Kyong Min Ro

The previous research introduced an innovative retrofitting technique for reinforced concrete beams using modularized steel plates. This technique enhances structural performance, offering a lightweight solution compared to conventional retrofitting methods using steel plates, and accommodates construction errors. However, a challenge arises due to the lack of integrity between unit steel plates. To address this, this study proposes a novel method of connecting each steel plate with bolts. The experimental results show that retrofitted beams achieved a maximum load of 311.9 kN, roughly 1.6 times that of non-retrofitted specimens, with the ductility of retrofitted beams being 3.3 times that of the non-retrofitted beams. Additionally, there was a 25% increase in load capacity for beams retrofitted with interconnected steel plates compared to those without connections between unit steel plates.

This study investigated the seismic performance of reinforced concrete columns retrofitted with Replaceable Steel Links (RSLs), focusing on the effects of varying sliding slot lengths and torsional loads. The RSL system, known for its simple construction and effective seismic performance, was analyzed to assess the feasibility of substituting damaged steel links post-earthquake, using the system as a structural fuse. The experimental results highlighted the role of sliding slot lengths in delaying the initiation of shear cracks, especially under eccentric lateral loads. The RSL system exhibited notable torsional resistance, showing only a 10% decrease in maximum load capacity, even with a two-fold increase in the eccentric distance. Furthermore, with an increase in sliding slot length, the difference in cumulative energy dissipation attributable to augmented eccentric distances reached approximately 50%, indicating a notable impact of sliding slot length on the system’s ability to resist torsion. Consequently, it is recommended that the length of the sliding slot be based on the specific seismic design objectives when employing the RSL system as a structural fuse. The post-experiment inspection revealed no deformation in the steel plates, and the buckled steel links could be effortlessly replaced by loosening the high-tensile bolts in the slots. These findings demonstrate the RSL system’s efficiency as a structural fuse.

The adequacy of retrofitting with concrete jacketing is influenced by the bonding between the old section and jacketing section. In this study, five specimens were fabricated, and cyclic loading tests were performed to investigate the integration behavior of the hybrid concrete jacketing method under combined loads. The experimental results showed that the strength of the proposed retrofitting method increased approximately three times compared to the old column, and bonding capacity was also improved. This paper proposed a shear strength equation that considers the slip between the jacketed section and the old section. Moreover, a factor was proposed for considering the reduction in the shear capacity of the stirrup resulting from the slippage between the mortar and stirrup utilized on the jacketing section. The accuracy and validity of the proposed equations were examined through a comparison with the ACI 318-19 design criteria and test results.

The concrete jacketing method for retrofitting old reinforced concrete (RC) columns should secure confinement and seismic performance under torsion as well as unidirectional later loads. In a previous study, a hybrid concrete jacketing method was proposed using steel wire mesh (SWM), steel grid reinforcement (SGR), which can replace reinforcement of existing concrete jacketing method, and using steel fiber non-shrinkage mortar (SFNM). These details can simplify the retrofitting process of the existing concrete jacketing method, and seismic performance was evaluated by conducting a cyclic loading test under unidirectional loading. In this paper, the torsional behavior of RC columns retrofitted with the hybrid concrete jacketing method was investigated. Four specimens were fabricated and conducted cyclic loading tests under two types of loading schemes, unidirectional and bidirectional loading, to examine the effect of the loading path. The strength and energy dissipation capacity of retrofitted columns with hybrid concrete jackets increased approximately eight times compared to the old RC columns under torsional loading. Therefore, the hybrid concrete jacketing method can improve torsional resistance.

In modular structures, prefabricated modular units are joined at the construction site. Modular structures must ensure splicing performance by connecting modular units sufficiently. The bolted connection using steel plates may suffer from alignment issues and corrosion problems. In a precast concrete (PC) modular system, there is difficulty grouting the sleeves when splicing reinforcing bars. This study proposed a PC modular beam using a bolted connecting plate to deal with issues in typical steel modules and PC modules. The structural performance was evaluated by flexural and shear tests on two monolithic beams and two proposed PC specimens. The test results showed that the structural performance of the PC modular specimen was 88% of that of the monolithic reinforced concrete (RC) beam specimen and 102% of the strength calculated by ACI 318-19. Therefore, the proposed PC modular system using bolted connecting plates can solve the problems observed in typical steel and PC modules and improve the structural performance.

