Fatigue Load Model Research Articles

Fatigue study on improved composite box girder bridge with corrugated steel webs

The purpose of the study is to estimate the fatigue behavior of an improved composite box girder bridge with corrugated steel webs (ICBGBCSW) based on a three-dimensional vehicle-bridge coupled vibration system (VBCVS). In the VBCVS, the benchmark of the ICBGBCSW widely used in the highway bridges of Gansu Province of China was adopted as the bridge model, and a 6-axle fatigue load model established based on the traffic data of China was adopted as the fatigue vehicle loading. Based on the established VBCVS, the critical fatigue details of the ICBGBCSW were determined and the effects of pavement roughness condition (PRC) and vehicle speed on the stress ranges experienced by critical fatigue details were first investigated. Then, the fatigue live of critical fatigue details were evaluated based on the deterministic analysis method and reliability analysis method. The results show that the fatigue life of the ICBGBCSW can reach more than 375 years from a deterministic perspective under the current traffic conditions of Gansu Province of China and that the fatigue reliability index of the ICBGBCSW sharply decreases to the target fatigue reliability index of 2 if the PRC stays in the “Poor” class and the “Very poor” class for eight years and two years, respectively. The results obtained in the study can provide a technique support for the wide application of the ICBGBCSW and provide a reference for the bridge management agency to make plans for bridge pavement maintenance.

Mean stress effect in high‐frequency mechanical impact (HFMI)‐treated welded steel railway bridges

AbstractThe need for new railway bridges is driven by the growing volume of transportation demands for both passenger and freight traffic on railway networks. In the design of these bridges, the fatigue limit state is a criterion that usually limits the allowable applied load level and thus also the utilization of the high strength of the steel material. Therefore, improving the fatigue performance of welded details by high‐frequency mechanical impact (HFMI) treatment leads to a more efficient design. However, the fatigue performance of HFMI‐treated welds is known to be affected by the mean stress and this needs to be considered in the design of treated welded details in steel bridges. This is rather straightforward if the bridge is subjected to cycles from one type of train but becomes cumbersome when several different sets of trains (e. g. axle loads, axle distances) cross the bridge. In this article, a factor to take the mean stress effect (including self‐weight and traffic load variations) into account is derived from traffic data measured in Sweden. Moreover, the mean stress effect is also predicted using the different fatigue load models in the Eurocode. These models either consist of one‐load patterns such as LM71, SW/0, and SW/2 or are composed of different trains with different combinations. It was found that the mean stress effect is underestimated by the first group of models. On the other hand, the mean stress predicted by the light traffic mix is found to be close to that calculated using real traffic data, while other mixes (standard and heavy) underestimate the mean stress effect. Therefore, a correction factor to account for the mean stress effects in real traffic is derived (called here λHFMI). This factor can be used to correct the design stress range for fatigue verification of HFMI‐treated welded details in railway bridges.

Fatigue design and assessment guidelines for high-frequency mechanical impact treatment applied on steel bridges

Structural design of bridges in Europe should be carried out in accordance with Eurocode regulations. However, there is no guideline demonstrating how the fatigue design is to be conducted when high-frequency mechanical impact (HFMI) is used to enhance welded joints in steel bridges. The aim of this paper is to present the different design rules and equations and apply them to some example bridges enhanced by HFMI treatment. Fatigue verification of some welded details in these bridges is carried out via either “damage accumulation” or “λ-coefficients” methods in the Eurocode. Four fatigue load models are used in the fatigue verification (FLM3 and FLM4 for road bridge assessment, and LM71 and traffic mix, for railway bridge assessment). The effect of steel grade, mean stresses, self-weight, variation in stress ratios, and maximum stress on the treatment efficiency is considered in both examples. It is found that HFMI treatment causes a significant increase in fatigue lives in all studied cases.

