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This report presents results from a laboratory study and field implementation of acoustic emission monitoring of fatigue cracks in cover-plated steel bridge girders. The acoustic monitoring successfully detected growing fatigue cracks in the lab when using both source location and a state of stress criteria. Application of this methodology on three field bridges also proved successful by detecting a propagating crack in two of the bridges and an extinguished crack in a third bridge. Researchers tested a double angle retrofit, designed by the Minnesota Department of Transportation, both in the lab and in the field of girder with fatigue cracks in the top flange. This retrofit does not require removal of concrete deck, and only involves bolting the retrofit to the bridge girder web. The double angle retrofit applied to laboratory test girder resulted in a reduction of flange stresses by 42 percent. Field implementation of the retrofit had mixed success. On one bridge, stress ranges in the cracked flange was reduced by 43 percent. However, on a second test bridge, the reduction was only 8 percent, likely due to the inadequate space for proper installation of the retrofit.

This document serves as a summary of the 1987 Minnesota State Bicycle Transportation System Plan.

Fatigue cracking often occurs in composite bridges with unstiffened girder web gaps at the ends of transverse stiffeners. In this project, researchers sought to better understand bridge diaphragm deflection behavior and advance the ability to estimate web gap distortional stress. Trends from the parametric studies led to general observations that may assist in identifying fatigue-prone bridges. Variables that influence diaphragm deflection behavior include girder spacing, bridge skew, span length, and deck thickness. Transverse load distribution properties appear to play the most significant role in determining the magnitude of differential deflection. Parameter study stress trends indicate that out-of-place distortional stress in fatigue prone web gaps primarily depends on web gap properties, bridge span length, and angle of skew. Differential deflection and web gap dimension trends apply to a varied spectrum of bridge configurations. The research resulted in a method to assess bridge girder differential deflection and distortional stress in actual steel bridges without complex analysis and modeling. Proposed procedures for evaluating out-of-plane stress should prove practical and aid in screening, identifying, and assessing bridges vulnerable to distortion-induced fatigue cracking.

Their initial curvature make steel-cured girder bridges more susceptible to lateral-torsional bucking during construction. Critical in assessing the strength and fatigue life of the bridge components, predicting stresses in the main girders and the crossframes proves more complex than in straight bridges. In this project, researchers investigated the correlation between measured and computed results in a two-span, four-girder, continuous composite steel curved girder bridge with skew supports. A previous phase involved computing the stresses through a linear elastic grillage finite element computer project and comparing the results with a typical third-party curbed girder analysis program. The project's second phase further investigated the correlation between measured and computed stresses by running two additional live load tests on the bridge. This report summarizes research to investigate the behavior of the curved girder bridge system through all phases of construction, as well as to a series of live load field tests. In addition, researchers investigated the effects of change in temperature on the bridge behavior and tracked any changes in behavior of the bridge system over time and under service load conditions.

This report summarizes the findings of a project with the following goals: to implement a field instrumentation and monitoring program for a typical multi-girder steel bridge on skew supports that may be susceptible to web-gap distortion; to assess the frequency and magnitude of the distortional fatigue stresses at the web-stiffener connections; and to evaluate the impact of these stresses on fatigue life. Measurements from 12 independent strain gauges were continuously monitored and recorded for more than three months on Minnesota Department of Transportation (Mn/DOT) bridge #27734. Truck loading tests also were conducted. Predicted web-gap fatigue life based on the long-term monitoring data from Mn/DOT bridge #27734 ranges from 45 to 75 years. Comparison of web-gap stresses with primary design stresses reveals that web-gap distortional stresses are comparatively high. The report also highlights a detailed finite element study to better understand the web-gap stress mechanism and to compare experimental results with theoretical predictions. Study results have important implications for investigators of distortion-induced web-gap fatigue. They indicate that the actual stress at the so-called hotspot may be as much as twice the stress measured at the strain gauge. The report includes a method for estimating girder deflections and web-gap stress.

