• 저자: 엄태성, 임종진, 문성환, 윤성환, 정재철
• Abstract :This study investigated the flexural behavior of prestressed concrete-encased steel composite beams (PSC composite beam) with hollow core. The PSC composite section consisted of the precast composite element, made of a steel section (H or box section) and U-shaped concrete encasement, and the cast-in-place concrete slab. By prestressing strands at the bottom of the precast element, the cracking resistance was enhanced. In addition, a void was formed in the web to reduce the beam weight. Six full-scale PSC composite beams were tested under positive bending and their flexural behaviors before and after casting the cast-in-place slab were investigated. The test results have shown that the initial strains of the steel section and reinforcing bars that occurred at the prestressing stage agreed well with the predictions by the existing PSC design theory. During the subsequent flexural testing, all beams were failed by concrete crushing at the compression end or slab. The stiffness and strength of the PSC composite beams agreed well with the effective stiffness and plastic moment strength for encased composite members, respectively, calculated in accordance with AISC 360-16. At the interface across the cast-in-place slab, relative slip did not occur and the interface shear resistance was contributed by both the vertical stirrups of the precast concrete encasement and headed studs of the steel element engaged or embedded within the slab.

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• 저자: 이호준, 박홍근, 황현종, 김창수
• Abstract: Existing experimental results show that carefully designed RC column-to-steel beam (RCS) joints have high seismic resistance comparable to those of traditional steel or RC frames with seismic details. However, such high seismic performance requires complicated and expensive strengthening details, which causes difficulties in fabrication and construction. This study investigated simpler strengthening methods, targeting moderate seismic capacity. Four interior RCS joint specimens were tested under cyclic lateral loading. The major test parameters were the strengthening details of the beam–column joint such as face bearing plates, transverse beams, and headed studs. The test specimens with the proposed methods exhibited good deformation capacity exceeding 4.0% story drift ratio after yielding of the beams and beam–column connection. However, the peak load did not reach the nominal strength based on the beam plastic moment due to early joint damage. The failure modes and test strengths were compared with the predictions of ASCE design guidelines. On the basis of the results, design recommendations for the simplified connection details were proposed.

• 저자: 엄태성, 멀무히유발트, 임종진
• Abstract: In this study, the seismic performance of exterior beam-to-column connections with SD600 beam reinforcements was investigated. Six specimens designed to exhibit beam yield mechanism were tested under cyclic loading. Rectangular spirals were used for shear reinforcement at the beam-column joint where the SD600 beam reinforcements were anchored by 90º standard hooks. Concrete strengths (32.2 and 52.2 MPa), reinforcement yield strengths (442 and 662 MPa), the use of steel fiber concrete were considered as test variables. The tests showed that although the development lengths of SD600 steel bars specified in design codes were not satisfied at the joint with the limited depth, bond slip and anchorage failure did not occur in most specimens. The deformation capacities were larger than 5% in drift ratio, but in three specimens out of five specimens SD600 steel bars used for beam reinforcements eventually ruptured due to the deficient post-yield hardening behavior. The bond and shear behavior at the joint was improved by the use of higher compressive strength concrete and steel fiber concrete.

• 저자: 엄태성, 임종진
• Abstract: In this study, the flexural behavior of noncompact and slender concrete-filled U-section (CFU) beams was investigated focusing on local buckling of U-section walls and shear transfer across the beam-to-slab interface. Seven large-scale CFU beam specimens were tested under negative bending. The width-to-thickness ratios of U-section walls (i.e., flanges and webs) and stiffening details for preventing local buckling were considered as the test variables. The tests showed that, due to excessive slab reinforcement, all specimens were failed by shear failure at the composite interface of beam and slab after flexural yielding. The ultimate strength and deformation capacity of CFU beams were degraded by flange and web local buckling. Buckling modes of U-section walls were affected by the stiffening details. The ultimate strengths of noncompact and slender CFU beams were compared with the nominal strengths calculated in accordance current standards. In addition, the flexural behavior of CFU beams affected by the local buckling and partial debonding of U-section walls and slip behavior at the composite interface was further investigated through fiber-based section analyses based on the effective stress–strain models of materials.

• 저자: 임종진, 김연희, 엄태성
• Abstract: Concrete-encased steel angle (CEA) column is an encased composite column where structural angles are used for longitudinal and transverse reinforcements. In this study, the compressive behavior of CEA columns was investigated, focusing on the use of high-strength concrete and noncompact and slender steel angles. Ten CEA column specimens (concrete strength fc’ = 33 and 66 MPa and steel yield strength Fy = 454–552 MPa) were tested under compressive loading. The section classification of steel angles and the spacing of transverse ties were considered as test variables. The tests showed that the cover concrete spalled off at peak loads and local buckling occurred in steel angles. Due to the use of high-strength concrete, the compressive strength (i.e., peak loads) was increased but the post-peak degradation behavior became steeper. The peak and residual strengths of CEA columns were examined focusing on the effects of cover failure, confinement by transverse ties, and local buckling of steel angles. Based on the results, empirical equations to estimate the peak and residual strengths were proposed and the validity was assessed by comparisons with the test results.

