An improved time-dependent reliability model for aging RC bridges based on GPD load distribution

wangxi xu,Yongjiu Qian,Conghe Jin,Wanting Gong 대한토목학회 2024 KSCE Journal of Civil Engineering Vol.28 No.6

To accurately assess the time dependent reliability of a bridge structure, this work modelled the load using the Generalized Pareto distribution (GPD) and improved the traditional time dependent reliability model. The improved model was validated by a numerical case study with the Monte Carlo simulation (MCS). In addition, the effects of load distributions, load growth rates, live-to-constant load ratios, and resistance degradation functions on the accuracy of the reliability probability were analyzed. Finally, the improved time dependent reliability calculation model was applied to analyze the behaviors of a simply-supported beam bridge. The results confirmed that the improved time dependent reliability model was more accurate than the Generalized Extreme Value (GEV) model, as the GPD model focused on the tail loads that were crucial for structural failure beyond the threshold. The improved time dependent reliability model reduced computational errors caused by uncertainties in load distribution since the computation results were not affected by the overall load distribution. Compared with the GEV model, the improved time dependent reliability model was more sensitive to the load growth rate, and it overcame the shortcomings of the conservative structural safety evaluation. It was worth noting that the accuracy of the improved time dependent model was related to factors such as load growth rate and evaluation base period, but was not affected by the live-to-constant load ratio. To ensure accuracy of structural assessment, we recommend that the base period T should not exceed 10 years. The improved time dependent reliability model demonstrated its ability to accurately capture the time dependent reliability, which can provide valuable insights for safety operation and maintenance strategies of bridges.

시공단계를 고려한 PSC 사장교의 풍하중 거동 해석

이재석,최규천 한국풍공학회 2004 한국풍공학회지 Vol.8 No.2

A study for the transverse behavior of segmentally erected PSC cable-stayed bridge under static wind loads is presented. To account for the nonlinear behavior of PSC cable-stayed bridge under static wind load, the geometric nonlinearity such as the cable sag and the change of the configuration of the structure, the material nonlinearity and the time-dependent properties of materials are considered. For the time-dependent properties of materials, creep, shrinkage, aging of concrete and relaxation of prestressing steel and cable are considered. A cable-stayed bridge is modeled as an assemblage of the frame elements consisting of concrete and reinforcing steel fibers, prestressing tendon segments and three-dimensional curved cable elements. At each erection step of the PSC cable-stayed bridge, time-dependent nonlinear analysis on the transverse behavior of the bridge under static wind load is carried out. The numerical results show that the transverse deflections and bending moments of the deck and the tower of the bridge under construction can be greater than those of the bridge after completion. And the time-dependent properties of material are found not to be negligible in considering the transverse behavior of PSC cable-stayed bridge under wind load. 이 논문에서는 PSC(Prestressed concrete) 사장교의 시공단계를 고려한 비선형 해석을 통하여 PSC 사장교의 시공 중과 시공 후에 발생할 수 있는 풍하중에 의한 주탑과 상판의 횡방향 거동을 검토하였다. PSC 사장교의 시간의존적 비선형 거동을 해석하기 위해 재료의 비선형성과 기하학적 비선형성은 물론 재료의 시간의존적 특성으로서 콘크리트의 크리프(creep), 강도증가(aging), 건조수축(shrinkage)과 PS 강재(Prestressing Steel)와 케이블의 이완(relaxation)을 고려하였다. PSC 사장교의 상판과 주탑은 콘크리트와 철근 화이버로 이상화한 화이버 모델 뼈대요소와 뼈대요소에 연결된 PS 강재요소로 이상화하였고, 케이블은 곡선 케이블요소로 이상화하였다. 정적 풍하중이 작용하는 PSC 사장교를 대상으로 시공단계를 고려한 비선형 해석을 수행한 결과, 시공 중에 발생하는 풍하중에 의한 PSC 사장교의 횡방향 거동이 시공이 완료된 후보다 큰 경우가 있으므로 각 시공단계마다 풍하중에 의한 PSC 사장교의 정적 거동에 대한 검토가 필요함을 확인하였다. 또한 풍하중이 작용하는 PSC 사장교의 횡방향 거동을 정확히 예측하기 위해서는 재료의 시간의존적 특성의 영향이 고려되어야 함을 알 수 있었다.

