• Steel and Composite Structures
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• 제48권1호
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• pp.19-25
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• 2023

Due to high application of concrete structures in construction industry, however, the quality improvement is essential. One of the new ways for this purpose is adding the nanoparticles to the concrete. In this work, vibration analysis of concrete beams reinforced by graphene oxide (GO) nanoparticles based on mathematical model has been investigated. For the accuracy of the presented model, the experimental study is done for comparing the compressive strength. Since the nanoparticles can not be solved in water without any specific process, at the first, GO nanoparticles should be dispersed in water by using shaker, magnetic striker, ultrasonic devices and finally mechanical mixer. For modelling of the strucuture, sinusoidal shear deformation beam theory (SSDBT) is utilized. Mori-Tanak model model is utilized for obtaining the effective properties of the beam including agglomeration influences. Utilizing the energy method and Hamilton's principal, the motion equations are calculated. The frequency of the concrete beam is obtanied by analytical method. Three samples with 0.02% GO nanoparticles are built and its compressive strength is compared which shows a good accuracy with maximum 1.29% difference with mathematical model and other papers. The aim of this work from the theoretical study is investigating the effects of nanoparticles volume percentage and agglomeration, length and thickness of the beam on the frequency of the structure. The results show that the with enhancing the GO nanoparticles, the frequency is increased. For example, with enhancing the volume percent of GO nanoparticles from zero to 0.08%, the compressive strength is increased 48.91%. and 46.83%, respectively for two cases of with and without agglomeration.

• Structural Engineering and Mechanics
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• 제88권2호
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• pp.159-168
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• 2023

Cost-effective high precision hybrid elements are presented in a hierarchical form for dynamic analysis of plates. The costs associated with controlling the vibrations of ferromagnetic plates can be minimized by adequate determination of the amount of electric current and magnetic field. In the present study, the effect of magnetic field and electric current on nonlinear vibrations of ferromagnetic plates is investigated. The general form of Lorentz forces and Maxwell's equations have been considered for the first time to present new relationships for electromagnetic interaction forces with ferromagnetic plates. In order to derive the governing nonlinear differential equations, the theory of third-order shear deformations of three-dimensional plates has been applied along with the von Kármán large deformation strain-displacement relations. Afterward, the nonlinear equations are discretized using the Galerkin method, and the effect of various parameters is investigated. According to the results, electric current and magnetic field have different effects on the equivalent stiffness of ferromagnetic plates. As the electric current increases and the magnetic field decreases, the equivalent stiffness of the plate decreases. This is a phenomenon reported here for the first time. Furthermore, the magnetic field has a more significant effect on the steady-state deflection of the plate compared to the electric current. Increasing the magnetic field and electric current by 10-times results in a reduction of about 350% and an increase of 3.8% in the maximum steady-state deflection, respectively. Furthermore, the nonlinear frequency decreases as time passes, and these changes become more intense as the magnetic field increases.

• Structural Engineering and Mechanics
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• 제89권1호
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• pp.33-46
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• 2024

The present research delves into the analysis of nonlinear free and forced vibrations of porous functionally graded (FG) shallow shells reinforced with oblique stiffeners, which are embedded in a nonlinear elastic foundation (NEF) subjected to external excitation. Two distinct types of PFG shallow shells, characterized by even and uneven porosity distribution along the thickness direction, are considered in the research. In order to model the stiffeners, Lekhnitskii's smeared stiffeners technique is implemented. With the stress function and first-order shear deformation theory (FSDT), the nonlinear model of the oblique stiffened shallow shells is established. The strain-displacement relationships for the system are derived via the FSDT and utilization of the von-Kármán's geometric assumptions. To discretize the nonlinear governing equations, the Galerkin method is employed. The model such developed allows analysis of the effects of the stiffeners with various angles as desired, in addition to the quantitative investigation on the influence of the surrounding nonlinear elastic foundations. To numerically solve the problem of vibrations, the 4th-order P-T method is used, as this method, known for its enhanced accuracy and reliability, proves to be an effective choice. The validation of the present research findings includes a comprehensive comparison with outcomes documented in existing literature. Additionally, a comparative analysis of the numerical results against those obtained using the 4th Runge-Kutta method is performed. The impact of stiffeners with varying angles and material parameters on the vibration characteristics of the present system is also explored. The researchers and engineers working in this field may use the results of this study as benchmarks in their design and research for the considered shell systems.

