID specimens up to 15% and 22% under the cyclic load, but has a negligible effect on SD columns due to enough ratio of transverse and longitudinal rebars in SD specimens. As a result, confinement of specimens by HCWM could be regarded as a simple and effective method to improve the seismic characteristics of RC columns, particularly those members constructed with a minimal number of transverse rebars.

Explainable machine learning using real, synthetic and augmented fire tests to predict fire resistance and spalling of RC columns

This paper presents the development of systematic machine learning (ML) approach to enable explainable and rapid assessment of fire resistance and fire-induced spalling of reinforced concrete (RC) columns. The developed approach comprises an ensemble of three novel ML algorithms namely; random forest (RF), extreme gradient boosted trees (ExGBT), and deep learning (DL). These algorithms are trained to account for a wide collection of geometric characteristics and material properties, as well as loading conditions to examine fire performance of normal and high strength RC columns by analyzing a comprehensive database of fire tests comprising of over 494 observations. The developed ensemble is also capable of presenting quantifiable insights to ML predictions; thus, breaking free from the notion of “black-box” ML and establishing a solid step towards transparent and explainable ML. Most importantly, this work tackles the scarcity of available fire tests by proposing new techniques to leverage the use of real, synthetic, and augmented fire test observations. The developed ML ensemble has been calibrated and validated for standard and design fire exposures and one-, two-, three- and four-sided fire exposures thus; covering a wide range of practical scenarios present during fire incidents. When fully deployed, the developed ensemble can analyze over 5,000 RC columns in under 60 s; thus, providing an attractive solution for researchers and practitioners. The presented approach can also be easily extended for evaluating fire resistance and spalling of other structural members under varying fire scenarios and loading conditions and hence paves the way to modernize the state of this research area and practice.

ASSESSMENT OF NOMINAL SHEAR STRENGTH OF REINFORCED CONCRETE COLUMN

: SNI 2847-2019 and ACI 318-19 have regulated the use of materials in the design for shear strength. This study presents an evaluation of nominal shear strength in Reinforced Concrete (RC) column from SNI 2847-2019 and ACI 318-19 by variants of the RC columns experimental database. RC column variants consist of normal strength and high strength RC column with a total of 162 specimens. Furthermore, the database also provides a variety of shear span to depth ratio and axial-load ratio. In this study, the calculation was carried out the mean and the coefficient of variation of measured to calculated strength ratio which is according to SNI, ACI, and other experimental models. Both SNI 2847-2019 and ACI 318-19 have 2 different (concrete nominal shear strength) equations. The calculation results show that the equation is more conservative in both SNI and ACI compared to other nominal shear strength equations. Overall, the combination of both normal strength and high strength RC columns, the calculation using ACI 318-19 with the first nominal shear strength equation can be said to be more conservative than any other calculations models. This study also shows that by using the ACI 318-19 model in the second equation which is used the size effect modification factor , the results do not have a significant effect because the value is close to 1.

Effects of tie detailing configurations on reinforcement buckling and seismic performance of high-strength RC columns

The premature buckling and subsequently low-cycle fatigue fracture of longitudinal bars is one of the main failure modes of reinforced concrete (RC) columns under earthquake loading. The use of high-strength steel bars (HSSB) in RC columns reduces rebar diameter or increases tie spacing, thus increases the vulnerability of bar buckling. In view of this, this paper studies the effects of transverse reinforcement detailing (referred hereafter also as ‘tie detailing’) configurations on reinforcement buckling and then on the seismic performance of high-strength RC columns. Five rectangular RC columns with different tie detailings and axial load ratios were tested under constant axial load and reversed horizontal load. All three tie detailings studied have the same large rectangle closed-hoop, while small hoops/ties are different: including two cross-ties (Type Ⅰ), a small rhombus closed-hoop (Type Ⅱ), or two small rectangle closed-hoops (Type Ⅲ). The axial load ratio includes 0.06 and 0.18. The anti-buckling constraint mechanisms of different tie detailings are proposed, and then the global buckling length of longitudinal bars in RC columns with different tie detailings are calculated. On this basis, the effects of tie detailing on bar buckling and on the seismic performance of RC columns are studied. The observed results of bar buckling verify the reliability of the proposed anti-buckling constraint mechanisms and reinforcement buckling length calculation model. Type Ⅲ tie detailing provide better anti-buckling constraint, and thus reduces buckling length of longitudinal bars of RC columns than that with Type Ⅰ & Type Ⅱ tie detailings. Meanwhile, the use of Type Ⅲ tie detailing enhances the seismic performance of RC columns (ultimate drift ratio and hysteretic dissipated energy) than that with Type Ⅰ & Type Ⅱ tie detailings. The effects of tie detailing on seismic performance of RC columns are more significant under high axial load ratio. As the longitudinal bars are more susceptible to buckle under high axial compression, thus Type Ⅲ tie detailing can more prominently play its advantages of delaying bar buckling and improving the seismic performance of RC columns.