The effective retrofit strategy of non-seismic designed structures is required to improve the strength and ductility. Jacketing is the most typical method which can enhance seismic resistance capacity, but there are many disadvantages due to its enlarged jacket section. This study developed a non-welded concrete-filled steel tube system (NoWS) which was installed using bolts in the form of tube-type steel-encased beams and columns; the NoWS could increase the capacity of seismic resistance strength and the ductility of members. One reinforced concrete (RC) frame specimen with non-seismic details and one NoWS were manufactured, and cyclic loading tests were conducted until critical failure was observed in the column after reaching maximum load. As experimental results, the specimen retrofitted with NoWS had excellent seismic performance, showing resistance to lateral load, effective stiffnesses, and energy dissipation capacity approximately 2–3 times greater than the results of an RC frame specimen.

Recently, interest in using precast concrete (PC) modules has increased due to their better seismic performance than steel modules. However, they must be joined by additional elements to ensure structural integration between the modules. The essential aim of the precast concrete module is to ensure structural performance with appropriate connection methods. However, the technical problem of connecting PC modules still needs to be improved. This study proposed a PC modular beam system for improved structural and splicing performance, and simple construction. This modular system consisted of modules with steel plates inserted to improve integrity of modules, ease of construction, and low cost. The structural performance of the proposed PC modular beam system was evaluated by flexural test on one reinforced concrete (RC) beam specimen consisting of a monolithic beam, and two PC specimens with the proposed modular system. The results demonstrated that the proposed PC modular beam system achieved approximately 80% of the structural performance compared to the monolithic specimen, with approximately 1.3-fold greater ductility.

Buildings constructed with non-seismic details are at risk of damage and collapse when an earthquake occurs due to a lack of strength, stiffness, and ductility. For reinforced concrete (RC) moment-resisting frames, seismic retrofitting methods that can enhance strength or ductility should be applied. However, such strategies have many disadvantages related to constructability, serviceability, securing integrity, and cost. In this paper, a welded concrete-filled steel tube (WCFST) system was examined in order to resolve the problems of the existing seismic retrofitting methods for RC moment-resisting frames. To evaluate the seismic performance of the proposed system, two specimens, one with non-seismic details and another reinforced with a WCFST seismic system, were manufactured for the cyclic loading tests. As a result of the experiments, the specimen retrofitted with the WCFST system showed maximum load, effective stiffness, and energy dissipation capacity values approximately 3, 2, and 2.5 times greater, respectively, than those obtained from the existing reinforced concrete frame specimen. The experimental results indicate that the proposed WCFST system is expected to be effective at improving the seismic performance by enhancing both the strength of the existing reinforced concrete frame structures and the dissipation of the seismic energy.

This paper evaluated the validity of a design method for anchorage zones in post-tensioned concrete members using Grade 2400 strands from American Concrete Institute (ACI) Building Code, ACI 318-19 and Association of State Highway and Transportation Officials (AASHTO) codes. For this purpose, the behaviors of anchorage zones in ten specimens were investigated and compared with ACI 318-19 and AASHTO codes, considering the jacking force and ultimate flexural load. The anchor shape, number of strands, and reinforcement details of the anchorage zone were considered as variables. The test results showed that ACI 318-19 and AASHTO codes could not predict where the maximum bursting force would occur. In addition, they calculated bursting force by summation of the pre-stressing forces from the individual strands, whereas the experimental results were not linearly proportional to the number of strands. The ACI 318-19 and AASHTO standards were inadequate to calculate bearing force of post-tensioned members using high-strength strands. Furthermore, additional stress in the anchorage zone occurred as the flexural load increased, which may have been caused by a failure in the anchorage zone. The experimental results indicated that the method used to predict stress and design of anchorage zone in current codes should be improved to consider stress distribution and additional stress in post-tensioned concrete members using high-strength strands.