• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.5B
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• pp.563-572
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• 2006

In designing the coastal structures, the accurate estimation of wave forces on them is very important. Recently, the empirical formulae such as Goda formula are widely used to estimate wave forces, as well as 2-D hydraulic and numerical model tests. But, sometimes, these estimation methods mentioned above seem to be unreasonable to predict 3-D structure of wave pressure on the coastal structures with 3-D plane arrangement in the real coastal area. Especially, in case of consideration of phase difference at harbor and seaward sides of the large-sized coastal structures like a composite breakwater, it is easily expected that the real wave pressures on each section of coastal structure have 3-D distribution. A new numerical model of 3-D Large Eddy Simulation, which is applicable to permeable structure, is developed to clarify the 3-D structure of wave pressures acting on coastal structure. The calculated wave forces on 3-D structure installed on the submerged breakwater show in good agreement with the measured values. In this study, the composite breakwater is adopted as a representative structure among the large-sized coastal structures and the 3-D structure of wave pressures on it is discussed in relation to the phase difference at harbor and seaward sides of it due to wave diffraction and transmitted wave through rubble mound.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.5A
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• pp.861-872
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• 2006

The element free Galerkin method is extended by the local partition of unity method to model the cohesive cracks in two dimensional continuum. The shape function of a particle whose domain of influence is completely cut by a crack is enriched by the step enrichment function. If the domain of influence contains a crack tip inside, it is enriched by a branch enrichment function which does not have the LEFM stress singularity. The discrete equations are obtained directly from the standard Galerkin method since the enrichment is only for the displacement field, which satisfies the local partition of unity. Because only particles whose domains of influence are influenced by a crack are enriched, the system matrix is still sparse so that the increase of the computational cost is minimized. The condition for crack growth in dynamic problems is obtained from the material instability; when the acoustic tensor loses the positive definiteness, a cohesive crack is inserted to the point so as to change the continuum to a discontiuum. The crack speed is naturally obtained from the criterion. It is found that this method is more accurate and converges faster than the classical meshless methods which are based on the visibility concept. In this paper, several well-known static and dynamic problems were solved to verify the method.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.1A
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• pp.191-203
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• 2006

The vibration characteristics of composite stiffened laminated plates with stiffener is presented using the assumed natural strain 9-node shell element. To compare with previous research, the stiffened plates are composed of carbon-epoxy composite laminate with a symmetric stacking sequence. Also, the result of the present shell model for the stiffener made of composite material is compared with that of the beam model. In the case of torsionally weak stiffener, a local buckling occurs in the stiffener. In this case, the stiffener should be idealized by using the shell elements. The current investigation concentrates upon the vibration analysis of rectangular stiffened and unstiffened composite plates when subjected to the in-plane compression and shear loads. The in-plane compression affect the natural frequencies and mode shapes of the stiffened laminated composite plates and the increase in magnitude of the in-plane compressive load reduces the natural frequencies, which will become zero when the in-plane load is equal to the critical buckling load of the plate. The natural frequencies of composite stiffened plates with shear loads exhibit the higher values than the case of without shear loads. Also, the intersection, between the curves of frequencies against in-plane loads, interchanges the sequence of some of the mode shapes as a result of the increase in the inplane compressive load. The results are compared with those available in the literature and this result shows that the present shell model for the stiffened plate gives more accurate results. Therefore, the magnitude, direction type of the in-plane shear and compressive loads in laminated composite stiffened plates should be selected properly to control the specific frequency and mode shape. The Lanczos method is employed to solve the eigenvalue problems.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.5B
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• pp.559-573
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• 2008

In this study, we newly proposed 3-D nonlinear wave driver utilizing the Navier-Stokes Eq. the numerical integration of which is carried out using SPH (Smoothed Particle Hydrodynamics), an internal wave generation with the source function of Gaussian distribution and an energy absorbing layer. For the verification of new 3-D nonlinear wave driver, we numerically simulate the sloshing problem within a parabolic water basin triggered by a Gaussian hump and uniformly inclined water surface by Thacker (1981). It turns out that the qualitative behavior of sloshing caused by relaxing the external force which makes a free surface convex or uniformly inclined is successfully simulated even though phase error is visible and an inundation height shrinks as numerical simulation more proceeds. For the more severe test, we also simulate the nonlinear shoaling and refraction over uniform beach of wedge shape. It is shown that numerically simulated waves are less refracted than the linear counterpart by Hamiltonian ray theory due to nonlinearity, energy dissipation at the bottom and side walls, energy loss induced by breaking, and the hydraulic jump occurring when breaking waves encounter a down-rush by the preceding wave.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6C
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• pp.321-329
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• 2008

In this study, a series of column test as a way in order to make up for the weakness point of the conventional acceleration method were conducted to both propose the suction board drain method and grapes the specific improvement character of this method as a result of a sort of plastic drain board and a phase of vacuum pressure conditions. On this occasion, the study focused on computing the effective factors of the fittest Suction board drain method affected by each condition through confirming the settlement generated during the test, the water content reduction and stress increase effect occurred arising from the test, and the ratio of consolidation related to the improvement period. In accordance with the shape of core and that whether the core is attached to the filter(pocket or adhesion), the castle type of adhesion and the column type of pocket are more efficient than the others as a consequence of the test to find out the improvement effect depending on each drainage such as a castle type, coil type, harmonica type, column type of pocket and a castle of the adhesion. In case of the step suction pressure, the shorter the period of $-0.8\;kg/cm^2$ as a final step of the suction pressure is, the better the improvement is. In addition, the correlation between degree of consolidation per each suction pressure level and duration of application was drawn as a curve and the point of inflection on this curve was provided to determine the duration period to maximize the consolidation.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.5A
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• pp.649-656
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• 2008

