Now showing 1 - 7 of 7
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    Experimental investigation of shear-extension coupling effect in anisotropic reinforced concrete membrane elements
    (05-12-2022)
    Kosuru, Ratnasai
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    Performance based analysis under seismic loads using the finite element method for wall-type reinforced concrete (RC) members in buildings and in important structures like liquid retaining structures, nuclear containment structures, offshore concrete gravity structures etc., necessitates the understanding of the non-linear behaviour of the constituent membrane elements. The current orthotropic formulation of the softened membrane model (SMM) can be strictly used only when the reinforcement is symmetric to the principal axes of applied stresses. When the reinforcement is asymmetric, shear strain is generated due to the normal stresses in the principal axes of applied stresses, which is referred to as shear-extension coupling. An anisotropic formulation is required to capture the generated shear strain. The current study quantifies the shear strain due to asymmetry in reinforcement, by testing panels under biaxial tension-compression using a large-scale panel testing facility. A model for the shear strain is proposed based on the tests data. The paper presents the experimental programme, important test results and the modelling of shear strain. Expression developed for the shear strain can be incorporated in the solution algorithm of the SMM for improved prediction of the shear behaviour of a membrane element. This further aids in accurate prediction of the seismic performance of the important structures mentioned earlier.
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    Publication
    Seismic performance of strengthened reinforced concrete columns
    (01-10-2020)
    Murugan, Komathi
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    Post-earthquake reconnaissance studies have reported severe damage to reinforced concrete buildings. In a moment resistant framed building, columns are the critical members of the load path. Shear failure of a column is brittle, leading to quick degradation of the lateral strength and vertical load carrying capacity. The reported study focuses on seismic strengthening of a short shear-critical column by concrete jacketing technique. For a shear-critical jacketed column, the integrity of the new and old concrete depends on any slippage at their interface. The investigation on the effects of three different interfaces such as surface roughening, providing dowel bars or bent shear connector bars in addition to surface roughening, on the seismic performance of jacketed columns, is presented in this paper. Large-scale column specimens were tested to shear failure under lateral monotonic and cyclic loads, in presence of estimated service level axial loads. The details of the experiments and the comparison of their results in terms of shear strength, stiffness and behavior are included. The specimens with different interfaces were found to be similar in their lateral load resistance, showing negligible difference in strength. However, the specimens with bent bars showed higher stiffness compared to the specimens with other two types of interfaces. The strengths of the specimens were predicted using codal provisions and a strut-and-tie model. Their behavior was traced using a piecewise linear model. The predictions were compared with the test results. The proposed method of analysis for shear deformation can be used in developing non-linear shear hinge properties for short jacketed columns, in a pushover analysis of a building.
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    Publication
    Formulation of a generalized truss analogy for the analysis of shear behavior of short jacketed columns
    (01-09-2023)
    Murugan, Komathi
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    Short columns in a story of a reinforced concrete building tend to fail in shear during an earthquake. They can be strengthened using concrete jacketing. A performance-based evaluation requires the modeling of nonlinear lateral load versus deformation behavior of such a column. While previous studies are available to analyze the behavior of a flexure-critical column, computationally simple method of analysis of the shear behavior of a short column is lacking in the literature. A generalized truss analogy is proposed to predict the shear behavior of short columns beyond diagonal cracking and till failure. Its formulation satisfies equilibrium of forces in the concrete strut and reinforcing bar ties, compatibility of strains in the concrete and ties of the web region of a column, and suitable constitutive relationships of the materials. First, the formulation and computation algorithm as applicable to an original column, are presented in this paper. Next, the method is extended to columns strengthened by concrete jacketing. Its application to a composite jacketed section is implemented using a sandwich model. The validation of the proposed method is presented, comparing the predictions with the lateral load versus deflection behavior curves obtained from the tests of five large-scale shear-critical column specimens (two original and three jacketed), tested as part of this research. The good correlation of the predicted and test results confirmed the application of the proposed method for the nonlinear analysis of shear for short columns.