New fatigue load models for assessing railway bridges in Europe

Train axle load measurements performed between 2012 and 2019 at 87 locations in the Dutch railway network are used to evaluate fatigue of railway bridges. The theoretical fatigue damage using realistic influence lines is determined for the measured loads and for the load model from Eurocode EN 1991-2 Annex D. It appears that this load model is (very) conservative for most cases, but unconservative for a few cases. Large differences in fatigue relevant loads are observed between different tracks. Two alternative load models that more closely represent the measured railway traffic in terms of trains and fatigue damage are presented, together with a third model based on historical train types. These models use track-specific traffic characteristics. Uncertainties are quantified and safety factors are established from a reliability analysis.

Fatigue load model of orthotropic steel deck for port highway in China

Vehicle load is an important factor for the fatigue damage of the orthotropic steel decks. The traffic vehicles for port highway in China are mainly multi-axle container trucks, which show different characteristics from conventional highways. Based on the multi-period traffic flow data collected by weight-in-motion system of a bridge over the sea in Zhejiang, China, this study has investigated the traffic load characteristics of the port highway bridge. Eight typical vehicles of fatigue load spectrum are classified according to the number of axles and wheelbase. The distribution characteristics of the gross vehicle weight (GVW) and axle weights of typical vehicles are analyzed by lane. Depended on the equivalent fatigue damage principle, the vehicle load spectrum for fatigue evaluation of the port highway bridges is established. The results show that the average proportion of the trucks (weighing greater than 3 tons) in the total traffic flow is 73.2%, and trucks with more than four-axle accounted for 89.0%. Heavy vehicles are mainly concentrated on the outer lanes, and the probability of vehicles driving in the heavy lane increases with the increase of gross vehicle weight. The maximum equivalent gross vehicle weight is 62.5 tons, which appeared in V7 in the exiting island direction. The simplified fatigue vehicle model in the heavy lane for Chinese port highway bridges is proposed, based on the six-axle vehicle which contributed the most fatigue damage. The proposed fatigue load model can better represent the actual traffic load level of port highway bridges in China.

How conservative is the Norwegian fatigue load model for service life estimation of railway bridges?

This study presents correction factors for the Norwegian fatigue load model for railway bridges. The fundamental idea of a correction factor is to ensure that the load model induces the same fatigue damage as actual traffic. Load measurements from traffic on a representative line on the Norwegian railway network are used to propose correction factors for improved fatigue life estimation with freight and passenger trains. Correction factors improve the accuracy of the estimated fatigue life and reduce the conservatism of the load model. The validity and conservatism of the most damaging train assumption for fatigue load modeling is also considered. The suitability and consequences of using the proposed correction factors for other lines and historic traffic are discussed.

Fatigue Performance Analysis of an Existing Orthotropic Steel Deck (OSD) Bridge

Orthotropic steel deck (OSD) bridges are lightweight constructions which are convenient, especially for the achievement of long spans. Conversely, due to the stress concentration in correspondence to the numerous and unavoidable welded construction details, this bridge typology is prone to fatigue cracking under the effect of cyclic loading with high-stress amplitudes. Existing OSD bridges are particularly vulnerable to fatigue damage accumulation because of the dated standards adopted at the time of their design and the fact that heavy lorries have increased in travel frequency and weight. In the present paper, a case study of a northern Italian existing highway viaduct, built in the 1990s, is presented and analyzed. The fatigue damage accumulation was carried out according to the fatigue load models for road bridges reported in Eurocode EN 1991-2 and the assessment criteria indicated in EN 1993-1-9. The stress amplitude, in correspondence to the critical details of the bridge, is assessed by means of detailed finite-element calculations carried out with the software MIDAS GEN®. The amplitude and frequency of the travelling weights are assessed based on real traffic monitoring from the highway. Moreover, an automatic “rain-flow” algorithm is implemented, which is able to detect each nominal stress variation above the fatigue limit. In general, the bridge is not fully compliant with today’s standards when considering the entire duration of the prescribed life of the design. Countermeasures, like lane number reductions and lane reshaping, are critically analyzed since their effectiveness is questionable as far as the reduction in heavy traffic is concerned. Other interventions, like the replacement of the pavement in order to improve the stress redistribution upon the connection details below the wheel footprint, and continuous bridge inspections or monitoring, look more promising.

