Ductile Detailing Research Articles

Performance of reinforced concrete columns confined with spiral reinforcement

SummaryThe study proposes a practical strengthening method for improving the seismic behavior of reinforced concrete square columns using spiral as lateral reinforcement in moderate seismic zone regions. Two specimens one without ductile detailing and the other with ductile detailing designed as per Indian standard specifications and six specimens with spiral ties (in one direction and cross directions) at 25°, 35°, and 45° of inclination were tested under constant axial load and cyclic lateral loads to study the feasibility of the provision of spiral ties as a seismic‐strengthening method. The important variable used was the angle of inclination of the spiral bars. Test results revealed that the column specimen strengthened with cross spirals at 25° had almost similar performance and ductility behavior when compared with the control specimen with ductile detailing and specimen strengthened with cross spirals at 35° had better performance and ductility behavior when compared with the control specimen without ductile detailing and can be adopted in moderate seismic regions. Considering the enhanced behavior, such as the strength, stiffness, and ductility, along with the construction easiness, for better performance of columns in moderate seismic zones, angle of inclination of 35° is sufficient.

Seismic performance of geopolymer concrete beam-column joints under reverse cyclic loading

All structural elements of the building are subjected to reverse cyclic loading during earthquakes, which causes large inelastic deformations in beam-column joints. Use of ductile concrete and presence of confinement in the joint region can reduce the seismic failure of beam-column joints. Cement industries produce a massive amount of greenhouse gases during its production and lead to global warming. Cement concrete is brittle in nature. It is necessary to reduce the production of cement for maintaining environmental sustainability. The present study investigates the behaviour of steel fibre reinforced geopolymer concrete beam-column joints under reverse cyclic loading. Steel fibres were added to GGBS-Dolomite geopolymer concrete to reduce its brittle nature. Enhanced strength properties were identified geopolymer concrete due to the presence of steel fibres. Parametric studies were conducted by finite element methods and observed similar load–deflection behaviour between GPC specimens (without ductile detailing) and cement concrete specimens in which ductile detailing provided as per IS 13920–2016, with reduced cost/strength ratio. Provision of ductile reinforcements creates congestion problem at the joints, which can be reduced by the use of geopolymer concrete made from GGBS and dolomite.

Effect of superstructure modeling assumptions on the seismic performance of seismically isolated buildings

AbstractRecent studies on the seismic performance of seismically isolated buildings with braced frames designed using the minimum criteria of the ASCE 7‐16 standard resulted in observations that seismically isolated buildings have unacceptable collapse performance due to either insufficient isolator displacement capacity or excessive inelastic structural deformations. The collapse performance and the overall performance could be improved by increasing the displacement capacity of the isolators and by limiting inelastic action in the superstructure while still providing ductile detailing. These observations may have been dependent on the modeling assumptions for the superstructure. This article investigates the effect of various modeling methods that are commonly used in research and design practice on the seismic performance of a six‐story braced seismically isolated building with two different isolation systems, and a comparable nonisolated six‐story building, all designed to meet the minimum criteria of ASCE 7‐16. Performance measures considered in this study include story drift ratios, residual story drift ratios, floor accelerations, isolator horizontal displacement, and collapse. The study found that for seismically isolated buildings and for the modeling assumptions considered, only the inclusion of the effect of gravity frames has a beneficial effect of reducing the occurrence of small to moderate peak floor accelerations while there is insignificant effect on the other performance measures and particularly the computed probabilities of collapse in the MCER. Also, the different modeling assumptions were found to have a more important effect on the seismic performance of nonisolated buildings than that of comparable seismically isolated buildings.

