The authors regret that due to errors in typing, Eq. (2) and the corresponding texts should be replaced by: where ∆M is the bending moment range and the second moment of area [Formula presented], both in unit width. The results in the paper are not affected by the typing error. The authors would like to apologise for any inconvenience caused. ... Read more

Steel Orthotropic Bridge Decks (OBDs) are deck plates stiffened by open or closed stiffeners. Due to the high strength/weight ratio, this type of deck plate is a popular choice for long-span and movable bridges where dead weight is important. OBDs are subjected to many traffic load cycles during the service life. The stress ranges are relatively high, with the highest stress concentrations often at welded connections. Therefore, the design of welded connections is often governed by fatigue resistance. The discovery of fatigue cracks worldwide indicates that our knowledge of fatigue resistance design in OBDs is limited. For example, unexpected cracks were observed only after several years of use in the Severn Bridge in the United Kingdom (opened in 1966), the Van Brienenoord Bridge in the Netherlands (opened in 1990), and the Humen Pearl River Bridge in China (opened in 1997). With increasing traffic, higher fatigue resistance is required. The main challenges for infrastructure worldwide are the design of safe and economical new OBDs and the accurate assessment of existing ones. The current standards for fatigue design of bridges, EN 1993-1-9: 2005, AASHTO LRFD Bridge Design Specifications: 2020, and JSSC recommendations: 2012, primarily rely on the nominal stress method or the hot spot stress method. The problems with these methods are that the nominal stress in OBDs is not well defined and that the resistances (detail categories) for the hot spot stress method are based on standard plate connections rather than the details in OBDs.

This dissertation aims to provide a comprehensive explanation of the behaviour of critical welded connections in OBDs stiffened by continuous trapezoidal stiffeners based on experimental investigations and Finite Element Analysis (FEA). This investigation focuses on the most critical welded connections, specifically the welded connections between the stiffener and the deck plate, as well as the welded connections between the crossbeam and the stiffener, utilising the author’s experiments and results from the literature. The studied details in this dissertation are summarised below:

Detail Description
C2a Stiffener-to-deck plate weld, weld toe crack in stiffener
C2b Stiffener-to-deck plate weld, weld root crack in weld
C1a Stiffener-to-deck plate weld, weld toe crack in deck plate
C1c Stiffener-to-deck plate weld, weld root crack in deck plate at crossbeam
C4d Crossbeam-to-stiffener weld at stiffener bottom, weld root crack
C3a Crossbeam-to-stiffener weld, weld toe crack in stiffener at lower end of the weld

Illustrations of the cracks are shown on the cover page and in Figure 1.5.

The author carries out experimental investigations on a 5.1 × 9.4 m2 full-scale OBD specimen with a 20 mm thick deck plate and 16 mm thick webs of crossbeams for details C1c (Chapter 5), C4d (Chapter 6) and C3a (Chapter 7), and on nineteen 350 × 200 × 200 mm3 small-scale stiffener-to-deck plate connections for details C2a and C2b (Chapter 3). The 20 mm thick deck plate is used because a thicker deck plate (≥ 14 mm) is becoming common in newly designed OBDs as a response to cracks found in thinner deck plates (10 mm or 12 mm). Either automatic or manual welding is used for the deck plate welds in small- and full-scale specimens (Chapters 3 and 5).

Six corresponding detail categories are established, covering the deck plate thicknesses from 10 mm to 20 mm. Among them are two connection details: C1c (Chapter 5) and C3a (Chapter 7), which were not included in either the first or the second generation of EN 1993-1-9: Eurocode 3: Design of steel structures - Part 1-9: Fatigue. The surface extrapolation approach is used to calculate the hot spot stress for the details: C2a (Chapter 3), C1a (Chapter 4), C1c (Chapter 5), and C3a (Chapter 7). The force equilibrium (pair) approach is used to calculate the structural stress for detail C2b (Chapter 3) and the nominal stress for detail C4d (Chapter 6). Fatigue resistance is evaluated using multiple failure criteria for full-scale experiments instead of using the first visible crack as the main failure criterion. The fatigue resistances of the studied welded details are relatively high compared with the fatigue resistances in the standards. The main reasons for this are: a steep stress gradient towards the hot spot (Chapters 3, 4, 5, and 7), a possibility to redistribute loads from a weakened component to adjacent parts (Chapters 5, 6, and 7), connections loaded in cyclic compression or out-of-plane bending or in a combination of the two instead of cyclic axial tension (Chapters 3 to 7), a relatively thin plate thickness of stiffener (Chapters 3 and 7), and strict requirements for the geometry of the weld between stiffener and deck plate (Chapters 3, 4, and 5). Additionally, automatic welding shows a higher and more consistent fatigue performance for details C2a and C2b (Chapter 3).