Acoustic Emission Monitoring of High-Strength Concrete Columns Subjected to Compressive Axial Loading.

Acoustic Emission (AE) nondestructive tests have attracted great interest for their use in the determination of structural properties and behavior of reinforced concrete (RC) elements. One of the applications this method can contribute to is in high-strength concrete (HSC) columns. These elements have a great advantage in the lower stories of high-rise buildings. However, the premature failure of the concrete cover and the brittleness nature of the failure is of a concern for engineers. This paper presents a study on the AE monitoring of HSC columns subjected to compressive axial loading. The study consists of four large-scale reinforced HSC columns with different confinement reinforcement and height. It is shown that the AE distributions in the columns are categorized by three stages. Moreover, the levels of loads reached at the first AE macro event are similar to the lower range levels of the nominal axial compressive strengths of the tested specimens, while the majority of macro AE events are located at the concrete cover. Based on the results of this study, AE monitoring can provide indications for the damage and load levels attained by reinforced high-strength concrete columns subjected to compressive axial loading.

Experimental Investigation on the Force-Crack Quantification Model for HSRC Columns with Flexure\u2013Shear and Shear Failure Modes

A total of six full-scale high strength reinforced concrete (HSRC) columns were tested under axial and cyclic lateral loading. The specified concrete compressive strength was 70 MPa and the specified yield strength was 685 MPa and 785 MPa for the longitudinal and transverse reinforcements, respectively. The main variables considered in the study are the transverse reinforcements ratio and axial load ratio. Although such HSRC columns have gradually transformed in use and scope, the damage assessment method is less understood. The main purpose of this study is to propose a damage assessment model for HSRC columns. An analytical backbone curve model for predicting force–deformation behavior of HSRC columns is described. Column stiffness is also measured from the experiment to obtain stiffness reduction factors that are necessary to calculate member deformation. Based on experiment results, a new limiting value of residual crack width is defined to determine damage level. This study uses specified residual crack width of 0.15 mm, 0.30 mm, and 1.00 mm in the damage assessment model. The new limiting value of residual crack width is also used to determine the performance points of structural members. Finally, a new drift ratio limit of each damage level is also proposed. Experiment results are presented and used to investigate the application of the proposed damage assessment model.

Experimental investigation on the seismic behavior and damage states of reinforced high strength concrete columns

Eight one-third scale concrete columns designed based on the latest seismic provisions for Special Moment Frame systems were experimentally investigated. To do so, the specimens were subjected to lateral cyclic loading and axial loads up to 35% of the axial capacity of columns, other test variables included the concrete strength, spacing, and diameters of transverse reinforcement. Accordingly, damage states were developed accounting for the specimens and attributed to the backbone curve by which the reinforced High-Strength Concrete (HSC) columns capacity and the performance levels were represented. Moreover, the range of drift ratios and damage index limits were proposed and utilized versus the performance levels of reinforced HSC columns used in Special Moment Frame systems. Finally, the correlation between the calculated drift ratio and observed damages such as initial crack, concrete cover spalling, longitudinal bar buckling, and concrete core crushing were studied. The results indicate that reinforced HSC columns have a more reliable performance at Immediate Occupancy and Damage Control levels compare to other levels.

