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 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.     

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.     

Thermal Effects on Skewed Steel Highway Bridges and Bearing Orientation

An investigation is conducted to characterize and quantify external effects in composite steel highway bridges under thermal loading. Based on the results of a literature review, including thermal and thermoelastic analyses as well as current design code provisions, a simple but realistic thermal loading is developed for winter and summer conditions for AASHTO load and resistance factor design (LRFD) Zone 3. Three cases of bearing orientation, representative of current design practice, are examined. Parametric studies are then conducted. Hypothetical bridges are designed for a range of different span lengths, section depths, widths, and skews. Each bridge model is tested under all three constraint cases and both winter and summer thermal loading. Variations in structural response with each parameter are plotted, and the relative influence of each parameter is discussed. Design equations to predict the observed displacements and restraint forces at the bearings are then developed by a systematic regression procedure. The applicability of these proposed design equations is demonstrated by examples.     

Full-Scale Test of Continuity Diaphragms in Skewed Concrete Bridge Girders

Continuity diaphragms used in prestressed girder bridges on skewed bents have caused difficulties in detailing and construction. The results of the field verification for the effectiveness of continuity diaphragms for skewed, continuous, and prestressed concrete girder bridges are presented. The current design concept and bridge parameters that were considered include skew angle and the ratio of beam spacing to span (aspect ratio). A prestressed concrete bridge with continuity diaphragms and a skewed angle of 48° was selected for full-scale test by a team of engineers from Louisiana Department of Transportation and Development and the Federal Highway Administration. The live load tests performed with a comprehensive instrumentation plan provided a fundamental understanding of the load transfer mechanism through these diaphragms. The findings indicated that the effects of the continuity diaphragms were negligible and they can be eliminated. The superstructure of the bridge could be designed with link slab. Thus, the bridge deck would provide the continuity over the support, improve the riding quality, enhance the structural redundancy, and reduce the expansion joint installation and maintenance costs.     

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.     

Truck Loading and Fatigue Damage Analysis for Girder Bridges Based on Weigh-in-Motion Data

Based on data collected by weigh-in-motion (WIM) measurements, truck traffic is synthesized by type and loading condition. Three-dimensional nonlinear models for the trucks with significant counts are developed from the measured data. Six simply supported multigirder steel bridges with spans ranging from 10.67 m (35 ft) to 42.67 m (140 ft) are analyzed using the proposed method. Road surface roughness is generated as transversely correlated random processes using the autoregressive and moving average model. The dynamic impact factor is taken as the average of 20 simulations of good road roughness. Live-load spectra are obtained by combining static responses with the calculated impact factors. A case study of the normal traffic from a specific site on the interstate highway I-75 is illustrated. Static loading of the heaviest in each truck type is compared with that of the American Association of State Highway and Transportation Officials standard design truck HS20-44. Several important trucks causing fatigue damage are found.     

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.     

Influence of Secondary Elements and Deck Cracking on the Lateral Load Distribution of Steel Girder Bridges

The AASHTO LRFD load distribution factor equation was developed based on elastic finite element analysis considering only primary members, i.e., the effects of secondary elements such as lateral bracing and parapets were not considered. Meanwhile, many bridges have been identified as having significant cracking in the concrete deck. Even though deck cracking is a well-known phenomenon, the significance of pre-existing cracks on the live load distribution has not yet been assessed. The purpose of this research is to investigate the effect of secondary elements and deck cracking on the lateral load distribution of girder bridges. First, secondary elements such as diaphragms and parapets were modeled using the finite element method, and the calculated load distribution factors were compared with the code-specified values. Second, the effects of typical deck cracking and crack types that have a major effect on load distribution were identified through a number of nonlinear finite element analyses. It was established that the presence of secondary elements may produce load distribution factors up to 40% lower than the AASHTO LRFD values. Longitudinal cracking was found to increase the load distribution factor by up to 17% when compared to the LRFD value while the transverse cracking was found to not significantly influence the transverse distribution of moment.     

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.     

Investigation of Large Web Fractures of Welded Steel Plate Girder Bridge

Constructed in 1972 with ASTM A36 (250 MPa) steel, a highway bridge in Maryland is comprised of seven welded steel plate girders of a constant web depth of 2,286 mm (90 in.). In March 2003, the web fractures of two steel girders were discovered in a three-span continuous superstructure unit. A full-height web fracture occurred in an interior girder at a cross frame connection plate; and a partial-height web fracture occurred in an exterior girder at an intermediate transverse stiffener next to a cross frame. The investigation of the girder fractures involved fracture surface examination, material testing, fracture mechanics analysis, and comprehensive finite-element modeling for fracture driving forces. The fracture mechanics analysis indicated that a brittle web fracture could occur at a high stress level with either a surface crack or a through-thickness crack of certain dimensions. Finite-element analysis using a global model and submodels investigated three possible causes: (1) localized distortion of the unsupported web gap due to the lateral forces of cross frame members; (2) fabrication induced out-of-flatness of the web plate under in-plane loading; and (3) residual stresses at the fracture origin area due to the stiffener-to-web welds. The investigation concluded that one or a combination of these can result in the high local tensile stresses triggering a brittle web fracture with certain crack dimensions at the fracture origin area. Several retrofit concepts were investigated for their effectiveness in reducing stresses in the fracture origin area. Bridge inspections in the subsequent 6 years after the web fractures have not reported any other cracks in the bridge.     

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 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.     