Thermal fatigue load models for fatigue design of steel box girder bridges

Coupling with traffic loads, thermal loads can also contribute to the fatigue damage cumulation of bridges. Referring to traffic and wind fatigue load models, the concepts of two kinds of probabilistic thermal fatigue load models for the first system of steel box girder bridges were proposed. Specifically, the basic forms of the two models are 1) equivalent vertical temperature gradient models based on the damage equivalence rule and 2) the effect ratio curves based on a statistical analysis of thermal actions. Then, two design probabilistic thermal fatigue load models following the given procedures are set up using an extended dataset based on a 350-day long-term in-suit temperature monitoring, and parametric analysis on three damage equivalence factors including geometric features, design life, and degree of indeterminacy was conducted. To validate the proposed models and related approaches, two kinds of fatigue details in a case steel box girder bridge were evaluated. The results show that thermal effects on fatigue damage cumulation are significant and should be taken into consideration, and the proposed thermal fatigue load models are applicable for fatigue design and evaluation.

Investigation on fatigue cracks of diaphragm cutout in bridge orthotropic steel deck

Fatigue cracks were found at the diaphragm cutouts in the orthotropic steel deck (OSD) of a self-anchored suspension bridge while the in-service time of the bridge was less than 10 years. The causes of fatigue cracks of the diaphragm cutout are investigated in this study. The fatigue cracks initiated from the intersections of the arc curves and the straight lines, which form the geometrical discontinuity to the cutout edge. The in situ monitoring and finite element analysis under fatigue load model indicate that the cracking region in the cutout is in bidirectional compression. However, the thermal-structural analysis and tests show that the rib-to-diaphragm welding process produces high tension residual stress at the cracking points of the cutouts. The cutouts are under tension-compression cyclic loading because of the coupled stress, which is derived from the vehicle-induced stress and the welding residual stress. The results show that the strain energy method can present a reasonable fatigue life estimation for the diaphragm cutouts with full consideration of the residual stress.

Systematic derivation of safety factors for the fatigue design of steel bridges

This paper presents a probabilistic framework to derive the safety factors for fatigue of steel and composite steel concrete road bridges. Engineering models are used for the design and the safety factor is derived in such a way that the design meets the target reliability set by international Eurocode and ISO standards, estimated using measured data and advanced probabilistic models. Engineering model uncertainties and dynamic amplification factors are established through comparison of measurements and models. The value of visual inspections is quantified based on observations from practice and expert opinions. The safety factors are derived for Eurocode’s Fatigue Load Model 4 and Eurocode’s tri-linear S-N curve. The study shows that the safety factors for fatigue as currently recommended by the Eurocodes need to be raised.

Mean Stress Effect in High-Frequency Mechanical Impact (HFMI)-Treated Steel Road Bridges

High-frequency mechanical impact (HFMI) is a post-weld treatment method which substantially enhances the fatigue strength of steel weldments. As such, the method enables a more efficient design of bridges, where fatigue is often the governing limit state. Road bridges are typically trafficked by a large variety of lorries which generate load cycles with varying mean stresses and stress ranges. Unlike conventional welded details, the fatigue strength of HFMI-treated welds is known to be dependent on mean stress in addition to the stress range. The possibility of considering the mean stress effect via Eurocode’s fatigue load models (FLM3 and FLM4) was investigated in this paper. Moreover, a design method to take the mean stress effect into account was proposed by the authors in a previous work. However, the proposed design method was calibrated using limited traffic measurements in Sweden, and as such, may not be representative of the Swedish or European traffic. In this paper, larger data pools consisting of more than 873,000 and 446,000 lorries from Sweden and the Netherlands, respectively, were used to examine the validity of previous calibration in both countries. The comparison revealed no significant difference between the data pools with regards to the mean stress effect. Additionally, previous calibration provided the most conservative mean stress effect and was considered adequately representative for both countries. The proposed design method was further validated using four composite case study bridges. It was also found that the mean stress effect was mainly influenced by the self-weight, while variation in the mean stress due to traffic had a minor influence on the total mean stress effect. Furthermore, it was found that the mean stress effect could not be accurately or conservatively predicted using FLM3 or FLM4.

Train Classification Using a Weigh-in-Motion System and Associated Algorithms to Determine Fatigue Loads.

