Distortion-induced Fatigue Research Articles

Distortion-Induced Fatigue Cracking in a Seismically Retrofitted Steel Bridge

This study examines the effect of a seismic retrofit on distortion-induced fatigue cracking in a steel girder bridge in the Midwestern United States. The seismic retrofit of the bridge was deemed necessary following a structural review initiated by the Federal Highway Administration (FHWA) in response to the 1989 Loma Prieta Earthquake. Following the review, the bridge was modified using a conventional seismic retrofit strategy to meet the 1995 FHWA standards for survivability and seismic performance. Upon completion of the retrofit, subsequent inspections identified new cracks (at the location of the retrofit) in the webs of longitudinal girders at transverse stiffener locations in an area known as the web gap. This study investigates the influence of the seismic retrofit strategy on crack formation by comparing principal stresses in the web-gap region of the original and retrofitted bridge under dead, live and thermal loading. A three-dimensional linear-elastic finite element analysis of the bridge before and after the retrofit is presented with results obtained from a commercial finite element software package. Comparisons of stresses before and after the retrofit indicate a measurable increase in stress near the web-gap region attributable to thermal and live loads following the retrofit. In addition, several viable repair strategies to limit these web-gap stresses and subsequent distortion-induced fatigue are presented. The results to date clearly demonstrate the importance of considering fatigue-sensitive details in seismic-retrofit strategies.

Inhibiting Distortion-Induced Fatigue Damage in Steel Girders by Using FRP Angles

AbstractThe idea of retrofitting web stiffener ends in steel bridge girders susceptible to distortion-induced fatigue using adhesively bonded fiber-reinforced polymer (FRP) angles is introduced in this study. The proposed retrofit method is relatively cheap and easy to use and does not require deck removal or any other severe modification to the steel girder. Fatigue tests were conducted on specimens designed to model the conditions in the region between a web stiffener and a flange in a steel girder bridge. Fatigue life increases on the order of several hundred percent were achieved by implementing the proposed retrofit. The effect of the retrofit on the hot-spot stress in the critical weld detail can be seen in the experimental strain data and in a subsequent finite-element (FE) analysis of the tested specimen geometry. Further research is recommended to assess the performance of the retrofit on full-scale girders and to develop guidelines for fatigue verification and design of the FRP angle and adhesiv...

Investigation and Retrofit of Distortion-Induced Fatigue Cracks in a Double-Deck Cantilever-Suspended Steel Truss Bridge

This paper discusses a comprehensive investigation and retrofit for extensive fatigue cracks in the end connections of floor beams on a double-deck, cantilever-suspended steel truss bridge. The investigation involved three-dimensional (3D) finite-element analyses using global and local models, field measurement of strains and displacements due to live load and temperature, laboratory testing of steel samples for material properties and high-stress low-cycle fatigue characteristics, as well as development of an effective retrofit based on the analytical, experimental, and field testing results. It was concluded that the cracks were a result of distortion induced fatigue in the floor beam web due to interactive deformations of the global structural system under live load and temperature variations. The retrofit entailed removing the fatigue susceptible weld terminations and reinforcing local areas of the floor beam web for out-of-plane distortion. Construction of the fatigue retrofit was completed in July 2011. The repaired structure has performed satisfactorily since then.

An investigation of distortion-induced fatigue cracking under variable amplitude loading using 3D crack propagation analysis

The distortion-induced fatigue cracks in the welded details of the Söderström Bridge are analytically and numerically investigated by performing 3D crack propagation analyses with variable amplitude fatigue loading. In the crack propagation analyses, the effects of bridge loading are defined on the basis of the field measurements in order to simulate crack growth and predict the residual fatigue life of the studied detail as accurately as possible. The effect of crack closure and crack direction while considering the most common criteria is also studied. The results are compared with those obtained from the crack propagation analyses with constant amplitude fatigue loading presented in Aygül et al. [1].The results of the crack growth simulations with variable amplitude fatigue loading have generally shown good agreement with the real crack formation and reveal that the crack growth rates are different in different directions. The crack behaviour in the damaged detail is mainly controlled by the loading and geometrical arrangement of the detail components. There is generally a significant difference between constant and variable amplitude fatigue crack growth analyses and the variable amplitude fatigue crack growth analyses yield more conservative results. The main reason for this difference is the bridge loading and the number of stress cycles defined in the analyses. The crack direction criteria studied in this investigation showed basically the same crack formation and crack growth rate.

