• Proceedings of the Korean Fiber Society Conference
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• 1987.06b
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• pp.11-11
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• 1987

Among other poly(4,4'-terephthanilidealkylamide)s (PTAA's), poly (4,4'-terephthanilideadipamide) (PTAd) gave clear critical concentration curves. For PTAA's with methylene units more than 6, the critical concentration (C*) seemed to be beyond the solubility limit of H₂SO₄. Under shearing conditions, the nematic domains were easily oriented and stretched in the direction of shear , and a fibrillar structure resulted. At low frequencies, a monotollous reduction of loss tangent (tan) was observed as concentration increased. At high frequencies, however, tan was increased above C* again, and showed maximum at saturation concentration (Cs). With increasing temperature, viscosity of isotropic and anisotropic phases was normally decreased, while viscosity of biphases was increased. Plot of complex viscosity (If) against temperature based on rheological measurements exhibited a good correlation with phase diagram constructed by polarizing microscope observations. Rheological parameters suggested the optimum dope concentration of PTAd with inherent viscosity 2.02 at 30oc is in the vicinity of 19.2 wt%, which seemed to agree well with spinning experiments (around 19.4 wt%). In general, effects of spinning and annealing conditions on the mechanical properties of PTAA fibres were most pronounced in PTAd fibre spun from anisotropic spinning dope .

• Structural Engineering and Mechanics
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• v.58 no.5
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• pp.799-823
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• 2016

A finite element model for the non-linear dynamic analysis of a reinforced concrete (RC) containment shell of a nuclear power plant subjected to extreme loads such as impact and earthquake is presented in this work. The impact is modeled by using an uncoupled approach in which a load function is applied at the impact zone. The earthquake load is modeled by prescribing ground accelerations at the base of the structure. The nuclear containment is discretized spatially by using 20-node brick finite elements. The concrete in compression is modeled by using a modified $Dr{\ddot{u}}cker$-Prager elasto-plastic constitutive law where strain rate effects are considered. Cracking of concrete is modeled by using a smeared cracking approach where the tension-stiffening effect is included via a strain-softening rule. A model based on fracture mechanics, using the concept of constant fracture energy release, is used to relate the strain softening effect to the element size in order to guaranty mesh independency in the numerical prediction. The reinforcing bars are represented by incorporated membrane elements with a von Mises elasto-plastic law. Two benchmarks are used to verify the numerical implementation of the present model. Results are presented graphically in terms of displacement histories and cracking patterns. Finally, the influence of the shear transfer model used for cracked concrete as well as the effect due to a base slab incorporation in the numerical modeling are analyzed.

• Steel and Composite Structures
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• v.19 no.6
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• pp.1333-1354
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• 2015

A higher order analytical solution for static analysis of a truncated conical composite sandwich panel subjected to different loading conditions was presented in this paper which was based on a new improved higher order sandwich panel theory. Bending analysis of sandwich structures with flexible cores subjected to concentrated load, uniform distributed load on a patch, harmonic and uniform distributed loads on the top and/or bottom face sheet of the sandwich structure was also investigated. For the first time, bending analysis of truncated conical composite sandwich panels with flexible cores was performed. The governing equations were derived by principle of minimum potential energy. The first order shear deformation theory was used for the composite face sheets and for the core while assuming a polynomial description of the displacement fields. Also, the in-plane hoop stresses of the core were considered. In order to assure accuracy of the present formulations, convergence of the results was examined. Effects of types of boundary conditions, types of applied loads, conical angles and fiber angles on bending analysis of truncated conical composite sandwich panels were studied. As, there is no research on higher order bending analysis of conical sandwich panels with flexible cores, the results were validated by ABAQUS FE code. The present approach can be linked with the standard optimization programs and it can be used in the iteration process of the structural optimization. The proposed approach facilitates investigation of the effect of physical and geometrical parameters on the bending response of sandwich composite structures.

• Structural Engineering and Mechanics
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• v.35 no.6
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• pp.759-772
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• 2010

Seismic isolation is a well-known method to mitigate the earthquake effects on structures by increasing their fundamental natural periods at the expense of larger displacements in the structural system. In this paper, the seismic response of isolated and fixed base vertical, cylindrical, liquid storage tanks is investigated using a Finite Element Model (FEM), taking into account fluid-structure interaction effects. Three vertical, cylindrical tanks with different ratios of height to radius (H/R = 2.6, 1.0 and 0.3) are numerically analyzed and the results of response-history analysis, including base shear, overturning moment and free surface displacement are reported for isolated and non-isolated tanks. Isolated tanks equipped by lead rubber bearings isolators and the bearing are modeled by using a non-linear spring in FEM model. It is observed that the seismic isolation of liquid storage tanks is quite effective and the response of isolated tanks is significantly influenced by the system parameters such as their fundamental frequencies and the aspect ratio of the tanks. However, the base isolation does not significantly affect the surface wave height and even it can causes adverse effects on the free surface sloshing motion.

• Structural Engineering and Mechanics
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• v.35 no.6
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• pp.735-758
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• 2010

The paper proposes two methodologies for damage identification from measured natural frequencies of a contiguously damaged reinforced concrete beam, idealised with distributed damage model. The first method identifies damage from Iso-Eigen-Value-Change contours, plotted between pairs of different frequencies. The performance of the method is checked for a wide variation of damage positions and extents. The method is also extended to a discrete structure in the form of a five-storied shear building and the simplicity of the method is demonstrated. The second method is through smeared damage model, where the damage is assumed constant for different segments of the beam and the lengths and centres of these segments are the known inputs. First-order perturbation method is used to derive the relevant expressions. Both these methods are based on distributed damage models and have been checked with experimental program on simply supported reinforced concrete beams, subjected to different stages of symmetric and un-symmetric damages. The results of the experiments are encouraging and show that both the methods can be adopted together in a damage identification scenario.

