• Steel and Composite Structures
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• v.51 no.6
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• pp.697-712
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• 2024

In this paper, the dynamic flexural stiffness of concrete-filled steel tubular (CFST) members is investigated based on vibration modal testing and a Bayesian model updating procedure. To reflect the actual service states of CFST members, a 3-stage modal testing procedure is developed for 6 circular CFST beam-columns, in which the modal parameters of the specimens under varying axial load levels are extracted. In the model updating procedure, a Timoshenko beam element model is first established, in which the influence of shear deformation and rotational inertia are incorporated. Subsequently, a 2-round Bayesian model updating strategy is proposed to calculate the dynamic flexural stiffness of the specimens, which could effectively consider the influence of physical constraints in the updating process and achieve reasonably well results. Analysis of the updating results shows that with the increase of the axial load level, degradation of the flexural stiffness is significantly influenced by the load eccentricity. It shows that the cracking of the core concrete is the primary reason for the flexural stiffness degradation of CFST beam-columns. Finally, based on comparison with equations proposed by several design standards, the calculation methods for the dynamic flexural stiffness of CFST members is recommended.

• Steel and Composite Structures
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• v.51 no.4
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• pp.363-375
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• 2024

Within the optimization field, addressing the intricate posed by fluidic pressure loads on functionally graded structures with frequency-related designs is a kind of complex design challenges. This paper thus introduces an innovative density-based topology optimization strategy for frequency-constraint functionally graded structures incorporating Darcy's law and a drainage term. It ensures consistent treatment of design-dependent fluidic pressure loads to frequency-related structures that dynamically adjust their direction and location throughout the design evolution. The porosity of each finite element, coupled with its drainage term, is intricately linked to its density variable through a Heaviside function, ensuring a seamless transition between solid and void phases. A design-specific pressure field is established by employing Darcy's law, and the associated partial differential equation is solved using finite element analysis. Subsequently, this pressure field is utilized to ascertain consistent nodal loads, enabling an efficient evaluation of load sensitivities through the adjoint-variable method. Moreover, this novel approach incorporates load-dependent structures, frequency constraints, functionally graded material models, and polygonal meshes, expanding its applicability and flexibility to a broader range of engineering scenarios. The proposed methodology's effectiveness and robustness are demonstrated through numerical examples, including fluidic pressure-loaded frequency-constraint structures undergoing small deformations, where compliance is minimized for structures optimized within specified resource constraints.

• Earthquakes and Structures
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• v.27 no.4
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• pp.263-283
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• 2024

This research introduces a new composite system that utilizes multiple moving masses to identify cracks in structures resembling beams. The process starts by recording displacement time data from a set of these moving masses and converting this information into a relative time history through weighted aggregation. This relative time history then undergoes wavelet transform analysis to precisely locate cracks. Following wavelet examinations, specific points along the beam are determined as potential crack sites. These points, along with locations on the beam susceptible to cracked point due to support conditions, are marked as crack locations within the optimization algorithm's search domain. The model uses equations of motion based on the finite element method for the moving masses on the beam and employs the Runge-Kutta numerical solution within the state space. The proposed system consists of three successive moving masses positioned at even intervals along the beam. To assess its effectiveness, the method is tested on two examples: a simply supported beam and a continuous beam, each having three scenarios to simulate the presence of one or multiple cracks. Additionally, another example investigates the influence of mass speed, spacing between masses, and noise effect. The outcomes showcase the method's effectiveness and efficiency in localizing crack, even in the presence of noise effect in 1%, 5% and 20%.

• Steel and Composite Structures
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• v.52 no.4
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• pp.435-450
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• 2024

The geometry change of railway tracks significantly influences the safety and ride comfort of high-speed trains. This paper presents an analytical method to map the thermal deformations of a long-span arch bridge to the geometry of CRTS Type I double-block ballastless tracks for high-speed railways. A mechanical model of the bridge-track coupled system was developed to derive analytical formulae of the deformations of the track. The analytical formulae explicitly consider the mechanical properties of the bridge-track coupled system and the temperature profile. A three-dimensional finite element model was established to evaluate the predictions obtained from the analytical formulae. The results show that the analytical formulae provide accurate predictions of the track deformations caused by the thermal deformations of bridges. This research will promote the design, evaluation, and operation of high-speed railway bridges for improved safety and ride comfort in engineering practices.

