Chang Sik Choi

Flexural Capacities of One-Way Hollow Slab with Donut Type Hollow Sphere

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.

Discrete-Continuous Configuration Optimization Methods for Structures Using the Harmony Search Algorithm

This paper proposes an efficient structural optimization methods based on the harmony search (HS) heuristic algorithm that treat integrated discrete sizing and continuous geometric variables. The recently developed HS algorithm was conceptualized using the musical process of searching for a perfect state of harmony. It uses a stochastic random search instead of a gradient search so derivative information is unnecessary. A benchmark truss example is presented to demonstrate the effectiveness and robustness of the new method, as compared to current optimization methods. The numerical results reveal that the proposed method is a powerful search and design optimization technique for structures with discrete member sizes, and may yield better solutions than those obtained using current methods.

Behaviour of Concrete Filled Steel Square-Tube Stub Column with Steel-Fiber Reinforced High Strength Concrete

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.

A Study on Shear Reinforcement of Exterior Joints for Flat Plate

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.

Retrofit of Artificially Perforated Shear Wall in Existing Dominant Wall Structure

A series of three shear wall specimens were tested under constant axial stress and reversed cyclical lateral loading in order to evaluate the seismic retrofit that had been proposed for the shear wall with the opening induced by remodeling. One of these specimens was tested in the as-built condition and the other two were retrofitted prior to testing. The retrofit involved the use of carbon fiber sheets and steel plates (a thickness of 3mm) over the entire face of the wall. The test results showed that the failed specimens had shear fractures and that two different types of retrofitting strategies had different effects on the strengths of each specimen.

Punching Shear Design Method of Voided Slabs

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.

Structural Performance of Exterior Beam-Column Joints with Large Diameter Headed Bars

Generally, a conventional standard hook is used for the reinforcements 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.

Ductility of Ultra-High Performance Concrete and its Correlation with Tensile Strength Increase

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.

Evaluation of Bond Strength between Steel Fiber Reinforced Ultra-High Strength Concrete and Rebar

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.

Temperature Estimation Method of Hollow Slab at Elevated Temperature

철근콘크리트를 적용한 구조부재의 경우, 다른 구조재료에 비해 콘크리트의 열확산계수가 낮고, 고온에서 완전붕괴에 도 달하기까지 에너지 소산능력이 높아 우수한 내화구조로 평가 받고 있다. 그러나 중공 슬래브와 같이 구조체 중량감소를 위 해 별도의 중공형성체를 슬래브 복부에 설치하는 경우, 화재 발생 시 중공형성체의(플라스틱, 스티로폼 등) 재료적 특성과 구조적 변화에 따라 Solid 슬래브와는 다른 열적 특성을 나타 낼 것이다. 특히, 슬래브 단면 감소로 인해 콘크리트의 축열 효과 감소, 중공부 내의 공기층 형성으로 인한 슬래브의 차열 성 증가, 온도 상승 시 중공부 내부의 압력 상승에 따른 폭렬 현상 등 Solid 슬래브와는 다른 특성이 예상된다. 하지만 국 내·외적으로 중공 슬래브에 관한 내화성능 및 그에 따른 구 조적 거동특성에 대한 연구는 미흡한 실정이다(Chung et al., 2012, 2011). 구조물이 화재등과 같은 비정상적 온도에 노출되는 경우, 구조재료의 열적 특성 및 고온 상태에서의기계적 성능은 건 축물의 화재피해 판정 및 재사용 여부를 결정하는데 중요한 요소가 된다. 이러한 잔류 강도 평가에 있어 ACI 216R (2007) 및 Eurocode 2 (2004)에서는 화재에 노출된 콘크리트 부재의 온도상승으로 인한 피해를 부재의 유효단면 감소로 가정하여 강도의 손실을 추정하는 방법을 제시하고 있다. 이 러한 강도 추정 방법은 화재에 노출된 콘크리트부재의 정확 한 온도추정을 기반으로 하고 있으며 화재 시 콘크리트 부재 단면의 온도 추정 방법은 Wickström(1986)의 방법과 Hertzs Abstract