Application of new damage indicator-based sequential law for remaining fatigue life estimation of railway bridges

Miner’s rule is generally accepted as the fatigue criteria for life estimation of railway bridges. Similarly, it has always been acknowledged as a simplification that is easy to use in design where detailed loading history is unknown. But in the case of existing railway bridges where the detailed loading history is known, Miner’s rule might provide incorrect results because of its omission of load sequence effect. Recently, a new damage indicator-based sequential law has been proposed to capture the load sequence effect more precisely. However, application of this sequential law to estimate the remaining fatigue lives of existing railway bridges has not been properly studied. Therefore, the major objective of this paper is to estimate the remaining fatigue life of a railway bridge using the sequential law and hence introduces a new approach to estimate remaining fatigue life of riveted railway bridges. This approach is specially based on combination of real stress histories, sequential law and fully known Wöhler curve. The practice of sequential law to obtain the remaining fatigue lives due to both uniaxial and multiaxial stresses are described by estimating remaining life of a considered railway bridge. Further, the obtained fatigue lives are compared with Miner’s estimations. Hence it could be concluded that it is advisable to apply the sequential law for assessment of remaining fatigue lives of riveted railway bridges, where the detailed stress histories are known.     

Assessment of the dynamic and fatigue behaviour of the Panaro railway steel bridge

The functionality maintenance of European infrastructures like bridges is acquiring more and more importance due to the huge economic losses related to the interruption of their regular service. In particular, fatigue represents one of the most common failure modes occurring in steel and steel–concrete composite bridges: most failures in steel structures are related to fracture and fatigue. Railway bridges endure millions of stress cycles during their life and they are expected to be highly vulnerable to such phenomena. Phenomena like ‘vibration induced’ and ‘distortion induced’ fatigue are still partially uncovered by actual design codes and they represent critical aspects for the assessment of existing bridge remaining life and for the design of new bridges. The European research project FADLESS ‘Fatigue damage control and assessment for railways bridges’, funded by the Research Fund for Coal and Steel, aims at defining innovative technical guidelines for the assessment and control of existing and new bridges with regard to fatigue phenomena induced by vibrations and distortions produced by train passages. The project combines experimental and numerical techniques to study fatigue cracking induced by vibration and distortion phenomena taking into account the dynamic train–bridge interaction effects and the actual traffic spectra on European railway lines. In the present paper, preliminary analyses performed on the Italian case study, the Panaro Bridge, are being reported upon. Results obtained by standard fatigue assessment according to Eurocode rules were compared with the actual fatigue cracks found in the deck secondary components. A preliminary critical review of adopted fatigue assessment methodologies was carried out. Moreover, experimental tests were designed and performed on the bridge in order to identify global and local vibration modes and to evaluate the strain time-histories under train passages of critical details. Finally, experimental global/local mode shapes were compared to numerical results from the preliminary FE bridge model.     

Fatigue tests on riveted steel elements taken from a railway bridge

Assessment of structural integrity and remaining life are essential tools for the management of ageing infrastructures, especially bridges. Compared to bolted or welded structures, little attention has been devoted to the fatigue assessment of riveted details. To fill this gap, extensive experiments are conducted on a short-span two-lane riveted steel-girder railway bridge near Sacile, Italy. In service since 1918, it was dismantled in 2006 and moved to a structural laboratory. Within a fatigue assessment framework, first physical and physical–chemical tests were performed, characterising the material properties; then, static, cyclic and fatigue full-scale tests were carried out. The experimental investigation allowed to test in particular the safe condition of the short riveted diaphragm connections of the bridge, and to compare the current fatigue design curves with experimental results. Moreover, the current practice to equate the fatigue behaviour of rivets to that of non-preloaded bolts proved to be a safe comparison.     