Repairing Distortion-Induced Fatigue Cracks in Steel Bridge Girders Using Angles-with-Plate Retrofit Technique. I: Physical Simulations

Repairing Distortion-Induced Fatigue Cracks in Steel Bridge Girders Using Angles-with-Plate Retrofit Technique. I: Physical Simulations

Investigation of distortion-induced fatigue cracked welded details using 3D crack propagation analysis

The behaviour of distortion-induced fatigue cracks in welded details in an existing bridge was studied analytically by performing crack propagation analysis based on linear elastic fracture mechanics. The real load history of the bridge was obtained from strain measurements. These loads were utilised to examine the crack growth rate and the residual service life of the cracked detail. Moreover, the effectiveness, accuracy and applicability of the crack propagation analysis on bridge structures were investigated by simulating a complex case of fatigue cracking using several crack propagation analyses.The results of the analyses revealed that the fatigue crack in the studied details had significantly different crack growth characteristics in different directions. In the thickness direction, for instance, the crack was seen to propagate at a certain rate, which increased with the propagated crack from the beginning and, as expected, the crack propagation rate decreased when the crack grew longer. The crack was subsequently arrested half way through the thickness of the plate. In the longitudinal direction, the crack was not, however, arrested in the same way as in the thickness direction and it continued to propagate at a reduced yet constant crack growth rate. The results also revealed that, even though distortion-induced fatigue cracking was usually caused by a mixed-mode condition (i.e. a combination of modes I, II and III), the governing propagation mode is still mode I. Furthermore, it was also observed that the contribution of modes II and III to crack propagation was very little and dependent on the location of the propagated crack front, as well as the geometrical configuration of the cross-beam.

Repairing Distortion-Induced Fatigue Cracks in Steel Bridge Girders Using Angles-with-Plate Retrofit Technique. II: Computer Simulations

This paper presents the results from computer simulations of 914 mm (36 in.) deep girder-cross frame subassemblies to study the effects of distortion-induced fatigue and to evaluate the effectiveness of a newly proposed cost-effective retrofit measure. The proposed retrofit measure consists of adding steel angles connecting the girder web and the transverse connection plate (CP), and a steel plate on the back side of the girder web. The retrofit measure is intended to reduce stress demand at the welds, to restrain the web-gap region from deforming in the out-of-plane direction, and to distribute lateral forces transferred by cross-frames over a wider region of the web. Parametric studies were carried out to determine the optimal configuration to prevent growth of fatigue cracks of various lengths in the web-gap region. It was found that the proposed retrofit measure reduced peak stress demands with respect to the unretrofitted configuration by 50% or more, making it likely that it will prevent fatigue crack reinitiation. The parametric studies showed that the proposed retrofit measure became more effective in reducing the peak stress demand as the stiffness of the elements increased, with the lowest average stress demand occurring when the angles and backing plate were assigned thicknesses equal to 2.67 times the thickness of the web. Experimental verification of this study is presented in a companion paper.

Innovative retrofit technique for distortion-induced fatigue cracks in steel girder web gaps

Cracking in web gaps of steel girder bridges is often difficult and expensive to repair, especially at the top web gap. A commonly-used retrofit technique involves creating new load path between a transverse connection plate and the top flange of the girder, by means of bolted angles on both sides of the transverse connection plate. This technique often requires removing portions of the bridge deck to create the bolted connection at the top flange, which is an approach that can incur significant expense and inconvenience to the traveling public. This study was aimed at evaluating the effectiveness of a newly-developed retrofit technique, in which connection is made between the transverse connection plate and the girder web, through use of bolted angles on both sides of the transverse connection plate and a backing plate on the opposing face of the girder web. The retrofit was evaluated through extensive structural testing and finite element modeling. Testing was performed on a 9.1-m [30-ft] long test bridge system, comprised of three 910-mm [36-in.] deep girders and a concrete deck. The system was loaded to produce distortion-induced fatigue cracking, and then the bridge was retrofitted with the newly-developed stiffener-to-web repair technique. Results of the testing were compared to findings from finite element analyses, as well as findings from structural tests performed on 2.7-m [9-ft.] long segments of similar girders tested under distortion-induced fatigue. Results have indicated that the newly-developed retrofit technique has significant potential for effectively controlling distortion-induced fatigue cracking in web gaps of steel girder bridges without requiring disruption to the concrete deck.

