Fatigue Performance of Stringer-to-Floor-Beam Connections in Riveted Railway Bridges

The behavior of double-angle stringer-to-floor-beam connections in riveted railway bridges is examined experimentally. A series of static and fatigue tests were performed on three full-scale bridge parts taken from an old riveted railway bridge. The results of the static tests reveal that the amount of end moment developed in these connections as a result of their rotational stiffness could be considerable. As a result of the cyclic variation in this moment, fatigue damage might develop in these connections. This damage was, however, observed to have a fairly low propagation rate and did not immediately reduce the load-carrying function of the connections.     

Probabilistic Fatigue Evaluation of Riveted Railway Bridges

A probabilistic fatigue assessment methodology for riveted railway bridges is presented. The methodology is applied to a typical, short-span, riveted U.K. railway bridge under historical and present day train loading. On the loading side, the problem is randomized through dynamic amplification and traffic volume; on the resistance side, the S-N curves and the cumulative damage model are treated probabilistically. Model uncertainty is represented by the ratio between actual and calculated stresses, the latter obtained through finite element analysis. Annual response spectra for a fatigue-critical connection are developed through Monte Carlo simulation, which show that there is a continual and accelerating increase in the mean stress range experienced by the connection with time. S-N curves proposed in United States and United Kingdom codes are used in combination with Miner’s rule, to estimate the remaining fatigue life of the connection for different target failure probabilities. Parametric studies revealed that fatigue life estimates exhibit the highest sensitivity to detail classification, to S-N predictions in the region of high endurances, and to model uncertainty. This highlights the importance of field monitoring for old bridges approaching the end of their useful life.     

Estimating Fatigue Life of Bridge Components Using Measured Strains

The design fatigue life of a bridge component is based on the stress spectrum the component experiences and the fatigue durability. Changes in traffic patterns, volume, and any degradation of structural components can influence the fatigue life of the bridge. A fatigue life evaluation reflecting the actual conditions has value to bridge owners. Procedures are outlined in the AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel Bridges to estimate the remaining fatigue life of bridges using the measured strain data under actual vehicular traffic. This paper presents the methodology with an actual case study of Patroon Island Bridge. The Patroon Island Bridge consists of ten spans. Spans 3 through 9 are considered the main spans and consist of steel trusses and concrete decks. Spans 1, 2, and 10 are considered approach spans and consist of plate girders. The overall bridge length is 1,795 feet. Strain data from critical structural members were used to estimate the remaining fatigue life of selected bridge components. The results indicate that most of the identified critical details have an infinite remaining safe fatigue life and others have a substantial fatigue life. Cracked floor beams were not addressed in this analysis, but have been recommended for retrofitting or replacement.     

Fatigue Resistance of HPS-485 W (70 W) Welded Butt-Splice Connections Using Narrow Gap Improved Electroslag Welding

The writers investigated the performance of narrow gap improved electroslag weld (NGI-ESW) procedures in welded butt splices using high-performance steel grade HPS-485 W (70 W) through comparison against similar specimens fabricated using submerged arc welding (SAW). Five NGI-ESW and five SAW specimens were tested in fatigue at very high stress ranges, and all achieved run-out at or above 2 million cycles. Two each of the specimens created using SAW and NGI-ESW were tested an additional 3 million cycles; the NGI-ESW specimens did not experience fracture, whereas one SAW specimen failed in the base metal and another completed the additional cycles without failure. All intact specimens were statically tested to failure and results were compared. All 10 specimens performed considerably better than predicted by the AASHTO (2007) fatigue life equation. The NGI-ESW specimens performed at least as well as the SAW specimens under fatigue and static testing, suggesting that inclusion of the NGI-ESW process in AWS D1.5 may be appropriate when used with HPS-485 W (70 W). Additionally, testing supports extension of current code provisions to the use of NGI-ESW in fracture-critical applications.     