• Journal of Korean Society of Steel Construction
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• v.29 no.2
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• pp.111-122
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• 2017

In this paper, the applicability of a 1900mm-deep concrete-filled U-shaped composite beam to composite ordinary moment frames (C-OMFs) was investigated based on existing test results from smaller-sized specimens and supplemental numerical studies since full-scale seismic testing of such a huge sized beam is practically impossible. The key issue was the web local buckling of concrete-filled U section under negative bending. Based on 13 existing test results compiled, the relationship between web slenderness and story drift capacity was obtained. From this relationship, a 1900mm-deep mega beam, fabricated with 25mm-thick plate was expected to experience the web local buckling at 2% story drift and eventually reach a story drift over 3%, thus much exceeding the requirements of C-OMFs. The limiting width to thickness ratio according to the 2010 AISC Specification was shown to be conservative for U section webs of this study. The test-validated supplemental nonlinear finite element analysis was also conducted to further investigate the effects of the horizontal stiffeners (used to tie two webs of a U section) on web local buckling and flexural strength. First, it is shown that the nominal plastic moment under negative bending can be developed without using the horizontal stiffeners, although the presence of the stiffeners can delay the occurrence of web local buckling and restrain its propagation. Considering all these, it is concluded that the 1900mm-deep concrete-filled U-shaped composite beam investigated can be conservatively applied to C-OMFs. Finally, some useful recommendations for the arrangement and design of the horizontal stiffeners are also recommended based on the numerical results.

• Journal of the Korean Geotechnical Society
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• v.32 no.10
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• pp.41-53
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• 2016

Stability of the braced earth wall in the composite ground, which is composed of the jointed base rocks and the soil strata depends on the earth pressure acting on it. In most cases, the earth pressure is calculated by the empirical method, in which base rocks are considered as a soil strata with the shear strength parameters of base rocks. In this case the effect of the joint dips of the jointed base rocks is ignored. Therefore, the calculated earth pressure is smaller than the actual earth pressure. In this study, the magnitude and the distribution of the earth pressure acting on the braced wall in the composite ground depending on the joint dips of the base rocks and the ratio of soil strata and base rocks were experimentally studied. Two dimensional large-scale model tests were conducted in a large scale test facility (height 3.0 m, length 3.0 m and width 0.5 m) by installing 10 supports in a scale of 1/14.5. The test ground was presumed with the base rock ratio of the composite ground of 65%:35% and 50%:50% and with the joint dips for each base rock layer, $0^{\circ}$, $30^{\circ}$, $45^{\circ}$ and $60^{\circ}$, respectively. And then finite element analyses were performed in the same condition. As results, the earth pressure on the braced wall increased as the base rock layer's joint dips became larger. And earth pressure at the rock layer increased as the rock rate became larger. The largest earth pressure was measured when the base rock rate was 50% (R50) and the rock layer's joint dips was $60^{\circ}$. Based on these results, a formular for the calculation of the earth pressure in the composite ground could be suggested. Distribution of earth pressure was idealized in a quadrangular form, in which the magnitude and the position of peak earth pressure depended on the rock ratio and the joint dips.

