• Structural Engineering and Mechanics
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• v.65 no.6
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• pp.731-739
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• 2018

This paper presents an optimal pattern for distributing stiffness along a framed tube structure through an analytic equation, which may be used during the preliminary design stage. Most studies in this field are computationally intensive and time consuming, while a hand-calculation method, as presented here, is a more suitable tool for sensitivity analyses and parametric studies. Approach in development of the analytic model is to minimize the mean compliance (external work) for a given volume of material. A variational statement of the problem is made, and a specified deformation-profile is obtained as the necessary condition for a minimum; enforcing this condition, stiffness is then computed. Due to some near-zero values for stiffness, the problem is modified by considering a lower bound constraint. To deal with this constraint, the design domain is assumed to be divided into two zones of constant stiffness and constant curvature; and the problem is restated in terms of these concepts. It will be shown that this methodology allows for easy computation of stiffness through an analytic and dimensionless equation, valid in any system of units. To show practicality of the proposed method, a tubed-system structure with uniform stiffness distribution is redesigned using the proposed model. Comparative analyses of the results reveal that in addition to simplicity of the proposed method, it provides a rather high degree of accuracy for real-world problems.

• Journal of the Earthquake Engineering Society of Korea
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• v.11 no.2 s.54
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• pp.69-79
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• 2007

In this paper, a technique to detect structural damage and estimate its severity using peaks and zeros of frequency response functions (FRFs) is developed. The peaks in FRFs represent the natural frequencies of the structure and the zeros provide additional information. The characteristics of peaks and zeros are defined and the calculation procedure to obtain the peaks and zeros from the relationship between frequency response function and stiffness and mass matrices are clearly explained. A structural system identification theory which is utilizing the sensitivity of stiffness of a structural member to eigenvalues, i.e., peaks and zeros, is established. The proposed method can identify damage location and its severity, with natural and zero frequencies, by estimating structural stiffness of the structure in the process of making a analytical model The accuracy and feasibility is demonstrated by numerical models of a spring-mass system and a beam structure.

• Computational Structural Engineering
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• v.9 no.3
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• pp.169-177
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• 1996

The focus of this study is on the problem of the design of structure of undetermined topology. This problem has been regarded as being the most challenging of structural optimization problems, because of the difficulty of allowing topology to change. Conventional approaches break down when element sizes approach to zero, due to stiffness matrix singularity. In this study, a novel nonlinear programming formulation of the topology problem is presented. Its main feature is the ability to account for topology variation through zero element sizes. Stiffness matrix singularity is avoided by embedding the equilibrium equations as equality constraints in the optimization problem. Although the formulation is general, two dimensional plane elasticity examples are presented. The design problem is to find minimum weight of a plane structure of fixed geometry but variable topology, subject to constraints on stress and displacement. Variables are thicknesses of finite elements, and are permitted to assume zero sizes. The examples demonstrate that the formulation is effective for finding at least a locally minimal weight.

• Structural Engineering and Mechanics
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• v.10 no.1
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• pp.27-36
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• 2000

The Oktalok nodal connection system is an aesthetic and efficient system. It has been widely used throughout Australia. The paper will briefly introduce the concept and application of the Oktalok nodal system. The existing design method is based on the assumption that the joints are pin-ended, i.e., the rotational stiffness of the joints is zero. However the ultimate capacity of the frame may increase significantly depending on the rotational stiffness of the joints. Stiffness tests and finite element simulations were carried out to determine the rotational stiffness of the Oktalok joints. Column buckling tests and non-linear finite element analyses were performed to determine the member capacity of columns with semi-rigid end conditions. A simple formulae for the effective length factor of column buckling is derived based on the above experimental and theoretical investigations.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.6
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• pp.80-87
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• 1998

In this study, the effects of residual shaft bow and flexible bearings of a single disk rotor are investigated. The stiffness coefficients of a shaft with initial deflection are different from those of a straight shaft. The stiffness coefficients are calculated using Castigliano theorem considering initial deflections. The stiffness coefficients, which are obtained in this study, are in good agreement with FEM results. The speed which causes zero amplitude is shown to be the square root of the ratio of residual bow amplitude to unbalance eccentricity in the case of rigid bearings and isotropic flexible bearings, but not in anisotropic bearings.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2006.05a
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• pp.349-352
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• 2006

