Orthotropic Steel Bridge Decks Research Articles

Influence of weld seam on the compaction characteristics of steel bridge deck pavement asphalt mixture

• The influence of weld seam on the compaction characteristics of SBDP was clarified. • The simulation results were verified with the field compaction results. • Optimization suggestions for SBDP compaction quality monitoring were given. • The findings contribute to the lean construction of SBDP. Many weld seams are generated to connect the orthotropic steel bridge decks (OSBDs), which poses a potential threat to the compaction quality of steel bridge deck pavement (SBDP) asphalt mixture. To clarify the influence of weld seam on the compaction characteristics of SBDP asphalt mixture, the coupled finite element method- discrete element method (FEM-DEM) model with weld seam was established to simulate the SBDP asphalt mixture compaction process, and the simulation results were verified with the field compaction ones. Then, the influence of different weld seam heights on the compaction quality was analyzed and the coupling effect under different compaction speeds was also investigated. Moreover, the impact of weld seam on the particle bonding failure mechanism was further examined. Results show that the weld seam height has little effect on the SBDP asphalt mixture compaction quality under low compaction speed, while large compaction speed would reduce the SBDP asphalt mixture compaction quality, and the existence of weld seam would exacerbate this adverse effect and prompt the particle bonging failures. The SBDP crack propagation process above the weld seam is divided into the initiation, propagation, acceleration and stagnation stages. Furthermore, optimization suggestions for SBDP asphalt mixture compaction quality monitoring were given: for the 25 mm and 30 mm thick SBDP lower layer, the weld seam height is recommended not more than 2 mm and 3 mm, respectively; the compaction speed is recommended not more than 3 km/h and 4 km/h, respectively. The findings contribute to proving the theoretical guidance and technical support for the lean construction of SBDP.

Fatigue test and life evaluation of rib-to-deck connections in orthotropic steel bridge decks

This study presents an experimental and analytical investigation of the fatigue performance of novel double-welded rib-to-deck (RD) connections in orthotropic steel bridge decks (OSDs). To better evaluate the fatigue of this kind of novel RD connection, eleven specimens with both outside grooves and inside fillet welds were fabricated and tested, and five specimens with only a single outside groove weld were fabricated and tested for comparison. First, the effects of the internal fillet weld, test stress amplitude, and penetration of the groove weld on the fatigue lives and crack form of the specimens were analysed. The test results showed that internal fillet welding could enhance the fatigue life of the specimens. The test stress amplitude had a considerable influence on the fatigue life and crack mode, while the penetration of the outside groove weld of the double weld had a moderate influence. Then, the master S-N curve method was used to evaluate the fatigue lifetimes of the specimens, and the results were compared with the measured values. The results show that most of the measured lifetimes were distributed in the scattering band of the master S-N curve, but the distributions were relatively scattered. Therefore, for most double-ribbed specimens with double-welded connections, the lifetimes were evaluated directly using the mean value, while for double-ribbed and single-ribbed specimens with single welded connections, the fatigue lives were estimated using the scattering band boundary lines that offset ±2 and ±3 times the standard deviation (σ) from the master S-N curve, respectively.

CFRP strengthening of fatigue cracks at U-rib to diaphragm welds in orthotropic steel bridge decks: Experimental study, optimization, and decision-making

This paper investigated fatigue strengthening of U-rib to diaphragm welds in orthotropic steel bridge decks (OSDs) using the carbon fiber-reinforced polymer (CFRP). Fatigue testing and CFRP strengthening were performed on a full-scale OSD specimen with several U-rib to diaphragm welds. A strengthening configuration was proposed during the experiment. The testing results show that the attachment of CFRP plates on the cracked U-rib wall performed well in reducing both the local fatigue stresses and the average crack growth rate. Then, a multi-objective optimization and decision-making framework was proposed to realize the optimum design of the strengthening configuration. Considered the critical factors to the strengthening efficiency and cost, the material elastic moduli, CFRP size, and strengthening timing were chosen as design variables while maximizing the strengthened fatigue performance and minimizing the total cost were together regarded as the optimization targets. Considering the uncertainty of macrocrack propagation, Monte Carlo Simulation was used to predict the lifecycle fatigue life distribution. Then, a probabilistic assessment criterion for the strengthed fatigue performance was derived by theoretical derivation. Repeated numerical simulations were conducted to obtain the strengthened stress intensity factors using a multi-scale OSD model. Based on these simulated data, a surrogate model of the optimization objective was developed by Response Surface Methodology, which successfully considered the interaction effects between multiple design variables. Finally, the optimum CFRP strengthening configurations were derived by genetic algorithm and multi-attribute decision-making. This study is convinced to not only advance our understanding of CFRP strengthening of U-rib to diaphragm welded joints but also facilitate the optimum application of CFRP material in maintaining deteriorating steel structures.