Assessment of Bridge Remaining Fatigue Life through Field Strain Measurement

With the aging of existing steel bridges and the accumulated stress cycles under traffic loads, assessment of remaining fatigue life for continuing service has become more important than ever, especially for decisions on structure replacement, deck replacement, or other major retrofits. Experience from engineering practice indicates that fatigue analysis based on specification loads and distribution factors usually underestimates the remaining fatigue life of existing bridges by overestimating the live load stress ranges. Fatigue evaluation based on field-measured stress range histograms under actual traffic load proves to be a more accurate and efficient method for existing bridges. This paper describes the application of such a method in assessing the remaining fatigue life of bridge structures. Current AASHTO specifications for fatigue evaluation of existing bridges are reviewed and compared. Case studies of three major highway bridges are discussed. Finally, a procedure is proposed for evaluating fatigue life of existing bridges through field strain measurement.     

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.     

Truck Models for Improved Fatigue Life Predictions of Steel Bridges

A new fatigue load model has been developed based on weigh-in-motion (WIM) data collected from three different sites in Indiana. The recorded truck traffic was simulated over analytical bridge models to investigate moment range responses of bridge structures under truck traffic loadings. The bridge models included simple and two?equally continuous spans. Based on Miner’s hypothesis, fatigue damage accumulations were computed for details at various locations on the bridge models and compared with the damage predicted for the 240-kN (54-kip) American Association of State Highway and Transportation Officials (AASHTO) fatigue truck, a modified AASHTO fatigue truck with an equivalent effective gross weight, and other fatigue truck models. The results indicate that fatigue damage can be notably overestimated in short-span girders. Accordingly, two new fatigue trucks are developed in the present study. A new three-axle fatigue truck can be used to represent truck traffic on typical highways, while a four-axle fatigue truck can better represent truck traffic on heavy duty highways with a significant percentage of the fatigue damage dominated by eight- to 11-axle trucks.     

Fatigue Behavior and Retrofit Investigation of Distortion-Induced Web Gap Cracking

This paper studies a Kansas Department of Transportation welded plate girder bridge that developed fatigue cracks at small web gaps close to the girder top flange. Repair had been previously performed by softening the connection plate end with a slot retrofit, but cracks were recently found to have reinitiated at some of the repaired details and are again propagating. A comprehensive finite-element method study was performed to investigate the cracking behavior observed in the bridge and to recommend appropriate measures for future bridge retrofit. The analytical results show that stresses developed at the top flange web gaps could exceed yielding under the loading of an HS15 fatigue truck. The current slot repair used in the bridge was found to have introduced higher magnitude fatigue stresses in the web gap. To achieve a permanent repair of the bridge, it is recommended that a welded connection plate to flange attachment be used during future bridge retrofit. The web gap details should be able to withstand unlimited number of load cycles once this additional repair is performed.     

High-Cycle Fatigue of Diagonally Cracked RC Bridge Girders: Laboratory Tests

Large numbers of conventionally RC deck–girder bridges are in the national highway system. Diagonal cracks have been identified in many of these bridges, which are exposed to millions of load cycles during service life. The anticipated life of these bridges in the cracked condition under repeated service loads is uncertain. Laboratory experiments were performed on full-size girder specimens to evaluate possible deterioration in shear capacity under repeated loading. Specimen variables included: T and inverted-T configurations, stirrup spacing, and flexural reinforcing details. Test results indicated bond deterioration increased diagonal crack displacements, and analysis methods to predict the shear capacity of diagonally cracked reinforced concrete girders subjected to high-cycle fatigue damage are provided. The AASHTO-LRFD shear provisions conservatively predicted shear capacity for the fatigued specimens without stirrup fractures, and shear capacity predictions from computer analysis program Response 2000 were very well correlated with experimental results for fatigued test specimens when the input concrete tensile strength was reduced to nearly zero.     

Rapid Ranking Procedures for Gusset Plate Connections in Existing Steel Truss Bridges

Evaluation and rating of steel truss bridge connections has become imperative for many transportation agencies after the recent collapse of the I-35W Bridge in Minneapolis. Detailed engineering capacity calculations of gusset plate connections are time consuming and thus expensive. Large numbers of connections are in the national inventory and must be evaluated. A screening process and a simplified rapid screening process are proposed for ranking gusset plate connections in steel truss bridges to help bridge engineers identify possible vulnerable connections and aid field inspections. The procedures consider member demands relative to the connection geometric proportions for four different parameters: fasteners, plate tension, plate compression, and overall horizontal shear. The methods are demonstrated for two bridges, including the collapsed I35W Bridge, and clearly identify connections U10 and L11 as vulnerable for three of the four parameter types (fasteners were not identified as vulnerable for these connections). The ranking approach is not proposed as a substitute for thorough, detailed, and expert assessment of the connections, but rather allows rating engineers to more quickly prioritize detailed evaluations in an ordered systematic way from the most likely vulnerable connections to the least likely vulnerable connections. This technique may be considered analogous to performing screening tests on a new patient to indicate the likely medical condition prior to conducting more sophisticated and costly investigations.     

