• Journal of the Korea Concrete Institute
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• v.19 no.6
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• pp.803-810
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• 2007

An analytical method capable of predicting various stress-strain responses in axially loaded concrete confined with FRP (fiber reinforced polymers) composites in a rational manner is presented. Its underlying idea is that the volumetric expansion due to progressive microcracking in mechanically loaded concrete is an important measure of the extent of damage in the material microstructure, and can be utilized to estimate the load-carrying capacity of concrete by considering the corresponding accumulated damage. Following from this, an elastic modulus expressed as a function of area strain and concrete porosity, the energy-balance equation relating the dilating concrete to the confining device interactively, the varying confining pressure, and an incremental calculation algorithm are included in the solution procedure. The proposed method enables the evaluation of lateral strains consecutively according to the related mechanical model and the energy-balance equation, rather than using an empirically derived equation for Poisson's ratio or dilation rate as in other analytical methods. Several existing analytical methods that can predict the overall response were also examined and discussed, particularly focusing on the way of considering the volumetric expansion. The results predicted by the proposed and Samaan's bilinear equation models correlated with observed results with a reasonable degree, however it can be judged that the latter is not capable of predicting the response of lateral strains correctly due to incorporating the initial Poisson's ratio and the final converged dilation rate only. Further, the proposed method seems to have greater benefits in other applications by the use of the fundamental principles of mechanics.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.10
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• pp.1057-1068
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• 2014

This paper proposes elastic-plastic constitutive relations for a composite material with two phases-inclusion and matrix phases-using a homogenization scheme. A thermodynamic framework is employed to develop non-local plasticity constitutive relations, which are specifically represented in terms of the second-order gradient terms of the internal state variables. A combined two back-stress evolution equation is also established and the degradation of the state and internal variables is expressed by continuum damage mechanics in terms of the damage factor. Then, deformation localization is analyzed; the analysis results show that the proposed model yields a wide range of shear band formation behaviors depending on the evolution of the specific internal state variables. The analysis results also show good agreement with the results of simplified Rice instability analyses.

• Geomechanics and Engineering
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• v.15 no.4
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• pp.1017-1027
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• 2018

To explore the influence of coal thickness on the mechanical behavior and the failure characteristics of rock-coal-rock (RCR) mass, the experimental investigation of uniaxial compressive tests was conducted first and then a systematic numerical simulation by particle flow code (PFC2D) was performed to deeply analyze the failure mechanical behavior of RCR specimens with different coal thicknesses in conventional compression tests. The overall elastic modulus and peak stress of RCR specimens lie between the rock and the coal. Inter-particle properties were calibrated to match the physical sample strength and the stiffness response. Numerical simulation results show that the deformation and strength behaviors of RCR specimens depend not only on the coal thickness, but also on the confining pressure. Under low confining pressures, the overall failure mechanism of RCR specimen is the serious damage of coal section when the coal thickness is smaller than 30 mm, but it is shear failure of coal section when the coal thickness is larger than 30 mm. Whereas under high confining pressures, obvious shear bands exist in both the coal section and the rock section when the coal thickness is larger than 30 mm, but when the coal thickness is smaller than 30mm, the failure mechanism is serious damage of coal section and shear failure of rock section.

• Tunnel and Underground Space
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• v.19 no.4
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• pp.304-317
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• 2009

Most crystalline rocks have much higher compressive strength than tensile strength and show brittle failure. In-situ rock mass, strong enough in general sense, often fails in brittle manner when subjected to high stress exceeding strength in due of geometrically induced stress concentration or of high initial stress. Therefore, it is necessary to verify the brittle failure characteristics of rock and rock mass for proper stability assessment of underground structures excavated in great depths. In this study, damage controlled tests were conducted on biotite-granite and granitic gneiss, which are the two major crystalline rock types in Korea, to obtain the strain dependency characteristics of the cohesion and friction angle. A Cohesion-Weakening Friction-Strengthening (CWFS hereafter) model for each rock type was constructed and a series of compression tests were carried out numerically while varying confining pressures. The same tests were also conducted assuming the rock is Mohr-Coulomb material and results were compared.

