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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 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.

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.

Traffic signs and signals are often supported by flexible cantilevered structures that are susceptible to wind-induced vibration and fatigue. The latest version of the design specifications published by the American Association of State Transportation Officials (AASHTO) now considers fatigue as a limit state. However, most of the fatigue classifications for welded details were not based on full-scale testing, and are thought to be overly conservative. This research will address the fatigue resistance of two common mast arm-to-pole connections used in the state of Minnesota. The resistance attained experimentally aligned with current predictions using AASHTO procedures, except for in-plane loading of box connection details. As a consequence of specimen design, a variety of tube-totransverse plate connections were also tested using multi-sided tube cross-sections with different tube diameters, tube thicknesses, as well as base plate thicknesses. The standard tube-to-transverse plate connection exhibited Category K2 resistance, two categories lower than the E' specified by AASHTO. This resistance was enhanced to Category E' through impact treatment or Category E by doubling the base plate thickness. Gusset plates could not prevent cracking of the tube at the base plate, but the tips of the gusset plate exhibited Category E resistance.

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

An appendix to report 2003-16, "Effects of Increasing Truck Weight on Steel and Prestressed Bridges." Chapter A1 - Introduction Chapter A2- Background Chapter A3 - Field Testing and Analytical Modeling Chapter A4 - Effect of Increasing Truck Weight on Tested Bridges Chapter A5 - Effect of Increasing Truck Weight on Other Minnesota Bridges Chapter A6 - Summary and Conclusions

An appendix to Report 2003-16, "Effects of Increasing Truck Weight on Steel and Prestressed Bridges." Chapter B1 - Introduction Chapter B2 - Background Chapter B3 - Field Testing and Analytical Modeling Chapter B4 - Effect of Increasing Truck Weight on Steel Bridges Chapter B5 - Summary and Conclusions

An appendix to Report 2003-16, "Effects of Increasing Truck Weight on Steel and Prestressed Bridges." Chapter C1 - Introduction and Research Approach Chapter C2 - Background Chapter C3 - Field Testing Chapter C4 - Fatigue in Bridge Decks Chapter C5 - Overstressing in Bridge Decks Chapter C6 - Extension of the Results to the Bridge System Chapter C7 - Summary, Conclusions and Recommendations