Fatigue reliability analysis of steel bridge details based on field-monitored data and linear elastic fracture mechanics

This article presents the fatigue reliability analysis of welded details of a 40-year old steel box-girder bridge, based on the linear elastic fracture mechanics (LEFM) and the long-term stress monitoring. The fatigue crack growth is described by using the LEFM, based on which the ultimate limit state is established as a function of the crack size. Effective stress ranges are obtained from daily stress range histograms, and lognormal probability density functions (PDFs) are used to model the uncertainties in stress data. Details at discontinuous backing bar splices, which have not been explicitly listed in the American Association of State Highway and Transportation Officials (AASHTO) specification, and the welded details at the base of bridge bents are evaluated on their fatigue reliabilities. It is found that the backing bar splice detail may have high fatigue reliability, considering that low stress ranges and small number of cycles occurred in the monitoring period. For the weld detail at the base of bent, stress ranges higher than the constant amplitude fatigue limit (CAFL) existed, and currently the reliability indexes are all below the target value, showing that maintenance or retrofit actions may be required. For the details at the upper base of bridge bents, sufficient reliabilities are expected throughout the service life.     

Fatigue and fracture simulation of welded bridge details through a bi-linear crack growth law

This paper deals with the application of a probabilistic fracture mechanics approach to predict the fatigue life of welded steel details in the presence of cracks under bridge spectrum loading. It is based on a recently proposed bi-linear relationship to model fatigue crack growth and incorporates a failure criterion to describe the interaction between fracture and plastic collapse. The formulation leading to the expected number of cycles to failure is first outlined, followed by a simple example on a butt-welded detail. Uncertainty modelling, especially on fatigue crack growth parameters, is undertaken with the aid of recently published data in support of the bi-linear crack growth relationship. Results pertaining to fatigue reliability and fatigue crack size evolution are presented using simulation with Latin Hypercube Sampling, and emphasis is placed on a comparison between linear and bi-linear crack growth models. The latter is found to lead to higher fatigue life estimates and significantly different crack size distributions, both of which have implications on inspection schemes for steel bridge components.     

Durability Monitoring for Improved Service Life Predictions of Concrete Bridge Decks in Corrosive Environments

Abstract: The development of an effective strategy for the inspection and monitoring of the nation's critical bridges has become necessary due to aging, increased traffic loads, changing environmental conditions, and advanced deterioration. This article presents the development of a probabilistic mechanistic modeling approach supported by durability monitoring to obtain improved predictions of service life of concrete bridge decks exposed to chlorides. The application and benefits of this approach are illustrated on a case study of a reinforced concrete barrier wall of a highway bridge monitored over 10 years. It is demonstrated that service life predictions using probabilistic models calibrated with selected monitored field data can provide more reliable assessments of the probabilities of reinforcement corrosion and corrosion‐induced damage compared to using deterministic models based on standard data from the literature. Such calibrated probabilistic models can help decision makers optimize intervention strategies as to how and when to repair or rehabilitate a given structure, thus improving its life cycle performance, extending its service life and reducing its life cycle cost.     

Life-Cycle Performance of RC Bridges: Probabilistic Approach

This article addresses the problem of reliability assessment of reinforced concrete (RC) bridges during their service life. First, a probabilistic model for assessment of time-dependent reliability of RC bridges is presented, with particular emphasis placed on deterioration of bridges due to corrosion of reinforcing steel. The model takes into account uncertainties associated with materials properties, bridge dimensions, loads, and corrosion initiation and propagation. Time-dependent reliabilities are considered for ultimate and serviceability limit states. Examples illustrate the application of the model. Second, updating of predictive probabilistic models using site-specific data is considered. Bayesian statistical theory that provides a mathematical basis for such updating is outlined briefly, and its implementation for the updating of information about bridge properties using inspection data is described in more detail. An example illustrates the effect of this updating on bridge reliability.     

细长正交异性钢桥面板的疲劳寿命评估

在多种焊接节点中产生的疲劳裂缝,也产生在大量细长正交异性钢桥面板中。一部分裂缝在桥梁投入使用短短几年之后便会出现,所以对于桥面焊接节点的疲劳寿命评估较复杂。这同时也是局部压应力会随着很多因素而随机变化的原因,尤其是那些在车胎、公路和钢结构的动力交互作用面上。     