Fatigue and Fracture of Steel Girders

This paper presents an overview of materials selection, design, and detailing of steel girders for fatigue and fracture limit states. The historical context of the fracture control plan for bridges is presented. A discussion of fracture toughness of structural steel and weld metal is presented along with typical Charpy and fracture-toughness test data, including the new high-performance steel A709 HPS 485W. Fatigue of cover plate details and distortion-induced cracking are discussed. Methods of dealing with variable-amplitude loading are then compared to test data.     

Fatigue Behavior of Composite-Reinforced Glulam Bridge Girders

While composite-reinforced glulam beams have been used in several bridge demonstration projects, knowledge of their fatigue behavior is quite limited. In this study, the response of full- and partial-length fiberglass composite-reinforced glulam beams under fatigue cycling followed by quasi-static bending to failure is examined. To mimic anticipated in-service conditions, a hygrothermal cycling regime was developed that replicates the effective stress history of a 50-year service life with a 55-day period in a moisture-controlled kiln. In addition, some of the beams had initial delaminations introduced between the reinforcing and the wood similar to those observed in field investigations of reinforced glulam bridge girders. For the partial-length reinforced beams, reinforcing with both confined and unconfined ends was considered. The results of the postfatigue tests to failure were compared with the expected strength. In addition, the stiffness of the beams was monitored during the fatigue cycling. It was found that, with the exception of the unconfined, partial-length reinforced beams, all specimens had a residual strength that compared favorably with the expected strength. Further, neither the preconditioning nor the fatigue cycling had an appreciable impact on the stiffness of the reinforced beams. The unconfined, partial-length reinforced beams did not perform well under fatigue loading and do not seem to be a viable alternative for use as reinforced glulam bridge girders.     

Identifying Effective and Ineffective Retrofits for Distortion Fatigue Cracking in Steel Bridges Using Field Instrumentation

It is estimated that nearly 90% of all fatigue cracking is the result of out-of-plane distortion or other unanticipated secondary stresses at fatigue-sensitive details. Neither design specifications nor evaluation specifications provide any guidance on how to evaluate the in-service potential for fatigue cracking at these details. Often, as a result, the effectiveness of various retrofit procedures is questionable and ill fated. There are many examples where implemented retrofit procedures did not work and fatigue cracking reinitiated or continued. Implementation of one or two prototype retrofits is an attractive alternative to ensuring effective retrofits are developed where many details have to be retrofitted. Field instrumentation and testing is an effective means to determine the effectiveness and behavior of a given retrofit strategy. Behavior that may not be anticipated can be identified prior to installing retrofits on a large scale, thereby preventing future problems. This paper provides guidance on how to instrument these details and examines one example where initial retrofit strategies did not work, as demonstrated by their performance through field instrumentation.     

Fatigue of Diagonally Cracked RC Girders Repaired with CFRP

Fiber-reinforced polymers (FRP) are becoming more widely used for repair and strengthening of conventionally reinforced concrete (RC) bridge members. Once repaired, the member may be exposed to millions of load cycles during its service life. The anticipated life of FRP repairs for shear strengthening of bridge members under repeated service loads is uncertain. Field and laboratory tests of FRP-repaired RC deck girders were performed to evaluate high-cycle fatigue behavior. An in-service 1950s vintage RC deck-girder bridge repaired with externally bonded carbon fiber laminates for shear strengthening was inspected and instrumented, and FRP strain data were collected under ambient traffic conditions. In addition, three full-size girder specimens repaired with bonded carbon fiber laminate for shear strengthening were tested in the laboratory under repeated loads and compared with two unfatigued specimens. Results indicated relatively small in situ FRP strains, laboratory fatigue loading produced localized debonding along the FRP termination locations at the stem-deck interface, and the fatigue loading did not significantly alter the ultimate shear capacity of the specimens.     