Fatigue Analysis of Long-Span Steel Truss Arched Bridge Part I: Experimental and Numerical Study of Orthotropic Steel Deck

The orthotropic steel deck is sensitive to fatigue, and a number of cracks have been found in existing bridges. Based on the long-span Guangzhou Mingzhu Bay steel arched bridge, this paper focus on the cracking process, fatigue mechanism, and fatigue performance evaluation of an orthotropic steel bridge deck under traffic load. A finite element model of a three-U-rib and three-span bridge deck was first established to investigate the stress state and the most unfavorable wheel loading position under the longitudinal wheel load. Then, four full-scale single-U-rib specimens were fabricated with high-strength lower alloy structural steel Q370qD in compliance with construction standards. High-cycle loading was subsequently implemented according to the Specification for Design of Highway steel bridge (JTG D64-2015), and the crack initiation, propagation process, and fatigue failure modes were studied. The results showed the stress at structural concern points is larger than in other locations, which was located around 35 mm from the welding seam of the U-rib and the lower end of the diaphragm plate. The Mingzhu Bay steel bridge deck meets the fatigue design requirements. However, the bottom of the welding seam between the U-rib and diaphragm plate is a dangerous fatigue position, and attention should be paid to the welding quality at this position during construction.

Parameter Optimization and Bending Performance Analysis of a Corrugated Steel Plate-UHPC Composite Bridge Deck with PZ Shear Connectors

Composite bridge decks consisting of steel plates and concrete are used to minimize fatigue cracking and damage of asphalt pavement of orthotropic steel bridge decks subjected to wheel loads. This paper presents a methodology to optimize the parameters and mechanical performance of a corrugated steel plate-ultrahigh-performance concrete (UHPC) composite bridge deck with panel zone (PZ) shear connectors using the improved non-dominated sorting genetic algorithm (NSGA-II). A 3-D finite element model is established in ANSYS Workbench software to conduct numerical parameter analysis, and ABAQUS software is used to analyze the optimized model’s mechanical performance. The results show a significant improvement in the mechanical performance of the optimized composite bridge deck, with a 27.66% decrease in the maximum stress in the UHPC layer. The vertical displacement of the optimized composite bridge deck increases sharply when the load reaches 90% of the ultimate load, and a significant change in the strain is caused by the sliding displacement between the UHPC layer and steel plate. The bottom part of the corrugation yields first in the middle span, followed by failures of the ribs. In addition, as the tensile stress increases, damage occurs first in the UHPC layer near the PZ shear connectors in the shear bending area and the bottom in the bending area, expanding significantly to the entire bending and loading areas. The results indicate that the parameter optimization of the corrugated steel plate-UHPC composite bridge deck using NSGA-II improves the structure’s bending performance.

Probabilistic Statistics-Based Endurance Life Prediction of Bridge Structures.

With the massive construction of bridge infrastructures, bridge health monitoring systems have gradually matured in application and research, but previous research has primarily focused on structural damage detection and bridge safety warnings based on valid data. The structural details of steel bridge panels and structural systems are determined by the coupling effects of many intrinsic and extrinsic uncertainties, such as material properties, structural characteristics, manufacturing processes, and random traffic loads. The evaluation of fatigue is a difficult task. This article first builds a big data platform, utilizing its high-efficiency parallel computing capability and highly fault-tolerant distributed file system to achieve second-level monitoring data processing; ensuring real-time data cleaning, data analysis, and safety warning; and building a big data analysis and processing platform with high reliability, high availability, high storage efficiency, and high scalability of bridge health monitoring. The big data platform chooses HDFS for offline data storage and Spark for data analysis and modelling after comparing and analysing the benefits and drawbacks of various big data technologies. Kafka is used for caching real-time data, and Spark-streaming is used for reading data and real-time processing. Finally, the platform's superiority and reliability in terms of offline computing performance, real-time online performance, scalability, and fault tolerance are confirmed through experimental analysis; the optimal data cleaning method is derived by comparing and analysing monitoring data noise, jump point, and drift phenomena. This part of the research is based on bridge temperature data with stable signals and bridge strain data with fluctuating signals, taking into account the influence of different data types; the corresponding data missing repair algorithms are proposed for different types of data to form a complete and general data patching method process. The probabilistic fracture mechanics theory, in comparison to the traditional deterministic fatigue assessment method, can better reflect the essential uncertainty of fatigue problems and is an effective way to assess the fatigue performance of orthotropic steel bridge decks. The goal of data patching is to ensure data recovery accuracy of over 90%, with no patching repair required for monitoring data with too much missing data. The endurance life of bridge structures is predicted using a big data probabilistic statistics approach based on a variety of factors such as material properties, construction characteristics, manufacturing processes, and random traffic loads.

