• Steel and Composite Structures
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• v.32 no.2
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• pp.199-212
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• 2019

This paper presents an investigation on seismic behavior of out-of-code Q690 circular high-strength concrete-filled thin-walled steel tubular (HCFTST) columns made up of high-strength (HS) steel tubes (yield strength $f_y{\geq}690MPa$). Eight Q690 circular HCFTST columns with various diameter-to-thickness (D/t) ratios, concrete cylinder compressive strengths ($f_c$) and axial compression ratios (n) were tested under the constant axial loading and reversed cyclic lateral loading. The obtained lateral load-displacement hysteretic curves, energy dissipation, skeleton curves and ductility, and stiffness degradation were analyzed in detail to reflect the influences of tested parameters. Subsequently, a simplified shear strength model was derived and validated by the test results. Finally, a finite element analysis (FEA) model incorporating a stress triaxiality dependent fracture criterion was established to simulate the seismic behavior. The systematic investigation indicates the following: compared to the D/t ratio and axial compression ratio, improving the concrete compressive strength (e.g., the HS thin-walled steel tube filled with HS concrete) had a slight influence on the ductility but an obvious enhancement of energy dissipation and peak load; the simplified shear strength model based on truss mechanism accurately predicted the shear-resisting capacity; and the established FEA model incorporating steel fracture criterion simulated well the seismic behavior (e.g., hysteretic curve, local buckling and fracture), which can be applied to the seismic analysis and design of Q690 circular HCFTST columns.

• International Journal of High-Rise Buildings
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• v.13 no.1
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• pp.85-95
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• 2024

A composite member made of concrete-filled steel tubes (CFT columns) has been recognized for its fire resistance due to the thermal mass effect of concrete inside the steel tube, as shown in various studies. In this study, the fire resistance performance of reinforced CFT columns under constant axial load was evaluated using finite element analysis with ABAQUS. For this purpose, the variables including cross-section size, steel tube thickness, and concrete cover thickness were set, and the temperature distribution in the column cross-section exposed to a standard fire was investigated using heat transfer analysis. Ultimately, a P-M interaction curve was obtained by evaluating the overall residual strength of columns, and the fire resistance time was determined by evaluating axial displacement-time responses due to the reduction in load capacity during fire through stress analysis.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.6
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• pp.937-946
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• 1986

In this paper, the plane stress fracture toughness of low carbon steel with 3mm thickness is investigated for various specimen widths and crack ratios using the J integral. The experiments is carried out for the compact tension(CT) specimen on an Instron machine. For materials that may be approximated by the Ramberg and Osgood stress strain law, the relevant crack parameter like the J integral and load line displacement are approximately normalized. Crack driving forces in terms of J integral is computed using the above estimation scheme. Abtained results are summarized as follows. (1) The plane stress fracture toughness, J$_{c}$, is almost constant in the range 50-70mm of width. Hence J$_{c}$ can be obtained by using smaller specimen than ASTM standard. (2) Yoon's and Simpson's formular which considers crack growth in obtaining J integral show more consevative J than Rice's and Merkle's (3) J$_{c}$ is almost constant in the range 0.499-0.701 crack ratios tested. J$_{c}$ obtained by using Kumar's formular is 28.14kgf/mm for base metal specimen and 32.51kgf/mm for annealed. (4) Comparison of the prediction with actual experimental measurements by Yoon's formular show good agreement for several different-size specimens.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.105-110
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• 2000

