Papers by Author: Chang Sik Choi

Abstract: Generally, a conventional standard hook is used for the reinforcement's anchorage. However, this results in steel congestion, and it makes fabrication and construction difficult. Using a headed bar offers a potential solution for these problems and may also ease fabrication, construction and concrete placement. But, in current design code of the headed bar, it had limitation about the yield strength and the diameter of rebar etc. It hard to use the large diameter headed bar in the reinforced concrete structure. This paper presents the cyclic responses of four reinforced concrete exterior beam-column joints, which are anchored with large diameter headed bars or hooked bars. To evaluate the anchorage capacity of large diameter headed bars, specimen variables were set with anchorage detail, side cover thickness of concrete, and transverse reinforcement. Also, structural performance of beam-column joints is evaluated and compared with each other. The behavior of joints with headed bars are as good as, or better than those companion joints with 90-degree hooked bars. Test results show that the large diameter headed bar has enough anchorage capacity in exterior beam-column joints. Test results show side cover of concrete improved the anchorage capacity of the bars and transverse reinforcement enhanced the anchorage capacity and ductility of joints.

Abstract: This study presents punching shear design method of voided slab in accordance with arrangement of voids around columns. According to previous studies, the slab-column connection of voided slabs is weaker than that of the solid slab due to the lack of cross-sectional area of concrete by voids. In this study, it is assumed that the arrangement of voids exert influence on the punching shear strength of voided slabs. To verify the assumption, finite element analysis was conducted related with previous test results. The variable of FE analysis was a distance between voids and column face. Based on FE analysis and test results including previous studies, punching shear design method is suggested which can consider the arrangement of voids around columns. The suggested design method is based on the punching shear design method in ACI-318. As a result, it can predict the punching shear strength of voided slabs according to arrangement of voids around column.

Abstract: In order to design reinforced concrete member using steel fiber ultra high strength concrete, current structural design methods should be re-evaluated because it has significant difference in material characteristics compared with normal concrete. In this study, bond strength of steel fiber reinforced ultra-high strength concrete was evaluated. For this purpose, direct pull out test specimens were constructed with variables of cover thickness, compressive strength of matrix and fiber inclusion ratio. According to the test, bond strength were sensitively varied with cover thickness and fiber inclusion. Because bond strength was determined by tensile strength of concrete. Comparing test results with theoretical methods suggested by Tepfers, specimens without steel fiber show good agreement with analytical method, because this method were based on elasticity. And other empirical equations were evaluated with other previous researches.

Abstract: In this study, ductility of members with ultra-high performance concrete was investigated using moment-curvature analysis for the verification of safety under large deformation of ultra-high performance concrete structural members. For the analysis of members with ultra-high performance concrete, mathematical stress-strain model was selected among the results conducted by other researchers on the compressive and tensile behavior of high strength concrete and fiber reinforced concrete. According to the investigation on ductility of members with ultra-high performance concrete, decrease of ductility was observed with increase of tensile strength of concrete under the same reinforcement ratio. Members with 2~3% of reinforcement ratio, which usually be used in the field engineering, show the decrease of ductility with increase of fiber volume fraction. As a results of parametric study, limitation of maximum reinforcement ratio ( or limitation of net tensile strain ) suggested by current design code is not safe when using ultra-high performance concrete.

Abstract: Recently, various types of slab systems which can reduce self-weight of slabs have been studied as the height and width of building structures rapidly increase. A hollow slab system is widely known as one of the effective slab system which can reduce self-weight of slab. According to previous studies, the hollow slab with donut type hollow sphere had enough flexural strength. On the other hand, there were some differences in flexural stiffness and deflection between general RC slab and donut type hollow slab. The deflection of hollow slab was influenced by the shapes of hollow spheres due to its different second moment of inertia and amount of concrete surrounding bottom re-bar. Especially, the cracking behaviors also influence the deflection of slab. General RC slab deflection has been sufficiently investigated and some models to consider the crack behavior such as crack width, crack space, tension stiffening effect, bond-slip relationship. However, there is no study about the crack behavior on the hollow slab. Therefore, the purposes of this paper are to presents the cracking behavior of donut type hollow slab and to evaluate the deflection considering the crack behavior under flexural load.