Experimental verification of seismic performance of built-up beam-to-column connections with ductile detail

The sections of hot-rolled H-beams produced by steel manufacturers in South Korea do not fully meet the requirements of recent buildings. Although custom-made built-up H-beams meet field requirements, they require a highly reliable manufacturing process through batch production and quality control. To this end, a steel manufacturer in South Korea recently constructed a production line for manufacturing built-up H-beams with uniform quality, and has attempted to address the problems of inefficient sections of hot-rolled members and the lack of large sections with a beam depth of 900 mm or greater. Furthermore, the steel manufacturer has attempted to produce highly ductile connection details that can be applied to large H-beam sections with a maximum depth exceeding that of existing hot-rolled H-beams to ensure seismic performance. In this study, real-scale structural performance tests were conducted on the beam-to-column connections of large built-up H-beams with ductile connection details suitable for site conditions. Built-up H-beams with a beam depth of 1000 mm were selected as specimens. The authors used a beam-end seismic welding detail and horizontal stiffener reinforcement detail, which were verified in a previous experimental study on beams with conventional depths. The test results were described in accordance with the connection performance verification process presented in the AISC standard and the seismic design code adopted in South Korea. The structural stability of the applied connection details was analyzed.

Seismic vulnerability assessment of reinforced concrete school building in Nepal

The school building built with reinforced concrete in Nepal confront high seismic risk during past seismic events. The vast extent structural damage and loss of human life’s and property was due to the poor enforcement of the standards, lack of ductile detailing and poor construction materials and practices in Nepal. The effect of earthquake excitation on buildings was ignored after the latest earthquake in 2015. Most of the school structures were used in full-fledged without performing any seismic evaluation. Seeing these situations, especially in Nepal, a vulnerability assessment of such school structures is essential. The seismic risk owing to future earthquake can be minimize by proper vulnerability assessment of the school structures. To this end, the purpose of the study is to evaluate seismic performance of school structure through both vulnerability and fragility assessment. The main research issues are explored through analytical method. For this, structural response parameters are analytically studied through linear and non-linear analyses by finite element program-based software. In dynamic analysis, numerical building models were subjected to three synthetic earthquakes Gorkha, El-Centro and Kobe. The fragility function of case study building structure was plotted with the probability of failure at every 0.1 g interval of PGA, which provided technical base for seismic vulnerability assessment of school buildings. The result indicates that the selected school building is found to be vulnerable compared with the standard international guidelines. The results of this study are more useful for different governmental authorities, emergency response organization who are directly involved for proper planning and implementation.

Comparative assessment of seismic collapse risk for non-ductile and ductile bridges: a case study in China

Risk assessment plays an important role in quantifying earthquake loss and has been widely used in the field of seismic engineering. However, current studies of the risk assessment rarely consider the different failure modes of piers (i.e., flexure failure, flexure-shear failure and shear failure), which will lead to an excessive estimation for the seismic performance and collapse safety of the reinforced concrete girder bridges. This paper focuses on studying the seismic behavior of bridges considering different failure modes and comparing the seismic collapse risk of the bridges with and without ductile detailing. The numerical analysis is performed in an open source finite element (FE) software framework, Open Sees. Nonlinear shear springs are introduced into the FE models of reinforced concrete (RC) columns to accommodate the shear and flexural-shear effect of the bridges. The accuracy of the FE models is validated by using the existing results of static cyclic-loading experiments. Case studies are conducted for two categories of the girder bridges, which are designed based on new and old seismic design criteria in China, respectively. The collapse fragility curves are developed by using the incremental dynamic analysis method, which considers the uncertainties of the ground-motion characteristics and the structural material parameters. Based on the seismic hazard curve of a designated region, a comparative assessment of the seismic collapse risk is performed regarding mean annual collapse rate for the non-ductile and ductile bridges. The main results of the paper are: (1) The FE model with the nonlinear shear spring provides an acceptably accurate solution for simulating the initial stiffness, shear failure point, strength degradation slope and residual shear strength of the RC columns; (2) The structural collapse safety is shown to be related not only to the collapse probability of the bridges but also to the occurring probability of the ground motion intensity exceeding a given threshold; (3) The mean annual collapse rate of the ductile girder bridges is significantly smaller than that of the non-ductile girder bridges. The results can be used as a reference to implement specific policies for appraising and mitigating the seismic risk of existing RC bridges.