• 저자: 엄태성, 조승리, 임종진
• Abstract: This study investigated the behavior of bolted end plate connection for structural angle sections. For the splice of angles under tensile loading, thick end plates attached to the end of angles are connected by one single high-tension bolt. To improve connection performance and reduce required plate thickness, plate washers made of the same material as that of the end plates are used under the bolt head and nut. Tensile tests of twenty-two specimens with different connection details were conducted. The tests showed that the governing failure mode was bolt rupture and the connection performance was affected by the thickness of the plates. The minimum thickness of end plate and plate washer determined by the yield line theory was conservative. Due to prying action, the connection strength (i.e., the tensile load of angles) was reduced to about 85% of the tensile strength of high-tension bolt. The force transfer behavior of the end plate connection with and without plate washer was investigated in detail through finite element analysis.

• 저자: 임종진, 이승환, 엄태성
• Abstract: This study investigated the behavior of end-plate connections for column splices with construction tolerance. To adjust misalignment between upper and lower columns, oversize holes were drilled on the end plate and plate washers were then placed to bridge over the oversize holes under the head or nut. Six columns with the misaligned connection in the plastic hinge region were tested under cyclic loads. Test variables considered were misalignments, end-plate details (i.e., thickness and stiffener), and the flexural capacity of columns. The test results showed that although plastic deformations occurred significantly near the connection with misalignment, the connection performance of the columns was good: prying action and plate deformation at the connection were minimized although the connection strength of the end plate was only 60% of the required strength because the plate washers provided additional resistance. In addition, slip deformation on the faying surface was negligible due to the preloads of fully tightened bolts. The stiffened end plate was effective in reducing the required thickness of end plate, whereas the shim plate between the top and bottom end plates slightly degraded the slip resistance. The force-transfer mechanism at the end-plate connection under axial compression was investigated and verified through finite-element analysis.

• 저자 : 임종진, 엄태성, 김진용, 김진원

• Abstract : This study investigated the flexural behavior of concrete-encased steel angle girders (PSRC girders). Inside the concrete section, high-strength steel angles (Fy = 605 MPa) with unequal legs were used as longitudinal reinforcement, and diagonal and vertical members were bolt-connected to the longitudinal angles as transverse ties. Flexural tests were performed for three PSRC girder specimens with different reinforcement details. The tests showed that the PSRC girders all developed the peak loads exceeding the nominal strengths; however, the ductility was limited as rupture occurred near bolt holes in the longitudinal angles. Overall, the concrete damage and cracking of the PSRC girders were comparable to those in conventional reinforced concrete beams, and cover spalling and bond deterioration along the longitudinal angles were not severe. The strength, stiffness, and ductility estimated in accordance with current design codes were compared with the test results. Given the investigation results, the application and design considerations of PSRC girders were discussed.

• 저자 : 임종진, 엄태성, 김진원

• Abstract : This study investigated the compressive behavior of concrete-encased high-strength steel angle columns (Fy ¼ 718 MPa). Axial compressive tests were performed on six composite columns with different angle sections and confinement details. The results showed that the failure mode of the columns was local buckling of the angle legs and crushing of the confined core. Local buckling and concrete confinement significantly affected the compressive strength and ductility. The test strengths were compared with the theoretical strengths computed using the plastic stress distribution method. For analytical investigation, effective stress–strain models of the steel angle and concrete are proposed to account for the local buckling, spalling, and confinement. Through the strain-compatibility analysis based on the proposed models, the contributions of the angles, unconfined cover, and confined core to the compressive strength varying with deformation were investigated. Based on the investigation results, recommendations for the design and detailing of concrete-encased high-strength steel angle columns are given.

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• 얇은 절곡단면을 사용한 콘크리트 충전형 합성부재를 위한 관통볼트 단부강판모멘트접합부의 거동

• 저자: 엄태성, 임종진, 이정민

• Abstract: This study investigated the behavior of a bolted end plate moment connection for the beam-column joint of filled composite members. To increase the moment resistance at the joint with bolt arrangement limited, U-shaped end plate and high-strength through bolts were used for the connection. Cyclic loading tests of three exterior beam-column connections were performed to investigate the behavior of the proposed end plate connection. Bolt arrangement and the use of the panel zone stiffener were considered as the test parameters. The tests showed that the U-end plate moment connection secured a robust moment transfer at the joint, without significant deformations of the end plate and through bolts. However, the ductility was limited due to the premature rupture at the welded joint to the end plate and low-cycle fatigue failure of thin-walled plates in the beam (i.e. inelastic local buckling and subsequent rupture). The effective stiffness of the specimens were reduced by the slip of through bolts, whereas the panel zone stiffener alleviated stiffness reduction. Based on the test results, design and detailing recommendations of the end plate connection for filled composite members with thin-walled sections were suggested.