Accelerated evaluation of membranes for pavement structures on orthotropic steel bridge decks

ABSTRACT Membranes of sufficient bonding characteristics could improve the integrity of the multi-layer structures on orthotropic steel deck bridges (OSBDs), enhancing thus the structural response of these systems and, ultimately, their service life. In this research, full-scale experiments were performed at the LINTRACK accelerated pavement testing facility of the Delft University of Technology to evaluate the performance of two surfacing systems commonly used in the Netherlands, giving emphasis on assessing the interface response of membranes with the surrounding materials. Results indicated that the tensile strains remain almost uniform at the top of porous asphalt, in both transverse and longitudinal directions, as no appreciable loss in stress-carrying capabilities was seen even at the end of the testing program. The sections exhibited similar behaviour in terms of strains, with some differences in strains indicating the impacts of membranes at interfaces. The importance of membranes of the desired bonding characteristics was also reflected by the relative displacement measurements. The relative interlayer slip had been higher in the transverse direction than the longitudinal one, with slightly higher displacements in one of the test sections. Overall, no cracking was observed on either section, and the current findings support the use of membranes between surfacing layers on OSBDs.

Experiment and numerical simulation investigation for crack growth of deck-to-rib welded details in orthotropic steel bridge decks

The cracks with different shape ratios propagating along the deck direction are extremely harmful for the deck-to-rib welded details in orthotropic steel decks (OSDs). To provide an effective and reliable fatigue life assessment method for OSDs, the 2D and 3D crack propagation simulations were carried out to solve the crack (including the penetrating crack) propagation problems based on linear elastic fracture mechanics (LEFM), respectively. Firstly, the fatigue experiment was carried out for the investigation of crack propagation characteristics and fatigue life evaluation of deck-to-rib weld details. Secondly, a solid finite element model (FEM) was established. The simulated hot spot stresses at the weld toe and the simulated nominal stresses at the weld root were compared with those of the experiment to verify the accuracy of the FEM. Thirdly, based on LEFM and the two-level method, the finite element method and the finite element/boundary element method were presented to analyze the penetrating crack with growth using the 2D stain model and 3D quarter-elliptical crack growth using 3D models with different initial shapes of the fatigue specimen respectively. Finally, the 2D and 3D numerical crack propagation morphologies, stress intensity factor ranges, fatigue life and crack growth rates were analyzed and discussed. The results indicate that the fatigue crack at the weld root of the specimen is an I-II-III mixed-type crack dominated by the I-type crack. The simulated fatigue life of 3D models is accurate with an error of about 10%. The fatigue life is mainly consumed in the initial stage of crack propagation. The fatigue life of the penetrating crack propagation simulated using the 2D stain model is only 60% of that of the 3D quarter-elliptical crack propagation. The initial shape of the crack is flatter, the fatigue life of the specimen is shorter, and the crack growth rate of it is faster. The penetrating cracks seriously affect the service life of the steel bridge decks. Therefore, it is necessary to detect the OSDs in time and take measures to prevent the further propagation of the crack.