Analysis of Time-Dependent Deformation of Expanded Polystyrene (EPS) Geofoam as a Flexible Pavement Subgrade Material

Park Ki-Chul(박기철),Babu Ramaraj(바부 라마라즈),Chang Yong-Chai(장용채) 한국토목섬유학회 2010 한국지반신소재학회 논문집 Vol.9 No.3

본 연구의 주된 목적은 반복적인 하중조건하에서 연성포장체의노반재료로써, EPS블럭의 시간의존적 변형을 연구하는데 있다. 반복하중 조건에서의 EPS블럭의 거동을 이해하기 위해서 본 연구에서는 실험적 방법 및 수치모형을 제시하였다. 이 수치모델링을 위해서 반복하중 조건하에서 관찰된 변형거동과 단위중량, 변형 그리고 적용된 하중과의 상관관계의 조사가 행해졌다. 실험결과는 핀들레이 이론과 반복하중 조건에서의 연구결과의 모델 분석등과 비교되었고, 보완된 핀들레이 모델과 본 연구에서 제시한 모델은 반복하중에서의 EPS블럭의 변형거동을 나타내는데 이용될 수 있는 것으로 기대된다. The main objective of this study is to investigate the time-dependent deformation of EPS blocks under repeated loading conditions which is the one of the flexible pavement structure. The study comprised of both the experimental work and analytical modeling in order to understand the behavior of EPS blocks under repeated loading. The analytical modeling included the selection of a suitable model for describing the deformation behavior observed under repeated loading conditions, investigating the relationship among the unit weight, deformation and applied stress, analyzing the effect of repeated load on deformation. The test results were compared with the Findley’s theory and model analysis with the results of this research under repeated loading conditions. Both Modified Findley’s model and the proposed model can be adopted to illustrate the deformation behavior of EPS blocks under repeated loads.

Long-term behavior of segmentally-erected prestressed concrete box-girder bridges

Hedjazi, S.,Rahai, A.,Sennah, K. Techno-Press 2005 Structural Engineering and Mechanics, An Int'l Jou Vol.20 No.6

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.

Nonlinear dynamic response of a simply supported rectangular functionally graded material plate under the time-dependent thermalmechanical loads

Y. X. Hao,W. Zhang,J. Yang,S. Y. Li 대한기계학회 2011 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.25 No.7

An analysis on nonlinear dynamic characteristics of a simply supported functionally graded materials (FGMs) rectangular plate subjected to the transversal and in-plane excitations is presented in the time dependent thermal environment. Here we look the FGM Plates as isotropic materials which is assumed to be temperature dependent and graded in the thickness direction according to the power-law distribution in terms of volume fractions of the constituents. The geometrical nonlinearity using Von Karman’s assumption is introduced. The formulation also includes in-plane and rotary inertia effects. In the framework of Reddy’s third-order shear deformation plate theory,the governing equations of motion for the FGM plate are derived by the Hamilton’s principle. Then the equations of motion with twodegree-of-freedom under combined the time-dependent thermomechanical loads can be obtained by using Galerkin’s method. Using numerical method, the control equations are analyzed to obtain the response curves. Under certain conditions the periodic and chaotic motions of the FGM plate are found. It is found that because of the existence of the temperature which relate to the time the motions of the FGM plate show the great difference. A period motion can be changed into the chaotic motions which are affected by the time dependent temperature.

Traffic-load-induced dynamic stress accumulation in subgrade and subsoil using small scale model tests

Tang, Lian Sheng,Chen, Hao Kun,Sun, Yin Lei,Zhang, Qing Hua,Liao, Hua Rong Techno-Press 2018 Geomechanics & engineering Vol.16 No.2

Under repeated loading, the residual stresses within the subgrade and subsoil can accelerate the deformation of the road structures. In this paper, a series of laboratory cyclic loading model tests and small-scale model tests were conducted to investigate the dynamic stress response within soils under different loading conditions. The experimental results showed that a dynamic stress accumulation effect occurred if the soil showed cumulative deformation: (1) the residual stress increased and accumulated with an increasing number of loading cycles, and (2) the residual stress was superimposed on the stress response of the subsequent loading cycles, inducing a greater peak stress response. There are two conditions that must be met for the dynamic stress accumulation effect to occur. A threshold state exists only if the external load exceeds the cyclic threshold stress. Then, the stress accumulation effect occurs. A higher loading frequency results in a higher rate of increase for the residual stress. In addition to the superposition of the increasing residual stress, soil densification might contribute to the increasing peak stress during cyclic loading. An increase in soil stiffness and a decrease in dissipative energy induce a greater stress transmission within the material.

