Seismic Behavior Of Reinforced Concrete Columns Research Articles

Seismic retrofitting of RC columns using stainless steel grid-reinforced UHPC jackets in plastic hinge zone

This study aims to propose a new seismic retrofitting method for reinforced concrete (RC) columns in buildings with insufficient or flawed design in early standards by locally using stainless steel grid-reinforced ultra-high performance concrete (UHPC) jackets in the plastic hinge zone. For this purpose, pseudo-static loading tests were conducted on four rectangular columns with economical reinforcement ratios, including three retrofitted columns and one traditional RC column, and the effects of the number of stainless steel grid layers and the height of UHPC jackets were investigated. Additionally, a fiber-based finite element model was developed for the retrofitted columns and its accuracy was verified. The influence of other parameters not considered in the experiment, such as the axial compression ratio, UHPC jacket thickness and concrete compressive strength on the seismic performance of the retrofitted columns were also analyzed. The results demonstrated that the proposed retrofitting method effectively improves the seismic behavior of RC columns in a quick and cost-effective manner. The combination of stainless steel grids and UHPC jackets provided stable confinement for the internal RC columns, significantly reducing the damage in the plastic hinge zone and the residual displacements. Compared with the RC column, the ductility, bearing capacity and energy dissipation of the retrofitted columns were enhanced. Especially, the ductility coefficient of the specimens using a two-layer stainless steel grid increased by 27.19 %–36.56 %.

The Assessment of the Influence of Soil Type on the Nonlinear Seismic Response of a Corroded Reinforced Concrete Portal Frame

Background: Most researchers have focused on the effect of rebar corrosion on the seismic behaviour of reinforced concrete (RC) columns. On the other hand, the influence of chloride corrosion of reinforced concrete in combination with soil type on the seismic behaviour of RC columns has not been investigated. Objective: This paper investigates the influence of soil characteristics on the seismic vulnerability of reinforced concrete structures. The case study in focus is a reinforced concrete portal frame with columns exhibiting corrosion at ground level. Methods: Chloride ingress initiates corrosion and reduces the cross-sectional area of the corroded concrete steel. In addition, this research considers two different soil types, namely rocky and loose, as relevant factors in the analysis. Firstly, the seismic loads are determined according to the Moroccan seismic building code RPS2011. Then, the evaluation of the evolution of the diameter of corroded bars due to chloride induced corrosion is investigated. Finally, pushover analysis is used to determine the location of the performance point on the capacity curve. Results: The study also includes an investigation of the ductility characteristics of corroded reinforced concrete structures and the inter-storey displacement in different soil conditions. The results show that the inter-storey displacement decreases with increasing age of the structure. In addition, it is observed that the base shear force and roof displacement at the point of failure are more sensitive in loose soils compared to rocky soils. Conclusion: The results indicate that consideration of soil type can significantly alter the expected seismic performance, resulting in higher vulnerability values.

Seismic behaviour of RC columns retrofitted with textile-reinforced mortar (TRM) optimized by short PVA fibres

Textile-reinforced mortar (TRM) optimized by short polyvinyl alcohol (PVA) fibres was used as a strengthening material for seismically deficient reinforced concrete (RC) columns. Seven RC columns, including a reference column, a column strengthened with the inorganic matrix added short PVA fibres, and five columns strengthened with the optimized TRM, were subjected to low-frequency cyclic loading. The influences of the reinforcing ratio of carbon textile and axial load ratio on the seismic performance of the jacketed columns were studied. The results showed that the shear capacity and deformability of RC column respectively increased by 54.3 ∼ 55.2 % and 40.34 ∼ 78.62 % after strengthening with the optimized TRM system. Increasing the reinforcing ratio of carbon textiles in the strengthening layer, the displacement ductility and energy dissipation of the jacketed column were further enhanced. As the axial load ratio increased, the shear capacity of the jacketed column increased; however, the corresponding ductility and energy-dissipation ability decreased. The relationship between the shear contribution and material configuration of the TRM was established. The shear capacity of the TRM-strengthened columns can be estimated by a modified shear model considering the reinforcement ratio of textile and the volume content of PVA fibres.