Field testing and finite element analysis of steel bridge retrofits for distortion-induced fatigue

Fatigue-crack formation in the web-gap region of multi-girder steel bridges has been a common occurrence due to differential deflections between girders resulting in diaphragm forces that subject the web-gap to out-of-plane distortion. However, bridge design specifications do not directly deal with distortion induced stresses. The objective of this paper is to examine the behavior of web-gap distortion of a skewed multi-girder steel bridge through field testing and finite element studies. The field test involved measurements of strains for eight load cases by using strain gages placed in the web-gap region and in the cross bracing adjacent to the web-gap. The results of the field test were used as a basis for the verification of developed finite element models. Critical spots in the web-gap were examined utilizing the finite element analysis. Retrofit methods that include the provision of a connection plate between the stiffener and the girder top flange, loosening of the bolts connecting the cross-bracing to the stiffener, and supplementing a stiffener plate opposite to the original stiffener side were investigated. The results indicated that the connection plate addition and loosening of bolts alternatives were effective in reducing induced strains and stresses in the web-gap region. The finite element study on the impact of web-gap height variation indicated an inverse relationship between web-gap height and induced strains and stresses with the shortest web-gap height resulting in the highest strains due to increased bending by diaphragm forces in the web. Influence surfaces for strains, diaphragm force and relative out-of-plane displacement provide insight into the behavior of web-gap distortion, and can be used to provide quick estimate of the effect of truck load positioning. Expressions developed that relate vertical stresses and relative out-of-plane displacements combined with measurements of out-of-plane displacements by transducers can be utilized for prediction of induced stresses at other critical web-gap regions of the bridge and at critical locations in the web gaps of similar bridges.

Parametric Analysis of Cross-Frame Layout on Distortion-Induced Fatigue in Skewed Steel Bridges

The effects of skew angle, cross-frame spacing, cross-frame layout, cross-frame stiffness, and load placement on the potential for distortion-induced fatigue damage in steel bridges was investigated by performing a suite of more than 1,000 analysis jobs of high-resolution three-dimensional finite-element models. Susceptibility to fatigue damage was quantified in terms of computed stress demand in the web gap region of the girders. Bridge configurations with three different cross-frame layouts were evaluated, including configurations with cross-frames placed parallel to skew angle (skewed-parallel) and perpendicular to the girder line, both staggered (skewed-staggered) and unstaggered (skewed-unstaggered). Skew angles of configurations evaluated ranged between 0 and 50°, and cross-frame spacing ranged from 2.29 to 9.14 m (7.50 to 30.0 ft). Influence and envelope surfaces were constructed to show the relationship between load placement, location of the maximum web gap stress, and the magnitude of the maximum web gap stress. It was found that maximum web gap stress always occurred when loads were positioned directly above the intersection of a cross-frame and girder web. The parametric study showed that cross-frame stiffness and spacing had a significant effect on the susceptibility to distortion-induced fatigue damage; greater cross-frame stiffness resulted in higher web gap stresses, and increased cross-frame spacing resulted in increased web gap stresses. It was also found that the bridge configuration was key to determining the location of the web gaps where damage is most likely to occur. In skewed-parallel and skewed-unstaggered layouts, maximum web gap stresses were identified in top web gaps, whereas in skewed-staggered configurations, maximum stresses occurred in bottom web gaps. It was found that in configurations with staggered cross-frames, maximum web gap stresses tended to occur in regions of support, where cross-frames are often placed back-to-back along the skewed alignment.

Optimization of retrofitting distortion-induced fatigue cracking of steel bridges using monitored data under uncertainty

This paper focuses on optimization of retrofitting distortion-induced fatigue cracking of steel bridges using monitored data under uncertainty. The optimization problem has two competing objectives: (i) maximization of the fatigue reliability of the connection details after retrofitting and (ii) minimization of the cut-off area. The geometrical restrictions, predefined maximum tensile stresses, and minimum remaining fatigue life of the connection details after retrofitting are all taken into account as constraints. The fatigue reliability assessment with monitored data is based on the formulation used in the AASHTO specifications. The original monitored data may be modified by using a proposed cut-off size adjustment factor (SAF) to represent the fatigue stress ranges at the identified critical locations after retrofitting. The nonlinear relationships between the cut-off size and SAF are established. The proposed approach is illustrated by using an existing steel tied-arch bridge monitored by the Advanced Technology for Large Structural Systems (ATLSS) Engineering Research Center at Lehigh University.

Effects of Cross-Frame Placement and Skew on Distortion-Induced Fatigue in Steel Bridges

Because of detailing practices common before the mid-1980s, many bridges are highly susceptible to distortion-induced fatigue. This research explored the influence of cross-frame placement and skew angle in bridges subject to distortion-induced fatigue. Forty high-resolution, three-dimensional finite element analyses of a bridge with multiple cross-frame and skew configurations were performed to examine the relationships between skew angle, cross-frame placement, and stresses on distortion-induced fatigue susceptibility. Bridges with skew angles of 0°, 20°, and 40° and cross-frames spaced at 4.58 m (15 ft) and 9.15 m (30 ft) were investigated. Cross-frame configurations examined included staggered perpendicular to the girder line as well as parallel to the support skew. The analyses found maximum stresses in the web-gap occurred in positive moment regions but not necessarily in regions of highest differential deflection. In configurations with cross-frames placed parallel to the skew angle, maximum stress demand was in the top web-gap but it was in the bottom web-gap region when cross-frames were staggered. Increased spacing between cross-frames correlated with slightly increased maximum web-gap stresses for parallel-to-skew cross-frame arrangements and decreased maximum web-gap stresses for perpendicular-to-girder line cross-frame arrangements. Skew angle had a minimal effect on web-gap stresses in the bridge studied.