Fatigue reliability model for steel railway bridges

Railway bridges are exposed to repetitive high stress due to the live load which may lead to failure even when the stress level is lower than the allowable stress. Therefore, components and connections need to be analysed for possible damage caused by fatigue. The basic approach for estimating the remaining fatigue life of a structure element is to use S–N curves. However, those laboratory specimens tested with a constant-amplitude stress range show a wide range variability in the results. This means that fatigue resistance has to be considered as a random variable. If load and resistance parameters are random variables, structural performance should be measured in terms of reliability. The objective of this study is to present a reliability model for the fatigue limit states demonstrated on a typical steel railway bridge. The results from the reliability analysis for the fatigue limit state are presented for various time periods from 10 to 100 years and three cases of operating conditions. In each considered case of load, the lowest reliability indices were obtained for the riveted angle in the stringer-to-floor-beam connection which means that this member has the highest probability of fatigue crack development in the entire bridge.     

Stochastic analysis of fatigue of concrete bridges

A novel method is proposed in this work for the assessment of the remaining fatigue life and fatigue reliability of concrete bridges subjected to random loads. The fatigue reliability of a bridge is a function of the fatigue damage accumulation; a stochastic fatigue damage model (SFDM) with physical mechanism is introduced for deriving the fatigue damage process. In order to implement the probabilistic analysis, based on the probability density evolution method (PDEM), the generalised density evolution equation (GDEE) for the remaining fatigue life is developed. Finally, a prestressed concrete continuous beam bridge located in China is illustrated. The random fatigue load acting on the bridge is modelled as the compound Poisson process, and the simulation of the random load uses the stochastic harmonic function (SHF) method. To simplify the reliability analysis, an equivalent constant-amplitude (ECA) load process is introduced based on energy equivalence. By employing SFDM, the finite element analysis of the bridge under the fatigue loading is performed. Then, the fatigue damage accumulation process of the bridge under the fatigue loading is obtained. Through solving the probability density evolution equation for the remaining fatigue life, the probability density functions (PDFs) of the remaining fatigue life evolving with time is obtained. The fatigue reliability is then calculated by integrating the PDF of the corresponding remaining life.     

钢桥疲劳评估研究进展

介绍近期国内外关于钢桥疲劳研究的新进展,主要是热点应力法、临界距离理论法、连续介质损伤力学和断裂力学法在钢桥疲劳评估中的应用。分析现行设计规范推荐的疲劳评估法存在的不足,提出钢桥疲劳三阶段评估法。在整体-局部模型和局部精细化模型中采用热点应力、临界距离理论与断裂力学法是三阶段疲劳评估的关键内容。三阶段疲劳评估法具有良好的普遍适用性,可提高疲劳寿命评估的效率和准确性。     

Beurteilung bestehender Stahltragwerke: Empfehlungen zur Abschätzung der Restnutzungsdauer

Assessment of existing steel structures: recommendations for estimation of remaining fatigue life. The growing importance of sustainability affects new markets for the construction industry; the rehabilitation and refurbishment of existing buildings and civil engineering works is getting more and more into the focus of construction activities. This also applies to bridges as well where the quality management comprising the full life of a bridge has a long tradition. For bridges in particular the enormous increase of freight‐volume on roads and rails and the growing age of the bridges cause the question, whether the existing structures are still fit for use and whether or not they can be retrofitted and if so how that could be done. For decision making concerning measures for maintenance and refurbishment of a bridge or its substitution by a new bridge the value of the residual service life is very important. Therefore the bridge authorities have been cooperating for a long time to develop appropriate methods for determining the residual safety and service life of existing bridges. Also the International Technical‐Scientific Organisations as the ECCS and fip as well as Standard Organisations as ISO and CEN have cared for an international exchange of views and the harmonisation of methods. For steel bridges in particular the Technical Committee 6 – Fatigue – of the ECCS has taken over the task to develop a technical guidance for the assessment of existing steel bridges on the grounds of national experiences and the present state of the art [1]. This guidance has been prepared to serve as a basis for CEN/TC 250 – the CEN‐Technical Committee responsible for the structural Eurocodes – for future further development of these Eurocodes to include the assessment and retrofitting of existing structures. The result of the development of ECCS‐TC 6 is a technical‐scientific report which has been published by the Commission of the EU (DG‐Research – Joint Research Centre – JRC) in the frame of a cooperation agreement between the JRC and the ECCS. This report may be used as a technical recommendation as long as the revision of the Eurocodes has not been carried out. This contribution gives the main information from the JRC‐ECCS‐report [1], that can be downloaded free of charge from the e‐bookshop of the Commission.     