桥上CRTS Ⅱ型底座板裂缝宽度时变可靠性研究

该文开展了不同环境温度下的桥上CRTS Ⅱ型底座板裂缝宽度时变可靠性研究。具体包括：比选确定了考虑钢筋锈蚀的底座板受拉钢筋有效截面积计算模型,建立了底座板纵向裂缝宽度极限状态函数,开展了列车荷载、桥梁变形和温度作用对底座板的作用效应分析;基于热力学传导公式,发展了基于气象信息的温度作用概率模型关键参数计算方法;考虑不同环境温度条件,开展了北京地区某桥上CRTS Ⅱ型底座板60年服役期内裂缝宽度时变可靠性分析。研究发现：底座板纵向裂缝宽度可靠性随服役时间的增长呈现下降趋势;正温度梯度较负温度梯度对底座板更为不利;裂缝截面处钢筋锈蚀会降低底座板裂缝宽度可靠性,且随着锈蚀程度的提高可靠指标下降速率明显加快;低温环境下底座板的裂缝宽度可靠性较低,应对低温环境下的底座板开裂失效模式加以重视。     

Structural behavior of strengthened bridge deck specimens under fatigue loading

The deterioration of concrete bridge decks that have been directly damaged by traffic loads affects their durability, safety, and function. It is therefore necessary to strengthen the damaged concrete structures. Even though there have been many experiments performed to investigate the static behavior of strengthened structures, few experiments or analyses have considered their fatigue behavior.

In this study, fatigue tests were conducted on bridge decks strengthened using various fiber-reinforced polymer plastics, such as carbon fiber sheet, glass fiber sheet, and grid-type carbon fiber reinforced plastic. All of the strengthened specimens were shown to have an improved resistance to crack propagation and better stress distributions. The Weibull distribution was adopted to analyze the fatigue life of the decks. The fatigue life limits of the strengthened bridge decks were determined at higher stress levels, and the grid-type carbon reinforced plastic specimens proved to be the most effective.     

正交异性钢桥面铺装层疲劳寿命的断裂力学分析

计算和分析正交异性钢桥面铺装层表面裂缝应力强度因子,在此基础上应用Paris扩展公式预测铺装层疲劳寿命。将奇异单元布置在铺装层表面裂缝前沿,建立正交异性钢桥面系三维断裂力学有限元模型,计算铺装层表面裂缝的应力强度因子;分析裂缝应力强度因子随轴载作用位置的变化规律,确定了带裂缝铺装层轴载作用的最不利荷位;以最不利荷位作为轴载作用的标准荷位,计算应力强度因子随裂缝扩展深度的变化,并数值拟合得到了应力强度因子与裂缝深度的关系式;将应力强度因子的深度关系式代入Paris公式,积分得到铺装层的疲劳寿命。计算结果表明,基于钢桥面铺装层带裂缝工作的事实,应用断裂力学方法预测钢桥面铺装层疲劳寿命是可行的。     

Zuverlässigkeitsorientierte Bewertung von Maßnahmen gegen die frühe Trennrißbildung für wasserundurchlässige,massige Betonbauwerke

Reliability oriented valuation of measures against early through cracks in watertight massive concrete buildings. The imperviousness and hence the serviceability of structures subjected to water pressure may be endangered by through cracks. For the avoidance of such cracks many counter‐measures exist. For the valuation of such measures, the authors have developed an engineering model. Because this model is deterministic, it fails to cope with the uncertainties of cracking. Hence, it had to be amplified to a probabilistic one. With the latter, the reliability‐based valuation of crack‐avoiding measures becomes possible via the probability of cracking. The decisions of contractor can thereby be supported. The fact that through cracking is accompanied by many uncertainties requires their embedment into a Monte‐Carlo Method. Reliability analyses were performed for the axially restrained bar which represents a cut‐out from the crack prone region of structural element. The relationship between the cracking probability and the so‐called crack index (tensile strength/tensile stress) could be established and approximated by FORM. The application of this relationship for the proofs according to the German guideline for water‐impervious structures is outlined.     

Probabilistic fatigue analysis of shop and field treated tubular truss bridges

This article examines the extent to which post-weld treatment by needle peening can improve the fatigue performance of tubular truss bridges. To do this, the various potential crack sites on several variants of a typical bridge are analyzed using a probabilistic, fracture mechanics-based model. Systems reliability theory is then used to determine the reliability of the entire untreated or treated bridge. The results of this work show that: considering phase effects may result in large reductions in the design stress ranges for these structures, a significant increase in the treatment benefit can be achieved if the treatment is applied after the dead load stresses are introduced, and weld root cracking does not appear to be the critical failure mode for these structures, so long as a strategy of partial treatment is employed.     