Extending the Fatigue Life of Riveted Coped Stringer Connections

Fatigue cracking occurs at the copes of stringer–floorbeam connections of older, riveted steel bridges. Some cracks are quite long and raise serious questions regarding the remaining fatigue life of the subject bridges. Damage limitation methods (DLMs) have been used to increase the fatigue life of these stringers, but the effectiveness of the DLMs for these riveted connections had never been evaluated by tests. Therefore, tests were conducted to evaluate the fatigue life of coped, riveted stringer–floorbeam connections, and the effectiveness of DLMs. Fatigue cracks in the coped stringer–floorbeam connection were initially developed to establish crack initiation requirements and the rate and extent of crack growth. Once a significant crack was noted, one of several DLMs was applied, and the specimens were retested to determine the effectiveness of the DLM in controlling cracking. These DLMs included the drilled hole, the inserted bolt and the removed rivet methods. The relative effectiveness of the methods is described, and a design procedure is proposed for improving their performance.     

Deflection Requirements for Bridges Constructed with Advanced Composite Materials

Many bridges in the United States have reached or are approaching the end of their useful lives. Since the 1940s, salt and other deicing agents applied to highways and bridges, coupled with inadequate maintenance funding, have led to the premature deterioration of many bridges. Growth of the United States economy and population has increased vehicular traffic volume and loads. For these reasons, the need exists for large-scale rehabilitation, strengthening, widening, and replacement of bridges. The financial cost to society to replace these bridges or to rehabilitate them conventionally is staggering. What is needed are low cost, durable methods of strengthening current bridges, extending their lives so that state departments of transportation may spread out the process of eventual replacement. In constructing new bridges, better materials or designs are needed so we may avoid tomorrow the consequences we are experiencing with today’s bridges. Polymer matrix composites offer that potential. A design methodology developed for composite bridges is based on limiting live load deflections. This paper focuses on the establishment of deflection limitations for bridges constructed with advanced composite materials, based on limiting response accelerations produced by the passage of truck traffic.     

Design for Shear in Curved Composite Multiple Steel Box Girder Bridges

Modern highway bridges are often subject to tight geometric restrictions and, in many cases, must be built in curved alignment. These bridges may have a cross section in the form of a multiple steel box girder composite with a concrete deck slab. This type of cross section is one of the most suitable for resisting the torsional, distortional, and warping effects induced by the bridge’s curvature. Current design practice in North America does not specifically deal with shear distribution in horizontally curved composite multiple steel box girder bridges. In this paper an extensive parametric study, using an experimentally calibrated finite-element model, is presented, in which simply supported straight and curved prototype bridges are analyzed to determine their shear distribution characteristics under dead load and under AASHTO live loadings. The parameters considered in this study are span length, number of steel boxes, number of traffic lanes, bridge aspect ratio, degree of curvature, and number and stiffness of cross bracings and of top-chord systems. Results from tests on five box girder bridge models verify the finite-element model. Based on the results from the parametric study simple empirical formulas for maximum shears (reactions) are developed that are suitable for the design office. A comparison is made with AASHTO and CHBDC formulas for straight bridges. An illustrative example of the design is presented.     

Fatigue Response of New Graphite/Epoxy-Concrete Girder

A new graphite/epoxy/concrete (G/E/C) cross section was developed and tested under fatigue loading with constant amplitude for one million cycles. The cross section consisted of a G/E box element, a G/E channel element, a concrete slab, and a concrete box formed by the walls of the G/E elements. Epoxy resin and vertical steel stirrups provided shear connection between G/E elements and the concrete slab. The results showed that the epoxy interface slipped after 150,000 cycles of fatigue loading. Softening of the girder continued for another 350,000 cycles of loading, after which the stiffness and strains stabilized. The failure testing of the girder after fatigue loading showed that the load and displacement capacities were only moderately reduced by fatigue loading.     

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.     

Collapse of Steel Bridges

The study concerns bridge collapses focusing on metal structures. It is based on literature and news research, due to the lack of extensive compendiums of this unpleasing but important topic. At first, a short overview of the occidental history of metal bridges is given presenting the historic context for the described incidents. It is followed by a classification of the most common causes of bridge failure, which include structural and design deficiencies, corrosion, construction and supervision mistakes, accidental overload and impact, scour, lack of maintenance or inspection, and force majeure. Some significant historic examples are described. Changes and investigations initiated by the described cases are also mentioned. The work concludes that without the disaster that represents each bridge collapse, we would have neither the structural behavior knowledge nor the relatively high safety of today.     

Stability Bracing Requirements for Steel Bridge Girders with Skewed Supports

Cross frames and diaphragms are critical elements for the stability of I-shaped steel bridge girders during construction. The AASHTO specifications are relatively vague with regards to the stability design requirements of the braces. Spacing limits that have been used in past AASHTO specifications have been removed from the Load and Resistance Factor Design Specification, which instead requires the bracing to be designed by a rational analysis. Whereas the AASHTO specification does not define what constitutes a rational analysis, stability bracing systems must possess adequate stiffness and strength. The commercially available software packages that are typically used in bridge design generally do not have the capabilities to determine the adequacy of the bracing from a stability perspective. This paper outlines the stability bracing requirements for bridges with normal and skewed supports. The effects of support skew on the stiffness and strength requirements for stability bracing are addressed. Solutions that are available for systems with normal supports were modified to account for the effects of the support skew angle. Two orientations of the intermediate bracing were considered: parallel to the skew angles and perpendicular to the longitudinal girder axis. The solutions are presented and compared with finite-element results. The design solutions have good agreement with the finite-element solutions.