• Journal of Korean Society of Steel Construction
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• v.16 no.5 s.72
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• pp.529-541
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• 2004

Concrete filled steel tube structures were recently used in constructing high-rise buildings due to their effectiveness. Studies on concrete filled steel tubes have been focused on the experiments of uni-axial compression and bending and eccentric compression. There were also a few studies that investigated CFT member behavior under combined compression and torsion. The behavior of a circular CFT column under combined torsion and compression was theoretically investigated, considering the confinement of steel tubes on the concrete, the softening of the concrete, and the spiral effect, which were the dominant factors that influenced compression and torsion strength. The biaxial stress effects due to diagonal cracking were also taken into account. By applying those factors to compatibility and equilibrium conditions, the basic equation was derived, and the equation could be used to incorporate the torsional behavior of the entire loading history of the CFT member.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.10
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• pp.1163-1169
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• 2012

Rubber, a hyperelastic material, is the main material used in tires. During the operation of a car, the tire receives various types of loads. The accumulation of strain energy due to such loads induces tire failure. Generally, because rubber materials used for tires have stress softening characteristics, unlike metals, test methods used for metals cannot be applied to rubber. Therefore, in this study, for the evaluation of the fatigue properties of two types of specimens that have different material components, a tensile test and a fatigue test according to the extended strain range dissimilar to ASTM D4482 are performed, and fatigue life equations are proposed based on the test results.

• Land and Housing Review
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• v.2 no.2
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• pp.183-188
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• 2011

Recently, the depth of tunnel constructed is getting deeper, which increases difficulty in construction works. Deliberate tunneling techniques are needed as the span and length of tunnels are increased. As one of the technical developments for tunnel, U-shaped and reinforced spider lattice girders are developed by optimizing the spider used in 95mm lattice girder as tunnel steel ribs. In order to evaluate the load bearing capacity of the lattice girder, the 4-point flexural tests are carried out. For the laboratory tests, straight specimens are made for the existing lattice girder and the new lattice girder. The results of the flexural tests showed that the maximum load bearing capacity of the new lattice girders was higher than the traditional one. The load-displacement behavior of the test specimens showed the elasto-plastic behavior in the existing lattice girder and the stress softening behavior in the new lattice girder. It was found that the load bearing capacities are changed depending on the location of the loading points.

• Journal of the Korean institute of surface engineering
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• v.32 no.2
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• pp.144-156
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• 1999

Thermal Barrier Coating with Functional Gradient Materials (FGM-TBC) can play an important role to protect the parts from harmful environments in high temperatures such as oxidation, corrosion, and wear and to improve the efficiency of aircraft engine by lowering the surface temperature on turbine blade. FGM-TBC can increase the life spans of product and improve the operating properties. Therfore, in this study the evaluations of mechanical and thermal properties of FGM-TBC such as fatigue, oxidation and wear-resistance at high temperatures have been conducted. The samples of both the TBC with 2, 3, 5 layers (YSZ/NiCrAlY) to be produced by Air Plasma Spray method (APS) and the bulk TBC with 6 layers to be produced by Plasma Assisted Sintering method (PAS) were used. Furthermore, residual stress, bond strength, and thermal conductivity were evaluated. The average thickness of the APS was 500$\mu\textrm{m}$ to 600$\mu\textrm{m}$ and the average thickness of the PAS was 3mm. The hardness number of the top layer of APS was 750 Hv to 810Hv and that of PAS was 950 Hv to 1440Hv. The $ZrO_2$ coating layer of APS was composed of tetragonal structure after spraying as the result of XRD analysis. As shown in the results of the high temperature wear test, the 3 layer coating of APS had the best wear resistance at $800^{\circ}C$ and the 5 layer coating of APS had the best wear resistance at $600^{\circ}C$. But, these coatings had the tendency of the low-temperature softening at $300^{\circ}C$. The main mechanism of wear was the adhesive wear and the friction coefficient of coatings was increased as increasing the test temperatures. A s results of thermal conductivity test, the ${\Delta}T$ of the APS coating was increased as number of layer and the range of thermal conductivity of the PAS was $800^{\circ}C$ to $1000^{\circ}C$.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.6
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• pp.597-605
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• 2009

