• 콘크리트학회논문집
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• 제13권5호
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• pp.415-422
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• 2001

본 연구에서는 반복하중을 받는 철근콘크리트보의 피로현상에 피로의 지속으로 인한 크리프 특성을 고려함으로서 실제의 반복사용하중상태에서 크리프와 피로가 공존하는 것을 반영하여 모델을 구성하였다. 반복크리프 모델에서, 철근 콘크리트 보의 휨손상 진행에 따른 압축영역의 손상속도를 정확히 대표할 수 있는 반복크리프 지수(n)의 특성을 도입하여 평가하였다. 이 지수는 크리프특성의 지수 n$_1$과 피로특성의 지수 n$_2$로 구분하였다. 모델의 적정성을 평가하기 위하여 철근콘크리트보의 피로실험을 수행하였으며, n$_2$의 경우 보통강도에서 고강도까지 콘크리트의 강도와 하중크기를 주요변수로 실험하였다. 본 실험결과로부터 반복하중에 의한 크리프모델의 지수를 결정하였다. 본 연구에서 제시된 반복크리프모델은 철근콘크리트 보의 피로손상의 누적과 처짐거동을 잘 설명할 수 있는 것으로 나타났으며, 앞으로 이들 부재의 피로거동해석에 유용하게 적용될 수 있을 것으로 사료된다.

• Steel and Composite Structures
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• 제30권6호
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• pp.535-550
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• 2019

Prestressed concrete (PC) bridges using corrugated steel webbing have emerged as one of the most promising forms of steel-concrete composite bridge. However, their long-term behavior is not well understood, especially in the case of large-span bridges. In order to study the time-dependent performance, a large three-span PC bridge with corrugated steel webbing was compared to a similar conventional PC bridge to examine their respective time-dependent characteristics. In addition, a three-dimensional finite element method with step-by-step time integration that takes into account cantilever construction procedures was used to predict long-term behaviors such as deflection, stress distribution and prestressing loss. These predictions were based upon four well-established empirical creep prediction models. PC bridges with a corrugated steel web were observed to have a better long-term performance relative to conventional PC bridges. In particular, it is noted that the pre-cambering for PC bridges with a corrugated steel web could be smaller than that of conventional PC bridges. The ratio of side-to-mid span has great influence on the long-term deformation of PC bridges with a corrugated steel web, and it is suggested that the design value should be between 0.4 and 0.6. However, the different creep prediction models still showed a weak homogeneity, thus, the further experimental research and the development of health monitoring systems are required to further progress our understanding of the long-term behavior of PC bridges with corrugated steel webbing.

• 콘크리트학회논문집
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• 제22권4호
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• pp.567-574
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• 2010

지금까지 환경 친화적 재료로서 황토에 관한 연구는 포틀랜드 시멘트를 부분적 대체하거나, 완전 대체하는 연구로 진행되어 왔다. 기존의 대부분의 연구에서는 압축강도, 건조수축, 크리프 등 황토 콘크리트의 역학적 성질에 초점이 맞춰졌다. 이 연구에서는 황토 콘크리트로 제작된 보 실험체의 휨 성능을 실험하였다. 이번 실험에서는 시멘트를 20% 대체하는 활성 황토를 사용한 콘크리트와 시멘트를 100% 대체하는 활성 황토 콘크리트가 사용되었다. 단순보 실험은 2점 정적 재하 하중으로 실험하였다. 휨 강도, 균열 모멘트, 처짐, 연성도 등의 결과를 일반 포틀랜드 시멘트 콘크리트로 제작된 보의 실험 결과와 비교하였다.

• Nuclear Engineering and Technology
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• 제28권1호
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• pp.17-26
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• 1996

프레팅마모 기인 연료봉 손상을 방지할 수 있는 노내 연료봉 지지조건은 잔여 지지격자스프링 변위량 또는 연료봉 /지지격자 갭에 의해 평가될 수 있다. 핵연료 설계 인자들이 프레팅마모 손상에 미치는 영향을 평가하기 위해 연소도의 함수로서 노내 연료봉 지지조건을 모사할 수 있는 방법론을 사용하여 GRID-FORCE프로그램을 개발하였다. 이 프로그램에서는 노내 연료봉 지지조건에 영향을 주는 주요 인자로서 피복관 크립, 초기 스프링 변위, 초기 스프링힘 그리고 스프링힘 조사이완이 고려된다. 이 주요 인자들에 대한 민감도 분석 결과, 초기 스프링 변위, 스프링힘 조사이완, 피복관 크립 순으로 노내 연료봉 지지조건에 영향을 주는 것으로 나타났다. 이 프로그램을 실제 노내에서 발생한 프레팅마모 기인 연료봉 손상에 적용한 결과를 토대로 판단해 볼 때 이 프로그램을 새로 개발된 피복관 재질 및 /또는 새로 개발된 지지격자 설계가 프레팅마모 기인 연료봉 손상을 방지할 수 있는 설계여유도를 효과적으로 평가할 수 있음을 알 수 있다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2002년도 봄 학술발표회 논문집
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• pp.303-308
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• 2002

