• Structural Engineering and Mechanics
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• 제41권1호
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• pp.139-155
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• 2012

Structural health monitoring systems are often installed on bridges to provide assessments of the need for structural maintenance and repair. Damage or deterioration may be detected by observation of changes in bridge characteristics evaluated from measured structural responses. However, construction materials such as concrete and steel cables exhibit certain time-dependent behaviour, which also results in changes in structural characteristics. If these are not accounted for properly, false alarms may arise. This paper proposes a systematic and efficient method to study the time-dependent effects on the dynamic properties of cable-stayed bridges. After establishing the finite element model of a cable-stayed bridge taking into account geometric nonlinearities and time-dependent behaviour, long-term time-dependent analysis is carried out by time integration. Then the dynamic properties of the bridge after a certain period can be obtained. The effects of time-dependent behaviour of construction materials on the dynamic properties of typical cable-stayed bridges are investigated in detail.

• 국제초고층학회논문집
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• 제6권1호
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• pp.83-89
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• 2017

Due to the time-dependent properties of materials, structures, and loads, accurate time-dependent effects analysis and precise construction controls are very significant for rational analysis and design and saving project cost. Elevation control is an important part of the time-dependent construction control in supertall structures. Since supertall structures have numerous floors, heavy loads, long construction times, demanding processes, and are typically located in the soft coastal soil areas, both the time-dependent features of superstructure and settlement are very obvious. By using the time-dependent coupling effect analysis method, this paper compares Shanghai Tower's vertical deformation calculation and elevation control scheme, considering foundation differential settlement. The results show that the foundation differential settlement cannot be ignored in vertical deformation calculations and elevation control for supertall structures. The impact of foundation differential settlement for elevation compensation and pre-adjustment length can be divided into direct and indirect effects. Meanwhile, in the engineering practice of elevation control for supertall structures, it is recommended to adopt the multi-level elevation control method with relative elevation control and design elevation control, without considering the overall settlement in the construction process.

• Computers and Concrete
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• 제13권4호
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• pp.483-499
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• 2014

Most of the methods presented in the literature to define the target service stresses (Objective Service Stage, OSS) of cable-stayed bridges rarely include the time-dependent phenomena effects. Nevertheless, especially in concrete structures, this assumption might be on the unsafe side because time-dependent phenomena usually modify service stresses. To fill this gap, this paper studies the time-dependent phenomena effects into service stresses of concrete cable-stayed bridges. After illustrating the important role of these phenomena in an asymmetrical cable-stayed bridge without backstay, a new method to include their effects into the OSS is presented. An important issue to be considered in this method is the target time in which the OSS is defined to be achieved. The application of this method to two different structures showed the convenience of defining the OSS to be achieved at early times because that way the envelope of service stresses is reduced.

• Structural Engineering and Mechanics
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• 제38권2호
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• pp.157-169
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• 2011

In this paper the time-dependent closed-form static solution of the suspended pre-stressed biconcave and biconvex cable trusses with unmovable, movable and elastic or viscoelastic yielding supports subjected to various types of vertical load is presented. Irvine's forms of the deflections and the cable equations are modified because the effects of the rheological behaviour needed to be incorporated in them. The concrete cable equations in the form of the explicit relations are derived and presented. From a solution of a vertical equilibrium equation for a loaded cable truss with rheological properties, the additional vertical deflection as a time-function is determined. The time-dependent closed-form model serves to determine the time-dependent response, i.e., horizontal components of cable forces and deflection of the cable truss due to applied loading at the investigated time considering effects of elastic deformations, creep strains, temperature changes and elastic supports. Results obtained by the present closed-form solution are compared with those obtained by FEM. The derived time-dependent closed-form computational model is used for a time-dependent simulation-based reliability assessment of cable trusses as is described in the second part of this paper.

• Structural Engineering and Mechanics
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• 제20권6호
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• pp.673-693
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• 2005

A general step-by-step simulation for the time-dependent analysis of segmentally-erected prestressed concrete box-girder bridges is presented. A three dimensional finite-element model for the balanced-cantilever construction of segmental bridges, including effects of the load history, material nonlinearity, creep, shrinkage, and aging of concrete and the relaxation of prestressing steel was developed using ABAQUS software. The models included three-dimensional shell elements to model the box-girder walls and Rebar elements representing the prestressing tendons. The step-by-step procedure allows simulating the construction stages, effects of time-dependent deformations of materials and changes in the structural system of the bridges. The structural responses during construction and throughout the service life were traced. A comparison of the developed computer simulation with available experimental results was conducted and good agreement was found. Deflection of the bridge deck, changes in stresses and strains and the redistribution of internal forces were calculated for different examples of bridges, built by the balanced-cantilever method, over thirty-year duration. Significant time-dependent effects on the bridge deflections and redistribution of internal forces and stresses were observed. The ultimate load carrying capacities of the bridges and the behavior before collapse were also determined. It was observed that the ultimate load carrying capacity of such bridges decreases with time as a result of time-dependent effects.