The plastic deformation of polycrystalline materials is induced by changes of the microstructure when the loading is beyond the critical state of stress. Constitutive models for the crystal plasticity have the common objective which relates microscopic single crystals in the crystallographic texture to the macroscopic continuum point. In this paper, a new consistent tangent stiffness for the anisotropic elasto-viscoplastic analysis of polycrystalline deformation is developed, which can be used in the finite element analysis for the slip-dominated large deformation of polycrystalline materials. In order to calculate the consistent tangent stiffness, the state function is defined based on the consistency condition between the elastic and plastic stress. The rate of shearing increment($\Delta{\gamma}^{\alpha}$) is calculated with satisfying the consistency condition. The consistency condition becomes zero when the trial resolved shear stress($\tau^{{\alpha}^*}$) becomes resolved shear stress($\tau^{\alpha}$) at every step. Iterative method is utilized to calculate the rate of shearing increment based on the implicit backward Euler method. The consistent tangent stiffness can be formulated by differentiating the rate of shearing increment with total strain increment after the instant rate of shearing increment converges. The proposed tangent stiffness is applied to the ABAQUS/Standard by implementing in the ABAQUS/UMAT.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.1
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• pp.12-17
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• 2018

In order to enhance the realism of a virtual environment, a method of maximizing the stiffness of the virtual environment model is needed, which maintains the stability of the haptic system. In our previous research, we proposed a haptic system with a first order hold, instead of a zero order hold, and showed that the maximum available stiffness of a virtual spring with the first-order hold is larger than that with the zero-order hold. However, in terms of real system implementation, the zero order hold is a more common and easy method. In this paper, we propose an extrapolation method and a high frequency zero-order-hold output method in order to obtain the stability region using a zero order hold, which is equivalent to the method using the first-order-hold. The simulation results shows that the stability range of the virtual spring becomes almost the same as that of the method using the first order hold when the sampling period of the high frequency zero-order-hold method is decreased. Moreover, the stability range of the proposed method is several times to several tens of times greater than that of the method using the zero order hold only. Therefore, it is expected that the proposed method can enhance the realism of rigid bodies in a virtual environment.

• Journal of Institute of Convergence Technology
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• v.2 no.2
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• pp.35-39
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• 2012

This paper presents the effect of data-hold methods on stability of haptic system with a virtual wall. When a human operator interacts with virtual wall, the lager the stiffness of the virtual wall is, the more realistic the operator feels that the virtual wall is. However, if the stiffness of the virtual wall becomes extremely large, the system may be unstable. When a virtual wall is designed, it is necessary to analyze the maximum available stiffness to guarantee a stable haptic interaction. The simulation model in this paper is developed based on the haptic device model, sampler, a virtual wall model, and data hold methods to compute the maximum stiffness for stability. The effectiveness of the simulation is evaluated through comparing the results of previous studies with the results of this simulation. In addition, the effects of two data hold methods, that is, zero-order hold (ZOH) and first-order hold (FOH) on the stability are analyzed and the values of the maximum available stiffness are compared through the simulation.

• International Journal of Highway Engineering
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• v.15 no.1
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• pp.95-102
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• 2013

PURPOSES: The existing method evaluating the existence of the hollows in concrete pavement does not consider the stiffness of pavement. In addition, the method uses unreasonable logic judging the hollow existence by the deflection caused by zero loading. In this study, the deflection of slab corner due to heavy weight deflectometer (HWD) was measured in concrete pavement sections where underground structures are located causing the hollows around them. METHODS: The modulus of subgrade reaction obtained by comparing the actual deflection of slab to the result of finite element analysis was calibrated into the composite modulus of subgrade reaction. The radius of relative stiffness was calculated, and the relationship between the ratio of HWD load to the radius of relative stiffness and the slab deflection was expressed as the curve of secondary degree. RESULTS: The trends of the model coefficients showing width and maximum value of the curve of secondary degree were analyzed by categorizing the pavement sections into three groups : hollows exist, additional investigation is necessary, and hollows do not exist. CONCLUSIONS: The results analyzed by the method developed in this study was compared to the results analyzed by existing method. The model developed in this study will be verified by analyzing the data obtained in other sections with different pavement structure and materials.

• Journal of Institute of Control, Robotics and Systems
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• v.20 no.4
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• pp.389-394
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• 2014

This paper presents the effect of a reflective force computed from a first-order-hold method on the stability of a haptic system. A haptic system is composed of a haptic device with a mass and a damper, a virtual spring, a sampler and a sample-and-hold. The boundary condition of the maximum virtual stiffness is analytically derived by using the Routh-Hurwitz criterion and the condition shows that the maximum virtual stiffness is proportional to the square root of the mass and the damper of a haptic device and also is inversely proportional to the sampling time to the power of three over two. The effectiveness of the derived condition is evaluated by the simulation. When the reflective forces are computed by using the first-order-hold method, the maximum available stiffness to guarantee the stability is increased several hundred times as large as when the zero-order-hold method is applied.