Experimental study and numerical analysis on mechanical behavior of T-shape stiffened orthotropic steel-concrete composite bridge decks

A new-type of orthotropic steel-concrete composite bridge deck system was developed, by casting the concrete overlay on the top of the orthotropic steel deck ribbed with T-shape steel members. To study its mechanical behavior (in terms of failure mode, load-deflection relationship, concrete crack initiation and propagation, strength, stiffness and so on), two new-type orthotropic steel-concrete composite bridge decks with different section dimensions were experimentally investigated and two reference decks (reinforced concrete deck and orthotropic steel deck) were also involved in the research for comparison. For the two new-type orthotropic steel-concrete composite decks, the average value of ultimate loads per width is 885.7kN, which is 2.35 and 1.61 times of that of the concrete and steel reference decks with almost the same section height. Experimental results proved that the composite deck can effectively control the crack initiation and propagation in the concrete and postpone the yielding of the steel bars and steel plates, due to the composite action between the concrete overlay and the underlying steel plate. Furthermore, the Finite Element (FE) model of the orthotropic steel-concrete composite deck was developed and validated by test results. A parametric study is conducted regarding to the stiffness of shear studs. With the validated FE model, stress distribution in the underlying steel plate and T-shape stiffeners and development of concrete cracking in the concrete overlay were characterized at different load levels.     

Probabilistic engineering analysis using the NESSUS software

The development of reliability-based design methods requires the use of general-purpose engineering analysis tools that predict the uncertainty in a response due to uncertainties in the model formulation and input parameters. Barriers that have prevented the full acceptance of probabilistic analysis methods in the engineering design community include availability of tools, ease of use, robust and accurate probabilistic analysis methods, and the ability to perform probabilistic analyses for large-scale problems. The goal of the reported work has been to develop a software tool that fully addresses these three aspects (availability, robustness and efficiency) to enable the designer to efficiently and accurately account for uncertainties as they might affect structural reliability and risk assessment. The paper discusses the NESSUS probabilistic engineering analysis software with specific sections on the reliability modeling and analysis process in NESSUS, the robust and accurate solution strategies incorporated in the available probabilistic analysis methods, and several application examples to demonstrate the applicability of probabilistic analysis to large-scale engineering problems.     

Fatigue life estimates for a slender orthotropic steel deck

Fatigue cracks in several types of welded joints and geometrical details have been reported to occur in a large number of slender orthotropic decks of existing steel bridges in many parts of the world. Some of these cracks are initiated very early, a few years after the bridge is brought into service. That is why it is said that fatigue life estimation for the welded joints of orthotropic decks is not an easy designing task. This is so because the random variation of local stresses is sensitive to many factors, in particular to those involved in the dynamic interaction between the vehicle’s tyres, the pavement and the steel structure.This paper presents and discusses the main causes for the observed cracks and the outcome of the ultimate fatigue life estimates for typical welded and geometrical details of a slender orthotropic deck with trapezoidal cross-section longitudinal ribs. This is the deck of an existing long-span steel bridge, which has been strengthened by adding a reinforced concrete layer fixed with shear studs to the steel deck top plate. This was done to avoid continued and extensive repair work and to enhance the fatigue life expectancy of the steel deck structure. The fatigue life estimation is done with the aid of refined numerical modelling and in situ experimental strain measurements and also by taking into account all the built-in structural alterations, changes in volume of traffic and in vehicles loading which have occurred during this bridge’s 32 years of service life.     

Feasibility of crack free reinforced concrete bridge deck from materials composition perspective: a state of the art review

Early age cracking on bridge deck has been the subject of many studies for years. Cracking is a major concern because it leads to premature deterioration of structures. Millions of dollars spent to repair the cracked bridge decks each year. To design an appropriate mixture for crack free bridge deck, it is important to study previous researches. This paper presents a comprehensive literature review of the performance of different materials compositions as well as methods have been used to reduce and control bridge deck cracks. Different material compositions and methods are discussed in terms of their performances as well as advantages and disadvantages.     

Trivariate analysis of soil ranking-correlated characteristics and its application to probabilistic stability assessments in geotechnical engineering problems