• Steel and Composite Structures
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• 제51권1호
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• pp.73-91
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• 2024

This paper has focused on presenting vibration analysis of trapezoidal sandwich plates with a damaged core and FG wavy CNT-reinforced face sheets. A damage model is introduced to provide an analytical description of an irreversible rheological process that causes the decay of the mechanical properties, in terms of engineering constants. An isotropic damage is considered for the core of the sandwich structure. The classical theory concerning the mechanical efficiency of a matrix embedding finite length fibers has been modified by introducing the tube-to-tube random contact, which explicitly accounts for the progressive reduction of the tubes' effective aspect ratio as the filler content increases. The First-order shear deformation theory of plate is utilized to establish governing partial differential equations and boundary conditions for the trapezoidal plate. The governing equations together with related boundary conditions are discretized using a mapping-generalized differential quadrature (GDQ) method in spatial domain. Then natural frequencies of the trapezoidal sandwich plates are obtained using GDQ method. Validity of the current study is evaluated by comparing its numerical results with those available in the literature. After demonstrating the convergence and accuracy of the method, different parametric studies for laminated trapezoidal structure including carbon nanotubes waviness (0≤w≤1), CNT aspect ratio (0≤AR≤4000), face sheet to core thickness ratio (0.1 ≤ ${\frac{h_f}{h_c}}$ ≤ 0.5), trapezoidal side angles (30° ≤ α, β ≤ 90°) and damaged parameter (0 ≤ D < 1) are carried out. It is explicated that the damaged core and weight fraction, carbon nanotubes (CNTs) waviness and CNT aspect ratio can significantly affect the vibrational behavior of the sandwich structure. Results show that by increasing the values of waviness index (w), normalized natural frequency of the structure decreases, and the straight CNT (w=0) gives the highest frequency. For an overall comprehension on vibration of laminated trapezoidal plates, some selected vibration mode shapes were graphically represented in this study.

• 대기
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• 제34권2호
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• pp.153-176
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• 2024

At 0843 UTC 30 May 2021, a commercial aircraft encountered severe turbulence at z = 11.5 km associated with the rapid development of Mesoscale Convective System (MCS) in the Gyeonggi Bay of Korea. To investigate the generation mechanisms of Near-Cloud Turbulence (NCT) near the MCS, Weather Research and Forecasting model was used to reproduce key features at multiple-scales with four nested domains (the finest ∆x = 0.2 km) and 112 hybrid vertical layers. Simulated subgrid-scale turbulent kinetic energy (SGS TKE) was located in three different regions of the MCS. First, the simulated NCT with non-zero SGS TKE at z = 11.5 km at 0835 UTC was collocated with the reported NCT. Cloud-induced flow deformation and entrainment process on the downstream of the overshooting top triggered convective instability and subsequent SGS TKE. Second, at z = 16.5 km at 0820 UTC, the localized SGS TKE was found 4 km above the overshooting cloud top. It was attributed to breaking down of vertically propagating convectively-induced gravity wave at background critical level. Lastly, SGS TKE was simulated at z = 11.5 km at 0930 UTC during the dissipating stage of MCS. Upper-level anticyclonic outflow of MCS intensified the environmental westerlies, developing strong vertical wind shear on the northeastern quadrant of the dissipating MCS. Three different generation mechanisms suggest the avoidance guidance for the possible NCT events near the entire period of the MCS in the heavy air traffic area around Incheon International Airport in Korea.

• 한국지진공학회논문집
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• 제28권1호
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• pp.47-57
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• 2024

Recently, in newly constructed apartment buildings, the exterior wall structures have been characterized by thinness, having various openings, and a significantly low reinforcement ratio. In this study, a nonlinear finite element analysis was performed to investigate the crack damage characteristics of the exterior wall structure. The limited analysis models for a 10-story exterior wall were constructed based on the prototype apartment building, and nonlinear static analysis (push-over analysis) was performed. Based on the finite element (FE) analysis model, the parametric study was conducted to investigate the effects of various design parameters on the strength and crack width of the exterior walls. As the parameters, the vertical reinforcement ratio and horizontal reinforcement ratio of the wall, as well as the uniformly distributed longitudinal reinforcement ratio and shear reinforcement ratio of the connection beam, were addressed. The analysis results showed that the strength and deformation capacity of the prototype exterior walls were limited by the failure of the connection beam prior to the flexural yielding of the walls. Thus, the increase of wall reinforcement limitedly affected the failure modes, peak strengths, and crack damages. On the other hand, when the reinforcement ratio of the connection beams was increased, the peak strength was increased due to the increase in the load-carrying capacity of the connection beams. Further, the crack damage index decreased as the reinforcement ratio of the connection beam increased. In particular, it was more effective to increase the uniformly distributed longitudinal reinforcement ratio in the connection beams to decrease the crack damage of the coupling beams, regardless of the type of the prototype exterior walls.