• 한국구조물진단유지관리공학회 논문집
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• 제5권3호
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• pp.213-223
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• 2001

This paper presents an analytical procedure for the time-dependent analysis of the multi-story prestressed concrete structure under the construction stage. To account for the actual structural behavior, the procedure considers the effects due to the construction interval and the time-dependent losses of prestress at every construction step on the entire structural response. A numerical study is performed to demonstrate the general validity of the approach and to quantitatively evaluate the effects resulted from the time-dependent behaviors during construction. Recommendations and conclusions are developed by comparisons with structural responses using the present and conventional methods of analysis. The comparative results show that both effects of sequential construction and time-dependent prestress losses should be considered for the construction stage analysis.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1991년도 가을 학술발표회 논문집
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• pp.41-44
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• 1991

Time dependent analysis of prestressed concrete beams subjected to pure torsion is studied. The present theory covers the behavior from the service load range to the ultimate stage. The tensile resistance of concrete is appropriately considered. The biaxial stress effects due to diagonal cracking are also taken into account. The time dependent aging, creep and shringkage effects are modelled by employing the equivalent nonmechanical torque concept. The present theory allows more accurate prediction of the service load behavior of pretressed concrete members.

• 아동학회지
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• 제31권6호
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• pp.1-13
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• 2010

This study examined the time-dependent effects of changes in private education on the trajectories of academic achievement among early adolescents. Five-wave data was employed from the KYPS; the participants being 2844 (1524 boys, 1320 girls). All were 4th graders in 2004 and became 8th graders in 2008. Latent growth curve model indicated that academic achievement significantly decreased over the five time points, but there were significant individual differences as well. Participants who reported higher initial levels of academic achievement tended to decrease in academic achievement more slowly over time compared to participants who reported lower levels of initial academic achievement. The effects of private education on academic achievement were at their most powerful when participants were in the 4th grade. Private educational experiences among 6th graders had relatively less influence upon the academic achievement of 7th graders.

• Journal of Gastric Cancer
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• 제15권4호
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• pp.238-245
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• 2015

Purpose: This study aimed to identify time-dependent prognostic factors and demonstrate the time-dependent effects of important prognostic factors in patients with advanced gastric cancer (AGC). Materials and Methods: We retrospectively evaluated 3,653 patients with AGC who underwent curative standard gastrectomy between 1991 and 2005 at the Korea Cancer Center Hospital. Multivariate survival analysis with Cox proportional hazards regression was used in the analysis. A non-proportionality test based on the Schoenfeld residuals (also known as partial residuals) was performed, and scaled Schoenfeld residuals were plotted over time for each covariate. Results: The multivariate analysis revealed that sex, depth of invasion, metastatic lymph node (LN) ratio, tumor size, and chemotherapy were time-dependent covariates violating the proportional hazards assumption. The prognostic effects (i.e., log of hazard ratio [LHR]) of the time-dependent covariates changed over time during follow-up, and the effects generally diminished with low slope (e.g., depth of invasion and tumor size), with gentle slope (e.g., metastatic LN ratio), or with steep slope (e.g., chemotherapy). Meanwhile, the LHR functions of some covariates (e.g., sex) crossed the zero reference line from positive (i.e., bad prognosis) to negative (i.e., good prognosis). Conclusions: The time-dependent effects of the prognostic factors of AGC are clearly demonstrated in this study. We can suggest that time-dependent effects are not an uncommon phenomenon among prognostic factors of AGC.

• 콘크리트학회논문집
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• 제11권4호
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• pp.3-11
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• 1999

The creep characteristics of concrete under tensile stress has been usually assumed to have the same characteristics as that under compressive stress in the time-dependent analysis of concrete structures. However, it appears from the recent experimental studies that tensile creep behavior is much different from compressive one. In particular, high sustaining tensile stress may cause time-dependent cracking and thus lead to tensile failure. It is, therefore, necessary to model the tensile creep behavior accurately for realistic time-dependent analysis of concrete structures. The present paper to have been focused to suggested more realistic model for the tensile creep behavior of concrete. The models are compared with tensile creep test data available in the literature. The proposed model may allow more refined analysis of concrete structures under time-dependent loading.