Girder Differential Deflection and Distortion-Induced Fatigue in Skewed Steel Bridges

The prevalence of fatigue cracking in steel bridge girders due to out-of-plane web distortion motivates development of procedures to evaluate the effects of distortional fatigue. In a previous study sponsored by the Minnesota Department of Transportation (Mn/DOT) the frequency and magnitude of distortional stresses on a typical skewed, steel bridge with staggered, bent-plate diaphragms were assessed. The results revealed a diaphragm deformation mechanism that causes distortional fatigue in the girder web gap, leading to simple, accurate estimates of fatigue stress if bridge properties and differential vertical deflection between girders are known. In the present study, linear finite element models are used to represent composite steel bridges and identify bridge parameters that influence relative deflection of adjacent girders. Parameters found to have a significant effect on differential deflection include girder spacing, angle of skew, span length, and deck thickness. These results are incorporated in a simple procedure that is intended for use in management schemes for skewed, steel-girder bridges, with staggered, bent-plate diaphragms, susceptible to web gap distortional fatigue.

Finite-Element Analysis of Steel Bridge Distortion-Induced Fatigue

Welded plate girder bridges built before the mid-1980s are often susceptible to fatigue cracking driven by out-of-plane distortion. However, methods for prediction of secondary stresses are not specifically addressed by bridge design specifications. This paper presents a finite-element study of a two-girder bridge that developed web gap cracks at floortruss-girder connections. The modeling procedures performed in this research provide useful strategies that can be applied to determine the magnitude of distortion-induced stresses, to describe the behavior of crack development, and to assess the effectiveness of repair alternatives. The results indicate severe stress concentration at the crack initiation sites. The current repair method used at the positive moment region connections is found acceptable, but that used at the negative moment region connections is not satisfactory, and additional floortruss member removal is required. Stress ranges can be lowered below half of the constant amplitude fatigue threshold, and fatigue cracking is not expected to recur if the proposed retrofit approach is carried out.

Measurement and Analysis of Distortion-Induced Fatigue in Multigirder Steel Bridges

Fatigue damage to multigirder steel bridges on skew can result from distortion caused by differential deflection of adjacent girders that impose out-of-plane bending of girder web gaps. Existing design procedures give recommendations to mitigate the effects of distortional fatigue but do not directly address secondary, out-of-plane deformations, nor do they provide guidance in determining the magnitude of out-of-plane stresses in girder webs. An experimental study was conducted to (1) implement a field monitoring program for a typical multigirder steel bridge on skew supports; (2) assess the frequency and magnitude of distortional fatigue stresses at web-stiffener connections; and (3) evaluate the impact of these stresses on fatigue life. Measurements from twelve strain gauges were continuously monitored and recorded for a period exceeding three months on Minnesota Department of Transportation Bridge #27734. Web-gap stresses in negative-moment regions were found to be much larger than flange stresses. The results of a detailed finite-element study indicate that actual strains at the web gaps may be much larger than the values measured at the strain gauge locations. This study also revealed the mechanism of web-gap distortion, suggesting an approximate method for predicting web-gap stress based on known girder differential deflection.

Finite Element Study of Distortion-Induced Fatigue in Welded Steel Bridges

Out-of-plane distortion-induced fatigue cracking is caused by relative rotation and displacement between longitudinal girders and transverse members framing into these girders. Procedures for determination of secondary stresses are not specified in the design or rating process. Appropriate finite element method procedures to analyze distortion-induced fatigue behavior are presented. A multigirder bridge developed web gap cracks near the girder bottom flange in a positive moment region. The affected diaphragm–girder connections were repaired by installing additional reinforcing splice plates to the web and attaching connection stiffeners to the flanges. Because no structural modifications were made to similar details in the bridge that had not developed fatigue cracks, concerns remain that these details may also be subjected to high-magnitude fatigue stresses that may lead to future cracking. By using finite element submodeling techniques, potential crack initiation sites in the bridge were identified and the corresponding distortion-induced stresses were determined. The most stressed detail reached yielding with an out-of-plane displacement of only a few thousandths of an inch. On the basis of the analytical results, a linear stress–displacement correlation was established for prediction of the secondary stresses. Repair analysis indicated that web gap stresses can be significantly reduced if a rigid stiffener-to-flange attachment is used. Thus, a bolted repair is recommended for the positive moment region connections, and a welded repair is recommended for the transition and negative moment region connections.