Seismic behavior of RC columns strengthened with near-surface-mounted aluminum alloy bars and CFRP wraps

This paper proposed a near-surface-mounted (NSM) strengthening technique by using aluminum alloy (AA) bars and carbon fiber-reinforced polymer (CFRP) wraps to improve the seismic behavior of square reinforced concrete (RC) columns. Eight strengthened concrete columns with square cross sections of 250 mm × 250 mm and 1600 mm shear span were tested under axial compression and reversal cyclic loading to investigate the effects of NSM reinforcement ratios (0.36–1.29%), layers of CFRP wraps (0 and 3), and axial compression ratios (0.1 and 0.4) on the seismic behavior. The failure mode, hysteretic response, load-bearing capacity, ductility, stiffness degradation, and energy dissipation capacity of those column specimens were analyzed. Results indicated that the lateral load-bearing capacity, stiffness, and cumulative energy dissipation capacity increased with the increase of the NSM reinforcement ratio. The CFRP wraps in the NSM technique can provide hoop confinement to avoid the buckling failure of NSM AA bars, and improve the seismic behavior by mitigating spalling of concrete. When increasing the axial compression ratio from 0.1 to 0.4, the lateral load-bearing capacity and cumulative energy dissipation capacity increased significantly, while the stiffness degradation became obvious. In addition, three-dimensional (3D) finite element (FE) models were developed and verified by comparisons in the deformation and lateral loads at characteristic points, as well as the strain distribution along NSM AA bars. The verified FE models can be used to analyze the damage of concrete and the stress distribution at the interface between epoxy and concrete.

Investigation of the contribution of the reinforcement tie to the seismic behavior of reinforced-concrete columns

In reinforced-concrete structural columns, special earthquake reinforcement ties are used as well as longitudinal and transverse reinforcement. Deficiencies related to all these reinforcements can cause different levels of damage to the reinforced-concrete structural elements after the earthquake. Within the scope of this study, a total of eight different structural models were created for the reinforcement-ties effect in reinforced-concrete columns, considering four different reinforcement-tie models and two different reinforcement materials. The period, seismic capacity and target displacement values were obtained for each structural model. In addition, demand, limit and capacity values for shear force were obtained and compared. Material strength and the amount of reinforcement ties used contributed significantly to the seismic behaviour of the structure.

Seismic Behavior of Existing Reinforced Concrete Columns with Non-Seismic Details under Low Axial Loads.

Reinforced concrete (RC) columns of old existing buildings are vulnerable to earthquakes because the hoops comprising their transverse reinforcement are widely spaced and anchored using 90° hooks. This study extensively evaluated the seismic behavior of RC columns with such non-seismic details. Experiments were conducted by applying lateral cyclic loads to five full-scale column specimens with various transverse reinforcement details subjected to low axial loads. The experimental results demonstrated that the internal transverse crosstie had a significant confinement effect in the non-seismic detailed columns with 90° hoop anchor hooks. In addition, the lateral load–drift relationships, ductilities, and energy dissipation capabilities of the columns were not significantly affected by the hoop spacing or anchor hook angle when a low axial load was applied up to a drift ratio of 3.5% before failure. The evaluation model based on ASCE/SEI 41-17 was then shown to approximate the initial stiffness, maximum strength, and post-peak strength reduction behavior of the non-seismically reinforced column. This study was based on the experimental behavior of single column members, and it needs to be extended to research on frame structures in which columns are connected to beams and slabs.

Experimental study on the seismic behavior of RC columns improved with central large-diameter butted steel rods

A reinforced concrete (RC) column improved with central large-diameter butted steel rods (CLBSR column) has been proposed to increase the limit value of the axial compression ratio. This paper presents the seismic behavior analysis of CLBSR columns subjected to low-cycle reciprocating loading. The CLBSR columns were also compared with the RC column improved with central large-diameter steel rods (CLSR column) and the RC column improved with central reinforcement bars (CRB column) under the same experimental conditions. The typical failure modes, hysteretic curves, skeleton curves, stiffness degradation, and energy dissipation capacity were all investigated. The improved PARK damage model obtained the corresponding relationships between the damage indices and the damage state of each column. The test results indicated that the CLBSR columns and CLSR columns had similar seismic behavior, but were slightly inferior to the CRB columns. Overall, the butt joint of large-diameter steel rods had little effect on the seismic behavior.