Creep of reinforced high‐strength concrete‐containing industrial by‐products silica fume and slag

AbstractThis study aimed to examine the creep of reinforced high‐strength concrete (HSC) containing industrial byproducts, silica fume (SF), and slag (SL). In total, 36 HSC specimens were prepared and tested for creep. Three specimens were used for the plain HSC creep test, 6 specimens were used for the reinforced HSC creep test, and 27 specimens were used for companion shrinkage tests. The applicability of existing creep models in predicting the creep of HSC containing SF and SL were evaluated. In addition, the effect of compressive strength and reinforcement ratio on reinforced HSC was investigated. Moreover, a new procedure to calculate the creep of reinforced HSC columns, based on the modified creep model and an existing creep recovery model, was developed. Finally, a parametric analysis was conducted to investigate the effects of compressive strength, reinforcement ratio, stress–strength ratio, loading age, and volume–surface ratio on reinforced HSC.

Force-displacement behavior of limited ductile high-strength RC columns under bidirectional earthquake actions

This paper endeavors to enhance the understanding of the non-linear force-displacement hysteretic behavior of reinforced concrete (RC) columns subjected to bidirectional earthquake actions with different loading paths and magnitudes of lateral displacement in the two axes. RC column testing in literature has traditionally been focused towards the unidirectional testing of both normal and high strength concrete columns or the bidirectional testing of normal strength concrete columns only. This study will address this important knowledge gap by experimentally investigating the biaxial lateral capacity of high-strength RC columns with limited ductile detailing. The results of this experimental study and other previous studies are also used to develop a simplified model for predicting the non-linear backbone force-displacement behavior of RC columns. The paper will firstly present the six full-scale column specimens, the test setup, and the loading procedure. The test results will then be presented and analyzed. Three different bidirectional loading protocols were adopted to study the effects of loading history. The important effects of axial load ratio on the collapse behavior (i.e. maximum drift) are also examined in detail. The paper will conclude with an overview of the proposed backbone model for predicting the force-drift behavior of RC columns subjected to bidirectional loading.

Strengthening and Repair of Reinforced Concrete Columns by Jacketing: State-of-the-Art Review

Sustainability necessitates the protection of infrastructure from any kind of deterioration over the life cycle of the asset. Deterioration in the capacity of reinforced concrete (RC) infrastructure (e.g., bridges, buildings, etc.) may result from localised damage sustained during extreme loading scenarios, such as earthquakes, hurricanes or tsunamis. In addition, factors such as the corrosion of rebars or ageing may also deteriorate or degrade the capacity of an RC column, thereby necessitating immediate strengthening to either extend or ensure its design life is not limited. The aim of this paper is to provide a state-of-the-art review of various strengthening and repair methods for RC columns proposed by different researchers in the last two decades. The scope of this review paper is limited to jacketing techniques for strengthening and/or repairing both normal- and high-strength RC columns. The paper also identifies potential research gaps and outlines the future direction of research into the strengthening and repair of RC columns.

Size Effect on the Seismic Performance of High-Strength Reinforced Concrete Columns with Different Shear Span-to-Depth Ratios

The size effect on the seismic performance of conventional reinforced concrete columns has been observed in terms of flexural failure and shear failure. Under earthquake loading, slender columns experience flexural failure, and short columns experience flexure-shear failure and shear failure. However, the effect of section size on the seismic performance of high-strength reinforced concrete columns under the conditions of different shear span-to-depth ratios requires further confirmation. For this purpose, six high-strength reinforced concrete columns with shear span-to-depth ratios of 2 and 4 were subjected to cyclic loading in this study. The experimental results indicated that relative nominal flexural strength, energy dissipation coefficient, factor of safety, and local factor of safety all exhibited a strong size effect by decreasing with increasing column size. Furthermore, the size effect became stronger as the shear span-to-depth ratio was increased, except for average energy dissipation coefficient. The observed changes in the factor of safety were in good agreement with the Type 2 size effect model proposed by Bažant. Thus, based on the local factor of safety and Bažant’s Type 2 model, the code equation for moment capacity of different shear span-to-depth ratios was modified to provide a consistent factor of safety regardless of column size.