The Micromechanics test is new test method which uses comparatively smaller specimen than that required in conventional material tests. There are several methods, such as small-specimen creep test, the continuous indentation test, and small punch(SP) test. Among them, the small punch(SP) test method has been applied to many evaluation fields, such as a ductile-brittle transition temperature, stress corrosion cracking, hydrogen embrittlement, and fracture properties of advanced materials like FGM or MMC. In this study, the small punch(SP) test is performed to evaluate the mechanical properties at high/low temperature from $-196^{\circ}C$ to $650^{\circ}C$ and the material degradation for virgin and aged materials of 9Cr1MoVNb steel which has been recently developed. The ${\Delta}P/{\Delta}{\delta}$ parameter defined a slope in plastic membrane stretching region of SP load-displacement curve decreases according to the increase of specimen temperature, and that of aged materials is higher than the virgin material in all test temperatures. And the material degradation degrees of aged materials with $630^{\circ}C$ -500hrs and $630^{\circ}C$ -1000hrs are $36^{\circ}C$ and $38^{\circ}C$ respectively. These behaviors are good consistent with the results of hardness($H_v$) and maximum displacement(${\delta}_{max}$).

• Journal of the Korean Geotechnical Society
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• v.23 no.4
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• pp.79-90
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• 2007

Behaviors of instrumented steel sheet piles which are installed in sand ground by vibratory hammer were investigated. Especially, stresses acting on the pile during vibratory driving, efficiency factor which reflects differences between theoretical driving force and actually delivered acting force, justifiability of rigidity of steel sheet pile, dynamic resistance characteristics of soil and penetration characteristics of sheet pile were analysed. According to the field test results it is justifiable that steel sheet pile behaves as a rigid body during vibratory driving. And it can be seen that maximum stress acting on sheet pile section is far less than tensile strength of the material. Value of the maximum section force at sheet pile head was 72% of that estimated from theoretical equation. Magnitudes of displacement amplitudes computed from displacement-time history curve corresponding to four penetration depths were in the range of 16 $\sim$ 75% of that specified by manufacturer.

• Journal of the Korean Geosynthetics Society
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• v.18 no.2
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• pp.23-36
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• 2019

In this paper, field measurement of the Block Type Mechanically Stabilized Earth (MSE) wall curved section was performed, and the reinforced area of the curved part is studied through the result. MSE method has been applied to various fields because of easy construction and excellent economic efficiency, so that it can be easily access in our life. However due to lack of compaction and stress concentration phenomenon, cracks and collapse occur in the curve of MSE wall, which is important for safety. The cause of collapse is lack of research on curved section, lack of design criteria, lack of construction due to economical efficiency and shortening of construction period, insufficient compaction space. In this study, therefore, it was examined the existing design and construction standards, analyzed the cause through accident examples of the curved section of the Block Type MSE wall. As a result, the horizontal displacement of the curved section was 90% higher than that of the straight section and 60% higher than that of the concave section. In the case of the convex section in the curved section reinforcement region, the maximum displacement is shown in the H/2 section in the horizontal direction from the center of the MSE wall, and the range of influence from H is shown. In the case of the concave section, the maximum displacement is shown in the center, The minimum displacement was confirmed in H/4 section in the horizontal direction from the center of the MSE wall. As a basic study on the reinforcement area rehabilitation through the actual construction of block type MSE wall, the behaviors of the straight part and the curved part were compared and analyzed. And analyzed the reinforced area in order to reduce the damage of the stress concentration phenomenon and secure the safety.

• Journal of the Korean Recycled Construction Resources Institute
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• v.7 no.3
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• pp.279-286
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• 2019

In this study, an experimental study on the tensile properties of steel fiber-reinforced ultra high strength concrete(UHSC) with a standard compressive strength of 180MPa was performed. Steel fibers with a volume ratio of 1% were mixed to prepare direct tensile strength specimens and prism specimens for the three-point bending test. The fabricated specimens were set up in the middle section of the specimen to induce cracks, and the test was carried out according to each evaluation method. First, the stress-strain curves were analyzed by performing direct tensile strength tests to investigate the behavior characteristics of concrete after cracking. In addition, the load-CMOD curve was obtained through the three-point bending test, and the inverse analysis was performed to evaluate the stress-strain curve. Tensile behavior characteristics of the direct tensile test and the three-point bending test of the indirect test were similar. In addition, the tensile stress-strain curve modeling presented in the SC structural design guidelines was performed, and the comparative analysis of the measured and predicted values was performed. When the material reduction factor of 1.0 was applied, the predicted value was similar to the measured value up to the strain of 0.02, but when the material reduction factor of 0.8 was applied, the predicted value was close to the lower limit of the measured value. In addition, when the strain was greater than 0.02, the predicted value by SC structural design guideline to underestimated the measured value.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.10
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• pp.739-744
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• 2017