This paper presents a methodology for classifying train passages into different types with a weigh-in-motion (WIM) system to allow the calibration of railway fatigue load models and identify individual vehicles from the measurements for the continuous calibration of railway WIM stations from in-service trains. The quality assurance of the measured responses is demonstrated using statistical methods. This paper discusses the measurement station, the method used for processing the raw data, the algorithm used to identify the train types and vehicles automatically, and the limits of the obtained load spectra. The measurement errors are demonstrated to be satisfying for use in fatigue load model calibration. Furthermore, this paper proposes actions for accurately obtaining the actual traffic conditions and describes the future work required in this area.

Fatigue damage evaluation of steel bridges considering thermal effect

With the change of external environmental factors, thermal loads are changing and causing fluctuating thermal stresses in critical fatigue details of steel bridges, which contribute to fatigue damage cumulation significantly. This paper proposed two fatigue evaluation methods considering the coupling effects of traffic load and thermal load: (1) Evaluation based on vehicular fatigue load model (VFLM) and thermal fatigue load model (TFLM). (2) Evaluation based on measurements of thermal load and traffic load. The analytical approach for obtaining the thermal fatigue load model was introduced, and the concept of the coupling stress model of ‘wave-surfer model’ was set up according to the mechanism of fatigue damage cumulation. Taking a steel box girder bridge as a case study, the collections of temperature data and traffic information were carried out. The two evaluation methods based on different load information were applied to two typical fatigue details, and the corresponding fatigue damage was calculated, which demonstrates that thermal effects should be taken into account for fatigue evaluation.

Incorporating the unevenness of lane truck loading into fatigue load modeling of multi-lane bridges

The current codes in practice define fatigue load models (FLMs) to characterize the bridge damage that is induced by traffic loads. However, most FLMs are specified to load on any single traffic lane, and the characteristics of the uneven traffic load distribution over multiple lanes under real-world conditions are not revealed. This study analyzed the uneven distribution characteristic of truck loading over multiple lanes based on multiple sets of weigh-in-motion (WIM) data and investigated its effect on the fatigue performance of bridges. A novel multi-lane FLM is proposed, comprising a standard fatigue truck model and multi-lane fatigue coefficients. The standard fatigue truck model is the same as FLM II of the Chinese design specification; however, the load value is adjusted using a calibrated coefficient correlated with the bridge length. The multi-lane fatigue coefficients help define the reduction coefficients for the load value and number of fatigue cycles when applying the standard fatigue truck model to a certain traffic lane. Finally, the proposed model and the code-based model are applied to evaluate the fatigue performance of typical steel–concrete composite girder bridges, and the results are compared with the estimations using realistic multi-lane WIM data. The proposed model yields similar estimations of the stress range and fatigue life as using WIM data, whereas the code-based model shows large deviations. The comparisons validate the feasibility and superiority of the proposed multi-lane FLM for the fatigue evaluation of bridges under uneven multi-lane truck loadings.

Data‐driven modeling for fatigue loads of large‐scale wind turbines under active power regulation

AbstractAdvanced control methods for coordinatively optimizing active power dispatch and fatigue load distribution of wind turbines (WTs) in mountain wind farms, require the fatigue load modeling that has not been fully studied. This study proposes a data‐driven modeling method for fatigue loads of large‐scale WT under active power regulation. Firstly, load simulations based on Monte Carlo approach are conducted to overcome the uncertainty of fatigue load caused by turbulent wind. Subsequently, the target components of WT are selected according to the fatigue load dataset distribution at various power setpoints and the sensitivity to active power. Specifically, the Jarque–Bera statistic is used to evaluate the distribution characteristics of datasets; the single‐value processing determines the valid value through the maximum probability of the Gaussian kernel density distribution of the dataset; the Morris screening is employed to select the valid value with high sensitivity to active power regulation. Afterwards, arbitrary polynomial chaos expansion and support vector regression are performed to establish the data model of fatigue loads, respectively. Finally, the proposed method is applied into a 1.5‐MW commercial WT model designed by the manufacturer through the Bladed software. The obtained data‐driven model establishes a basis of simultaneously optimizing active power dispatch and fatigue load distribution, making it possible to reduce energy cost in mountain wind farms.