• Steel and Composite Structures
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• 제51권4호
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• pp.391-406
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• 2024

The present research investigates the combination resonance behaviors of porous FG shallow shells reinforced with oblique stiffeners and subjected to a two-term excitation. The oblique stiffeners considered in this research reinforce the shell internally and externally. To model the stiffeners, Lekhnitskii's smeared stiffeners technique is utilized. According to the first-order shear deformation theory (FSDT) and stress functions, a nonlinear model of the oblique stiffened shallow shell is established. With regard to the FSDT and von-Kármán nonlinear geometric assumptions, the stress-strain relationships for the present shell system are developed. Also, in order to discretize the nonlinear governing equations, the Galerkin method is implemented. To obtain the required relations for investigating the combination resonance theoretically, the method of multiple scales is applied. For verifying the results of the present research, generated results are compared with previous research. Additionally, a comparison with the P-T method is conducted to increase the validity of the generated results, as this method has illustrated advantages over other numerical methods in terms of accuracy and reliability. In this method, the piecewise constant argument is used jointly with the Taylor series expansion, which is why it is named the P-T method. The effects of stiffeners with different angles, and the effects of material parameters on the combination resonance behaviors of the present system are addressed. With the findings of this research, researchers and engineers in this field may use them as benchmarks for their design and research of porous FG shallow shells.

• Computers and Concrete
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• 제34권2호
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• pp.179-193
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• 2024

The bending and buckling effect for carbon nanotube-reinforced composite (CNTRC) beams can be evaluated by developing the theory of third shear deformation (TSDT). This study examines beams supported by viscoelastic foundations, where single-walled carbon nanotubes (SWCNTs) are dispersed and oriented within a polymer matrix. Four patterns of reinforcement are used for the CNTRC beams. The rule of mixtures is assessed for the material properties of CNTRC beams. The effective functionally graded materials (FGM) properties are studied by considering three different uneven distribution types of porosity. The damping coefficient is considered to investigate the viscosity effect on the foundation in addition to Winkler's and Pasternak's parameters. The accuracy of the current theory is inspected with multiple comparison works. Moreover, the effects of different beam parameters on the CNTRC beam bending and buckling over a viscoelastic foundation are discussed. The results demonstrated that the O-beam is the weakest type of CNTRC beam to resist buckling and flexure loads, whereas the X-beam is the strongest. Moreover, it is indicated that the presence of porosity in the beams decreases the stiffness and increases deflection. In comparison, the deflection was reduced in the presence of a viscoelastic foundation.

• 한국해안·해양공학회논문집
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• 제26권1호
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• pp.49-64
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• 2014

해안 및 해양구조물 하부의 해저지반에 고파랑이 장시간 작용하는 경우 과잉간극수압(진동과잉간극수압과 잔류과잉간극수압의 합)이 크게 발생할 수 있고, 이어지는 유효응력의 감소에 따라 해저지반에 액상화가 발생될 수 있다. 일단, 지반액상화가 발생 및 진행되면 구조물의 침하 혹은 전도에 의해 종국적으로 구조물이 파괴될 가능성이 높아진다. 특히, 중력식구조물이 설치된 하부지반내에서는 파작용에 의한 큰 과잉간극수압과 작은 유효응력으로 부터 발생되는 지반액상화의 여부를 정확히 예측할 필요가 있고, 이러한 지반의 동적거동 특성은 설계에 충분히 반영되어야 한다. 본 연구에서는 2차원수치파동수로를 불규칙파동장으로 확장한 수치해석법을 적용하여 해저지반상 및 구조물의 표면상에서 시간변동의 동파압과 유속에 의한 전단응력을 산정하고, 그 결과를 지반의 동적거동을 정밀하게 재현할 수 있는 해저지반응답용의 수치해석프로그램 FLIP(Finite element analysis LIquefaction Program)에 입력치로 적용하여 해저지반내에서 과잉간극수압 및 유효응력의 시공간적인 변화, 이로 인한 액상화, 그리고 지반의 시간변형과 구조물의 시간변위를 정량적으로 평가한다. 이로부터 해저면상에서 전단응력을 고려한 경우 구조물 전면의 하부해저지반에서 액상화 가능성을 확인할 수 있었고, 액상화된 토립자는 흐름에 저항력을 상실하므로 세굴로 이어질 것으로 판단된다. 따라서, 태풍시 고파랑의 작용이 장시간 지속되는 경우 구조물의 전면에서는 지반액상화로 인한 지반강도의 현저한 저하로 구조물의 진동변위가 더욱 크게 발생되고, 더불어 구조물의 안정성에 영향을 미칠 것으로 예상된다.