Fatigue Behavior of Rib-to-Floor Beam Junctions with Separate Inner Stiffeners in Orthotropic Steel Bridge Decks

Fatigue Behavior of Rib-to-Floor Beam Junctions with Separate Inner Stiffeners in Orthotropic Steel Bridge Decks

Verification of the Reinforcement Effect of an SFRC Pavement under a Live Load Based on Visualization of Deformation of a Trough Rib of an Orthotropic Steel Deck Using MEMS IMUs and Contact Displacement Gauges

Because repetitive high strain caused by complicated deformation due to traffic load leads to fatigue damage in an orthotropic steel deck, reinforcement methods that suppress deformation are effective as a countermeasure. In Japan, a steel fiber–reinforced concrete (SFRC) pavement is often used as reinforcement to suppress deformation by improving the stiffness of the deck plate of the orthotropic steel deck. To appropriately evaluate the suppression of deformation, it is important to compare the deformation before and after reinforcement. In this study, the reinforcement effects of an SFRC pavement were verified based on visualizations of the deformation of a trough rib of an in-service orthotropic steel deck bridge before and after reinforcement by such a pavement. The visualization results showed that the deformation of the trough rib midway between the transverse ribs decreased after reinforcement, and the strain ranges at the weld joints between the deck plate and the trough rib decreased by approximately 18.5%–31.1%. On the other hand, at the intersection of the transverse rib, it was confirmed that the rotations of the lower flange of the trough rib increased after reinforcement. Accordingly, as inferred from the visualization results for the trough rib, the strain range of the web of the trough rib on the left side at the upper slit and that of the deck plate on the right side at the upper slit after reinforcement increased by approximately 10.6% and 47.5%, respectively.

Fatigue Optimization of Structural Parameters for Orthotropic Steel Bridge Decks Using RSM and NSGA-II

This study presents an optimization flow for structural parameters of orthotropic steel bridge decks (OSDs) based on the fatigue performance and the structural weight. The effects of structural parameters, namely, the thicknesses of the deck plate, the U-rib, the diaphragm, and the longitudinal diaphragm, on equivalent fatigue stress ranges were investigated using surrogate modelling based on the response surface methodology (RSM). Finally, the Pareto optimum front was obtained via the nondominated sorting genetic algorithm II (NSGA-II). The results show that the established surrogate response surface model can predict the local equivalent stress amplitudes well. The deck thickness is the most significant factor for the fatigue performance of the deck to U-rib weld and the U-rib to diaphragm weld. The objectives of minimizing the equivalent stress range and the total structural weight are truly conflicting, and the optimum solutions for structural parameters can be obtained from the Pareto optimum front.

Study on Shear Performance of Short Bolt Interface in ECC–Steel Bridge Deck Composite Structure

Aiming at the problem that orthotropic steel bridge deck and bridge deck pavement are prone to fatigue damage, Engineered Cementitious Composites (ECC) bridge deck pavement is used to replace concrete or asphalt in flexible bridge deck pavement. In order to deeply explore the shear resistance of the short stud interface in the ECC–steel composite structure and provide theoretical support for the practical application of the project, 16 static push-out tests were completed. The effects of stud diameter, height and arrangement spacing on the shear capacity of the medium and short ECC studs were studied. The failure modes, load–slip curves, load–strain curves and interface gap width curves of the components were analyzed. The test results showed that the shear force of the medium and short ECC bolts mainly produces two failure modes, bolt shearing and bolt root weld shearing, while the ECC plate has a local crushing area at the interface bolt root position, and no large cracks occur in other areas. The shear capacity of short bolts is significantly affected by the diameter of the bolts, but is less affected by the height and spacing of the bolts, and increases with the diameter of the short bolts. The length of the stud has an important influence on the stress on the surface of the ECC board. The longer the stud, the greater the tensile stress on the ECC surface. The shorter the peg, the more prone to eccentric compression the ECC plate is, and the longer the peg, the more prone to axial compression it is.

Effects of Stiffener Type on Fatigue Resistance of Steel-UHPC Composite Bridge Decks

Recent research showed that ultrahigh-performance concrete (UHPC) and orthotropic steel bridge decks could be integrated to form steel-UHPC composite sections offering superior load-carrying capacity and fatigue resistance. This study investigates the effects of longitudinal stiffener types on the fatigue resistance of steel-UHPC composite sections. Three types of longitudinal stiffeners were compared, including U-ribs, bulb flat ribs, and plate ribs. Finite element analysis and full-scale fatigue tests were performed to investigate the load-carrying capacity and fatigue resistance of the composite bridge decks with different stiffeners. Besides, the proposed steel-UHPC composite bridge decks were compared with conventional orthotropic steel decks. The results show that steel box girders with bulb flat rib composite deck have reasonable fatigue resistance, construction efficiency, and economic benefits. This study is expected to promote the design and evaluation of economic steel-UHPC composite bridge decks to achieve higher mechanical performance and long-term durability.