Influence of Buckling Restrained Reinforcement on Behavior of Columns

The present study proposes an alternative reinforcement material system, known as buckling restrained reinforcement (BRR), to improve the behavior of concrete columns loaded with earthquake-type loading in moderate seismic areas. BRR is made from circular steel tubes and placed in the main bars in the plastic-hinge-critical portion. Experimental investigations were carried out on half-scale models of concrete columns provided with and without BRR and loaded with a combination of axial loading and quasi-static reverse cyclic lateral loading. Performance improvement is observed in load–deformation characteristics, energy dissipation, and lower stiffness degradation for column specimens with the proposed BRR in comparison with specimens having light ductile detailing. Moreover, congestion of tie bars near the joint was also removed by providing BRR at the plastic hinge zone with wider tie spacing (>33%) than ductile detailing, thus reducing the confining reinforcement bars by 25% and reducing the construction challenges in the field.

Performance of Exterior Column Beam Connections as built at site

Comparing performance of ill detailed and ductile detailed connections as per IS: 13920 was the prime aim of this study. It was by and large observed during literature review that, usually investigations are done at laboratory casting and curing conditions. How-ever, site conditions are grossly different from laboratory conditions. It is a general experience that the ductile detailing is rarely followed at site. Thus it was felt necessary to investigate performance of such ill detailed constructions at site testing conditions. Specimen for this experiment were manufactured by site people at site conditions and cured at site conditions. A 6-storied MRF constructed in Satara (IS: 1893, Zone 3) was analyzed. An exterior column-beam connection from first slab was chosen for assessment. Design for seismic requirements was carried out referring to suggestions from latest revisions of IS :1893 and IS: 13920. Four 0.3:1 scaled down specimen from actually site sourced concrete and steel were constructed. Out of the four, two were detailed as per actual site practice. Two specimen were detailed as per IS:13920. Specimen were subjected to reverse cyclic displacement loading protocol. It was observed that latest revisions from IS codes ensure that beam fails prior to the connection. Overall performance characteristics were seen improved in case of ductile detailed connections.

Behaviour of GGBS-dolomite geopolymer concrete short column under axial loading

Geopolymer concrete was developed from Ground Granulated Blast furnace Slag (GGBS) and dolomite which are obtained from steel and rock industries. Optimum proportions of GGBS and dolomite were found out based on the maximum compressive strength. GGBS-Dolomite geopolymer concrete can reduce construction time and cost due to high early age strength. Experimental investigations were conducted to evaluate the behavior of GGBS-dolomite geopolymer concrete short columns under axial loading. GGBS-dolomite geopolymer concrete column showed better stress-strain behavior compared to cement concrete specimens. Load carrying capacity of geopolymer concrete specimens were improved by adding steel fibres at 0.25, 0.5 and 0.75% by volume of concrete. Ductility of geopolymer concrete was enhanced by 18%, 30% and 45% with the addition of steel fibres in the range of 0.25-0.75% by the volume of concrete. Parameters such as ultimate load, deflection, ductility and crack pattern of axially loaded short columns were evaluated using finite element method. It was observed that geopolymer concrete has 28% reduction in cost/ultimate load compared to ordinary concrete. GGBS-dolomite geopolymer concrete short columns without ductile detailing behaves similar to that of cement with ductile detailing (as per IS 13920-2016) with reduced cost/strength ratio.

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.

Impact of Slab Thickness on Reinforced Concrete Buildings using Fragility Curves

Earthquakes are the natural disaster occurring since years but during the last two decades they are causing huge looses whether it may economic or to life. This paper focuses to evaluate the seismic performance of various building confirming to Indian standard criteria for earthquake resistant design of structures and ductile detailing of reinforced concrete structures subjected to seismic Forces-code of practice, Bureau of Indian Standards, both as per the revised codes in the year 2016. Due to ground shaking, seismic loads are the governing load and thus it becomes necessary to assess the conditional probability of structural response. Use of HAZUS methodology is followed to construct seismic fragility curves as it is well-organized and defined approach. Spectral displacement plays the functional parameter to derive the expected damage for fragility. This work represented here is compiled by means of procedure for establishing the fragility curves for three typical Reinforced Concrete (RC) frame structures having variations resembling 3 storey intended for short-period structures, 6 storey used for medium-period structures and 12 storey representing long-period structures using SAP2000 as a software tool for analyzing the structure. Furthermore an attempt is made for focus on the variation of one of the major structural configuration i.e. slab thickness which is not certainly paid attention as compared to columns and beams. Slabs adds additional stiffness to the structure which can enlighten how it behaviour would be when subjected to ground excitation. As a result, the fragility curves are plotted to study the impact due to slab thickness in order they are carefully selected while design.