• 암석학회지
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• 제19권3호
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• pp.227-244
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• 2010

제주도 현무암에 포획된 첨정석 레졸라이트 포획암은 비교적 뚜렷하게 구별되는 세 종류의 조직으로 산출된다. 즉, 조립질의 프로토그라뉼라 조직, 쌍봉의 입자분포를 보이는 포피로클라스틱 조직, 그리고 세립질의 입자들이 신장되어 엽리를 보이는 마일로니틱 조직의 포획암으로 구별된다. 이들 조직은 프로토그라뉼라에서 포피로클라스틱, 마일로니틱 조직으로 가면서 입자크기는 점점 감소하고, 입자의 경계는 더욱 직선화되며, 삼중점이 더 빈번하게 산출되는 경향을 보인다. 특히 포피로클라스틱 및 마일로니틱 조직의 포획암은 킹크밴드를 가지는 거정의 반상쇄정(2-3 mm) 사이에 변형의 흔적이 없는 세립질의 입자($200-300\;{\mu}m$)로 구성되어 있으며, 반상쇄정 내 용리엽리가 구부러져 있는 조직적 특징을 보인다. 또한 프로토그라뉼라 조직에서 첨정석은 사방휘석과 항상 같이 접촉되어 산출되지만 마일로니틱 조직에서는 사방휘석과 관계없이 단독으로 뿌려져 산출되는 특성을 보인다. 이러한 조직적 특성은 포획암이 특정한 편압 하에서 동력재결정작용과 열에 의한 정적재결정작용을 경험하였음을 반영하고 있다. 첨정석 레졸라이트 포획암을 구성하고 있는 감람석, 사방휘석, 단사휘석의 mg#[$=100{\times}Mg/(Mg+Fe_t)$]는 입자의 크기나 조직적 차이와는 상관없이 거의 일정하다(Ol: 88-91, Opx: 89-92, Cpx: 90-92). mg#의 값, 감람석의 NiO (0.3~0.4 wt%)와 MnO (0.1~0.2 wt%)의 조성은 세계 다른 곳의 맨틀 레졸라이트 포획암내 감람석의 값과 유사하며, 20~25%의 부분용융을 경험한 잔류맨틀임을 나타내고 있다. 미량원소 및 희토류원소 조성은 주성분원소 조성에 비해 프로토그라뉼라, 포피로클라스틱, 마일로니틱 조직 사이에 좀 더 뚜렷한 차이를 보여주면서 미량원소 조성과 조직적 특성사이에 밀접한 관련성이 있음을 나타내고 있다. 첨정석 레졸라이트 포획암의 단사휘석과 사방휘석의 미량원소 및 희토류원소함량은 프로토그라뉼라에서 마일로니틱 조직으로 갈수록 증가하고 있으며, 특히 LREE와 불호정성이 큰 미량 원소일수록 현저하게 증가하는 경향을 나타낸다. 이는 첨정석 레졸라이트 포획암이 부분용융을 경험한 이후 LREE가 부화된 멜트 또는 유체에 의한 교대작용을 받았으며, 변형사건과 교대작용사이에 밀접한 관련성이 있음을 의미한다. 노두에서 관찰되는 조직의 상대적 산출비율이 이들 포획암이 기원된 상부맨틀에서의 상대적 산출비율이라고 가정한다면, 포피로클라스틱 및 마일로니틱 조직을 발달시킨 변형사건은 비교적 좁은 전단대 지역에 한정되었을 것으로 제시된다. 이러한 전단대는 제주도 아래 암석권질 상부맨틀에서 LREE가 부화된 멜트 또는 유체가 침투하기에 용이한 환경을 형성하였을 것이다. 마일로니틱 조직에서 더욱 현저하게 나타나는 미량원소부화특성은 입자의 크기가 작아지고 변형의 강도가 높을수록 멜트 또는 유체의 침투가 더욱 용이했기 때문일 것으로 해석된다.