• Computers and Concrete
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• v.27 no.5
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• pp.437-445
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• 2021

Retrofitting of structures has gained importance over the recent years. Particularly, Reinforced Cement Concrete (RCC) column strengthening has become a challenge to the structural engineers, owing to the risks and complexities involved in it. There are several methods of RCC column strengthening viz. RCC jacketing, steel jacketing and Fiber Reinforced Polymer (FRP) wrapping etc., FRP wrapping is the most promising alternative when compared to the others. The large research database shows FRP wrapping, through lateral confinement, improves the axial load carrying capacity of the columns under concentric loading. However, its confining efficiency reduces under eccentric loading. Hence a relative newer technique called Near Surface Mounting (NSM), in which Carbon FRP (CFRP) strips are epoxy grouted to the precut grooves in the cover concrete of the columns, has been thrust domain of research. NSM technique strengthens the column nominally under concentric load case while significantly under eccentric case. A novel configuration of NSM in which the vertical NSM (VNSM) strips are being connected by horizontal NSM (HNSM) strips was numerically investigated under both concentric and eccentric loading. It was found that the configuration with 6 HNSM strips performed better under eccentric loading than under concentric loading, while the configuration with 3 HNSM strips performed better under concentric loading than under eccentric loading. Hence an optimum of 4 HNSM strips is recommended as strengthening measure for the given column specifications. It was also found that Aluminum alloy cannot be used instead of CFRP in NSM applications owing to its lower mechanical properties.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.1
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• pp.9-14
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• 2017

In the present work, a mechanical performances under cyclic loading in RC (Reinforced Concrete) beams with normal steel and FRPH (Fiber Reinforced Plastic Hybrid) bar are investigated. For the work, RC beam members with $200{\times}200{\times}2175mm$ of geometry and 24 Mpa of design strength are prepared, and 4-point-bending tests are performed for evaluation of cracking, yielding, and ultimate loads. Through static loading test, 48.9kN and 36.0 kN of yielding loads are measured for normal RC and FRPH beam, respectively. They have almost same ultimate load of 50.0 kN. Typical tension hardening behavior is observed in FRPH beam, which is caused by the behavior of FRPH bar with tension hardening. In cyclic loading conditions, FRPH beam has more smaller crack width and scattered crack pattern, and it shows more elastic recovery than normal RC beam. The energy dissipation ratio in FRPH beam is 0.83, which is greater than 0.62 in normal RC beam and it shows more effective resistance to cyclic loadings.

• Steel and Composite Structures
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• v.44 no.6
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• pp.845-865
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• 2022

In this study, the performance of shear deficient reinforced concrete (RC) beams with rectangular cross-sections, which were externally bonded reinforced (EBR) with high strength CFRP and GFRP strips composite along shear spans, has been experimentally and analytically investigated under vertical load. In the study, the minimum CFRP and GFRP strips width over spacing were considered. The shear beam with turned end to a bending beam was investigated by applying different composite strips. Therefore various arising in each of strength, ductility, rigidity, and energy dissipation capacity were obtained. A total of 12 small-scaled experimental programs have been performed. Beam dimensions have been taken as 100×150×1000 mm. Four beams have been tested as unstrengthened samples. This paper focuses on the effect of minimum CFRP and GFRP strip width on behaviours of RC beams shear-strengthened with full-wrapping, U-wrapping, and U-wrapping+longitudinal bonding strips. Strengthened beams showed significant increments for flexural ductility, energy dissipation, and inelastic performance. The full wrapping strips applied against shear failure have increased the load-carrying capacity of samples 53%-63% interval rate. Although full wrapping is the best strengthening choice, the U-wrapping and U-wrapping+longitudinal strips of both CFRP and GFRP bonding increased the shear capacity by 53%~75% compared to the S2 sample. In terms of ductility, the best result has been obtained by the type of strengthening where the S5 beam was completely GFRP wrapped. The experimental results were also compared with the analytically given by ACI440.2R-17, TBEC-2019 and FIB-2001. Especially in U-wrapped beams, the estimation of FIB was determined to be 81%. The estimates of the other codes are far from meeting the experimental results; therefore, essential improvements should be applied to the codes, especially regarding CFRP and GFRP deformation and approaches for longitudinal strip connections. According to the test results, it is suggested that GFRP, which is at least as effective but cheaper than CFRP, may be preferred for strengthening applications.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.6
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• pp.409-416
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• 2019

An improved numerical model for non-linear analysis of reinforced concrete (RC) slabs subjected to blast loading is proposed. This approach considers a strain rate dependent orthotropic constitutive model that directly determines the stress state using the stress-strain relation acquired from the data obtained using the biaxial strength envelope. Moreover, the bond-slip between concrete and reinforcing steel is gradually enlarged after the occurrence of cracks and is concentrated in the plastic hinge region. The bond-slip model is introduced to consider the crack direction of the concrete under a biaxial stress state. Correlation studies between the numerical analysis and the experimental results were performed to evaluate the analytical model. The results show that the proposed model can effectively be used in dynamic analyses of reinforced concrete slab members subjected to explosive loading. Moreover, it was determined that it is important to consider biaxial behavior in the material model and the bond-slip effect.

• Structural Engineering and Mechanics
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• v.17 no.3_4
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• pp.357-378
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• 2004

Nonlinear dynamic analysis of a reinforced concrete (RC) frame under earthquake loading is performed in this paper on the basis of a hysteretic moment-curvature relation. Unlike previous analytical moment-curvature relations which take into account the flexural deformation only with the perfect-bond assumption, by introducing an equivalent flexural stiffness, the proposed relation considers the rigid-body-motion due to anchorage slip at the fixed end, which accounts for more than 50% of the total deformation. The advantage of the proposed relation, compared with both the layered section approach and the multi-component model, may be the ease of its application to a complex structure composed of many elements and on the reduction in calculation time and memory space. Describing the structural response more exactly becomes possible through the use of curved unloading and reloading branches inferred from the stress-strain relation of steel and consideration of the pinching effect caused by axial force. Finally, the applicability of the proposed model to the nonlinear dynamic analysis of RC structures is established through correlation studies between analytical and experimental results.