Effectiveness of a novel ductile detailing of reinforced concrete wall–flat slab joint

ABSTRACTAn experimental investigation on the behaviour of two identical structural wall–flat slab subassemblies was conducted by testing scaled down joint models: one with conventional reinforcement detailing at the joint and the other with a proposed novel reinforcement detailing at the joint region under lateral seismic type loading. A theoretical analysis was carried out by establishing the cleavage pattern and the state of stress at the joint region. As per the results, it was observed that the proposed novel reinforcement detailing exhibited improved ductility in the joint region ensuring increased energy dissipation which is desired under seismic loading.

Ductility demands and reduction factors for 3D steel structures with pinned and semi-rigid connections

A numerical investigation regarding local (uL) and story (uS) ductility demand evaluation of steel buildings with perimeter moment resisting frames (PMRF) and interior gravity frames (IGF), is conducted in this study. The interior connections are modeled, firstly as perfectly pinned (PP), and then as semi-rigid (SR). Three models used in the SAC steel project, representing steel buildings of low-, mid-, and high-rise, are considered. The story ductility reduction factor (RuS) as well as the ratio (QGL) of RuS to uL are calculated. uL and uS, and consequently structural damage, at the PMRF are significant reduced when the usually neglected effect of SR connections is considered; average reductions larger than 40% are observed implying that the behavior of the models with SR connections is superior and that the ductility detailing of the PMRF doesn\'t need to be so stringent when SR connections are considered. RuS is approximately constant through height for low-rise buildings, but for the others it tends to increase with the story number contradicting the same proportion reduction assumed in the Equivalent Static Lateral Method (ESLM). It is implicitly assumed in IBC Code that the overall ductility reduction factor for ductile moment resisting frames is about 4; the results of this study show that this value is non-conservative for low-rise buildings but conservative for mid- and high-rise buildings implying that the ESLM fails evaluating the inelastic interstory demands. If local ductility capacity is stated as the basis for design, a value of 0.4 for QGL seems to be reasonable for low- and medium-rise buildings.

Nonlinear modeling of concentrically braced frames

Concentrically braced frames (CBFs) comprise a large proportion of lateral-force-resisting systems used in steel building infrastructure around the world. Many categories of CBFs exist, including special, ordinary, and non-seismically detailed (i.e., current “R = 3” or pre-1988 construction) CBFs. Experimental testing of these different types of CBFs has shown that they have widely varying nonlinear behavior depending on the relative strengths of their yielding mechanisms and failure modes and level of ductile detailing. Numerical modeling of this range of behavior types is necessary to evaluate the seismic performance, including to quantify potential damage to special CBFs and the vulnerability of lower-ductility CBFs. Special CBFs have been the focus of many previous nonlinear modeling recommendations, including simulation of brace fracture, gusset-plate flexural strength and stiffness, and gusset-plate contribution to frame stiffness. However, recommendations for lower-ductility CBFs have not been well established. To provide important guidance for modeling these common systems, new recommendations based on the large quantity of available experimental data are proposed for simulating fracture of rectangular HSS braces with varying local slenderness, asymmetric load histories, and concrete in-fill; axial yielding of gusset plates; fracture of brace-to-gusset-plate welds; fracture of gusset-plate interface welds; post-fracture, secondary yielding mechanisms in gusset-plate interface connections; yielding beams in the chevron configuration; and buckling columns. These recommendations are validated using experiments of two-story chevron CBFs with yielding beams which simulated an existing, pre-1988 CBF (i.e., a nonductile CBF) and a subsequent repair where the braces and gusset plates were replaced.

