• Magazine of the Korea Concrete Institute
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• v.7 no.3
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• pp.139-148
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• 1995

The test results from three one fourth scale models using high strength Reinforced Concrete $f_x=704\;kg/cm^2,\;f_y=5.830\;kg/cm^2$ are presented. Such specimens are considered to represent the critical 3 storics of 60-story tall building of a structural wall system in area of high seismicity respectively. They are tested under inplane vertical and horizontal loading. The main varlable is the level of axial stress. The amounts of vertical and horizontal reinforcement are identical for the three walls testcd. The cross-section of all walls is barbell shape. The aspectratio($h_w/I_w$) of test specimen is 1.8. The aim of the study is to investigate the effects of levels of applied axial stresses on the inelastic behavior of high-strength R /C tall walls. Experimental results of high strength R /C tall walls subjected to axial load and simulated sels rnic loading show that it is possible to insure a ductlle dominant performance by promotmg flex ural yielding of vertical reinforcement and that axial stresses within $O.21f_x$ causes an increase in horizontal load-carrying capacity, initial secant st~ffness characteristics, but an decrease in displacement ductility. energy dissipation index and work damage index of high strength K /C tall walls

• Journal of the Korea Concrete Institute
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• v.14 no.5
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• pp.734-741
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• 2002

An experimental study was carried out for beam-column joints composed of RC column and RS beams. The purpose of this study is to examine the inelastic seismic behavior for the RC-RS connection. Two interior and one exterior beam-column assemblies with variable moment ratios were tested. Experimental results showed that strength and deformability except stiffness were satisfactory. It is considered that the lack of stiffness was due to the slipping of steel beam from RS beam. The behavioral characteristics of the RC-RS connection were evaluated according to the quideline suggested by Hawkins et al. Nominal strength at 5 ％ joint distortion was not satisfactory, but all the other requirements, such as strength preserving capability, energy dissipation, and initial stiffness and strength ratios after peak load were satisfactory compared with the guideline. Thus it was concluded that the RC-RS connections can maintain ductility with excellent energy-dissipating capacity if being provided with appropriate reinforced structural system such as RC core wall for the initial lateral stiffness.

• Magazine of the Korea Concrete Institute
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• v.2 no.2
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• pp.81-92
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• 1990

Concrete, a mixed material, has heterogeniety, anisotrophy and nonlinearity. Therefore, in its 'racture analysis, it is more reasonable to evaluate its fracture toughness by applying the concept of 'racture mechanics rather than the strength concept. Up to the present the concepts of fracture mechanics which were applied to concrete have been divided into two main classes. The one is the concept of linear elastic fracture mechanics and the other is the concept of elastic-plastic fracture mechanics. But it has been pointed out that there are many problems and irrationalities in applying the concept of linear elastic fracture mechanics to concrete. In this study, the J -integral method and the COD method mainly used in the analysis of nonlinear fracture mechanics, were introduced and the three point bending test was carried out for investigating the effects of the variation of the maximum aggregate size and notch depth on the fracture behavior and the crack growth of concrete, and the relationships of fracture energy and crack opening displacement. According to the results of this study the more the maximum aggregate size and the notch depth increased, the more the nonlinearity of load-deflection behavior was remarkable. The increase of the coarse aggregate size created the more ductility of concrete. Thus concrete showed the more stable fracture. As for the path of the crack growth, the more the coarse aggregate size increased, the more it was irregulary deviated from the straight line but it was not almost affected by the variation of the notch depth. Also, the fracture energy increased according as the coarse aggregate size increased and the notch depth decreased.

• Magazine of the Korea Concrete Institute
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• v.6 no.1
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• pp.120-130
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• 1994

The purpose of this study is to propose the analytical model for the hysteretic behavior of Reinforced Concrete bearn-column joints under various loading history. Discrete line elernents , YVith inelastic rotational spring was adopted to consider the movement of plastic hinging zone influenced by the details of longitudinal reinforcements. Also hysteretic model was constructed by excluding such variables which can not be utilized in dynamic analysis of Reinforced Concrete. structure that it will be adoptable in two-dimensional inelastic frame ardysis with 6-DOF. From the analysis of previous test results, it was found that stiffness deterioration caused by inelastic hysteretic loadings can be predicted by the functron of basic pinching coefficients, ductility ratio.and yield strength ratio of members. Strength degradation coefficients were newly proposed to explain the difference of inelastic behavior of members caused by spacing ratio of transverse steel and sectlon aspect ratio. The energy dissipation capacities calculated using the analytical model proposed in thls paper show a good agreements w~lh test results by an error of 10~20%.