The present study has been proposed a model for the in-plane shear behavior of reinforced(Engineered Cementitious Composite(ECC) panels under biaxial stress states. The model newly considers the high-ductile tensile characteristic of cracked ECC by its multiple micro-cracking mechanism, the compressive strain-softening characteristic of cracked ECC, and the shear transfer mechanism in the cracked interface of ECC element. A series of numerical analyses were performed, and the predicted curves were compared with experimental results. The proposed in-plane shear model, R-ECC-MCFT, was found to be well matched with the experimental results, and it was also demonstrated that reinforced ECC panel showed more improved in-plane shear strength and post peak behavior, in comparing with the conventional reinforced concrete panel.

• Geomechanics and Engineering
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• v.13 no.5
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• pp.715-742
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• 2017

Earth berms are often left in place to support retaining walls or piles in order to eliminate horizontal struts in excavations of soft soil areas. However, if the excavation depth is relatively large, an earth berm-supported retaining system may not be applicable and could be replaced by a multi-bench retaining system. However, studies on multi-bench retaining systems are limited. The goal of this investigation is to study the deformation characteristics, internal forces and interaction mechanisms of the retaining structures in a multi-bench retaining system and the failure modes of this retaining system. Therefore, a series of model tests of a two-bench retaining system was designed and conducted, and corresponding finite difference simulations were developed to back-analyze the model tests and for further analysis. The tests and numerical results show that the distance between the two rows of retaining piles (bench width) and their embedded lengths can significantly influence the relative movement between the piles; this relative movement determines the horizontal stress distribution in the soil between the two rows of piles (i.e., the bench zone) and thus determines the bending moments in the retaining piles. As the bench width increases, the deformations and bending moments in the retaining piles decrease, while the excavation stability increases. If the second retaining piles are longer than a certain length, they will experience a larger bending moment than the first retaining piles and become the primary retaining structure. In addition, for varying bench widths, the slip surface formation differs, and the failure modes of two-bench retained excavations can be divided into three types: integrated failure, interactive failure and disconnected failure.

• Geomechanics and Engineering
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• v.13 no.3
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• pp.459-474
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• 2017

To investigate the mechanical properties of Expanded Polystyrene (EPS) Beads Stabilized Lightweight Soil (EBSLS), Laboratory studies were conducted. Totally 20 sets of specimens according to the complete test design were prepared and tested with unconfined compressive test and consolidated drained triaxial test. Results showed that dry density of EBSLS ($0.67-1.62g/cm^3$) decreases dramatically with the increase of EPS beads volumetric content, while increase slightly with the increase of cement content. Unconfined compressive strength (10-2580 kPa) increases dramatically in parabolic relationship with the increase of cement content, while decreases with the increase of EPS beads volumetric content in hyperbolic relationship. Cohesion (31.1-257.5 kPa) increases with the increase of cement content because it is mainly caused by the bonding function of hydration products of cement. The more EPS beads volumetric content is, the less dramatically the increase is, which is a result of the cohesion between hydration products of cement and EPS beads is less than that between hydration products of cement and sand particles. Friction angle ($14.92-47.42^{\circ}$) decreases with the increase of EPS beads volumetric content, which is caused by the smoother surfaces of EPS beads than sand grains. The stress strain curves of EBSLS tend to be more softening with the increase of EPS beads content or the decrease of cement content. The shear contraction of EBSLS increases with the increase of $c_e$ or the decrease of $c_c$. The results provided quantitative relationships between physico-mechanical properties of EBSLS and material proportion, and design process for engineering application of EBSLS.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.41 no.2
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• pp.101-111
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• 2021