RETRACTED: Effect of EBR- and EBROG-GFRP laminate on the structural performance of corroded reinforced concrete columns subjected to a hysteresis load

The reinforced concrete (RC) columns infiltrated by chloride ions in the marine environment are much vulnerable under seismic loading and corrosion can severely reduce their load-bearing capacity. To improve the load-bearing capacity of RC columns subjected to different load conditions and hazardous environments using glass fibre reinforced polymers (GFRP) have been gained a great deal of attention. Therefore, this paper investigated the seismic behaviour of RC columns strengthened by GFRP laminate exposed to the different rebars corrosion levels. A total of 15 square RC columns were fabricated and tested under the lateral seismic load. A novel wrapping method was employed to perform different longitudinal rebar corrosion at levels of 0%, 5%, 10%, 15% and 20%. To introduce GFRP laminate on the specimens’ surfaces, two strengthening techniques were applied: Externally Bonded Reinforcement (EBR) and Externally Bonded Reinforcement on Grooves (EBROG), and the effect of these strengthening techniques on the hysteresis behaviour, energy dissipation, stiffness, ductility, load-bearing capacity, and deformation of specimens were studied. Additionally, new models were developed to predict the ductility, stiffness, and energy absorption of EBR- and EBROG-GFRP strengthened RC columns with various rebar corrosion levels. Results showed that using GFRP laminate improved the maximum load-bearing capacity, stiffness, ductility and energy absorption of corroded RC columns, and these properties were further improved using the EBROG technique. However, the difference between the improvement effect of EBR and EBROG techniques was declined with increasing the longitudinal rebars corrosion level. Additionally, the proposed models with high accuracy could be utilized as an efficient tool to anticipate the ductility, stiffness and energy absorption of EBR- and EBROG-GFRP strengthened RC columns exposed to different rebar corrosion levels.

Experiment investigation of the influence of reinforcing bar buckling on seismic behavior of RC columns

The inelastic buckling of longitudinal steel bar is one of the major damage states of reinforced concrete (RC) column under seismic loading, which may result in a reduction of section moment capacity and ductility. However, the relationship between bar buckling and deterioration in seismic behaviors of RC column has normally been proposed based on the theoretical analysis, direct experimental verification is scarce, and the differences between the buckling behavior of individual steel bar and that of longitudinal bar in RC column remain unclear. In this paper, a series of experiments on cantilever RC columns and individual bars were conducted to study the buckling behavior by means of the measured lateral buckling displacements of the bars. A new method was proposed to measure two components of the lateral buckling displacements of longitudinal bars and individual bars in orthogonal directions, the reliability and accuracy of the proposed method have been identified by the comparison of measured buckling displacement of corner bars in two identical RC specimens. Based on the measured lateral buckling displacements of reinforcing bar in RC columns and corresponding individual bar, the onset of buckling and the buckling direction of the two kinds of steel bars were analyzed. It is found that buckled main bars in RC columns behave differently with individual steel bars due to the constraint effect of cover concrete. The equivalent average stress of main bar was determined by creating a mapping between the buckling displacement of bar in RC section and the average stress of individual bar. On the basis of the buckling displacements and the equivalent average stress, the effects of bar buckling on the post-peak bearing capacity and the displacement ductility of RC column were investigated. The results show that the degradation of the post-peak lateral force of RC column is generally weaker than the strength deterioration of buckled steel bar due to the load redistribution between longitudinal reinforcement and surrounding core concrete.

Seismic behaviour of RC columns with lap-spliced crossties under various axial loads

Lap-spliced crossties are formed from the lap splicing of two J-shaped reinforcement bars (rebars) with a 180° hook on one end and a straight end on the other. In this study, cyclic lateral load testing of four large-scale reinforced-concrete (RC) column specimens was performed. Moreover, the integrity of lap-spliced crossties and the seismic performance of RC columns with lap-spliced crossties under various axial load ratios were examined. The results indicated the following. (i) Under various axial load ratios, column specimens with lap-spliced crossties were verified to possess favourable seismic performance and effectiveness in confining the concrete core. (ii) For all specimens with lap-spliced crossties, the straight ends of the J-shaped reinforcement bars did not slide toward the column interior. This indicated that the two J-shaped reinforcement bars were effectively combined into a crosstie with 180° hooks on both ends. (iii) Compared to conventional crossties, lap-spliced crossties are not only easier to install but also more effective in confining core concrete since lap-spliced crossties possess two effective seismic hooks while conventional crossties possess just one.