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    Performance of high-strength concrete as jacket material in strengthening applications
    (01-05-2020)
    Murugan, Komathi
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    A major advantage of high-strength concrete (HSC) is the reduction in size of members in a reinforced concrete building. The present research examines the application of HSC as a jacketing material to increase the shear strength of columns. Self-compacting concrete of two different strengths (high and normal) was used to jacket twelve shear-critical beam-column specimens. The details of specimen preparation, tests and results are discussed in this paper. It was observed that the use of HSC is suitable for increasing the shear capacity of columns with thin jackets, with improvement in dowel action of the added longitudinal bars and diagonal crushing strength of the concrete. There was no undesirable increase in stiffness as compared to the specimens jacketed with normal-strength concrete.
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    Tensile Behaviour of Corroded Strands in Prestressed Concrete Systems
    (01-09-2022)
    Giriraju, Resmi
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    Corrosion of strands can cause brittle failure of prestressed concrete bridge girders. The present study experimentally investigated the effect of chloride-induced corrosion on the tensile behaviour of 7-wire strands commonly used in prestressed concrete structures. Five prism specimens with embedded strands were subjected to impressed current corrosion in chloride environment. The corroded strands were extracted, and their section loss profiles were obtained using Computed Tomography (CT) scanning technique. Pitting factor was used to quantify the severity of corrosion in a strand. The strand specimens were tested under tension to generate the average stress versus average strain curves for the corroded regions. It was found that the ductility of a strand was completely lost even at an average section loss of about 12%. Analytical models for the mechanical properties of the tensile behaviour of a corroded strand were developed. An attempt was made to estimate the pitting factor from the average section loss, that can be used for field applications in the absence of section loss profile for a corroded strand. Finally, the developed models were substantiated by comparing with the results of the tension tests. They were also corroborated using test results and theoretical predictions available in the literature.
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    Effect of compressive strength of concrete on transmission length of pre-tensioned concrete systems
    (01-02-2020)
    Mohandoss, Prabha
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    This paper presents an experimental study on Lt in pre-tensioned concrete (PTC) members made of four values of compressive strength of concrete. Also, it presents a comparison between the formulations of transmission length (Lt) given in various codes and other literature. A test specimen consisted of a concrete prism (100 × 100 × 2000) mm with a prestressed seven–wire strand (12.7 mm diameter) at the center. The values of average compressive strength for concrete at transfer (fci) were 23, 28, 36, and 43 MPa. The challenges involved with the measurements of DEMEC readings, difference between the readings from surface–mounted discs and the inserts are discussed. The results indicate that Lt could decrease by 33% when the fci increases from about 23 to 43 MPa. Based on this data, a new bi-linear formulation to determine the Lt as a function of fci is proposed. Further, it is shown that higher estimates of Lt as per the available formulations, will lead to lower estimate of bursting tensile stress in concrete generated during transfer. Hence, a precise estimate of Lt as a function of the strength of concrete at transfer is expected to provide more rational design of transmission zone reinforcement.
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    Publication
    Modeling of Column Shear Hinges in Pushover Analysis and Experimental Validation
    (2024-08-01)
    Murugan, Komathi
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    A column is a critical member in a reinforced concrete framed building. It transfers lateral shear during an earthquake, in the presence of gravity loads. Failure of short wall-type shear-critical columns in the soft and weak open ground story (OGS) of a building during an earthquake can trigger pancake-type collapse. To avoid these types of failures, the weak columns in the OGS can be strengthened locally, in addition to a global retrofit strategy applied to the entire structure. This research focuses on the seismic strengthening of such short columns using the concrete jacketing technique. The study presented in this paper demonstrates a methodology to model the shear behavior of short columns before and after jacketing. This pertains to the nonlinear static analysis of a building under lateral loads, such as a static pushover analysis. The development of shear hinge properties for as-built and jacketed short columns is presented in this paper. These were based on a proposed generalized truss analogy, which was validated based on the tests of column specimens conducted as part of this research. Pushover analyses of a building model without and with jacketed columns in the OGS were conducted. Selected results of the analyses are presented in this paper. The modeling of the shear hinge properties for the short columns in the OGS demonstrated the brittle behavior under lateral loads before jacketing and the improvement in ductility after jacketing. The proposed method can be used in professional practice for evaluating buildings with short columns strengthened by concrete jacketing. Since all the columns in a story cannot be jacketed due to economic and functional considerations, the effect of selective jacketing of the columns on the behavior of the building under lateral loads is illustrated.