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Increased use of timber bridges in the U.S. transportation system has required additional research to improve the current design methodology of these bridges. For this reason, the U.S. Forest Service, Forest Products Laboratory (FPL), and the Federal Highway Administration have supported several research programs to attain the objective listed above. This report is a result of a study sponsored by the FPL, with the objective of determining how highway truckloads are distributed to girders of a glued-laminated timber bridge. The American Association of State Highway and Transportation Official (AASHTO) load and resistance factor design (LRFD) Bridge Design Specification provides live-load distribution provisions for glued-laminated girder timber bridges that were used in previous AASHTO Specifications. The AASHTO live-load distribution provisions were reviewed in this report. Field-test results were used to review the current AASHTO LRFD glued-laminated timber girder bridge-design specifications and to validate analytical results obtained by finite-element analyses. With the validated analytical models, parametric studies were performed to determine the worst-case live-load distribution factors that can be used to calculate the design moment and shear for glued-laminated timber girders. Simplified live-load distribution equations that can be used to determine these distribution factors were developed and are provided in this report. These equations take into account how load is distributed to the bridge girders, considering the effects of span length, girder spacing, and clear width of the bridge.
Increased use of timber bridges in the U.S. transportation system has required additional research to improve the current design methodology of these bridges. For this reason, the U.S. Forest Service, Forest Products Laboratory (FPL), and the Federal Highway Administration have supported several research programs to attain the objective listed above. This report is a result of a study sponsored by the FPL, with the objective of determining how highway truckloads are distributed to girders of a glued-laminated timber bridge. The American Association of State Highway and Transportation Official (AASHTO) load and resistance factor design (LRFD) Bridge Design Specification provides live-load distribution provisions for glued-laminated girder timber bridges that were used in previous AASHTO Specifications. The AASHTO live-load distribution provisions were reviewed in this report. Field-test results were used to review the current AASHTO LRFD glued-laminated timber girder bridge-design specifications and to validate analytical results obtained by finite-element analyses. With the validated analytical models, parametric studies were performed to determine the worst-case live-load distribution factors that can be used to calculate the design moment and shear for glued-laminated timber girders. Simplified live-load distribution equations that can be used to determine these distribution factors were developed and are provided in this report. These equations take into account how load is distributed to the bridge girders, considering the effects of span length, girder spacing, and clear width of the bridge.
In order to promote and increase the use of timber bridges in our nations transportation systems, the United States Department of Agriculture (USDA) and the Forest Products Laboratory funded research to develop design criteria to improve the design of glued-laminated timber bridges. This project is part of this research and is directed towards developing, and/or making recommendations for, acceptable live load deflection criteria, which are based on the actual structural performance of these types of bridges. Specifically, the relationship between live load deflection and the condition of the asphalt wearing surface is of particular interest. To accomplish this, eight glued-laminated timber girder bridges and four longitudinal glued-laminated timber deck bridges were selected for testing. The performance of the bridges was investigated under live loading and analyzed in conjunction with the condition of the wearing surfaces gathered from field inspections. Testing involved loading the structures with fully loaded tandem axle dump trucks and gathering global and differential deflection data. Field tests revealed that the majority of the asphalt wearing surface deterioration was primarily the result of differential deflections.
These standardized bridge plans are for superstructures consisting of treated timber. Seven superstructure types are included: five longitudinal and two transverse deck systems. Both HS20 and HS25 loadings are included, along with L/360 and L/500 deflection criteria.
"The provisions of these Specifications are intended for the design, evaluation and rehabilitation of both fixed and movable highway bridges. Mechanical, electrical, and special vehicular and pedestrian safety aspects of movable bridges, however, are not covered. Provisions are not included for bridges used solely for railway, rail transit or public utilities. For bridges not fully covered herein, the provisions of these Specifications mat be applied, as augmented with additional design criteria where required. These specifications are not intended to supplant proper training or the exercise of judgment by the Designer, and state only the minimum requirements necessary to provide for public safety. The Owner or the Designer may require the sophistication of design or the quality of materials and construction to be higher than the minimum requirements. The concept of safety through redundancy and ductility, and protection against scour and collision are emphasized. The design provisions of these Specifications employ the Load and Resistance Factor Design, LRFD, methodology. The factors have been developed from the theory of reliability based upon current statistical knowledge of loads and structural performance. Methods of analysis, other than those included in previous Specifications, and the modelling techniques inherent in them are included, and their use is encouraged. The commentary is not intended to provide a complete historical background concerning the development of these, or previous Specifications, nor is it intended to provide a detailed summary of the studies and research data reviewed in formulating the provisions of the Specification. However, references to some of the research data are provided for those who wish to study the background material in depth. The commentary directs attention to other documents that provide suggestions for carrying out the requirements and intent of these Specifications. However, those documents and this commentary are not intended to be a part of these Specifications."--Page1-1.