Numerical simulation of root-deck crack propagation of orthotropic steel bridge deck

The fatigue crack at the rib-to-deck joint that initiated at the weld root and propagated through the deck plate is one of the most common and unfavorable crack types on orthotropic steel deck (OSD). To evaluate the fatigue performance of OSD during its life-cycle in a more accurate way, a numerical simulation method to reproducing the behavior of this type of crack was established based on linear elastic fracture mechanics (LEFM) in this paper. Firstly, the crack growth at the weld root was provided by the fatigue test using the beach marking method. Then, by adopting the proposed crack propagation simulation method, the crack growth in the test was simulated and the fatigue cracks growth life was predicted. Finally, some factors that may have a great influence on crack propagation behaviors were analyzed, including the lack of penetration defects (LOP) at the weld root, the angle between the crack surface and the deck plate, initial crack depth as well as crack aspect ratio. Accordingly, a practical approach was put forward to estimate the fatigue life based on the test, which can be applied to actual structures that are manufactured by the same process of the specimens, and the inspection and maintnenance works can be benefitted.

Effect of adhesion failure and temperature on the mechanical behavior of orthotropic steel bridge deck

Orthotropic steel bridge decks (OSBDs) have been widely used in long span bridges with many advantages, however, the long-term performance of the asphalt wearing course is still questionable. There is a lack of understanding the causes of cracking and adhesion failure between the tack coat and the steel plate, etc. Therefore, this paper focuses on the adhesion failure between the asphalt wearing course and the steel plate, and the temperature influence on the mechanical behavior of the orthotropic deck. The five-point bending beam test model using the finite element model is used in this investigation in order to clarify the effect of ratio of adhesion failure and the temperature on the mechanical behavior of the OSBD. The ratio of adhesion failure varies from 0.1 to 0.5, and the temperature changes from 30oC to 60oC. The results reveal the influence of temperature and adhesion failure on the mechanical behavior of the OSBD

Dynamic Property Analysis of Orthotropic Bridge Deck with Local Fatigue Crack

With the increase in traffic volume and overweight vehicles, fatigue problems are highlighted. Especially for the orthotropic steel deck, the premature cracks seriously affect the service performance, operation, and service quality of the bridge. In this paper, the steel box‐girder segment model and deck substructure model considering several typical fatigue cracks were established to reveal the variation law of fatigue cracks on vibration frequency and deflection under vehicle traveling load. A natural frequency analysis was performed to evaluate the change in the vibration characteristics due to localized damage, together with a dynamic analysis that considered a vehicle traveling on the bridge. The results show that the initiation and propagation of several typical cracks have little influence on the lower modes of the overall structure and great influence on the high‐order modes of the substructure. The dynamic deflection response of the bridge changed under vehicle load as damage progressed. Therefore, it was possible to identify cracks of the orthotropic steel bridge deck and to provide guidance for fatigue crack detection and repair.

Enhancing fatigue resistance of rib-to-floorbeam welded connections in orthotropic steel bridge decks by using UHPC layer: An experimental study

This study investigates the fatigue performance of rib-to-floorbeam welded connection in orthotropic steel decks (OSDs) reinforced by using ultra-high-performance concrete (UHPC) overlay. Three full-scale rib-to-floorbeam welded connections were fabricated and tested, one specimen represents the conventional OSD type, and the other two specimens represent the UHPC reinforced OSD type, so that the effect of employing UHPC layer can be explored. One rear axle load of HL-93 fatigue truck provided in AASHTO specifications was considered in the fatigue tests. The behaviors including fatigue crack initiation and propagation process, fatigue failure mode, characteristic fatigue life, and vertical rigidity degradation were all observed in the fatigue tests. Experimental results indicated that, under the effect of the same load amplitude, three cracks were observed for the unreinforced specimen, while only one crack for UHPC reinforced specimen was detected. It was observed that using UHPC overlay significantly extended the fatigue life of the rib-to-floorbeam welded connections, where the crack initiation life (N0) and through U-rib thickness fatigue life (N1) were increased by 236 % and 268 %, respectively, under the same load range. Moreover, vertical rigidity was significantly enhanced by employing UHPC layer, which could be due to the fact that the maximum rigidity degradation of the unreinforced and UHPC reinforced specimens were 25% and 14% under the effect of 0.64 and 1.64 million load cycles, respectively.