Remaining fatigue life of steel railway bridges under enhanced axle loads

In this study, various fatigue damage models proposed by researchers have been briefly discussed and found that the models are problem specific and their efficacy needs to be checked for high cyclic fatigue cases such as in railway bridges. Towards this, field studies were conducted to obtain the strain responses from a steel bridge during the passage of scheduled trains and test train formation with enhanced axle loading. Instrumentation was carried out at critical locations to obtain the responses from the girder. Three different scenarios have been considered to avoid the influence of noise. Further, numerical simulation of the bridge subjected to train loading at different speeds was carried out using ANSYS to obtain synthetic data of strain response from the validated finite element model. Analysis was carried out for normal as well as for futuristic speed of the trains. Responses obtained from field measurements as well as from numerical investigations were used to calculate the damage indices. Based on the damage indices, remaining fatigue life of the bridge was evaluated. The present study can be helpful in assessing the health condition of the railway bridges and to check the suitability of further increase in axle load or speed of trains.     

Versuche an genieteten Brückenlängsträgern – Statisches Verhalten,Ermüdungsfestigkeit und Rissausbreitung

Tests on riveted bridge stringers – behaviour under static loads, fatigue strength and crack propagation. The assessment of the load‐carrying capacity and of the remaining fatigue life of existing railway and road bridges has gained an increasingly significant role over the last years. Thus, the problem of determining adequate strength values of riveted members of older steel bridges once again arose during the elaboration of the ONR 24008 specification. In the course of a research project initiated by the Austrian Federal Railways (ÖBB), static and fatigue tests were carried out on riveted members taken from a demolished railway bridge in Salzburg. The results of these tests gave further insight into the load carrying behaviour of built‐up riveted members, particulaly the progagation of fatigue cracks.     

Fatigue evaluation of steel-concrete composite deck in steel truss bridge——A case study

An innovative composite deck system has recently been proposed for improved structural performance. To study the fatigue behavior of a steel-concrete composite bridge deck, we took a newly-constructed rail-cum-road steel truss bridge as a case study. The transverse stress history of the bridge deck near the main truss under the action of a standard fatigue vehicle was calculated using finite element analysis. Due to the fact that fatigue provision remains unavailable in the governing code of highway concrete bridges in China, a preliminary fatigue evaluation was conducted according to the fib Model Code. The results indicate that flexural failure of the bridge deck in the transverse negative bending moment region is the controlling fatigue failure mode. The fatigue life associated with the fatigue fracture of steel reinforcement is 56 years. However, while the top surface of the bridge deck concrete near the truss cracks after just six years, the bridge deck performs with fatigue cracks during most of its design service life. Although fatigue capacity is acceptable under design situations, overloading or understrength may increase its risk of failure. The method presented in this work can be applied to similar bridges for preliminary fatigue assessment.     

Fatigue evaluation of riveted railway bridges through global and local analysis

This paper presents an overview of recent research efforts by the authors and co-workers on the fatigue assessment of old metallic railway bridges. The investigation focuses on the behaviour of riveted stringer-to-cross-girder connections in a typical, short-span bridge. A generic methodology, which is based on nominal stresses and the S-N method, is presented first, followed by a more detailed analysis using a recently developed fatigue assessment theory, which is based on local stress distributions. The discussion is made within a deterministic as well as a probabilistic context and typical results are presented in terms of fatigue damage and remaining fatigue life.     

A probabilistic fatigue strength model for brick masonry under compression

The paper presents the development of a model for the fatigue resistance of brick masonry under compression. The model takes into account the random nature of the fatigue strength phenomena and, as a consequence, proposes different model parameters as a function of the desired probability level. The model, based on a Weibull distribution, has been calibrated with the still limited experimental data on brick masonry under high cycle compression loading. Very good correlation is obtained. The results show that there is a clear trend of the experimental data to follow the Weibull distribution. It is also shown how the fatigue resistance of brick masonry is not only dependent on the magnitude of the stress cycles, but also on the magnitude of the minimum stress level. The paper also shows how the proposed model can be used in the reliability-based assessment of the fatigue performance of existing masonry arch bridges. Either the nominal probability of failure (reliability index) to fatigue or the remaining service life with a predefined probability level for an existing bridge can be predicted with the proposed method.     