• Structural Engineering and Mechanics
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• v.61 no.4
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• pp.483-496
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• 2017

Earthquake excitations may induce important amount of seismic energy into structures. Current design philosophy mainly deals with the plastic deformations of replaceable energy dissipating devices rather than damages accumulated on structural members. Since earthquake damage is substantially concentrated on these devices they could be replaced after severe earthquakes. In this study, the efficiency of steel cushion (SC) on seismic improvement of a vulnerable reinforced concrete (RC) frame is determined by means of several numerical simulations. The cyclic shear behaviors of SCs were determined by performing quasi-static tests. The test results were the main basis of the theoretical model of SCs which were used in the numerical analysis. These analyses were performed on three types of RC frames namely bare frame (BF), full-braced frame (F-BF) and semi-braced frame (S-BF). According to analysis results; implementation of SCs has considerable effects in reducing the storey shear forces and storey drifts. Moreover plastic energy demands of structural elements were reduced which indicates a significant improvement in seismic behavior of the RC frame preventing damage accumulation on structural elements. Full-braced frame having SCs with the thickness of 25 mm has better performance than semi-braced frame interms of energy dissipation. However, global energy dissipation demand of S-BF and F-BF having SCs with the thickness of 18 mm are almost similar.

• Steel and Composite Structures
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• v.38 no.6
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• pp.671-690
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• 2021

Seismic responses of RC core wall with two outriggers are investigated in this study. In the models analyzed here, one of the outriggers is fixed at the top of the building and the second is placed at different levels along the height of the system. Each of the systems resulting from the placement of the outrigger at different locations is designed according to the prescriptive codes. The location of the outrigger changes along the height. Linear design of all the structures is accomplished by using prescriptive codes. Buckling restrained braces (BRBs) are used in the outriggers and forward directivity near fault and far fault earthquake record sets are used at maximum considered earthquake (MCE) level. Results from nonlinear time history analysis demonstrate that BRB outriggers can change the seismic responses like force distribution and deformation demand of the RC core-walls over the height and lead to the new plastic hinge arrangement over the core-wall height. Plasticity extension in the RC core wall occurs at the base as well as adjacent to the outrigger levels. Considering the maximum inter-story drift ratio (IDR) demand as an engineering parameter, the best location for the second outrigger is at 0.75H, in which the maximum IDR at the region upper the second outrigger level is approximately equal to the corresponding value in the lower region.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.05a
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• pp.885-890
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• 2002

Pseudo dynamic test is an on-line computer control method to achieve the realism of shaking table test with the economy and versatility of the conventional quasi-static approach. Pseudo dynamic tests of four full-size RC bridge piers have been carried out to investigate their seismic performance. For the purpose of precise evaluation, the experimental investigation was conducted to study the seismic performance of the real size specimen, which is constructed for highway bridge piers in Korean peninsula. Since it is believed that Korea belongs to the moderate seismicity region, three test specimens were designed in accordance with limited ductility design concept. Another one test specimen was nonseismically designed according to a conventional code. Important test parameters were transverse reinforcement and lap splicing. Lap splicing was frequently used in the plastic hinge region of many bridge columns. Furthermore, the seismic design code is not present about lap splice in Korean Roadway Bridge Design Code. The results show that specimens designed according to the limited ductility design concept exhibit higher seismic resistance. Specimens with longitudinal steel lap splice in the plastic hinge region appeared to significantly fail at low ductility level.

• Journal of the Korea institute for structural maintenance and inspection
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• v.17 no.5
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• pp.49-58
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• 2013

Eight small scale circular reinforced concrete columns were tested under cyclic lateral load with constant axial load. Test specimens were designed with 4.5 aspect ratio. The selected test variables are longitudinal steel ratio, transverse steel ratio, yielding strength of longitudinal steel and axial load ratio. The test results of columns with different longitudinal steel ratio, transverse steel ratio and axial load ratio showed different seismic performance such as equivalent damping ratio, residual displacement and effective stiffness. It was found that the column with low strength of longitudinal steel showed significantly reduced seismic performance, especially for equivalent damping ratio and residual displacement. The regulation of flexural over-strength is adopted by Korea Bridge Design Specifications (Limited state design, 2012). The test results are compared with nominal strength, result of nonlinear moment-curvature analysis and the design specifications such as AASHTO LRFD and Korea Bridge Design Specifications (Limited state design).

• Journal of the Korea Concrete Institute
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• v.21 no.1
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• pp.65-74
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• 2009

Glass fiber reinforced polymer (GFRP) bars are sometimes used when corrosion of conventional reinforcing steel bar is of concern. In this study, a total of 36 beams and one-way slabs reinforced using GFRP bars were tested in flexure. Four different GFRP bars of 13 mm diameter were used in the test program. In most test specimens, the GFRP bars were lap spliced at center. All beams and slabs were tested under 4-point loads so that the spliced region be subject to constant moment. Test variables were splice lengths, cover thicknesses, and bar spacings. No stirrups were used in the spliced region so that the tests result in conservative bond strengths. Average bond stresses that develop between GFRP bars and concrete were determined through nonlinear analysis of the cross-sections. An average bond stress prediction equation was derived utilizing two-variable linear regression. A splice length equation based on 5% fractile concept was then developed. As a result of this study, a rational equation with which design splice lengths of the GFRP bars can be determined, was proposed.