Modified response spectrum analysis to compute shear force in tall RC shear wall buildings

Among various analysis methods, response spectrum analysis (RSA) is widely used to compute design forces of structures subjected to earthquake loading. Previous studies indicate that using a single response modification factor (R) to reduce elastic forces contributed from all modes in the RSA procedure is the primary cause for underestimating the design force demands on structures. In this paper, two modified response spectrum analysis (MRSA) methods, MRSAHE based on a higher-mode elastic approach and MRSAHI based on a higher-mode inelastic approach are proposed for computing design shear forces in tall reinforced concrete (RC) shear wall buildings. Here 15-, 20-, 31-, and 39-story tall buildings subjected to earthquake excitations in Bangkok are first designed using the RSA procedure, and then nonlinear response history analysis (NLRHA) is carried out. It is found that the RSA procedure provides good estimates for floor displacements and story drift ratios, but significantly underestimates force demands, when compared with NLRHA. The MRSAHE method provides good estimates of shear force for the 15- and 20-story buildings and conservative estimates for the 31- and 39-story buildings; while the MRSAHI method provides good estimates for all cases. However, MRSAHI requires a nonlinear inelastic structural model but MRSAHE does not. In both MRSA methods, the bending moment demands are computed in the same way as the conventional RSA procedure, with ductile detailing in RC walls or columns required at locations where the combined axial and bending strain exceeds a certain limit. In order to determine the locations that require ductile detailing in RC walls or columns, a novel method based on the equal displacement concept using strain from elastic analysis to predict inelastic strain is found to provide good agreement with inelastic strain computed from NLRHA.

Behavior of RC exterior beam column joint retrofitted using UHP-HFRC

The performance of beam column joint (BCJ) significantly affects the behavior of RC framed structures during the earthquake. The absence of ductile detailing in pre-seismic designed RC beam column joints lead to a greater extent of damage in the non-linear rotation. The retrofitting offers a greater flexibility to strengthen the damaged beam column joint. However, the limitations of retrofitting materials and the extent of initial damage in RC beam column joint affects the retrofitting performance. Therefore, an experimental investigation is carried out to evaluate the efficacy of ultra high performance hybrid fiber reinforced concrete (UHP-HFRC) in retrofitting, and the effect of initial damage on the performance of (UHP-HFRC) retrofitted exterior beam column joint. Park and Ang [1] damage indices model is used to defining the initial damage as complete, severe, moderate and slight. Exterior BCJ, initially damaged are then retrofitted using UHP-HFRC. The test results indicate that the UHP-HFRC retrofitting increases the load carrying capacity, energy dissipation, and ductility of retrofitted BCJ over control specimen. The initial damage level affects the performance of BCJ, as the initial damage level increases from slight to complete damage, the ductility, stiffness, strength retention at the higher drift, and energy dissipation decreases.

Effect of corrosion damage on seismic behaviour of existing reinforced concrete beam-column sub-assemblages

Environmental degradation of reinforced concrete (RC) in the form of reinforcement corrosion poses greater threat to both strength and serviceability of existing RC structures. If this corrosion damaged structures are situated in the zones of higher seismicity, there would be a great threat to their safety under earthquakes and hence it is extremely important to understand their seismic performance in order to formulate suitable remedial measures. In view of this, in present study, an exterior beam-column sub-assemblage of a residential building is taken up and is designed for two levels, namely (i) designed for seismic load without ductility detailing (non-ductile specimen), and (ii) designed for lower seismic load and with ductility detailing (ductile specimen) to represent the existing RC structures of different design evolutions. Two specimens are cast for each level of design and one specimen from each of these two categories is subjected to accelerated corrosion. The experimental investigations are carried out on the uncorroded and corroded beam-column sub-assemblages under reverse cyclic loading. Corrosion of reinforcement changed the damage progression from joint shear failure to longitudinal and splitting cracking along the reinforcement bars followed by fracturing of corroded reinforcement bars. The corroded specimens showed poor hysteretic performance in the form of huge in-cyclic strength degradation. Furthermore, the cumulative energy dissipated by corroded non-ductile and corroded ductile specimens are respectively only 0.4 times and one-seventh of that of the corresponding uncorroded specimens. Thus, the seismic performance of corrosion affected existing structure is an alarming issue and warrants immediate retrofit intervention.