Fatigue Crack Initiation and Propagation in Piles of Integral Abutment Bridges

Abstract: A new continuum damage modeling approach of “successive initiation” is used to determine the location of a thermomechanical fatigue crack initiation and the propagation path and rate in piles of integral abutment bridges. A global‐local modeling approach is introduced to determine the critical location in the pile where a crack is initiated using a 3‐dimensional nonlinear finite element model and to implement “successive initiation.” A simulated case study is used to showcase the multistep procedure. The results indicate that for a pile subjected to the maximum stress, the first fatigue‐induced crack initiates in the tip of the flange at the element immediately below the abutment. Several other cracks at different locations form in the flange of the pile while the initial crack continues to propagate in the flange to the web. The crack propagation rate increases as more cracks initiate in the flange. The propagation rate decreases when the crack reaches the web. Based on the case study presented, a crack could initiate in the pile in as little as 6 years, but it may take about 20 years for it to reach the web; however, final failure of the pile may not take place for several decades. The method can also be used as a guide in bridge foundation inspection and in the determination of the remaining life of an existing bridge.     

基于可靠度的组合桥梁设计及维护策略(英文)

Currently,the design practice of highway bridges around the world are moving towards limit states design,a reliability-based design procedure.Canadian Highway Bridge Design Code(CHBDC)is the first design code entirely developed based on limit states design philosophy,including foundations and FRP components.However,reliability of a structure decreases in service due to environmental attacks and material deterioration such as fatigue,corrosion and many other reasons.Therefore,the structure should be inspected periodically,and the reliability of the structure should be evaluated according to its age and field data.If its reliability is reduced to a certain level,a repair should be scheduled as well as some preventive maintenance measures should be implemented to prevent further deterioration.Recently,many research works have been conducted to investigate reliability-deterioration mechanisms for each type of infrastructure and its components,optimize the inspection and maintenance strategy,predict remaining service life,estimate its life cycle cost.This paper is focused on the study of reliability-deterioration mechanisms of slab on steel girder bridges due to fatigue and corrosion of steel girders as well as corrosion of reinforcement in the deck slab.Examples will also be given to illustrate the proposed life cycle management strategy for composite slab on steel girder bridges.     

Remaining fatigue life assessment of aging fixed steel offshore jacket platforms

Actual fatigue life of an existing offshore platform typically deviates from that intended by its designer because aging platforms commonly experience some degradations, mechanical damages, repairs and modifications, and come across demands for the service extension. The current multidisciplinary paper focuses on different aspects of the remaining fatigue life assessment of an aging offshore platform. In the course of a case study, it describes a hindcasting technique for quantitative estimation of the long-term wave climatology, methods for geotechnical/structural modelling of the platform foundation to evaluate the piles fatigue damages accumulated during their driving and in-place service and the outlines for a spectral fatigue modelling and analysis of the fatigue damages to welded structural joints. Potential limitations in the available codes of practice, regarding the fatigue life assessment of existing offshore structures, are highlighted. The reliability of the fatigue life evaluations, possible remedy measures and some fatigue integrity monitoring techniques are discussed.     

Schweißen ermüdungsbeanspruchter Altstahlkonstruktionen – Untersuchungen zum Einfluss der Vorbelastung auf die Lebensdauer der Schweißkerbdetails

Welding of existing steel structures – Effect of prior load cycles. When existing structures under fatigue load are to be strengthened or refurbished by welded joints fatigue assessment is required for new welds. For the analysis of existing structures or determining of their remaining fatigue life, experimental investigations were carried out in order to estimate the effect of prior load cycles on the material. Mild steel specimens were preloaded cyclically, then surface treatment was applied and a notch representative for a real weld of a structure was introduced. The results of fatigue tests were compared with fatigue tests on similar non‐preloaded specimens. It has been found that there is no significant effect of prior load cycles if the surface is treated prior to welding and enough fusion penetration depth is given. Therefore, fatigue assessment of new welds on old materials does not need to take into account prior load cycles.     