Finite element models are built using the commercial software Abaqus. The effective notch stress, averaged strain energy density factors, notch strain and fracture mechanics are used to account for the effects of penetration depth, load ratio, initial flaw, residual stress and weld profile on the fatigue behaviour of the welded connections. The Linear Elastic Fracture Mechanics (LEFM) gives good predictions for the fatigue resistances of both the weld toe (detail C2a) and the weld root (detail C2b). A LEFM hand calculation model is proposed for C1a with three different weld profiles (Chapter 4). The surface crack propagation of C1a calculated by the proposed model, together with the Paris’ equation, is validated against the experiment in the literature. Probabilistic fracture mechanics analysis for detail C1a is carried out using the same method. The calculation predicts the fatigue resistances well compared with the values obtained from the experiments in the literature. The eXtended Finite Element (XFE) method is used to study the crack propagation of C1c within the LEFM framework (Chapter 5). The crack arrest and the crack path of C1c for 10 mm, 12 mm, 16 mm, and 20 mm thick deck plates are correctly predicted (validated against experimental investigations).

The residual stress due to welding is numerically studied using Abaqus for detail C3a (Chapter 7), which is further used in the fatigue analysis using the Notch StrAin (NSA) and the Elastic-Plastic Fracture Mechanics (EPFM) for the fatigue initiation and the short crack propagation, respectively. An engineering framework is proposed for the three-dimensional fatigue crack propagation using the LEFM. The crack propagation is predicted using the XFE algorithm in Abaqus, which successfully predicts the long crack propagation of C3a. The fatigue initiation stage and the crack propagation stage in the available 2 stage model are analysed by the NSA and the LEFM, respectively. The author bridges the gap between the fatigue initiation and the long crack propagation by using the EPFM. The 2+ stage model is therefore proposed by the author, which provides a full range fatigue analysis of metallic structures with a specific focus on welded connections (Chapter 7). The proposed model realistically considers the geometric characteristics and different material properties (e.g. heat affected zone) at the initiation location, and quantitatively implements the welding-induced residual stress obtained by FEA.

Recommendations are given for the fatigue design of the details based on this study, which were used as input for the technical specification “TS 1993-1-901 — Fatigue design of orthotropic bridge decks with the hot spot stress method” as part of the second generation of Eurocodes. ... Read more

The present study proposes a novel model to modify master S-N curves of components according to their load redistribution capability reflected in different boundary conditions (BCs) based on the fracture mechanics analysis. To that end, a comprehensive numerical study was conducted on a Single Edge Notch Bend (SENB) specimen constrained with different kinematic BCs using discrete fatigue crack growth (FCG) simulation. It was observed that BCs indeed can have a significant effect on the crack growth behavior and consequently on the resulting fatigue life under the same nominal loading conditions. The proposed model was applied to the S-N curve of a T-welded joint, and the predicted fatigue life was validated against 3D FCG simulations. Finally, FCG tests were conducted on SENB specimens to experimentally corroborate the effect of BCs on the FCG rate. ... Read more

Steel Orthotropic Bridge Decks (OBDs) are widely used in long-span and movable bridges. Fatigue resistance analysis plays an important role in the design or assessment of OBDs. One possible fatigue failure is the crack initiating from the weld root of stiffener-to-deck plate connections at crossbeams. A full-scale experimental investigation in this study using a 20 mm thick deck plate with a dimension of 9.4 m × 5.1 m, including three crossbeams, represents the modern designed OBDs. The experiments show an arrest of crack propagation with a final crack depth of approximately 75% of the deck plate thickness. On the contrary, through thickness cracks develop in deck plates of 10 or 12 mm. Hot spot stress based fatigue detail categories (DC) using various failure criteria derived from the tests. Analysis with the effective notch stress shows that the DC has low sensitivity to the amount of weld penetration. The results of analyses with the eXtended Finite Element Method (XFEM), employed to analyse the fatigue crack propagation path and crack arrest, are in line with the experimental study. ... Read more

Steel Orthotropic Bridge Decks (OBDs) are widely used in the superstructures of movable and long-span bridges. There is a high concentration of stress at many welded connections, which are governing the fatigue performance of OBDs. The fatigue behaviour of the lower end of weld in crossbeam-to-stiffener connection with cut-outs is studied from multiple experimental results obtained from a 48m2 full-scale specimen, as well as finite element simulations. The fatigue resistance of the detail is recommended based on 8 experiments reported in the current study and experiments in the literature using the hot spot stress method. The influence of the load location on the hot spot stress is analysed. The effective notch stress method is used to evaluate the fatigue resistance with different weld profiles. ... Read more