Evaluation of traffic load models for fatigue verification of European road bridges

Fatigue verification of bridge structures requires information on the loads induced by heavy vehicles, which can be obtained from weigh in motion measurement (WIM) systems. The current fatigue load models applied in Europe are based on traffic load measurements in 1986. This paper evaluates the appropriateness of these models for today’s traffic by comparing their load effects with those of recent WIM databases, covering the years 2008 to 2018. A procedure is derived to determine the required size of the WIM database for a sound representation of the fatigue loads. The effects of traffic jams are evaluated and the required safety margins or partial factors are derived. As already concluded by others, it appears that the most frequently used fatigue load model is unable to represent the fatigue action effects of today’s European traffic. In addition, the paper demonstrates that the other fatigue load models are also inaccurate. A new fatigue load model is proposed that is easy in use and gives a significant improvement in accuracy compared to the existing models. The parameters in this model can easily be calibrated for other WIM databases.

Research on Random Fatigue Load Model of Highway Bridge by Vehicle Traffic Based on GMM

Research on Random Fatigue Load Model of Highway Bridge by Vehicle Traffic Based on GMM

Impact of Road Traffic Tendency in Europe on Fatigue Assessment of Bridges

Fatigue load models for road bridges given in the Eurocode EN1991-2 have been calibrated considering real traffic measurements that became available around 1990. Since then, traffic composition has evolved considerably, also considering the issuing of the 96/53/EC Directive, which legitimated member states, on an equal and not discriminatory basis, to allow the circulation of Long and Heavy Vehicles (LHVs). Thus, the appropriateness of fatigue load models to cover also the effects of these vehicles, which are longer, heavier and potentially more damaging than common Heavy Goods Vehicles (HGVs), became an issue. The aim of the study is to assess how the evolution of European traffic influences the fatigue assessment of bridges. To capture the essence of the problem, three different real traffic measurements are compared in terms of fatigue damage: the Auxerre (FR) traffic, adopted to define fatigue load models in EN1991-2; the Moerdijk (NL) traffic, characterized by a high percentage of LHVs; and the Igualada (ES) traffic. To assess the current relevance of fatigue load models LM2 and LM4 of EN1991-2, the aptitude of these models to adequately reproduce the effects caused by LHVs is discussed in detail. The results demonstrate that the Auxerre traffic is still the most onerous; that the Moerdijk traffic is generally more severe than the Igualada traffic, and that the fatigue load models of Eurocode do not require major updates. The study is further supplemented by investigating the suitability of the formulae provided in the Eurocodes for the damage equivalence factors λ2 and λ3 to express the influence of the total lorry volume on the fatigue damage. In that latter case, the conclusion is that the formulae proposed in the Eurocodes, based on the assumption of a linear fatigue strength S–N curve with constant conventional slope m, could lead to erroneous, even unsafe, estimates of the fatigue life, especially when details are characterized by constant amplitude fatigue limit ΔσD, thus calling for further improvements of the formulae themselves.

Istraživanje modela opterećenja za zamor mostova na autocestama u jugoistočnoj Kini

Istraživanje modela opterećenja za zamor mostova na autocestama u jugoistočnoj Kini

Influence of Traffic Load and Far-Field Seismic Excitation in Cables of Cable-Stayed Bridge

Bridges are always expose to the cyclic live loads in the form of moving traffic and wind loads. This could jeopardise the service life of the bridges peculiarly in fatigue. With the rise of seismic activities nowadays, bridges may become more susceptible to fatigue damage. Thus, in this study, the static and dynamic behaviour of a cable-stayed bridge in terms of fatigue of steel elements are addressed by focusing on the stress variation in stay cables due to the effects of moving traffic loads and ground motion excitations. Fatigue Load Model 4 (FLM 4) has been adopted as a dynamic response on the bridge subjected to moving load while a two set of ground motions considering horizontal and vertical motions from far-field faults has been used as seismic excitation on the bridge. The results of this study revealed the notable influence of traffic loads on stress variations of cable-stayed bridges with significant effects due to ground motions scaled to the expected return period.