Two different types of shear-friction tests were classified by external loadings and referred to as a push-off and a pull-off test. In a pull-off test, a tension force is applied in the transverse direction of the test specimen to produce a shear stress at the shear plane. This paper presents a method to evaluate shear transfer strengths of uncracked pull-off specimens. The method is based on the compression field theory and different constitutive laws are applied in some ways to gain accurate shear strengths considering softening effects of concrete struts based on Modified Compression Field Theory (MCFT) and Softened Truss Model (STM). The validity of the proposed method is examined by applying to some selected test specimens in literatures and results are compared with the predicted values. A general agreement is observed between predicted and measured values at ultimate loading stages in initially uncracked pull-off test specimens. A shear strength evaluation formula considering the effective compressive strength of a concrete strut was proposed, and the applicability of the proposed formula was verified by comparing with the experimental results in the literature.

• Computers and Concrete
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• v.24 no.1
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• pp.73-83
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• 2019

Concrete reinforced with fiber reinforced polymer (FRP) bars (FRP-RC) has attracted a significant amount of research attention in the last three decades. A limited number of studies, however, have investigated the effect of bond slip on the performance of FRP-RC columns under eccentric loading. Based on previous experimental study, a finite-element model of eccentrically loaded FRP-RC columns was established in this study. The bondslip behavior was modeled by inserting spring elements between FRP bars and concrete. The improved Bertero-Popov-Eligehausen (BPE) bond slip model with the results of existing FRP-RC pullout tests was introduced. The effect of bond slip on the entire compression-bending process of FRP-RC columns was investigated parametrically. The results show that the initial stiffness of bond slip is the most sensitive parameter affecting the compression-bending performance of columns. The peak bond stress and the corresponding peak slip produce a small effect on the maximum loading capacity of columns. The bondslip softening has little effect on the compression-bending performance of columns. The sectional analysis revealed that, as the load eccentricity and the FRP bar diameter increase, the reducing effect of bond slip on the flexural capacity becomes more obvious. With regard to bond slip, the axial-force-bending-moment (P-M) interaction diagrams of columns with different FRP bar diameters show consistent trends. It can be concluded from this study that for columns reinforced with large diameter FRP bars, the flexural capacity of columns at low axial load levels will be seriously overestimated if the bond slip is not considered.

• Geomechanics and Engineering
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• v.21 no.5
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• pp.433-445
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• 2020

The weakening and softening behavior of soft clay subjected to cyclic loading due to the build-up of excess pore water pressure is well-known. During the design stage of the foundation of highways and coastal high-rise buildings, it is important to study the mechanical behavior of marine soils under cyclic loading as they undergo greater settlement during cyclic loading than under static loading. Therefore, this research evaluates the cyclic stress-strain and shear strength of untreated and treated marine clay under the effects of wind, earthquake, and traffic loadings. A series of laboratory stress-controlled cyclic triaxial tests have been conducted on both untreated and treated marine clay using different effective confining pressures and a frequency of 0.5 and 1.0 Hz. In addition, treated samples were cured for 28 and 90 days and tested under a frequency of 2.0 Hz. The results revealed significant differences in the performance of treated marine clay samples than that of untreated samples under cyclic loading. The treated marine clay samples were able to stand up to 2000 loading cycles before failure, while untreated marine clay samples could not stand few loading cycles. The untreated marine clay displayed a higher permanent axial strain rate under cyclic loading than the treated clay due to the existence of new cementing compounds after the treatment with recycled tiles and low amount (2%) of cement. The effect of the effective confining pressure was found to be significant on untreated marine clay while its effect was not crucial for the treated samples cured for 90 days. Treated samples cured for 90 days performed better under cyclic loading than the ones cured for 28 days and this is due to the higher amount of cementitious compounds formed with time. The highest deformation was found at 0.5 Hz, which cannot be considered as a critical frequency since smaller frequencies were not used. Therefore, it is recommended to consider testing the treated marine clay using smaller frequencies than 0.5 Hz.