• Journal of Korean Society of Steel Construction
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• v.25 no.3
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• pp.243-254
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• 2013

Internally Confined Hollow RC(ICH RC) column consisted of concrete, transverse reinforcement, longitudinal reinforcement, and inner tube. It had good strength and ductility by core concrete was become triaxial confining state with transverse reinforcement and inner tube. There were two confining stress as external confining stress and internal confining stress in an ICH RC column. While external confining stress was researched by former researchers, internal confining stress has not researched. In this paper, confining stress of both Hollow RC column and ICH RC column was investigated using FEA program. Relation between theoretical confining stress and internal confining stress was drawn by analysis results. Modified failure condition equations of inner tube were suggested to base on failure condition equations of inner tube by former researcher. When thickness of inner tube was calculated by modified equations, it could be economic because thickness of inner tube was reduced 50% compared with former researcher equations in order to same confining stress.

• Computers and Concrete
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• v.22 no.3
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• pp.337-353
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• 2018

The present study focuses on examining the structural behaviour of steel-fibre-reinforced concrete (SFRC) beams under high rates of loading largely associated with impact problems. Fibres are added to the concrete mix to enhance ductility and energy absorption, which is important for impact-resistant design. A simple, yet practical non-linear finite-element analysis (NLFEA) model was used in the present study. Experimental static and impact tests were also carried out on beams spanning 1.3 meter with weights dropped from heights of 1.5 m and 2.5 m, respectively. The numerical model realistically describes the fully-brittle tensile behaviour of plain concrete as well as the contribution of steel fibres to the post-cracking response (the latter was allowed for by conveniently adjusting the constitutive relations for plain concrete, mainly in uniaxial tension). Suitable material relations (describing compression, tension and shear) were selected for SFRC and incorporated into ABAQUS software Brittle Cracking concrete model. A more complex model (i.e., the Damaged Plasticity concrete model in ABAQUS) was also considered and it was found that the seemingly simple (but fundamental) Brittle Cracking model yielded reliable results. Published data obtained from drop-weight experimental tests on RC and SFRC beams indicates that there is an increase in the maximum load recorded (compared to the corresponding static one) and a reduction in the portion of the beam span reacting to the impact load. However, there is considerable scatter and the specimens were often tested to complete destruction and thus yielding post-failure characteristics of little design value and making it difficult to pinpoint the actual load-carrying capacity and identify the associated true ultimate limit state (ULS). To address this, dynamic NLFEA was employed and the impact load applied was reduced gradually and applied in pulses to pinpoint the actual failure point. Different case studies were considered covering impact loading responses at both the material and structural levels as well as comparisons between RC and SFRC specimens. Steel fibres were found to increase the load-carrying capacity and deformability by offering better control over the cracking process concrete undergoes and allowing the impact energy to be absorbed more effectively compared to conventional RC members. This is useful for impact-resistant design of SFRC beams.

• Journal of the Korea Concrete Institute
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• v.19 no.4
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• pp.411-420
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• 2007

The applicability of headed bars in exterior beam-column joints under reversed cyclic loading was investigated. A total of ten pullout tests were first performed to examine pullout behavior of headed bars subjected to monotonic and cyclic loading with test variables such as connection type between head and bar stem (weld or no weld), loading methods (monotonic or cyclic loading), and head shape (small or large circular head and square head). Two full-scale beam-column joint tests were then performed to compare the structural behavior of exterior beam-column joints constructed using two different reinforcement details: i.e. $90^{\circ}$ standard hooks and headed bars. Both joints were designed following the recommendations of ACI-ASCE Committee 352 for Type 2 performance: i.e. the connection is required to dissipate energy through reversals of deformation into inelastic range. The pullout test results revealed that welded head to the stem did not necessarily result in increased pullout strength when compared to non-welded head. Relatively large circular head resulted in higher peak load than smaller circular and square head. Both beam-column joints with conventional $90^{\circ}$ hooks and headed bars behaved similarly in terms of crack development, hysteresis curves, and peak strengths. The joint using the headed bars showed better overall structural performance in terms of ductility, deformation capacity, and energy dissipation. These experimental results demonstrate that the headed bars using relatively small head can be properly designed far use in external beam-column joint.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.34 no.6
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• pp.1677-1685
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• 2014

