Papers by Author: Hyun Ki Choi

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: 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.

Abstract: The use of glass-fiber-reinforced polymer (GFRP) bars to replace steel reinforcement in concrete structures is a relatively new technique. GFRP bars possess mechanical properties different from steel bars, including high tensile strength combined with low elastic modulus and elastic brittle stress–strain relationship. Therefore, design procedures should account for these properties. This paper presents the experimental moment deflection relations of GFRP reinforced beam which are spliced. Test variables were lab-spliced length of GFRP rebar. A total of 6 concrete beams reinforced with steel and GFRP rebar tested. Three concrete beams reinforced with spliced GFRP rebar and 1 reference beams reinforced with non-spliced GFRP rebar was tested. All the specimens had a span of 4000mm, provided with 12.7mm nominal diameter steel and GFRP rebar. All test specimens were tested under 2-point loads so that the spliced region is subject to constant moment. The experimental results show that the splice length of GFRP increased with the ultimate load increasing and decreased with stiffness.

Abstract: This paper presents the punching shear capacities of biaxial hollow slab with donut type hollow sphere. 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 biaxial 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 has weakness in slab-column connection. In addition, the present code does not provide a clear computation method for the punching shear strength of hollow slab. In this study, the special type of cage was used to improve punching shear capacities and fix the hollow spheres in critical section. To verify the punching shear capacities of this biaxial hollow slab, punching shear tests were performed. Four test specimens were used for test parameters. One was conventional RC slab and three were hollow slabs. The test parameter was the areas of critical section which were determined by the number of hollow spheres in critical section.

Abstract: This paper presents the flexural capacities of one-way hollow slab with donut type hollow sphere. 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 biaxial hollow slab system is widely known as one of the effective slab system which can reduce self-weight of slab. A biaxial hollow slab has hollow spheres within slabs in order to reduce self-weight of slabs. The capacities of biaxial hollow slab are influenced by the shapes, volume and materials of hollow spheres. According to analytical studies, the hollow slab with donut type hollow sphere had good flexural capacities such as strength, stiffness and deflection. To verify the flexural capacities of this hollow slab, flexural tests were performed on the one-way hollow slabs. Five test specimens were used for test parameters. One was conventional RC slab and others were hollow slabs. The test parameters included two different shapes and materials of plastic balls. The shape parameters were donut and non-donut forms. And the material parameters were general plastic and glass fiber plastic.

Abstract: Unreinforced masonry buildings have significant portion of existing and historical buildings around the world. Recent earthquakes have shown the needs of seismic retrofitting for these types of buildings. There are many types of retrofitting materials for URM(unreinforced masonry buildings) such as shotcrete, ECC and FRPs. Many engineers use many types of fiber reinforced polymers because these types of material enhance the shear strength of wall without expansion of wall sectional area and additional weight of total structure. However, the complexity of mechanical behavior of masonry shear wall and the lack of experimental data of masonry wall which was retrofitted by FRPs may cause the problem that engineers hard to determine the retrofitting level. Determining and providing the information for retrofitting effect of FRPs for masonry shear wall, this paper investigate in-plane shear behavior of URM and retrofitted masonry shear walls using two types of different FRP materials. Specimens were designed to idealize the wall of low rise apartment which was built in 1970s Korea with no seismic reinforcements and have 1 aspect ratio. Retrofitting materials were carbon FRP and Hybrid sheet which have different elastic modulus and ultimate strain. Consequently, this study will evaluate the structural capacity of masonry shear wall and retrofitting effect of FRP sheet for in plane shear behavior comparing with evaluation method for reinforced concrete beam which was retrofitted by FRPs.

Abstract: The purpose of this research was to study the response of slab-column connections containing various types of shear reinforcement when subjected to the combination of gravitational and lateral cyclic loads. The three test specimens were full-scale representations of exterior slabcolumn connections of a prototype apartment building in Korea. The control specimen had no shear reinforcements, while the other specimens had CS-Bar and SS-Bar as shear reinforcements. The control specimen failed due to the punching shear around the slab-column connection at 4.0% drift. None of the specimens with shear reinforcement experienced punching shear failure up to 4.4% drift. The two types of slab shear reinforcements proved to be equally effective in resisting punching shear failure of these connections subjected to relatively low levels of gravity load. The presence of shear reinforcements significantly increased the lateral load ductility of the connections. The test results showed that the strength and ductility of the specimens with SS-Bar and CS-Bar were improved by 23% and 15% compared to the specimen without shear reinforcements.