• Special Issue of the Society of Naval Architects of Korea
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• 2007.09a
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• pp.99-106
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• 2007

The IACS Common Structural Rules are to be applied for double hull tanker of more than 150m length with contracted after 1 April 2006. The objectives of the rules are to make more robust, safer ship and to ensure transparency of the technical background. In compliance of CSR, we had carried out prescriptive rules scantling determination and 3-D hold FE analysis of AFRAMAX TANKER. Prescriptive rules scantling determines the minimum required scantling, hull-girder longitudinal bending and shear strength, hull girder ultimate strength, local strength of plate and stiffener, strength of primary supporting member and fatigue assessment of the longitudinal stiffener end connections to the transverse bulkhead. 3-D hold FE analysis assesses the structural adequacy of the vessel's primary hull structure and major supporting members using yielding and buckling failure modes. So we could verify the strength assessment of AFRAMAX TANKER applied CSR.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.13 no.2
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• pp.209-220
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• 2000

For non-linear analysis of stiffened shell structures, the total Lagrangian formulation is presented based upon the degenerated shell element. Geometrically correct formulation is developed by updating the direction of normal vectors and taking into account second order rotational terms in the incremental displacement field. Assumed strain concept is adopted in order to overcome shear locking phenomena and to eliminate spurious zero energy mode. The post-buckling behaviors of stiffened shell structures are traced by modeling the stiffener as a shell element and considering general transformation between the main structure and the stiffener at the connection node. Numerical examples to demonstrate the accuracy and the effectiveness of the proposed shell element are presented and compared with references' results.

• Journal of Welding and Joining
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• v.17 no.6
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• pp.25-31
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• 1999

Stainless steel sheets are commonly used for vehicles such as the bus and the train. These are mainly fabricated by spot-welding. By the way, its fatigue strength is lower than base metal due to high stress concentration at the nugget. edge of the spot-welding. By the way, its fatigue strength is lower than base metal due to high stress concentration at the nugget edge of the spot-welding point. Especially, it is influenced by welding conditions as well as geometrical factors of spot welded joint. Therefore, it is not too much to say that structural rigidity and strength of spot-welded structures is decided by fatigue strength of spot welded lap joint. Thus, it is necessary to establish a reasonable and systematic long life design criterion for the spot-welded structure. In this study, numerical stress analysis was performed by using 3-dimensional finite element model on IB-type spot-welded lap joint of 304 stainless steel sheet under tension-shear load. Fatigue tests were also conducted on them having various thickness, joint angle, lapped length, and width of the plate. From the results, it was found that fatigue strength of IB-type spot-welded lap joints was influenced by its geometrical factors, however, could be systematically rearranged by maximum principal stress ({TEX}$σ_{1max}${/TEX}) at the nugget edge of the spot-welding point.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1997.10a
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• pp.145-154
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• 1997

The results of optimum design by the deterministic approach adopted in the current design codes depend upon the safety levels of the applied code. But, it is now generally recognized that structural problems are nondeterministic and, consequently, that engineering optimum design must cope with uncertainties. Therefore, it is not an overstatement to affirm that the combination of reliability-based design procedures and optimization techniques is the only means of providing a powerful tool to obtain a practical optimum design solution. In the paper, reliability based optimum design procedure as a rational approach to optimum structural design is presented. The design constraints are formulated based on the ASD, LRFD and reliability theories. The reliability analysis is based on an advanced first-order second moment approach. Uncertainties in the structural strength and loading due to inherent variability as well as modeling and prediction errors are included in failure due to combined bending and shear. For the realistic reliability-based optimization of continuous steel box girder bridges, interactive non-linear limit state model is formulated based on the von Mises's combined stress yield criterion. Comparative results are presented when the ASD criteria are used for the optimum design of a structure under reliability constraints. In addition, this study comparatively shows the results of the optimum design for various criteria of design codes.

• Computational Structural Engineering : An International Journal
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• v.3 no.1
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• pp.1-7
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• 2003

For system identification of large structures, it is not practical to identify the entire structure due to the prohibitive computational time and difficulty in numerical convergence. This paper explores the possibility of performing system identification at substructure level, taking advantage of reduction in both the number of unknowns and the number of degrees of freedom involved. Another advantage is that different portions (substructures) of a structural system can be identified independently and even concurrently with parallel computing. Two substructural identification methods are formulated on the basis whether substructural approach is used to obtain first-order or second-order model. For substructural first-order model, identification at the substructure level will be performed by means of the Observer/Kalman filter Identification (OKID) and the Eigensystem Realization Algorithm (ERA) whereas identification at the global level will be performed to obtain second-order model in order to evaluate the system's stiffness and mass parameters. In the case of substructural second-order model, identification will be performed at the substructure level throughout the identification process. The efficiency of the proposed technique is shown by numerical examples for multi-storey shear buildings subjected to random forces, taking into consideration the effects of noisy measurement data. The results indicate that both the proposed methods are effective and efficient for damage identification of large structures.