Fatigue cracks in the stiffener-to-deck plate connections of orthotropic bridge decks, initiating from the weld toe or root and propagating into the stiffener or weld throat, are experimentally and numerically studied. A statistical analysis of the structural stress is carried out using the experimental data. Automatic welded specimens show a significantly higher fatigue resistance than manual welded ones for both details of the study. Including results in the literature, the characteristic fatigue resistances appear larger than the values in current standards and range between 100 and 160 MPa. A proposal for the fatigue resistance values is given for design purposes. The effective notch stress, averaged strain energy density factor, and fracture mechanics methods are employed to study the sensitivity of the weld toe and root cracks for different (geometrical) variations, such as the lack of weld penetration. Among them, the fracture mechanics method agrees best with the experimental data. With the increase of weld penetration ratios from 75% to 100%, the fracture mechanics predicted fatigue resistances remain approximately equal for the weld toe cracks and increase for the weld root cracks. ... Read more

This study derives the fatigue resistance of welded details in orthotropic decks using structural stress (hot-spot stress where possible) based on tests described in literature and tests by the authors. The data are supported with linear elastic fracture mechanics simulations. Details covered are the rib to deck weld, the crossbeam to deck weld and the deck butt weld. High fatigue resistances are found, caused by favourable loading modes (bending and compression) and reduced driving force with the growth of cracks. The technical specification TS 1993-1-901, part of the new generation of Eurocodes, is based on the results of this study. ... Read more

The orthotropic steel decks are one of the most widely used bridge decks, especially in moveable and long span bridges. Numerous cracks have been detected in existing bridges, mainly in the welded connections. Fatigue performance of the bridge decks governs the design. The crack at the rib-to-deck joint is one of the most representative types. Strict requirements of the configuration of this joint are given in design codes resulting in classification categories based on experimental findings. Fatigue life of the crack initiation and changes of the stress conditions are usually experimentally investigated. However, prediction of the crack propagation and the effect of the cracks on the local stress fields are not possible to be fully understood based on experiments only. In this paper, finite element models are refined around the crack tip using software programs Abaqus and FRANC3D to study the crack propagation of rib-to-deck joint between crossbeams. The results is compared with beach marking tests in literature. Good agreement is found between predicted results and test data for both the crack shapes and loading cycles. The largest SIF range is predicted in the middle of the developed crack. It is then moved to the position 0.19 (relative distance, normalized by the width of the specimen) away from the crack priori to the fracture. During propagation period, structural integrity is affected by the crack and stress redistribution phenomenon is observed. ... Read more

The orthotropic steel decks (OSDs) are widely used in bridge engineering to support traffic loads. A possible crack, initiating from the weld toe of rib-to-deck welded joint and growing into the deck plate, is studied using linear elastic fracture mechanics. A detailed FE model is created and the results are compared with the fatigue tests published. Good agreement is found between beach marks from experiments and calculated crack fronts in FE. An engineering approach with the crack shape simplified as a semi-ellipse is applied. Geometric correction factors for a hand calculation method is proposed based on the parametric analysis. Using the proposed correction factors, Monte Carlo simulation is carried out with failure criteria defined with respect of the crack depth reaching “50%” of the deck thickness, “75%” of the deck thickness, and the failure criterion “2A FAD” according to BS7910. Predicted results using the failure criterion “75%” show good agreement with experimental data, for 5%, 50%, and 95% survival probabilities. Effects of initial crack shapes and sizes are discussed using the improved hand calculation model. Lower fatigue resistance is found when the initial crack is shallow or large. In addition to the standard weld geometry in which the weld profile is represented by a straight line, concave and convex arc shape weld profiles are studied. Fatigue resistance is improved in the case with assumption of concave arc weld profile. The difference of fatigue resistance between the cases with a straight line and convex arc weld profiles is small. ... Read more

The orthotropic steel decks (OSDs) are one of the most widely used bridge components, especially in moveable and long span bridges. Numerous cracks have been detected in this type of deck in existing bridges, mainly in the welded joints. The fatigue performance of the bridge deck dominates its design. Among them, the crack at the rib-to-deck joint is one of the most representative types. At the crossbeam conjunction, high stress concentration makes the joint more sensitive to fatigue loading. In this paper, finite element models are built using software program Abaqus integrated with FRANC3D. The calculated stress at uncracked stage is validated with measured data obtained from laboratory tests. Afterwards, cracks are inserted at the weld root and the stress intensity factor ranges in mode I (ΔK _I ) are calculated. Parametric analysis with various cracks is carried out. General correction factors are calculated from the finite element calcualtion with the power fit values. ... Read more