Bridges

It is not often well enough noted when research techniques and practices cross over into real- world problem solving, bringing purpose and practice together in a pragmatic fashion. Research and practice do rely on each other for informative construction techniques and real- world application, whether successful or not, but they are separated by the bulk of time required for research to be verified and implemented and the necessity to practice road construction and repair in a concurrent manner. Recently though, a unique case involving a bridge, some fresh-poured concrete and a petulant flash- flood helped to spectacularly showcase how ongoing research projects, such as MnROAD, can help to effectively, practically and pragmatically solve situational crisis around the state of Minnesota.

Appendix A.3 referenced in TRS 1508, State of the Practice for Managing, Maintaining and Operating Culverts

The MnDOT Bridge Office sought out information on best practices regarding the use of drainage systems on bridges. Minnesota regulations strongly discourage the discharge of runoff directly into waterways; however, bridge drainage systems can lead to accelerated deterioration in bridge elements. MnDOT requested a synthesis to gather information to assist them in future development of guidance on design, detailing, specifications, construction, and maintenance procedures for bridge needs, focusing specifically on agencies with similar northern climates to the greatest extent possible.

The Minnesota Department of Transportation (Mn/DOT) conducted a comprehensive research program in response to the problem of bridge deck deterioration. This program placed emphasis on protecting newer decks as well as repairing damaged ones. Two basic approaches were applied to protecting bridge decks: 1. Prevent the penetration of chloride ions to the rebars by using special concrete overlays, deck sealers and waterproof membranes. 2. Use galvanized or epoxy coated rebars to protect the steel after chloride ions have contaminated the surrounding concrete. Bridge decks were constructed or repaired using the systems mentioned above. The decks covered by this study were built or repaired from 1974 to 1978 and were tested annually through 1981. Testing consisted of visual observations, electrical potential corrosion measurements, measuring depth of concrete cover over rebars, delamination detection and determining chloride ion content.

This study was conducted to develop a rational method for selecting the most appropriate type of traffic control at construction sites involving the closing of a one-lane bridge. The study consisted of a literature review, a survey of practices in other states, the observation of a number of actual construction sites, the collection and analysis of data from these construction sites and finally, the development of methods for estimating the capacity and measuring the effectiveness of various types of traffic control. As a result, a method for selecting an appropriate type of traffic was developed. This method takes into account variables such as sight distance, bridge length, traffic volumes and capacity, consideration of the costs and benefits of various types of control and an analysis of the geometric features of the geometric features of the approaches to the bridge.

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 project details the development and evaluation of an acoustic emission (AE) system for monitoring large scale structures, both in the lab and in the field. The system consists of acoustic emission sensors, preamplifiers, filers, an AE monitor, and a digital oscilloscope. The system has been applied successfully to both steel and concrete structures and used to detect brittle fracture and low-cycle fatigue failures in welded steel joints and crack propagation in cover-plated rolled bridge girders, in the field and in the laboratory. The AE system detected initial cracking during the flexural crack testing of two high-strength concrete prestressed bridge girders. The acoustic emission monitoring of bond tests also provided insight into the behavior of the bond between glass fiber reinforced polymer rebar and concrete.

This study sought to determine the dominant parameters that lead to premature transverse cracking in bridge decks and to make recommendations that help reduce cracking tendency in bridge decks. The project includes two main parts: a field study and a parametric study. The field study identified 72 bridges in the Minneapolis/St. Paul area and explored the correlation between the observed cracking of those bridges and available design, material, and construction-related data. The parametric study investigated the relative influence of the factors that affect transverse deck cracking through a controlled nonlinear analysis study. Variables included: shrinkage, end restraint, girder stiffness, supplemental steel bar cutoff, cross frames, splices, deck concrete modulus of elasticity, and temperature history. In addition, four bridges from the companion field study were modeled to compare the analytical results with the actual crack patterns. Based on these results and correlation with other research, the study identified the following dominant factors affecting transfer cracking: shrinkage, longitudinal restraint, deck thickness, top transverse bar size, cement content, aggregate type and quantity, air content, and ambient air temperature at deck placement. Recommended practical improvements to bridge deck construction, in order of importance, include: using additives to reduce shrinkage of the deck concrete, using better curing practices, and minimizing continuity over interior spans.

Timber in nail-laminated and stress-laminated bridges is often installed with moisture contents (MC) near the fiber saturation point. Post-installation moisture loss induces shrinkage in the timber components, which results in loosening of component fasteners. This research project sought to establish the equilibrium moisture content (EMC) of timber bridges in Minnesota. Researchers took seasonal MC measurements on six nail-laminated timber bridges to determine annual MC variations and moisture gradients in individual bridge components: three bridges from northern Minnesota in St. Louis County and three from southern Minnesota in Sibley County. An electrical resistance meter measured moisture content, with oven-dry and toluene distillation methods of MC determination as controls. The study found the average MC of bridge components in St. Louis County was 2 percent-11 percent higher than bridge components in Sibley. The study determined the average MC at a three-inch depth on three of the major bridge components as: • deck laminations 18 percent (Sibley) to 28 percent (St. Louis) • transverse stiffener beams 14 percent (Sibley) to 18 percent (St. Louis) • deck supports 17 percent (Sibley) to 27 percent (SI. Louis) . The results indicate that the regional microclimate may greatly affect MC. Results from this research will allow MC specifications to be determined before bridge installation, helping minimize post-installation moisture-related problems and optimize design calculations. In addition, results will provide necessary data for ongoing research on transverse load-sharing characteristics of longitudinally nail-laminated timber bridges. Finally, this information will provide a basis for inspecting MC in timber 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).