Fatigue analysis of long-span suspension bridges under multiple loading: Case study

Long-span suspension bridges are often subject to multiple types of dynamic loads, especially those located in wind-prone regions and carrying both trains and road vehicles. Fatigue assessment shall be performed to ensure the safety and functionality of the bridges. This paper proposes a framework for fatigue analysis of a long-span suspension bridge under multiple loading by integrating computer simulation with structural health monitoring system. By taking the Tsing Ma Bridge in Hong Kong as an example, a computationally efficient engineering approach is first proposed for dynamic stress analysis of the bridge under railway, highway and wind loading. The fatigue-critical locations are then determined for key bridge components, and databases of the dynamic stress responses at the critical locations are established. The time histories of dynamic stresses induced by individual loading during the design life of the bridge are generated based on the databases. The corresponding stress time histories due to the combined action of multiple loading are also compiled. Finally, fatigue analysis is performed to compute the cumulative fatigue damage over the design life of 120 years. The results indicate that it is necessary to consider the combined effect of multiple loading in the fatigue analysis of long-span suspension bridges.     

Fatigue assessment of highway steel bridges in presence of seismic loading

In this study a LEFM (Linear Elastic Fracture Mechanics) approach is used in a probabilistic context to evaluate the fatigue reliability of steel girder highway bridges in the presence of seismic loading. In the first part the fatigue damage is related to the traffic load produced by heavy trucks crossing the bridge; the second part deals with the fatigue damage related to seismic loading. Both damage typologies are analyzed using linear elastic fracture mechanics principles, and the time required for an initial crack propagation is calculated. Taking into account that the correlation between fatigue effects and seismic actions is not usually considered in the literature, this method could enable a better understanding of progressive damage phenomena due to fatigue related problems, and could give some new insights for increasing the remaining fatigue life of a large number of steel bridges in seismic zones.     

Bruchmechanik als Komplementärmethode zur Bestimmung der Restnutzungsdauer alter genieteter Stahlbrücken in Rumänien

Fracture Mechanics as a complementary method for determination of service life of old riveted steel bridges in Romania. A majority of existing railway steel bridges that has been built at the beginning of 20^th century are riveted structures. Many of the bridges are still in operation after several phases of repair and strengthening. The problem of the structures is the assessment of the present safety for traffic loads and the remaining service life. Replacement with new structures raises financial, technical and political problems. Along with the classical method of damage accumulation, a new approach based on the fracture mechanics principles is proposed. The paper includes a general presentation of the verification methods and a case study. In the following investigations of the old riveted railway steel bridge over the Mures River in Arad are presented. This structure has been dismantled in 2000, after 88 years in operation. Therefore a detailed analysis on main structural elements could be made. Many similar structures with the same age are still in operation. The conclusions can be extended to other countries from Middle and Southeast Europe, where the situation of the existing bridges is comparable.     

Fatigue-safety assessment of reinforced concrete (RC) bridges: application to the Brazilian highway network

During the assessment programme of Brazilian roadway bridges, it was noticed that two girder-slab reinforced concrete bridges, with cantilever girders on extremity spans, were a common structural solution adopted 40 years ago in Brazil. Particularities intrinsic to these systems like significant displacements expected on cantilevers associated with soil embankment settlement in bridge entrance can take to a sudden change in slope grades near its ends, provoking sometimes high impact factors. Besides that, traffic and axle load increase has been of concern because these bridges were designed with different scenarios 40 years ago. Therefore, these facts can affect the structural performance of these structures in face of 100 years life required on most current codes for new bridges. So, in this article, a safety assessment to this assembly of bridges is performed. For this study, a simulation model has been developed, which includes the most significant sources of uncertainty in the definition of traffic action and structural response of bridges. Database provided by dynamic tests of these bridges were used for this performance-based assessment. As a result, reliability indexes to fatigue and ultimate limit state of bending are calculated and compared with target values as specified by codes.