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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.

There are many wooden bridges in the United States. Their decks are often built of timber beams nailed together and covered with asphalt. The asphalt plays a mechanical role, and it provides environmental protection for the wood deck. The asphalt layer deteriorates and requires replacement. That leads to a faster deterioration of the deck, increased maintenance, and shorter bridge life. The flexibility of the deck is a probable cause of the fast deterioration of the asphalt. Low temperatures lead to deformations of the deck and may lead to cracks, which are propagated by mechanical and environmental effects. This project investigates stiffening the bridge deck by connecting a beam perpendicularly to the deck planks with metal bolts to reduce deformations of the deck. The additional beam is called a Transverse Stiffener Beam (or TSB). It can be incorporated as a part of new bridges or be attached to existing bridges. The investigations show the TSB significantly reduces deformations of the deck in most cases. The study indicates the positive effects of the TSB's should be expected in other applications. The magnitude of the effects can be analyzed with the computer program developed during this project.

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 research project resulted in a new, accurate way to assess fatigue cracking on Bridge 9340 on I-35, which crosses the Mississippi River near downtown Minneapolis. The research involved installation on both the main trusses and the floor truss to measure the live-load stress ranges. Researchers monitored the strain gages while trucks with known axle weights crossed the bridge under normal traffic. Researchers then developed two- and three-dimensional finite-element models of the bridge, and used the models to calculate the stress ranges throughout the deck truss. The bridge's deck truss has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system. The research helped determine that the fatigue cracking of the deck truss is not likely, which means that the bridge should not have any problems with fatigue cracking in the foreseeable future. As a result, the Minnesota Department of Transportation (Mn/DOT) does not need to prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project. The research also has implications for other bridges. The project verified that the use of strain gages at key locations combined with detailed analysis help predict the bridge's behavior. In addition, the instrumentation plan can be used in other similar bridges.

A 143-m (470-foot) span steel truss bridge, the Wabasha County Bridge crosses the Mississippi River at Wabasha, Minn. In November 1996, the Minnesota Department of Transportation (Mn/DOT) implemented a retrofit strategy to mitigate perceptible vibrations in several truss members at moderate and strong wind gusts. In this strategy, Mn/DOT installed a "central cord" of tubular members, halfway between top and bottom cords, to reduce the effective length of the truss members, thereby increasing the natural frequencies of vibration and reducing the amplitude of vibration and the associated strains. This report documents the monitoring and assessment program used to investigate the dynamic response and efficacy of the retrofit strategy for the Wabasha Country Bridge. Researchers determined amplitudes and frequencies of the vibration for the longest diagonal member. The measured frequencies are larger than those estimated before the retrofit and have resulted in reduced strains and displacements from vibration. Maximum strain levels at the quarter point of the member are estimated to be small after the retrofit, with peak values corresponding to 8.6 MPa (1.2 ksi).

A vertical-lift bridge, the Stillwater Bridge, Bridge 4654, opened in 1931 across the St. Croix River between Minnesota and Wisconsin. To assess the remaining fatigue life of this bridge, strain gages were installed on an interior floorbeam and on a tension chord of a typical through truss span. The maximum stress range was 32 MPa at the centerline of the floorbeam. The measured data were rationalized by performing an analysis of the floor system and truss. The greatest ratio of the maximum expected stress range (18 MPa) to the fatigue strength (31 MPa) is at the centerline of the severely floorbeams located at the ends of the spans. Therefore, fatigue cracking is not expected in the steel members of a typical truss and taking the trucks off the bridge will have no significant effect on the fatigue life of the steel members in a typical through truss span. As a staff paper, this publication is intended for internal use by the Minnesota Department of Transportation (Mn/DOT). Distribution is limited

Over time; the southbound exterior girder ends on each side of Pier 4 and Pier 26 of Bridge 27568 suffered significant corrosion damage that exposed transverse reinforcement; prestressing strands in the exterior side of the bottom flange and the sole plate anchorages. The girder ends were repaired in 2013 by encasing supplementary steel reinforcement in shotcrete over a 4 ft. length of the girder. The two repaired girders and two companion girders; removed when the bridge was replaced in 2017; were brought to the University of Minnesota and tested to failure in shear to determine the effectiveness of the repair. The laboratory testing showed that the repair was able to return the girders with significant corrosion damage to the strength of the companion girders; indicating that the repair was effective.