Any increase in legal truck weight would shorten the time for repair or replacement of many bridges. Five steel girder bridges and three prestressed concrete I-girder bridges were instrumented, load tested, and modeled. The results were used to assess the effects of a 10 or 20% increase in truck weight on bridges on a few key routes through the state. Essentially all prestressed girders, modern steel girders, and most bridge decks could tolerate a 20% increase in truck weight with no reduction in life. Unfortunately, most Minnesota steel girder bridges were designed before fatigue-design specifications were improved in the 1970's and 1980's. Typically, an increase in truck weight of 20% would lead to a reduction in the remaining life in these older steel bridges of up to 42% (a 10% increase would lead to a 25% reduction in fatigue life). Bridge decks are affected by axle weights rather than overall truck weights. Transverse cracks in bridge decks are primarily caused by shrinkage soon after construction and are not affected by increasing axle weight. However, decks with thickness less than 9 inches or with girder spacing greater than 10 ft may be susceptible to longitudinal flexural cracking which could decrease life.

Steel pier caps designed such that the longitudinal girders are continuous through the pier cap are subject to significant torsion due to differences in the girder end moments and may be susceptible to fatigue cracking. One such pier cap, part of Bridge 69832 on northbound Interstate 35 heading into the business district of Duluth, was instrumented, load tested, and modeled. Several similar pier caps had developed fatigue cracking at different details. The cracks are due to high stress ranges that occur in the corners of the box section. None of these cracks are presently a threat to the structural integrity of the pier caps. Most of the cracks are limited to the welds and will eventually arrest as they grow larger with minimal structural consequences. Therefore, the recommendation for these cracks is to inspect them carefully every two years and not repair them. However, holes must be drilled at least at one location where the cracks are presently in the web plates of the pier caps. Recommendations are presented for inspection of similar integral pier caps and for design of new steel pier caps.

This report investigates a method of repairing fatigued steel bridge girders using carbon fiber reinforced polymer (CFRP) strips. This type of repair would be used to prevent the propagation of cracks which could lead to failure of the bridge girders. The main advantage of using CFRP is it is lightweight and durable, resulting in ease of handling and maintenance. Therefore, it would not require the closing of traffic on the bridge during rehabilitation. Effective bond length was determined by a series of experimental tests with actual materials, as well as through the use of analytical equations. Finally, tests were conducted on full-scale cracked girders; the application of the CFRP strips to the steel girders resulted in significant strain reduction, except in the case of small cracks where it was difficult to clearly identify the benefits.

Distortion-induced fatigue cracking in unstiffened web gaps is common in steel bridges. Previous research by the Minnesota Department of Transportation (Mn/DOT) developed methods to predict the peak web gap stress and maximum differential deflection based upon field data and finite element analyses from two skew supported steel bridges with staggered bent-plate and cross-brace diaphragms, respectively. This project aimed to test the applicability of the proposed methods to a varied spectrum of bridges in the Mn/DOT inventory. An entire bridge model (macro-model) and a model encompassing a portion of the bridge surrounding the diaphragm (micro-model) were calibrated for two instrumented bridges. Dual-level analyses using the macro- and micro-models were performed to account for the uncertainties of boundary conditions. Parameter studies were conducted on the prototypical variations of the bridge models to define the sensitivity of diaphragm stress responses to typical diaphragm and bridge details. Based on these studies, the coefficient in the web gap stress formula was calibrated and a linear prediction of the coefficient was proposed for bridges with different span lengths. Additionally, the prediction of differential deflection was calibrated to include the influence of cross-brace diaphragms, truck loading configurations and additional sidewalk railings. A simple approximation was also proposed for the influence of web gap lateral deflection on web gap stress.

Prior to 1985, it was common practice to avoid welding the connection plates to the tension flange of the girders of steel bridges. However, extensive fatigue cracking has developed in the unstiffened web gaps because of out-ofplane distortion. A new retrofit option was investigated that uses a room-temperature-cured two-part epoxy (3M Adhesive DP460-NS) to join a small length of 3/4-inch thick steel angle to the tension flange and the connection plate. A field test on two skewed bridges showed that the adhesive-angle retrofit system decreased the out-of-plane strain range by 40 to 50% when the original strain range was more than 50 microstrains. The ten adhesive-angle retrofits remained in place and were in good condition after three and a half years, suggesting that the chosen adhesive had good environmental durability. A laboratory large-scale specimen test with 8 web gaps showed that the retrofit system stopped or retarded most cracks even without stop holes when the measured out-of-plane strains were approximately 600 microstrains. Coupon tests conducted to investigate the environmental durability of the chosen adhesive showed that the chosen adhesive is suitable for applications at room or low temperature, even with high relative humidity.