Analytical investigation on the load-moment characteristics of GFRP bar reinforced circular NSC and HSC columns

In this study, the efficiency of Glass Fibre Reinforced-Polymer (GFRP) bar reinforced normal strength concrete (NSC) and high strength concrete (HSC) columns in sustaining axial and flexural loads was analytically investigated. Experimental data from available literature were used as benchmarks for the analytical investigations conducted in this study. In addition, a comprehensive parametric study was carried out to investigate the effect of different parameters (i.e., compressive strength of concrete, mechanical properties and reinforcement ratio of GFRP bars, and slenderness ratio of the columns) on the performance of concrete columns reinforced with GFRP bars. It was observed that under concentric axial load, the improvements in the axial load carrying capacity due to increasing GFRP longitudinal reinforcement ratio were more pronounced in GFRP bar reinforced NSC columns than in GFRP bar reinforced HSC columns. It was also observed that HSC columns reinforced longitudinally with GFRP bars with small longitudinal reinforcement ratio or low tensile modulus of elasticity might experience a tensile failure of the GFRP bars located on the tension side of the column cross-sections, especially if the columns are subjected to a high level of axial load eccentricity.

Numerical Analysis of Shear Behavior of High-Strength Reinforced Concrete Columns

Although the utilization of high-strength concrete and high-strength steel can reduced column dimension at high-rise building, the column aspect ratio remain low. These column were tended to dominate by shear failure than flexure failure. The research discusses the numerical analysis of shear critical of High-strength reinforced concrete columns. The Uniaxial Shear Flexure Method (USFM) was used to observe this behavior and examined on the test result. This study showed that USFM method provided conservative prediction. Some modification was proposed in order to improve this method when estimate the shear behaviour of high-strength reinforced concrete column.

Unified Models for Post-Peak Failure Drifts of Normal- and High-Strength RC Columns

Displacement-based design of reinforced-concrete (RC) columns requires reliable estimation of the drift capacity beyond the elastic range. Although many post-peak drift models exist for normal-stre...

Experimental study of cyclic behavior of high-strength reinforced concrete columns with different transverse reinforcement detailing configurations

Detailing of transverse reinforcement is essential to ensure ductile behavior of reinforced concrete (RC) members, particularly for columns with large displacement demanding under high axial loading level. Requirements of reinforcing details are even important for high strength concrete due to its brittleness nature. This paper presents experimental study regarding cyclic behavior of high strength RC columns with different transverse reinforcement detailing layouts. The three high strength RC columns consisted of different transverse reinforcement detailing configurations, which were conventional closed-hoops, butt-welded hoops and single closed-hoop with cross-ties, respectively. The columns were made of high-strength concrete with compressive strength of 70 MPa and high-strength steel with yield stresses of 685 MPa and 785 MPa for longitudinal and transverse reinforcement, respectively. Cyclic displacement static tests subjected to high axial compression loading of 0.3Agf′c were conducted to verify the adequacy of transverse detailing. The performance of the column with conventional closed-hoops and that of the column with butt-welded hoops, which were compliant of ACI 318-14, were almost identical and met the performance criteria required by ACI 374. In contrast, the column with a single closed-hoop and cross-tie that was designed based on ACI 318-11 did not perform well as a ductile RC column. These results prove that the ACI 318-14 minimum requirements for confinement should be followed. In addition, butt-welded hoops are acceptable as a form of transverse reinforcement of a column, since no fracture was observed at the welded location. The current flexural and shear design equations for the RC columns are discussed and compared with the test results.