The enhancement of the service life of damaged or cracked structures is a major issue for researchers and engineers. The hierarchic void element based on the integrals of Legendre polynomials is used to characterize the fracture behaviour of unpatched crack as well as repaired crack with bonded composite patches by computing the stress intensity factors and stress contours at the crack tip. Since the equivalent single layer approach is adopted in this study, the proposed element is necessary to represent a discontinuous crack part as a continuum body with zero stiffness. Thus the aspect ratio of this element to represent the crack should be extremely slender. The sensitivity of numerical solution with respect to energy release rate, displacement and stress has been tested to show the robustness of zero stiffness element as the aspect ratio is increased up to 2000. The stiffness derivative method and displacement extrapolation method have been applied to calculate the stress intensity factors of Mode I problem. It is noted that the proposed hierarchical void element can be one of alternatives to analyze the patched crack problems.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6B
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• pp.635-642
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• 2008

The influence of unsubmerged resistance bodies in a channel turbulence flow on energy loss was investigated by hydraulic experiments. Square-shaped multi-piers were used for unsubmerged structure or rigid vegetation in an open channel. In experimental channel flows multi-piers were arranged in double or single row along the channel direction, and mean-concept uniform elevations were attained and measured with a set of discharges and channel slopes. Applying the experimental results to the Manning equation, the equivalent resistance coefficient n, which implicates flow resistance and energy loss due to bottom friction as well as drag, was evaluated with varying the interval of piers and the uniform water depth. And the experimentally evaluated n values were compared with the semi-theoretical formula of the equivalent resistance coefficient derived from momentum analysis including a drag interaction coefficient. From the comparisons it was found that the interaction effect of piers on flow resistance was significant for the overall energy losses in a channel flow. The n values decrease when the interval of piers in flow-direction is less than about 2.2 times of the pier width. And it was also found that the n values increase with the 2/3 power of water depth in the theoretical formula, since the drag interaction coefficient was found to be mostly dependent on the interval of piers.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.4A
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• pp.507-515
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• 2008

In this study, a method predicting the displacement response of structures from the measured dynamic strain signal is proposed by using mode decomposition technique. Evaluation of bridge stability is normally focused on the bridge completed. However, dynamic loadings including wind and seismic loadings could be exerted to the bridge under construction. In order to examine the bridge stability against these dynamic loadings, the prediction of displacement response is very important to evaluate bridge stability. Because it may be not easy for the displacement response to be acquired directly on site, an indirect method to predict the displacement response is needed. Thus, as an alternative for predicting the displacement response indirectly, the conversion of the measured strain signal into the displacement response is suggested, while the measured strain signal can be obtained using fiber optic Bragg-grating (FBG) sensors. As previous studies on the prediction of displacement response by using the FBG sensors, the static displacement has been mainly predicted. For predicting the dynamic displacement, it has been known that the measured strain signal includes higher modes and then the predicted dynamic displacement can be inherently contaminated by broad-band noises. To overcome such problem, a mode decomposition technique was used. Mode decomposition technique estimates the displacement response of each mode with mode shape estimated to use POD from strain signal and with the measured strain signal decomposed into mode by EMD. This is a method estimating the total displacement response combined with the each displacement response about the major mode of the structure. In order to examine the mode decomposition technique suggested in this study model experiment was performed.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.2A
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• pp.401-409
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• 2006

This study presents basic information on the mechanical properties and long-term deformations of high-strength steel fiber reinforced concrete(HSFRC). The Influence of steel fiber on modulus of elasticity, compressive, splitting tensile and flexural strength, and drying shrinkage and creep of HSFRC are investigated, and flexural fracture toughness is evaluated. Test results show that Test results show that the effect of steel fibers on the compressive strength is negligible, and the modulus of elasticity of HSFRC increased with the increase of fiber volume fraction. And the effect of fiber volume fraction($V_f$) and aspect ratio($l_f/d_f$) on tensile strength, flexural strength and toughness is extremely prominent. It is observed that the flexural deflection corresponded to ultimate load increased with the increase of $V_f$ and $l_f/d_f$, and due to fiber arresting cracking, the shape of the descending branch of load-deflection tends towards gently. Also, the effect of addition of various amounts of fiber on the creep and shrinkage is obvious. Especially, the effect of adding fibers to high-strength concrete is more pronounced in reducing the drying shrinkage than the creep.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.1A
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• pp.1-9
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• 2006

An analysis of the annual frequency of collapse(AF) is performed for each bridge pier exposed to ship collision. From this analysis, the impact lateral resistance can be determined for each pier. The bridge pier impact resistance is selected using a probability-based analysis procedure in which the predicted annual frequency of bridge collapse, AF, from the ship collision risk assessment is compared to an acceptance criterion. The analysis procedure is an iterative process in which a trial impact resistance is selected for a bridge component and a computed AF is compared to the acceptance criterion, and revisions to the analysis variables are made as necessary to achieve compliance. The distribution of the AF acceptance criterion among the exposed piers is generally based on the designer's judgment. In this study, the acceptance criterion is allocated to each pier using allocation weights based on the previous predictions. To determine the design impact lateral resistance of bridge components such pylon and pier, the numerical analysis is performed iteratively with the analysis variable of impact resistance ratio of pylon to pier. The design impact lateral resistance can vary greatly among the components of the same bridge, depending upon the waterway geometry, available water depth, bridge geometry, and vessel traffic characteristics. More researches on the allocation model of AF and the determination of impact resistance are required.