Abstract: High-strength concrete is widely used in construction field. The growth has been possible as a result of recent developments in material technology and a demand for high-strength concrete. High-strength concrete has different mechanical properties from normal-strength, as many researches mentioned about. However, the existing equations and procedures for prediction of ultra-high strength concrete are based on tests using normal-strength concrete, yet. In this study, experiments on ultra-high-strength steel fiber reinforced concrete beams with 2% volume fraction of steel fiber and 200MPa of compressive strength have been conducted. Test was conducted by two point loading with 2,000kN actuator for slender test specimen which have varied shear-span to depth ratio. Using test results with several assumptions, an empirical equation for flexural strength and shear strength of ultra-high-strength steel fiber reinforced concrete beams have been proposed.

Abstract: The paper presents an experimental study on the behavior of axial loaded concrete-filled steel square-tube stub columns with high strength fiber reinforced concrete until failure. Four specimens were tested to investigate the effect of high strength concrete on the load carrying capacity of the concrete-filled steel square-tube stub columns. The effect of the presence of steel fiber in high strength concrete which filled in the steel tube was also investigated. The main parameters in the tests were: (1) the strength of concrete (30 Mpa and 100 Mpa), and (2) the use of reinforcing steel fiber in concrete (plain high strength concrete and steel fiber high strength concrete). The main purpose of these tests were three-step: (1) to describe a series of tests on composite stub columns, (2) to analyze the influence of several parameters, and (3) to compare the accuracy of the predictions by using the specifications in the code (ACI and EC4 etc.) for the design of high-strength composite columns. Experimental results indicate that the high strength of concrete and use of steel-fiber in concrete had significant influence on both the axial compressive load capacity and the ductile of the steel square-tube stub columns.

Abstract: Generally because of the economic advantage and stable behavior in seismic loading, shearwalls combined with coupling slabs are widely used in high-rise apartment buildings. When analyzing such structures for lateral loads, however, the question of the actual stiffness and strength of the coupling slabs arises. For more accurate analysis approach, an experimental investigation was conducted with half-scale representations of the reinforced concrete shearwalls with the opening and coupled with slabs were subjected to cyclic loads. The test results of opening installed specimen, severe decrease of strength was observed. The decrease of strength of the shear walls by installation of openings shows a great deal of difference compared to previous researches. This is because flexural capacity of the slabs is working as coupling elements for the shear walls. The critical section of coupling slabs that works as coupling elements for shear walls was a little different from the results of previous researches.

Abstract: Remodeling is also an environmentally-friendly approach that reduces the amount of waste in construction site. Specifically, there are many attempts to make house more wide according to the merging of two old houses in apartment buildings. For making two houses into single housing area, openings shoud be needed or walls should be removed. However, removing shearwalls are very dangerous attempt because of reducing of lateral load capacity of buildings. Therefore many engineers prefer to leave shearwalls. However, there are insufficient studies about partial damage, that is, openings. In these cases the damaged shear walls need to be retrofitted by additional materials or members. In this research, four specimens were tested to investigate the capacity of the damaged wall and the retrofitted wall. The artificially damaged wall was prestressed by tendons to improve the shear capacity of the wall, and the other walls were retrofitted by adding steel plate at the surface for the same purpose. Consequently, these retrofitted walls had improved capacity and stiffness in both shear and flexure. Especially, the wall with steel plate showed ductile behavior after ultimate load and the prestressed wall had greater stiffness than the unstrengthened prototype wall.

Abstract: The installation of new opening is necessary for remodeling project, house merging type. Current structural design code cannot provide the deterministic way for designing the shear walls with openings. So many engineers prefer to retrofit the perforated wall area for the structural safety. For the safe design of retroftitting, for the perforated structural walls, we carried out nonlinear finite element analysis to find the coupling effect of remaing walls. Specifically, many types of shapes were considered and various area of rectangular openings were analyzed. For the effective retrofitting, door shape and such area about 20% of wall is appropriate for retrofitting of reinforced concrete shearwalls.