• Composites Research
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• v.27 no.4
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• pp.158-167
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• 2014

Final goal of this research is to establish the database for correlation factors which connects the test and analysis results of shear buckling allowables for composite plate. To accomplish the goal, extensive test and analysis works are required. In this paper, as the first step, a frame-type fixture for shear buckling test was designed and validated through the test and analysis. Final configuration of the fixture were determined via parametric study on the effect of specimen size, cross-sectional dimensions, and number of fastening bolts on the shear buckling load. Results of the study showed the designed frame-type fixture successfully induces the shear buckling of composite plate. However, there were deviations between the test results and analysis results for ideal case under pure shear load, which were mainly caused by the difference in plate sizes for both cases. The difference were larger in the plates with larger hole and simply supported boundary condition. It is concluded from the results that while the designed fixture can be used for the clamped plates with acceptable accuracy, it shows larger difference in the simply supported plates.

• Journal of the Korean Ceramic Society
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• v.40 no.1
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• pp.37-45
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• 2003

The dielectric and sintering characteristics of composites made by substituting Ca ion to Ba-site and Zr ion to Ti site in $(Ba{1-x}Ca_x)(Ti{0.96-yZr_ySn_{0.04})O_3$ $(0.15{\leq}x{\leq}0.20,\;0.09{\leq}y{\leq}0.14)$ were investigated. As the content of Ca was more than 15 mol%, composite was formed by precipitating the second phase whose main element was $CaTiO_3$ and the fraction of the second phase was increased. The curie temperature of composites was depended on Ca concentration, $-1.7^{\circ}C$ per mol% and the maximum dielectric constant of composite was decreased by the rate of 200/mol%. The substitution of Zr ion decreased the curie temperature by the rate of $10^{\circ}C$ per mol% and the maximum dielectric constant was decreased by 217/mol% due to the increase of diffuse phase transition. The density and insulation breakdown characteristics were improved by suppressing the abnormal grain growth due to the increase of second phase. We developed the composition of Y5U (EIA standard) condenser which had high breakdown voltage and dielectric constant by controlling diffuse phase transition by the addition of Zr ion into composite.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.825-831
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• 2017

In this study, we perform the design of dome geometry for the composite pressure vessel with applying the equation of Fulton and Vasiliev considering external load(thrusts). Variables of the dome geometry are opening radius ratio(${\rho}_0$) from 0.1 to 0.5 and thrust level from 40kN to 200kN. We conduct Finite Element Analysis(FEA) by using ABAQUS. As a result, the strain of the composite pressure vessel has shown strain gradient from inner to outer of dome surface. And the strain gradient may cause crack of resin inside the composite laminate. Strain gradient of Fulton dome is monotonously decreased as the ${\rho}_0$ increases, but the strain gradient of Vasiliev dome bas shown some different trend. when ${\rho}_0{\leq}0.1$, strain gradient of Fulton's is higher than Vasiliev's. But when 0.1<${\rho}_0$<0.35, strain gradient of Vasiliev's becomes higher than Fulton's. And in the case of $0.35{\leq}{\rho}_0$, strain gradient of Vasiliev's is higher than Fulton's. So the Vasiliev dome is more effective in ${\rho}_0{\leq}0.1$ condition and Fulton dome is more effective in $0.35{\leq}{\rho}_0$ condition. So, it's important for dome design to consider the crack of resin cause of the strain gradient.

• Composites Research
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• v.35 no.4
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• pp.269-276
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• 2022

In this paper, a composite stiffened panel was fabricated using a three-dimensional weaving method that can reduce the risk of delamination, and mechanical properties such as buckling load and natural frequency were investigated. The preform of the stringer and skin of the stiffened panel were fabricated in one piece using T800 grade carbon fiber and then, resin (EP2400) was injected into the preform. The compression test and natural frequency measurement were performed for the stiffened panel, and the results were compared with the finite element analyses. In order to compare the performance of 3D weaving structures, the stiffened panels with the same configuration were fabricated using UD and 2D plain weave (fabric) prepregs. Compared to the tested buckling load of the 3D woven panel, the buckling loads of the stiffened panels of UD prepreg and 2D plain weave exhibited +20% and -3% differences, respectively. From this study, it was confirmed that the buckling load of the stiffened panel manufactured by 3D weaving method was lower than that of the UD prepreg panel, but showed a slightly higher value than that of the 2D plain weave panel.