• Tunnel and Underground Space
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• v.30 no.4
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• pp.306-319
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• 2020

Characterizations of Excavation Damage Zone (EDZ), which is hydro-mechanical degrading the host rock, are the important issues on the geological repository for the spent nuclear fuel. In the DECOVALEX 2019 project, Task G aimed to model the fractured rock numerically, describe the hydro-mechanical behavior of EDZ, and predict the change of the hydraulic factor during the lifetime of the geological repository. Task G prepared two-dimensional fractured rock model to compare the characteristics of each simulation tools in Work Package 1, validated the extended three-dimensional model using the TAS04 in-situ interference tests from Äspö Hard Rock Laboratory in Work Package 2, and applied the thermal and glacial loads to monitor the long-term hydro-mechanical response on the fractured rock in Work Package 3. Each modelling team adopted both Finite Element Method (FEM) and Discrete Element Method (DEM) to simulate the hydro-mechanical behavior of the fracture rock, and added the various approaches to describe the EDZ and fracture geometry which are appropriate to each simulation method. Therefore, this research can introduce a variety of numerical approaches and considerations to model the geological repository for the spent nuclear fuel in the crystalline fractured rock.

• Journal of the Korea institute for structural maintenance and inspection
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• v.22 no.1
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• pp.129-136
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• 2018

The estimation of the fatigue design life for large welded structures is usually performed using the liner cumulative damage method such as Palmgren-Miner rule or the equivalent damage method. When a fatigue crack is detected in a welded steel structure, the residual service life has to be estimated base on S-N curve method and liner elastic fracture mechanics. In this study, to examine the 3D fatigue crack behavior and estimate the fatigue life of out-of-plane gusset fillet welded joint, the fatigue tests were carried out on the model specimens. Investigations of three-dimensional fatigue crack propagation on gusset welded joint was used the finite element analysis of FEMAP with NX NASTRAN and FRANC3D. Fatigue crack growth analysis was carried out to demonstrate the effects of aspect ratio, initial crack length and stress ratio on out-of-plane gusset welded joints. In addition, the crack behaviors of fatigue tests were compared with those of the 3D crack propagation analysis in terms of changes in crack length and aspect ratio. From this analysis result, SIFs behaviors and crack propagation rate of gusset welded joint were shown to be similar fatigue test results and the fatigue life can also be predicted.

• Steel and Composite Structures
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• v.1 no.4
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• pp.459-480
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• 2001

The damage suffered by steel structures during the Northridge (1994) and Kobe (1995) earthquakes indicates that the fully restrained (FR) connections in steel frames did not behave as expected. Consequently, researchers began studying other possibilities, including making the connections more flexible, to reduce the risk of damage from seismic loading. Recent experimental and analytical investigations pointed out that the seismic response of steel frames with partially restrained (PR) connections might be superior to that of similar frames with FR connections since the energy dissipation at PR connections could be significant. This beneficial effect has not yet been fully quantified analytically. Thus, the dissipation of energy at PR connections needs to be considered in analytical evaluations, in addition to the dissipation of energy due to viscous damping and at plastic hinges (if they form). An algorithm is developed and verified by the authors to estimate the nonlinear time-domain dynamic response of steel frames with PR connections. The verified algorithm is then used to quantify the major sources of energy dissipation and their effect on the overall structural response in terms of the maximum base shear and the maximum top displacement. The results indicate that the dissipation of energy at PR connections is comparable to that dissipated by viscous damping and at plastic hinges. In general, the maximum total base shear significantly increases with an increase in the connection stiffness. On the other hand, the maximum top lateral displacement $U_{max}$ does not always increase as the connection stiffness decreases. Energy dissipation is considerably influenced by the stiffness of a connection, defined in terms of the T ratio, i.e., the ratio of the moment the connection would have to carry according to beam line theory (Disque 1964) and the fixed end moment of the girder. A connection with a T ratio of at least 0.9 is considered to be fully restrained. The energy dissipation behavior may be quite different for a frame with FR connections with a T ratio of 1.0 compared to when the T ratio is 0.9. Thus, for nonlinear seismic analysis, a T ratio of at least 0.9 should not be considered to be an FR connection. The study quantitatively confirms the general observations made in experimental results for frames with PR connections. Proper consideration of the PR connection stiffness and other dynamic properties are essential to predict dynamic behavior, no matter how difficult the analysis procedure becomes. Any simplified approach may need to be calibrated using this type of detailed analytical study.