The influence of stirrup-confined action on the size effect of high-strength RC columns under small eccentric compression

The aim of the present study is to explore the influence of stirrup-confined action on size effect of high-strength reinforced concrete (RC) columns subjected to small-eccentric ( e 0 =0.25 h 0 ) compressive loading. A series of eccentric compressive tests of the mechanical performances of 12 geometrically similar high-strength RC columns with different stirrups ratios (in the stirrup ratios of 0%, 0.66%) were conducted, in which the maximum cross-section size of the high-strength RC columns was 800 mm×800 mm. Moreover, a meso-scale simulation method for the investigation of the global mechanical properties of RC columns was established. The numerical tests on the RC columns with larger stirrup ratios (1.2% and 2.4%) was then carried out complementarily, based on the fact that the present simulation results were consistent with the available test observations. Subsequently, the failure behavior of the RC columns under eccentric loadings was studied, involving the failure patterns, the nominal compressive stress-strain relationships, the load-carrying capacity, the nominal compressive strength and the post-peak softening behavior. The size effect of nominal compressive strength was also explored. Furthermore, the influencing mechanism of stirrup-confined action on failure behavior and size effect of high-strength RC columns under small-eccentric compressive loading was revealed. One can conclude from the results that: 1) the size effect exists in the nominal compressive strengths of the eccentrically loaded high-strength RC columns with the present four different stirrup ratios; 2) the bi-logarithmic plots of nominal compressive strengths for RC columns under eccentric loading follows closely the “size effect law (SEL)” proposed by Bažant; 3) the presence of the stirrup confinement changes the failure patterns, improves the nominal strengths and makes the post-peak behavior of columns more ductile, and finally it weakens the corresponding size effect; 4) a meso-scale numerical method conducted presents the mechanical behavior of eccentrically loaded RC columns well.

Size effect tests of high-strength RC columns under eccentric loading

The objectives of the study are to examine the possibility of existence of size effect in eccentrically loaded high-strength reinforced concrete (RC) columns, and discover the confinement effect of transversal reinforcement on the size effect. A series of eccentric compressive tests of 24 geometrically similar high-strength RC columns with different structural sizes (with ratio 1:2:4, and slenderness of 4.5), different eccentricities and different stirrups ratios were conducted. Two eccentricities (i.e., 0.6h0 and 0.25h0, for large and small-eccentric loadings respectively) and two groups of columns with and without stirrups at the mid-length were utilized. The widths of the square cross-section of the RC columns tested were between 200mm and 800mm, and the lengths varied from 900mm to 3600mm. The behavior of the RC columns, involving the failure patterns, the nominal compressive stress-strain relationships, the ductility, the nominal strength and the post-peak behavior of columns were observed and explored. The test results demonstrate the existence of size effect in both the large and small eccentrically-loaded RC columns. The presence of the transverse reinforcement changes the failure patterns, improves the nominal strengths and makes the post-peak behavior of columns less brittle. Compared to the columns with middle stirrups, the ones without stirrups at the mid-length present a stronger size effect for their higher brittleness. Moreover, the tested nominal strengths follow closely the “size effect law (SEL)” proposed by Bažant.

Mechanical behavior and size effect of moderate high-strength RC columns under monotonic and cyclic axial compression

The reliable design of reinforced concrete (RC) structures against external mechanical forces, including earthquake-induced, impact and other types of forces, necessitates a clear understanding of the mechanical behavior and size effect of moderate high-strength RC structural members under cyclic loading. This study presents the results of an experimental study on a series of geometrically similar moderate high-strength RC columns under monotonic and cyclic axial compression. A total of 16 moderate high-strength RC columns with different structural dimensions (in the ratio 1:2:3:4) were tested. The cross-sectional size of the columns was between 200mm and 800mm, and the length varied from 600mm to 2400mm. The overall mechanical performances of the moderate high-strength RC columns, including the failure patterns, the hysteretic curves, the nominal compressive stress-strain relationships, the peak load-carrying capacity, the energy-dissipation capacity, the nominal compressive strength, the concrete softening behavior and the buckling/necking of steel rebar were observed and explored. The test observations indicate the existence of size effect in relatively larger-sized moderate high-strength RC columns under both monotonic and cyclic axial compression, and the RC columns under cyclic loading pronounce a more obvious size effect. It is found that the bi-logarithmic plots of nominal compressive strengths for different moderate high-strength RC columns follows closely the “size effect law (SEL)” proposed by Bažant.