Mechanical behavior of the asphalt wearing surface on an orthotropic steel bridge deck under cyclic loading

Orthotropic steel decks are prone to fatigue problems and to delamination of the wearing surface. In addition, the mechanical behavior of the asphalt wearing surface on an orthotropic steel bridge deck is viscous and affected by the amplitude and frequency of moving loads. This paper presents an experimental investigation on the strain distribution and evolution in an orthotropic deck under cyclic loading. A five-point bending (5PB) test was used in this study. The tested orthotropic deck was composed of a 14-mm-thick steel plate, an epoxy bonding layer, and a 7-cm-thick epoxy asphalt mixture layer. Two types of tests performed at a fixed temperature were: i) cyclic loading with a small number of cycles at different frequencies and loading amplitudes, and ii) fatigue test. The strain was measured at different positions in the steel plate and in the asphalt wearing course. The results obtained from the first type of tests show the strain distribution in the orthotropic deck and the effect of frequency and loading amplitude on this distribution. When the loading amplitude varied from 20 kN to 40 kN, the behavior of the deck structure was observed to be quasi-linear except for several points in the asphalt wearing course where the strain amplitude was higher than 1000 µm/m. The viscoelastic property of the epoxy asphalt concrete was shown to have a significant influence on the deformation of the deck structure. During the fatigue test, the effect of the cyclic loading on the strain evolution in the orthotropic deck was analyzed. The results show that the accumulated strain in the steel plate was small and reversible while that in the asphalt wearing course was important and irreversible, which caused the cracks and damage in the specimen. Fatigue cracking at the top of the asphalt concrete layer and the debonding of the asphalt concrete layer from the steel plate at the support positions were also observed during the fatigue test.

Shear behavior of short headed studs in Steel-ECC composite structure

Engineering cementitious composite (ECC) is a promising candidate as the overlay material of orthotropic steel bridge deck (OSBD), to improve the durability and extend the service life. The mechanical performance of short headed stud connectors in steel-ECC composite structure has been limited studied. This study investigated the shear behavior of short headed studs embedded in ECC through experimental push-out tests and validated finite element (FE) analysis. The experimental and analytical results were analyzed accordingly in terms of shear strength, shear stiffness and failure mode. The push-out testing results showed that the existence of natural interfacial bond was beneficial for improving the initial stiffness of the specimen. The FE simulation results agreed well with the experimental ones. Base on the validated FE model, a parametric analysis was then conducted to investigate the influence of the stud diameter, stud height, stud strength, corrosion of stud, and compressive strength of ECC on the shear capacity of short headed studs. The analysis indicated that the diameter and tensile strength of headed stud had a significant improvement on the shear strength of headed studs, while the stud height and compressive strength of ECC exhibited the opposite trend. The corrosion of stud drastically reduced the shear performance of headed studs in terms of shear strength and ductility. It was also observed that AASHTO LRFD specification overestimated the shear strength of short headed studs embedded in ECC, while the Eurocode 4 and Chinese code were relatively conservative.

Review on fatigue life assessment methods for welded joints in orthotropic steel decks of long-span bridges

Orthotropic Steel Decks have been used in long-span bridges for several decades because of their high capacity to weight ratio. However, many fatigue related issues have been reported. This paper provides an overview of the main existing fatigue prediction models and discusses their relevance for the fatigue life assessment of Orthotropic Steel Bridge Decks (OSBDs). Several case studies have proven the importance of considering the combined effect of wind and traffic loadings to estimate the fatigue life of long-span bridges. The importance of incorporating welding residual stresses is also well documented while it is often disregarded in design practices. Reliability-based fatigue assessment methods make it possible to quantify how the sources of uncertainty related to loading conditions, welding residual stresses or fabrication defects can affect the fatigue reliability of OSBDs. Monte Carlo simulations are often used to perform probabilistic analyses, but machine-learning algorithms are very promising and computationally efficient. The shortcomings of the Palmgren-Miner rule are discussed and the need for alternative damage accumulation indexes is clear. A number of conclusions are drawn from the analysis of fatigue tests conducted on OSBDs.