Elastic deformation and fatigue damage can cause the permanent deformation of a kart's frame during turning, affecting the kart's driving performance. A kart's frame does not contain any suspension or differential devices and, therefore, the dynamic behavior caused by torsional deformation when driving along a curve can strongly affect these two kinds of deformations. To analyze the dynamic behavior of a kart along a curved section, the GPS trajectory of the kart is obtained and the torsional stress acting on the kart-frame is measured in real time. The mechanical properties of leisure and racing karts are investigated by analyzing their material properties and conducting a tensile test. The torsional stress concentration and frame distortion are investigated through a stress analysis of the frame on the basis of the obtained results. Leisure and racing karts are tested in each driving condition using driving analysis equipment. The behavior of a kart when being driven along a curved section is investigated through this test. Because load movement occurs owing to centrifugal force when driving along a curve, torsional stress acts on the kart's steel frame. In the case of a leisure kart, the maximum torsional stress derived from the torsional fatigue limit was found to be 230 MPa, and the torsional fatigue limit coefficient was 0.65 when driving at a speed of 40 km/h. Furthermore, the driving elements during the cornering of a kart were measured based on an actual auto-test after installing a driving measurement system, and the driving behavior of the kart was analyzed by measuring its vertical displacement.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.2C
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• pp.71-79
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• 2009

This study is to develope the resilient modulus prediction model, which is the function of mean effective principal stress and axial strain, for three types of railroad trackbed materials such as crushed stone, weathered granite soil, and crushed-rock soil mixture. The model consists of the maximum Young's modulus and nonlinear values for higher strain, analogous to dynamic shear modulus. The maximum value is modeled by model parameters, $A_E$ and the power of mean effective principal stress, $n_E$. The nonlinear portion is represented by modified hyperbolic model, with the model parameters of reference strain, ${\varepsilon}_r$ and curvature coefficient, a. To assess the performance of the prediction models proposed herein, the elastic response of a test trackbed near PyeongTaek, Korea, was evaluated using a 3-D elastic multilayer computer program (GEOTRACK). The results were compared with measured elastic vertical displacement during the passages of freight and passenger trains at two locations, whose sub-ballasts were crushed stone and weathered granite soil, respectively. The calculated vertical displacements of the sub-ballasts are within the order of 0.6mm, and agree well with measured values. The prediction models are thus concluded to work properly in the preliminary investigation.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.4
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• pp.391-397
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• 2017

Recently, digital image correlation (DIC) techniques have been used to measure dynamic deformation during tensile testing. The standard tensile test method measures the average displacement of the relevant specimen to calculate the true stress-strain curve. Therefore, the validity of the true stress curve is restricted to the stress incurred within the uniform stretching interval, i.e., the maximum stress corresponds to the starting point of the necking deformation. Alternatively, if DIC is used, the effective range of the strain and strain rate can be extended to the breaking point of the tensile specimen, because of the feasibility of measuring the local strain over the entire area of interest. Because of these advantages, many optical 3D measurement systems have been introduced and used in research and industry. However, the conventional 3D measurement systems are exceedingly expensive and time consuming. In addition, these systems have the disadvantage of a very large equipment size which makes their transport difficult. In this study, a 2D image correction method employing a 2D DIC measurement method in conjunction with a numerical analysis method is developed using a smartphone. The results of the proposed modified 2D DIC method yielded higher accuracy than that obtained via the 3D measurement equipment. In conclusion, it was demonstrated that the proposed 2D DIC and calibration methods yield accurate measurement results with low time costs.