Experimental Study on Fibre-Reinforced Concrete Beam-Column Joint with Ductile Detailing Under Reverse Cyclic Loading

The behaviour of a beam-column (BC) joint in a moment resisting framed structure is the key criteria when the structure is subjected to seismic loading. In the past many research have been carried out to grasp the mechanism of failure of beam-column joint and methods to enhance the performance of beam-column joint have been developed. The provision of ductile detailing as prescribed in IS-13920: 1993 has been followed. Improvement of the performance of joint region has been studied by using additional reinforcement with different fibres. In the present study the performance of the BC joint is studied in terms of load vs. displacement curve, stiffness degradation vs. drift angle curve and energy dissipation by the BC joint using different types of fibres in the specimen. The laboratory testing of the concrete cubes and cylinders reinforced with different percentages of fibre volume have been implemented. Based on the test results, the selection of the optimum quantity of fibre that can be used in beam-column joint for each type of fibre has been done. The testing of the specimens of BC joints is carried out in cyclic actuator of capacity 100 kN. The fibres which have been used in the present study are carbon fibre, steel fibre and glass fibre. Percentages of the fibres that had been added to the specimen are carbon fibre (0.75%), steel fibre (0.75%) and glass fibre (0.50%) respectively and the control specimen made from ordinary concrete made of M25 mix concrete. The performance of the specimens reinforced with different type of fibres is then compared and the specimen showing desirable performance under cyclic loading is found out. It has been observed that the application of carbon fibres could improve the strength and ductility of BC joints appreciably.

NONLINEAR FINITE ELEMENT ANALYSIS OF NONSEISMICALLY DETAILED INTERIOR RC BEAM-COLUMN CONNECTION UNDER REVERSED CYCLIC LOAD

This paper presents a nonlinear finite element analysis of non-seismically detailed RC beam column connections under reversed cyclic load. The test of half-scale nonductile reinforced concrete beam-column joints was conducted. The tested specimens represented those of the actual mid-rise reinforced concrete frame buildings designed according to the non-seismic provisions of the ACI building code. The test results show that specimens representing small and medium column tributary area failed in brittle joint shear while specimen representing large column tributary area failed by ductile flexure though no ductile reinforcement details were provided. The nonlinear finite element analysis was applied to simulate the behavior of the specimens. The finite element analysis employs the smeared crack approach for modeling beam, column and joint, and employs the discrete crack approach for modeling the interface between beam and joint face. The nonlinear constitutive models of reinforced concrete elements consist of coupled tension-compression model to model normal force orthogonal and parallel to the crack and shear transfer model to capture the shear sliding mechanism. The FEM shows good comparison with test results in terms of load-displacement relations, hysteretic loops, cracking process and the failure mode of the tested specimens. The finite element analysis clarifies that the joint shear failure was caused by the collapse of principal diagonal concrete strut.

Seismic Behavior of Under Confined Square Reinforced Concrete Columns

The role of confining reinforcement in dissipating a large amount of energy in reinforced concrete (RC) elements at the time of seismic events has been well demonstrated in previous studies. In India, this practice of providing special confinement in RC elements began to be followed after the ductile detailing code came into being in the year 1993. Hence, the structures built before 1990s in seismicaly active areas had no ductile detailing, as the design recommendation of pre 90s era do not consider adequate confinement requirements. The Present experimental study is a part of an ongoing project of upgrading these old reinforced concrete structures, which had no special confinement and are located in seismically active regions. Three full-scale columns with different amount of transverse reinforcement (volumetric ratio of 1.31, 0.33, and 0.22) were tested in a specially made testing setup. One specimen was designed as per the current Indian guidelines for ductile detailing (IS 13920:2016), while the other two specimens were designed as per the guidelines which existed before the Code on ductile detailing was implemented. The response of the columns under quasi-static reverse lateral cyclic loading (at constant axial load) was recorded in terms of lateral load v/s deflection, moment v/s curvature, energy dissipation, stiffness reduction and different ductility parameters. As expected, the results of the study show inferior seismic performance of under-confined columns as indicated by the key strength and ductility parameters. The results have though quantified the strength and ductility deficit of these under confined RC columns, which shall be of great help in designing suitable retrofit for the existing structures constructed with no ductile detailing features.