Seismic Performance of Reinforced Concrete Short Columns Subjected to Freeze–Thaw Cycles

Previous research shows that freeze–thaw cycles represent one of the most dangerous threats to reinforced concrete (RC) structures. However, there is almost no experimental data on the effects of freeze–thaw cycles on the seismic behavior of RC columns showing flexure-shear failure. In this study, three columns with the shear span-to-depth ratio of 2.5 were subjected to different numbers of freeze–thaw cycles (FTCs) and pseudo-static testing. The seismic performance indexes of the specimens were analyzed in terms of hysteretic behavior, skeleton curves, shear deformation, and energy dissipation. The test observations show that the failure patterns of the test columns altered from the flexure dominated to shear dominated, owing to the more severe deterioration in shear capacity induced by freeze–thaw attack than in flexure capacity. The test results also indicate that freeze–thaw cycles significantly decrease the ductility and energy dissipation of test columns, and they increase the contributions of shear deformation to the total deformation.

CFRP confinement in retrofitted RC columns via CSB technique under reversed lateral cyclic loading

This paper reports the results on an experimental investigation on the efficiency of confinement with carbon fiber reinforced polymer (CFRP) in retrofitting square reinforced concrete (RC) columns. The study was designed to evaluate the effect of the recently developed corner strip-batten (CSB) method on the seismic behavior of RC columns. In the CSB method, FRP battens are bonded, similar to flat coupons, at the sides of the section while each adheres on the CFRP corner strips at the two ends. For the purposes of this study, five large-scale RC columns were subjected to constant axial and cyclic lateral loads. The main parameter examined was the method employed for confining the columns. The RC columns were investigated in terms of their failure mode, hysteretic and envelope curve, enhanced lateral strength and ductility, equivalent damping ratio, stiffness degradation, and energy dissipation. The results indicated that RC columns retrofitted with CFRP composites exhibited a ductile behavior under cyclic lateral loading. Furthermore, seismic loading tests revealed that the transverse FRP sheets applied through recently developed CSB technique at the potential plastic hinge areas led to considerable enhancements in lateral strength, energy dissipation, and ductility. This is evidenced by enhancements of 51.6%, 30.3%, 27.2% and 24.6%, respectively, in the ductility, equivalent viscous damping ratio, energy dissipation, and lateral strength of columns retrofitted via the CSB technique, relative to the control column.

Seismic behavior of strengthened square reinforced concrete columns under combined loadings

This paper presents an experimental study examining the seismic behavior of square reinforced concrete (RC) columns subjected to combined loadings. In this study, nine full-scale square RC columns were made with the same structural properties. Five of them were strengthened with high-performance ferrocement laminate and bonded steel plates in order to investigate the effects of strengthening on the seismic behavior of RC columns under combined loadings. Finally, the failure mode, hysteretic load-displacement response, bearing capacity, energy dissipation, ductility and plastic hinge characteristics were recorded and analyzed. The results show that combined loadings reduced the bearing capacity and changed the failure modes and deformation characteristics. Strengthened RC columns had significantly improved bearing capacity and different failure modes under the same combined loadings in contrast to the non-strengthened RC columns. The vertical eccentricity had little effect on the mechanical properties of columns, while the horizontal eccentricity significantly weakened the mechanical properties of columns. The mechanical properties of uniaxial compression-bending-shear-torsion columns were the worst. The tri-linear model calculation parameters under different failure modes and combined loadings were fixed and validated by experimental results.

Quasi-static tests of RC columns under variable axial forces and rotations

The behaviour of reinforced-concrete (RC) elements subjected to axial force and rotation variations in conjunction with lateral displacement variation is considered a highly important topic, but only a few experimental studies investigating this topic have been performed. In this paper, five full-scale RC rectangular columns were tested by quasi-static testing (QST) to investigate the effects of large variation axial forces and rotations on the seismic behaviour of columns. Furthermore, the effects of variable axial forces and rotations on the seismic performance of RC columns in terms of the failure modes, hysteretic loops, skeleton curves, ductility factors, stiffness degradations and energy dissipations are presented and analysed. Based on the QST results, the axial force and rotation variations have significant effect on the seismic behaviour of RC columns. Generally, the variable axial forces and rotations may cause a distinct asymmetrical failure phenomenon in the specimens and reduce the lateral strength and ductility factor. Thus, RC columns should be well designed to account for the adverse effects of variable axial forces and rotations.