A governing differential equation (GDE) of PSC flexural member with constant eccentricity considering the long-term losses including concrete creep, shrinkage, and PS steel relaxation is derived based on the two approaches. The first approach utilizes the force and moment equilibrium equations derived based on the geometry of strains of the uniform and curvature strains while the second one utilizes the principle of minimum total potential energy formulation. The identity of the two GDE's is verified by comparing the coefficients consisting of the GDE's. The boundary conditions resulting from the functional analysis of the variational calculus are investigated. Rayleigh-Ritz method provides a way to get the explicit form of the continuous deflection function in which the total potential energy is minimized with respect to the unknown coefficients consisting of the trial functions. As a closure, the analytically calculated results are compared with the experiments and show good agreements.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1992년도 봄 학술발표회 논문집
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• pp.126-130
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• 1992

Recently, fatigue problem become a critical issue in the design of prestressed concrete bridges due to the increase of traffic volumes and use of high-strength materials. Most existing studies are mainly concerned with the fatigue behavior of component materials only such as concrete, reinforcing bars, and prestressing steels and few studies exist that deals with the fatigue behavior of bridge members. An improved analytic formulation for both uncracked and cracked prestressed concrete composite section with cyclic creep effect is developed to take into account the change of neutral axis with crack propagation. The procedure also enables to investigate serviceability limit states, deflection and crack width. The present study allows more realistic analysis and design of prestressed concrete composite girder bridges under fatigue loadings.

• Structural Engineering and Mechanics
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• 제81권5호
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• pp.617-631
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• 2022

In this paper the viscoelastic responses of four experimental steel-concrete composite beams subjected to highly variable environmental conditions are investigated by means of a finite element (FE) model. Concrete specimens submitted to stepped stress changes are also evaluated to validate the current formulations. Here, two well-known approaches commonly used to solve the viscoelastic constitutive relationship for concrete are employed. The first approach directly solves the integral-type form of the constitutive equation at the macroscopic level, in which aging is included by updating material properties. The second approach is postulated from a rate-type law based on an age-independent Generalized Kelvin rheological model together with Solidification Theory, using a micromechanical based approach. Thus, conceptually both approaches include concrete hardening in two different manners. The aim of this work is to compare and analyze the numerical prediction in terms of long-term deflections of the studied specimens according to both approaches. To accomplish this goal, the performance of several well-known model codes for concrete creep and shrinkage such as ACI 209, CEB-MC90, CEB-MC99, B3, GL 2000 and FIB-2010 are evaluated by means of statistical bias indicators. It is shown that both approaches with minor differences acceptably match the long-term experimental deflection and are able to capture complex oscillatory responses due to variable temperature and relative humidity. Nevertheless, the use of an age-independent scheme as proposed by Solidification Theory may be computationally more advantageous.

• 한국구조물진단유지관리공학회 논문집
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• 제25권2호
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• pp.43-48
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• 2021

프리캐스트 콘크리트는 일반적으로 건설 기간을 줄이고 시공 능력을 향상시키는 데 주로 사용되고 있다. 그러나 분할 과정에서 원래 구조 시스템과 다른 경계 조건과 구조적 거동을 적용하여 구조적 문제가 발생할 수도 있다. 이 연구에서는 시공 후 휨모멘트 및 크리프 증가로 인해 처짐과 균열이 발생한 프리캐스트 콘크리트 슬래브를 대상으로 검토하였으며, 이는 프리캐스트 콘크리트 슬래브의 지지 조건 및 구조거동에 대한 잘못된 적용에서 비롯된 것임을 알 수 있었다. 프리캐스트 콘크리트 슬래브 하부에 2 개의 지지대를 삽입하여 휨모멘트를 줄이고 보강 작업시 구조적 안전성을 확보하기 위해 잭킹력에 따른 캠버를 추정해야 한다. 따라서 기존 구조물의 처짐 및 균열을 확인하기 위해 역 해석을 통해 프리캐스트 콘크리트 슬래브의 다양한 지지 조건과 휨강성을 고려하였으며, 프리캐스트 콘크리트 슬래브의 잭킹력에 따라 캠버를 추정하고 안전한 구조물을 만드는 보강방법을 제안하였다.

• Structural Engineering and Mechanics
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• 제82권1호
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• pp.121-131
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• 2022

The main goal of this study is to prepare a program for analyzing High Strength Steel Fibrous Reinforced Concrete (HSSFRC) slabs and predict the response and strength of the slab instead of preparing a prototype and testing it in the laboratory. For this purpose, new equations are proposed to represent the material properties of High Strength Steel Fibrous Reinforced Concrete. The proposed equations obtained from performing regression analysis on many experimental results using statistical programs. The finite element method is adopted for non-linear analysis of the slabs. The eight-node "Serendipity element" (3 DoF) is chosen to represent the concrete. The layered approach is adopted for concrete elements and the steel reinforcement is represented by a smeared layer. The compression properties of the concrete are modeled by a work hardening plasticity approach and the yield condition is determined depending on the first two stress invariants. A tensile strength criterion is adopted in order to estimate the cracks propagation. many experimental results for testing slabs are compared with the numerical results of the present study and a good agreement is achieved regarding load-deflection curves and crack pattern. The response of the load deflection curve is slightly stiff at the beginning because the creep effect is not considered in this study and for assuming perfect bond between the steel reinforcement and the concrete, however, a great agreement is achieved between the ultimate load from the present study and experimental results. For the models of the tension stiffening and cracked shear modulus, the value of Bg and Bt (Where Bg and Bt are the curvature factor for the cracked shear modulus and tension stiffening models respectively) equal to 0.005 give good results compared with experimental result.

• 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.