• Structural Engineering and Mechanics
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• v.15 no.4
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• pp.381-394
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• 2003

While constructing multistorey buildings with reinforced concrete framed structures it is a common practice to provide parking space for vehicles at the ground floor level. This floor will generally consist of open frames without any infilled walls and is called an open-storey. From a post disaster damage survey carried out, it was noticed that during the January 26, 2001 Bhuj (Gujarat, India) earthquake, a large number of reinforced concrete framed buildings with open-storey at ground floor level, suffered extensive damage and in some cases catastrophic collapse. This has brought into sharp focus the need to carry out systematic studies on the seismic vulnerability of such buildings. Determination of vulnerability requires realistic structural response estimations taking into account the stochasticity in the loading and the system parameters. The stochastic finite element method can be effectively used to model the random fields while carrying out such studies. This paper presents the details of stochastic finite element analysis of a five-storey three-bay reinforced concrete framed structure with open-storey subjected to standard seismic excitation. In the present study, only the stochasticity in the system parameters is considered. The stochastic finite element method used for carrying out the analysis is based on perturbation technique. Each random field representing the stochastic geometry/material property is discretised into correlated random variables using spatial averaging technique. The uncertainties in geometry and material properties are modelled using the first two moments of the corresponding parameters. In evaluating the stochastic response, the cross-sectional area and Young' modulus are considered as independent random fields. To study the influence of correlation length of random fields, different correlation lengths are considered for random field discretisation. The spatial expectations and covariances for displacement response at any time instant are obtained as the output. The effect of open-storey is modelled by suitably considering the stiffness of infilled walls in the upper storey using cross bracing. In order to account for changes in soil conditions during strong motion earthquakes, both fixed and hinged supports are considered. The results of the stochastic finite element based seismic analysis of reinforced concrete framed structures reported in this paper demonstrate the importance of considering the effect of open-storey with appropriate support conditions to estimate the realistic response of buildings subjected to earthquakes.

• Structural Engineering and Mechanics
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• v.50 no.5
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• pp.589-603
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• 2014

Expressions for determining the value of the impact force as reported in the literature and incorporated into code provisions are essentially quasi-static forces for emulating deflection. Quasi-static forces are not to be confused with contact force which is generated in the vicinity of the point of contact between the impactor and target, and contact force is responsible for damage featuring perforation and denting. The distinction between the two types of forces in the context of impact actions is not widely understood and few guidelines have been developed for their estimation. The value of the contact force can be many times higher than that of the quasi-static force and lasts for a matter of a few milli-seconds whereas the deflection of the target can evolve over a much longer time span. The stiffer the impactor the shorter the period of time to deliver the impulsive action onto the target and consequently the higher the peak value of the contact force. This phenomenon is not taken into account by any contemporary codified method of modelling impact actions which are mostly based on the considerations of momentum and energy principles. Computer software such as LS-DYNA has the capability of predicting contact force but the dynamic stiffness parameters of the impactor material which is required for input into the program has not been documented for debris materials. The alternative, direct, approach for an accurate evaluation of the damage potential of an impact scenario is by physical experimentation. However, it can be difficult to extrapolate observations from laboratory testings to behaviour in real scenarios when the underlying principles have not been established. Contact force is also difficult to measure. Thus, the amount of useful information that can be retrieved from isolated impact experiments to guide design and to quantify risk is very limited. In this paper, practical methods for estimating the amount of contact force that can be generated by the impact of a fallen debris object are introduced along with the governing principles. An experimental-calibration procedure forming part of the assessment procedure has also been verified.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6D
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• pp.881-893
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• 2008

The major objective of this study is to investigate the fatigue damage factor evaluation of immovability crossing for railway turnout by the field test and qualitative analysis. From the field test results of the servicing turnout crossing and qualitative analysis with frictional wear which section stiffness decreased, it was evaluated fatigue life of servicing turnout crossing. Most design practices have not taken advantage of the advanced theories in the modern fracture mechanics and finite element analysis due to complexity of analysis as well as the large quantity of vaguely defined parameters in actual designs. This paper considers fatigue problems in turnout crossing using effective analytical and design tools from the field of qualitative constraint reasoning. A set of software modules was developed for fatigue analysis and evaluation, which is easily applicable in engineering practices of designers. The techniques enable the use complex analysis formulations to tackle practical problems with uncertainties, and present the design outcome in two-dimensional design space solution. Appropriate engineering assumptions and judgments in carrying out these procedures, often the most difficult part for practicing engineers, can be partially produced by using qualitative reasoning to define the trends and ranges, interval constraint analysis to derive the controlling parameters, as well as design space to account for practical experience.