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Minnesota has over 2,000 bridges that contain structural timber in the superstructure or the substructure. Historically, inspections for timber bridges have been mostly limited to visual inspection, hammer sounding and probing. These techniques have proven appropriate for advanced decay detection, but are inadequate for early stage or internal deterioration. During this project, new advanced inspection techniques and equipment were identified that were capable of improving the quality of timber bridge inspection. This equipment and technologies were introduced into routine bridge inspections through the development of standard inspection protocols, integration of the results into bridge data management software, development of a customized inspection manual, outreach training for MnDOT districts and state counties, recommendation of equipment purchases, and completion of an economic assessment on the use of advanced inspection techniques. Implementation of these inspection techniques will support the long-term service life of Minnesota's timber bridges and will improve the safety and reliability of Minnesota's bridges.

Current timber bridge inspection procedures used in Minnesota and across the United States are mostly limited to visual inspection of the wood components. Use of advanced techniques like stress wave timing, moisture meters, resistance drills will significantly improve the reliability of the inspections but these inspection techniques are time consuming. The objective of this project was to conduct vibration testing of dowel laminated timber bridge systems to better understand the potential for using vibration testing to assess the structural health and condition of bridges in Minnesota. A second key objective was to improve and automate the vibration testing system that is currently being used. This research showed that the forced vibration system developed is an effective tool for conducting forced vibration tests of timber bridges and that there is a noted increase in frequency during each successive stage of construction. A reliable means for assessing the peak frequencies and an identification of the mode still needs to be developed for this system to use the vibration response to predict the EI product for use in load ratings. Each bridge has a unique set of vibration characteristics that were identified using the automated system. These characteristics showed peaks in amplitude as the frequency of the vibration was increased from 0 - 35 Hz during testing. It is believed that monitoring of the characteristic vibration response for each bridge would be a means of identifying changes in structural health over time due to wood decay, accidents, vandalism, or lack of maintenance. Note: The phase I report is available at https://hdl.handle.net/11299/96712.

Modern timber bridges have shown that timber is a durable option for primary structural members in highway bridges and can perform satisfactorily for 50 years or longer when properly designed; fabricated and maintained. However; various cost assumptions have indicated that timber bridges are more expensive than concrete bridges. This project was undertaken to better understand the benefits and costs of using timber bridges as a viable substitute for other bridge construction materials and designs. Two demonstration construction projects were completed to develop comparative information. A steel girder with a transverse glulam deck bridge with a curbless; crash-tested railing system was built; and a spike-laminated longitudinal deck bridge was constructed. Both projects were completed and allowed for a good comparison to be developed both in terms of project-specific cost and the time required for bridge construction completion. These projects showed that the main advantage of a timber bridge is the speed of superstructure construction with the other costs similar to that of other materials. It is clear from previous case studies; interviews with engineers; contractors; and suppliers; and the projects that timber superstructures can be installed within days to weeks; compared to months for other materials.