Short discrete fibers compounded with concrete can enhance the tensile resistance and ductility of concrete. Recently, the effectiveness of the reinforcement has increased according to the increasing length of steel fiber. However, the lengthening of steel fiber requires reducing the ratio of the fiber content to remain the workability and quality of concrete. Thus, the present study evaluated the flexural performance of fiber reinforced concrete with less than l.0% fiber volume ratios of steel fiber, 30mm and 60mm long, and polypropylene fiber, being evaluated as a good reinforcing material with chemical stability, long-term durability, and cost effectiveness. Concrete with more than 0.25% steel and 0.5% polypropylene fibers improved the brittle failure of concrete after reaching cracking strength. Concrete reinforced with polypropylene exhibited deflection-softening behavior, but that with more than 0.5% polypropylene delayed stress reduction and recovered flexural strength by 60 to 80% after cracking strength. In conclusion, concrete reinforced with more than 0.75% polypropylene could improve structural flexural performance. In particular, energy absorption capacity of reinforced concrete with 1.0% polypropylene fiber was similar to that with 0.5% and 0.7% steel fibers.

• Journal of the Microelectronics and Packaging Society
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• v.26 no.4
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• pp.163-169
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• 2019

Sn-Pb solder alloys in electronics rapidly has been replaced to Pb free solder alloys because of various environmental regulations such as restriction of hazardous substances directive (RoHS), European Union waste electrical, waste electrical and electronic equipment (WEEE), registration evaluation authorization and of chemicals (REACH) etc. Because Sn58%Bi (in wt.%) solder alloy has low melting point and higher mechanical properties than that of Sn-Pb solder, it has been studied to manufacture electronic components. However, the reliability of Sn58%Bi solder could be lowered because of the brittleness of Bi element included in the solder alloy. Therefore, we observed the microstructures of Sn58%Bi composite solders with various contents of Sn-decorated multiwalled carbon nanotube (Sn-MWCNT) particles and evaluated bonding strength of the FR-4 components assembled with Sn58%Bi composite solder. Also, microstructures and bonding strengths of the Sn58%Bi composite solder joints were evaluated with the number of reflows from 1 to 7 times, respectively. Bonding strengths and fracture energies of the Sn58%Bi composite solder joints were measured by die shear test. Microstructures and fracture modes were observed with scanning electron microscope (SEM). Microstructures in the Sn58%Bi composite solder joints were finer than that of only Sn58%Bi solder joint. Bonding strength and fracture energy of Sn58%Bi composite solder including 0.1 wt.% of Sn-decorated MWCNT particles increased up to 20.4% and 15.4% at 5 times in reflow, respectively.

• Journal of the Korea Concrete Institute
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• v.17 no.2 s.86
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• pp.263-271
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• 2005

HPFRCC(High Performance Fiber Reinforced Cementitious Composite) is a class of FRCCs(Fiber Reinforced Cementitious Composites) that exhibit multiple cracking. Multiple cracking leads to improvement in properties such as ductility, toughness, fracture energy, strain hardening, strain capacity, and deformation capacity under tension, compression, and bending. These improved properties of HPFRCCs have triggered unique and versatile structural applications, including damage reduction, damage tolerance, energy absorption, crack distribution, deformation compatibility, and delamination resistance. These mechanical properties of HPFRCCs become different from the kinds and shapes of used fiber, and it is known that the effective size of fiber in macro crack is different from that in micro crack. This paper reports an experimental findings on the mechanical properties of HPFRCCs reinforced with the micro fiber(PP50, PVA100 and PVA200) and macro fiber(PVA660, SF500). Uniaxial compressive tests and three point bending tests are carried out in order to compare with the mechanical properties of HPFRCCs reinforced with micro fibers or hybrid fibers such as compressive strength, ultimate bending stress, toughness, deformation capacity and crack pattern under bending, etc.,