Rehabilitation Study of Toutle River Tied-Arch Bridges

The Toutle River tied-arch bridges (Br. No. 5/140 E & W) are located on Interstate 5 in western Washington State and were open to traffic in 1969. Approximately 33,000 vehicles a day cross these bridges; 22% are trucks. Large deflections result with the passage of these trucks, causing the bridges to vibrate in the first mode of vibration. A strengthening and stiffening study was conducted to minimize deflection, vibration, and fatigue cracking. Several stiffening methods were analyzed. The addition of diagonal tension members between each vertical hanger and addition of compression members placed near the quarter points of the bridges emerged as the most promising options. The bridges also experience fatigue cracking. The floor-beam webs have cracks at most tie-chord connections and at exterior longitudinal stringer connections. Stiffening the arches would decrease the fatigue stress range in the tie chord. Retrofitting the floor system to accommodate thermal movements would minimize distortion-induced fatigue in the floor beams. At the bridge bearings, extensive cracking is present at the fillet welds connecting the upper bearing blocks to the tie chord. A bearing replacement was recommended. Adding post-tensioning strands to the tie chord was also recommended as a safeguard against sudden failure. Estimated retrofit costs were also developed.

Removal of Diaphragms from Three-Span Steel Girder Bridge

Many multigirder steel bridges built before 1985 are experiencing distortion-induced fatigue cracking at diaphragm-girder connections. A means of eliminating the cyclic stresses that cause those fatigue problems is to remove the interior diaphragms (diaphragms not at the girder supports). The effects of removing interior channel diaphragms from an existing three-span bridge with rolled steel girders are reported in this paper. The effects of removing the diaphragms are evaluated using field measurements of girder stresses and deflections made before and after the diaphragms were taken out. Results from tests with trucks of known weight indicate that removing the diaphragms results in increases up to 15% in the maximum bottom flange stresses at the most heavily loaded girder. Comparisons of measured and calculated stresses show that the conservatism of common design practice is not significantly reduced by removing diaphragms, and the effects of removing interior diaphragms are judged insignificant.

Steel Orthotropic Decks: Developments in the 1990s

Recent developments in the field of steel orthotropic decks, which made possible the design of several recently completed highway bridges with record-breaking spans, are summarized. The more efficient, but fatigue sensitive, decks with closed ribs were prevalently used. Extensive research on the fatigue of closed-rib orthotropic decks was conducted, and the results were used in new deck design provisions in the United States and in Europe. Empirical design of details subject to distortion-induced fatigue has been adopted in the AASHTO Specifications. Recent fatigue cracks and failures in the deck plates of closed-rib orthotropic decks in the Netherlands brought to light the stress concentrations in the deck plate at rib intersections with the floor beams that were not previously recognized in the design. This observation is discussed, and a solution is proposed for future designs. Orthotropic decks with open ribs are free from second-order, local stress concentrations, and their fatigue performance has been generally satisfactory. Such decks have been extensively used on railway bridges in Europe. Also, several long-span highway bridges with open-rib decks have been built recently. Because of their good performance record, a more frequent use of the decks with open ribs on future bridges may be indicated. Surfacing research has shown that deck cracking is directly related to a composite action with the orthotropic deck, which must have adequate rigidity for good surfacing performance. Elastic properties of surfacings need more research. The possibilities for further development are included in the discussion.

Laboratory Tests of Bolted Diaphragm-Girder Connections

Many multigirder steel bridges built before 1985 are experiencing distortion-induced fatigue cracking at diaphragm-girder connections. Fatigue cracks in the welds and base metal in hundreds of diaphragm-girder connections in bridges in the Birmingham, Ala. area have been discovered. Many of the welded connections have been replaced with angles bolted to the diaphragm and girder web. The replacement angles have not performed satisfactorily due to fatigue cracking in the connection angle. Laboratory tests of three existing and one proposed replacement angle were conducted to determine the fatigue limit for the replacement angles and the most appropriate replacement angle to use on future repairs. Primarily, surface strain measurements and vertical girder deflections due to fatigue loading were recorded during fatigue tests of the laboratory model to evaluate the performance of the connections. Results indicate that the appropriate bending stress fatigue limit is 165 MPa and the proposed new replacement angle should be used for future repairs.