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.

Ductility design of RC columns. Part 2: extent of critical zone and confinement reinforcement

ABSTRACTThe current Hong Kong concrete design code (HKConcrete2013), which is a non-seismic design code, includes provisions of the design of confinement zone in reinforced concrete (RC) columns. Whether or not these provisions can be adopted in the development of the new design code of structures for seismic resistance remains uncertain. A thorough scientific investigation with a comprehensive statistical analysis on the seismic behaviour of RC columns under different levels of the axial compression ratio (ACR) and concrete confinement has been conducted and presented in the companion paper [1], where the use of the ACR limits for controlling the seismic performance of various types of RC columns is proposed. The present paper addresses the important aspect of the confinement zone length and the design of confinement reinforcement in RC columns under different levels of the ACR. The theoretical background of the confinement zone is presented and the requirements of the prevailing design standards for des...

Seismic Behaviour of RC Columns Strengthened with Steel Bar/Wire Mesh Mortar

In order to enhance the seismic behavior of reinforced concrete (RC) columns more efficiently, a thought to strengthen concrete columns by using steel bar/wire mesh mortar （FS）was proposed. An experimental study including five RC square columns strengthened with FS and steel bar mat mortar (S), respectively, under constant axial loading and lateral cyclic loading was carried out. Seismic bearing capacity, ductility, failure modes and hysteretic characteristics of all columns were tested, and the effect of reinforcement ratio and strengthening method to the tested columns was analyzed. The results showed that the energy dissipation capacity of FS strengthened columns was 73% higher than that of the S strengthened column, though the reinforcement ratio of the former was only 3.02% higher than that of the latter.

Seismic Behavior of RC Columns with Interlocking Spirals under Combined Loadings Including Torsion

Reinforced concrete (RC) columns of skewed and curved bridges with unequal spans or column heights can be subjected to combined loading including axial, flexure, shear, and torsion loads during earthquakes. This combination of seismic loading can induce the complex flexural and shear failure of bridge columns. Seismic performance of bridge RC columns is largely controlled by the level of confinement provided by transverse reinforcement. Interlocking spirals are commonly used in noncircular RC bridge columns because they can provide more effective confinement than rectangular hoops and simplify the column fabrication. However, the seismic behavior of columns with interlocking spirals has been studied only to a limited extent with respect to the hysteretic behavior of pure torsion and the interaction between flexure, shear, and torsion. The presence of torsion significantly affects the inelastic flexural response of RC members under seismic loadings and results in brittle failure modes. This paper presents an experimental study on the inelastic behavior of the RC columns with double interlocking spirals under combined action of cyclic flexural and torsional moments. The columns were designed with aspect ratio (H/B=5.5) and tested under various loading conditions: cyclic pure torsion, and combined cyclic flexure and torsion. The hysteretic torsional and flexural response, damage distribution, stiffness degradation and ductility characteristics with respect to various torsion-to-bending moment (T/M) ratios are discussed. The significant confinement of interlocking spirals to core concrete and its effect on the torsional resistance under combined loadings is also highlighted. Finally, interaction diagrams were established based on experimental results.

Seismic Behavior of RC Columns with Bond-Critical Regions: Criteria for Bond Strengthening Using External FRP Jackets

In 2003, an experimental research program was initiated at the American University of Beirut with the objectives of (1) evaluating the effectiveness of external fiber-reinforced polymer (FRP) confinement in improving the bond strength of spliced reinforcement in reinforced-concrete (RC) columns and its implications on the lateral load capacity and ductility of the columns under seismic loading; and (2) establishing rational design criteria for bond strengthening of spliced reinforcement using external FRP jackets. This paper presents a discussion of recent experimental results dealing with rectangular columns and the results of a pilot study conducted on circular columns with particular emphasis on aspects related to the bond strength of the spliced column reinforcement. A nonlinear analysis model is developed for predicting the envelope load–drift response, taking into account the effect of FRP confinement on the stress–strain behavior of concrete in compression. Results predicted by the model showed exc...