점증하중에 의한 압밀의 유한차분해석

이승현(Lee Seung-Hyun) 한국산학기술학회 2012 한국산학기술학회논문지 Vol.13 No.4

유한차분법을 적용한 압밀해석을 수행하였는데 순간하중이 재하되는 경우 유한차분법에 의해 예측되는 시 간별 침하량과 Terzaghi 방법에 의한 침하량 사이의 차이는 시간격자간격을 충분히 작게 하여 해결할 수 있음을 알 수 있었다. 점증하중에 대한 압밀해석을 위한 유한차분식을 유도하였는데 해석결과에 따른 과잉간극수압의 분포가 Olson의 이론해와 일치하였다. 점증하중이 작용하는 경우에 대해 예측한 시간-침하거동에 있어 유도된 유한차분식에 의한 결과와 Terzaghi 및 Olson 에 의한 결과 또한 거의 일치하였다. 다단 점증하중에 대한 해석결과 또한 신뢰성이 높은 것으로 보인다. Formulation of finite difference method for analyzing consolidation were carried out. It can be seen that the differences in settlement with time obtained by FDM and Terzaghi method are diminished by fine discretization of time increment. Excess pore pressures predicted by the derived finite difference equation were same as those calculated by Olson's method. Predicted time-settlement behavior from the derived finite difference method were almost same as those calculated by Terzaghi's method and Olson's method. Analysis results obtained from the assumed multi-step time dependent loading are thought to be reasonable.

Creep analysis of the FG cylinders: Time-dependent non-axisymmetric behavior

Mohammad Arefi,Mehrdad Nasr,Abbas Loghman 국제구조공학회 2018 Steel and Composite Structures, An International J Vol.28 No.3

In this paper history of stresses, strains, radial and circumferential displacements of a functionally graded thick-walled hollow cylinder due to creep phenomenon is investigated. The cylinder is subjected to an arbitrary non-axisymmetric two dimensional thermo-mechanical loading and uniform magnetic field along axial direction. Using equilibrium, strain-displacements and stress-strain relations, the governing differential equations of the problem containing creep strains are derived in terms of radial and circumferential displacements. Since the displacements are varying with time due to creep phenomenon, an analytical solution is not available for these equations. Thus, a semi-analytical procedure based on separation of variables and Fourier series together with a numerical procedure is employed. The numerical results indicate that the non-axisymmetric loading and the material grading index have significant effect on stress redistributions. Moreover, by proper selection of material for any combination of non-axisymmetric loading, one can arrive suitable response for the cylinder to achieve optimal design. With some simplifications, the results are validated with the existing literature.

Minimum thickness of flat plates considering construction load effect

Hyeon-Jong Hwang,Gao Ma,김창수 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.69 No.1

In the construction of flat plate slabs, which are widely used for tall buildings but have relatively low flexural stiffness, serviceability problems such as excessive deflections and cracks are of great concern. To prevent excessive deflections at service load levels, current design codes require the minimum slab thickness, but the requirement could be unconservative because it is independent on loading and elastic modulus of concrete, both of which have significant effects on slab deflections. In the present study, to investigate the effects of the construction load of shored slabs, reduced flexural stiffness and moment distribution of early-age slabs, and creep and shrinkage of concrete on immediate and time-dependent deflections, numerical analysis was performed using the previously developed numerical models. A parametric study was performed for various design and construction conditions of practical ranges, and a new minimum permissible thickness of flat plate slabs was proposed satisfying the serviceability requirement for deflection. The proposed minimum slab thickness was compared with current design code provisions and numerical analysis results, and it agreed well with the numerical analysis results.

지속하중을 받는 탄소섬유메쉬 보강 철근콘크리트 보의 장기거동에 관한 실험적 연구

윤현도 ( Yun Hyun-do ),서수연 ( Sung Soo-young ),김태용 ( Seo Soo-yeon ),성수용 ( Park Woan-shin ),박완신 ( Kim Tae-young ),한병찬 ( Han Byung-chan ) 한국구조물진단유지관리공학회 2003 한국구조물진단유지관리공학회 학술발표대회 논문집 Vol.7 No.2

Carbon fibre mesh (CFM) is recently has been recognized as a strengthening element in concrete structures in view of its excellent resistance to corrosion. In this experimental program, four beams strengthened in flexure with CFM were case for the study of the long term behavior under sustained loading. The beams were designed with total span of 2400mm and rectangular cross-section of 300mm width and 200 mm depth. The beams were simply supported and subjected to two pointed sustained loading. The beams were instrumented and monitored to observe the changes in the behavior due to creep and shrinkage of concrete. Observations were recorded regularly for nearly four months. Test results indicated that the reinforcing effect of CFM was small at instantaneous deformation range, but was considerable in long term behavior due to creep and shrinkage of the beam. An analytical method is developed to predict the long-term behavior of reinforced concrete beams strengthened in flexure with CFM. The calculated deflections, strains and curvatures compare reasonably with the experimental values.