An important issue in the probabilistic prediction modelling of multivariate soil properties (usually including cohesion, friction angle, and unit weight) is the measurement of dependence structure among these properties. The use of Pearson's correlation as a dependence measure has several pitfalls; therefore, it may not be appropriate to use probabilistic prediction models in geotechnical engineering problems based on this correlation. As an alternative, a copula-based methodology for prediction modelling and an algorithm to simulate multivariate soil data are proposed.In this method, all different random variables are transformed to a rank/uniform domain in order to form a copula function by applying cumulative distribution function transformations. The technique of copulas, representing a promising alternative for solving multivariate problems to describe their dependence structure by a ranked correlation coefficient, is highlighted. Two existing observed soil data sets from river banks are used to fit a trivariate normal copula and a trivariate fully nested Frank copula. The ranking correlation coefficient Kendall's τ and the copula model parameters are estimated. The goodness-of-fit test to choose the best-fitting model is discussed.A series of triplet samples (i.e., cohesion, friction angle, and unit weight) simulated from the trivariate normal copula with flexible marginal distributions are used as input parameters to evaluate the uncertainties of soil properties and to define their correlations. The influence of the cross-correlation of these soil properties on reliability-based geotechnical design is demonstrated with two simple geotechnical problems: (a) the bearing capacity of a shallow foundation resting on a clayey soil and (b) the stability of a cohesive-frictional soil in a planar slope. The sensitivity analysis of their correlations of random variables on the influence of the reliability index provides a better insight into the role of the dependence structure in the reliability assessment of geotechnical engineering problems.     

新型钢-UHPC组合桥面板抗弯承载力模型试验与理论分析方法

为综合解决正交异性钢桥面板疲劳开裂和桥面铺装易损两大难题,提出一种由波形顶板、超高性能混凝土(ultra-high-performance concrete,UHPC)结构层和改进螺旋线(modified clothoide,MCL)形组合销所构成的新型波形顶板-UHPC组合桥面结构.设计2类共12个足尺模型,对所提出...     

正交异性钢桥面板纵肋与桥面板连接细节的疲劳评估及修复措施

正交异性钢桥面板由于具有自重轻、极限承载力大等优点目前广泛应用于大、中跨径桥梁中,我国已建和在建的大跨径桥梁也大多采用正交异性钢桥面板.但由于正交异性钢桥面板结构构造复杂,受焊接残余应力影响大,钢桥面板直接位于车轮荷载的作用下,一些构造细节处极易发生疲劳开裂.以国内某大桥正交异性钢桥面板为例,针对纵肋与桥面板之间的疲劳...     

Spatial time-dependent reliability analysis of corroding pretensioned prestressed concrete bridge girders

Accelerated pitting corrosion tests have been performed to obtain spatial and temporal maximum pit-depth data for prestressing wires. This data is then used to develop probabilistic models of pitting corrosion and strength capacity of 7-wire strands. The probabilistic model of pitting corrosion for strands is then combined with a non-linear Finite Element Analysis and probabilistic models of corrosion initiation and propagation to study the spatial and temporal effects of pitting corrosion on a typical pretensioned prestressed concrete bridge girder. The limit states considered are flexural strength and serviceability. The spatial time-dependent reliability analysis takes into account the uncertainties and variabilities related to material properties, dimensions, loads and corrosion parameters as well as the spatial variability of pitting corrosion of prestressing strands. Including the spatial variability of pitting corrosion in the reliability analysis increased both the probability of strength and serviceability failure when compared with a mid-span sectional analysis.     

Sensitivity of uncertainties in performance prediction of deteriorating concrete structures

Deterioration models for the condition and reliability prediction of civil infrastructure facilities involve numerous assumptions and simplifications. Furthermore, input parameters of these models are fraught with uncertainties. A Bayesian methodology has been developed by the authors, which uses information obtained through health monitoring to improve the quality of prediction. The sensitivity of prior and posterior predicted performance to different input parameters of the deterioration models, and the effect of instrument and measurement uncertainty, is investigated in this paper. The results quantify the influence of these uncertainties and highlight the efficacy of the updating methodology based on integrating monitoring data. It has been found that the probabilistic posterior performance predictions are significantly less sensitive to most of the input uncertainties. Furthermore, updating the performance distribution based on ‘event’ outcomes is likely to be more beneficial than monitoring and updating of the input parameters on an individual basis.     

Life cycle cost management of concrete structures relative to chloride-induced reinforcement corrosion

Chloride-induced reinforcement corrosion is one of the major causes for the deterioration of concrete structures. This article has developed a performance-based life cycle cost management (LCCM) model for reinforced concrete structures relative to corrosion deterioration. This model is characterised by three features: (1) a chloride-induced probabilistic corrosion deterioration mechanism is used to predict the service life of concrete structures; (2) the performance of a concrete structure is measured by serviceability limit states and structural capacity limit states; and (3) a dual management methodology is adopted, in which management actions are optimised to maximise the life cycle performance of a concrete structure under limited budget constraints and minimum required serviceability constraints. Case studies of bridge decks and piers are provided to demonstrate the application of the proposed performance-based LCCM model.