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.     

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.     

Variable-Amplitude Fatigue Strength of Structural Steel Bridge Details: Review and Simplified Model

This paper reviews eight previous studies on the variable-amplitude (VA) fatigue strength of structural steel details: (1) four studies in the finite-life regime in which the number of cycles to failure of the specimens are equal to or shorter than the number of cycles at the intersection between the sloped S-N line and the VA fatigue limit, hereafter called the transition life; and (2) four studies in the infinite-life regime that is, at numbers of cycles greater than the transition life. The VA data correlate well with the constant-amplitude data when the former are plotted in terms of an equivalent root-mean-cube stress range. The effects of the following variables on the fatigue strength of structural details are discussed: block sequence in a stress range spectrum, spectrum size, type of spectrum, minimum stress, and type of steel. A so-called long-life factor quantifies how far a type of detail was cycled into the finite-life regime. Based on the results of the literature review, the writers recommend that the current AASHTO log-log bilinear equations for calculating the fatigue life be replaced with a single equation similar to the equation for predicting the fatigue crack growth rate in metals. This simplified model more accurately predicts the fatigue life near the VA fatigue limit.     

Determining Appropriate Fatigue Inspection Intervals for Steel Bridge Members

As part of the National Bridge Inspection Standards, owners of public bridge structures are required to perform a Fracture Critical Inspection on steel superstructures that contain primary structural elements having no load path redundancy, e.g., two girder systems. Such inspections are looking to identify damage or deterioration such as corrosion and fatigue cracking that may lead to failure of the critical member. The Oregon Department of Transportation is responsible for the inspection of 196 fracture critical structures that are subjected to widely varying service and environmental conditions. These conditions range from coastal bridges in a fairly corrosive environment with moderate traffic volumes, to large and complex structures in urban areas that experience large volumes of traffic, to very benign conditions in the sparsely populated eastern regions with very low traffic volumes. In response to these widely varying service conditions, Oregon has developed a method to better categorize steel superstructures for fatigue inspection priority and frequency. This method is not only proving to save unnecessary inspection costs but increasing the inspection quality by concentrating resources where they are most needed. This paper presents a simple and practical method of evaluating fatigue inspection periods.     

Influence of Surface Condition on the Inspection of Steel Bridge Elements Using the Time-of-Flight Diffraction Method

Prior studies on the time-of-flight diffraction (TOFD) method have focused primarily on ground smooth, clean surfaces of steel. In practice, however, the surface of an existing bridge element will be covered with rust or have several layers of paint. The main objective of this study was to evaluate the influence of the surface condition of steel elements (i.e., painted or rusted) on the ability of the method to accurately detect and size flaws. These objectives were met by performing a number of tests on plates with saw cuts or implanted fatigue cracks with different surface conditions. These included ground smooth and polished, rusted, and painted surfaces. The data show that rusted surfaces will reduce the amplitude of the ultrasonic signals, but they will not impair the ability of the TOFD method to detect and accurately size flaws. A painted surface will also cause a reduction in signal amplitude. More important, however, is the appearance of additional wave signals that could be interpreted as false indications. While these additional signals do not obscure the presence of actual flaws or affect the accuracy of the TOFD method to size the flaws, they make flaw detection more difficult. Based on the results of this study, recommendations for field inspection on rusted or painted surfaces using the TOFD method are provided.     

Fatigue Damage of Steel Bridges Due to Dynamic Vehicle Loads

This paper focuses on the fatigue damage caused in steel bridge girders by the dynamic tire forces that occur during the crossing of heavy transport vehicles. This work quantifies the difference in fatigue life of a short-span and a medium-span bridge due to successive passages of either a steel-sprung or an air-sprung vehicle. The bridges are modeled as beams to obtain their modal properties, and air-sprung and nonlinear steel-sprung vehicle models are used. Bridge responses are predicted using a convolution method by combining bridge modal properties with vehicle wheel forces. A linear elastic fracture mechanics model is employed to predict crack growth. For the short-span bridge, the steel-sprung vehicle caused fatigue failure up to 6.5 times faster than the air-sprung vehicle. For the medium-span bridge, the steel-sprung vehicle caused fatigue failure up to 277 times faster than the air-sprung vehicle.