Michael A. Ritter

Evaluation and Field Load Testing of Timber Railroad Bridge

Several spans of a 60-year-old open-deck timber railroad bridge on the Southern Pacific Railroad Line (now the Union Pacific) in Southwest Texas were field tested. The tests were conducted with the sponsorship and cooperation of the Association of American Railroads to determine the vertical live load distribution characteristics of the superstructure. The bridge was originally constructed with Douglas-fir larch solid sawn stringers but was rehabilitated on several occasions to allow comparisons to be made with respect to different rehabilitation options, including the use of a helper stringer and the use of glued laminated timber (glulam) stringers. The test spans measured approximately 4.1 m (13.5 ft) center-to-center of supports and included two closely “packed” chords, each consisting of four timber stringers (one test span included an additional helper stringer added to one chord). One chord was made up of glulam timber and the other was made up of solid sawn timber. The bridge superstructure was generally in satisfactory condition, with some stringer horizontal splitting noted over the bents. The bents were in reasonably good condition, but chord bearing was uneven on bent caps. Static and dynamic deflection load test data were obtained using a special test train. The test results indicate that the glulam chord performed better than the older sawn stringer chord, even when a helper stringer was added. Individual stringers within a chord did not always share the load equally.

TWO TEST LEVEL 4 BRIDGE RAILING AND TRANSITION SYSTEMS FOR TRANSVERSE TIMBER DECK BRIDGES

The Midwest Roadside Safety Facility, in cooperation with the Forest Products Laboratory, which is part of the U.S. Department of Agriculture’s Forest Service, and FHWA, designed two bridge railing and approach guardrail transition systems for use on bridges with transverse glue-laminated timber decks. The bridge railing and transition systems were developed and crash tested for use on higher-service-level roadways and evaluated according to the Test Level 4 safety performance criteria presented in NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features. The first railing system was constructed with glulam timber components, whereas the second railing system was configured with steel hardware. Eight full-scale crash tests were performed, and the bridge railing and transition systems were acceptable according to current safety standards.

Field Performance of Stress-Laminated Timber Bridges

Stress-laminated timber bridges were introduced into the United States in the late 1980s. Since then this concept has been used for more than 500 bridges constructed on roadways throughout the United States. To evaluate the performance of these bridges, the Forest Products Laboratory of the U.S. Department of Agriculture Forest Service and FHWA initiated a nationwide bridge-monitoring program. A field evaluation and the results obtained through that monitoring program for stress-laminated bridges that have been continuously monitored for 2 or more years after installation are presented. Included are discussions related to bridge design and construction, moisture content, stressing bar force, vertical creep, load test behavior, and condition evaluation. On the basis of the monitoring program results, the performance of stress-laminated timber bridges is generally satisfactory, although there are areas in which performance can be improved.

Wood in Transportation Program: An Overview

Research and demonstration bridge projects to further develop wood for transportation structures increased substantially in the United States in 1988 under a legislative action by the U.S. Congress known as the Timber Bridge Initiative. This program, renamed the Wood in Transportation Program, continues today and is administered by the Forest Service. FHWA became involved in timber bridge research in 1990. The FHWA program increased substantially under the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA). From 1992 to 1997, ISTEA authorized significant funding for timber bridge research, technology transfer, and demonstration bridges. The current transportation authorization, the Transportation Equity Act for the 21st century, does not contain a program for timber bridges similar to that of ISTEA; however, there are provisions under the fiscal year 1999 Transportation Appropriations Bill for advancing engineered wood and composites technology through research and demonstration projects to further develop the use of wood for transportation structures. As a result of these combined efforts, a large number of research projects have been initiated, and a number of demonstration bridges have been built under both programs. An overview of the research and the demonstration timber bridge programs is provided here.

Railing Systems for Use on Timber Deck Bridges

Bridge railing systems in the United States have historically been designed based on static load criteria given in the AASHTO Standard Specifications for Highway Bridges. In the past decade, full-scale vehicle crash testing has been recognized as a more appropriate and reliable method of evaluating bridge railing acceptability. In 1989, AASHTO published the Guide Specifications for Bridge Railings, which gave the recommendations and procedures to evaluate bridge rails by full-scale vehicle crash testing. In 1993, the National Cooperative Highway Research Program (NCHRP) published Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features, which provided criteria for evaluating longitudinal barriers. Based on these specifications, a cooperative research program was initiated between the University of Nebraska-Lincoln and the Forest Products Laboratory, and later the FHWA, to develop and crash test 11 bridge rails for wood deck bridges. The research that resulted in successful development and testing of 11 bridge railing systems for longitudinally and transversely laminated wood bridge decks in accordance with AASHTO Performance Level 1 and 2 (PL-1 and PL-2) requirements and Test Levels 1, 2, and 4 (TL-1, TL-2, and TL-4) requirements of NCHRP Report 350 are described here.

Development of Two Test Level 2 Bridge Railings and Transitions for Use on Transverse Glue-Laminated Deck Bridges

The Midwest Roadside Safety Facility, in cooperation with the United States Department of Agriculture Forest Service Forest Products Laboratory and FHWA, designed two bridge railing and approach guardrail transition systems for use on transverse glue-laminated timber deck bridges. The bridge railing and transition systems were developed and crash tested for use on medium-service roadways and evaluated according to the Test Level 2 safety performance criteria provided in NCHRP Report 350. The first railing system was constructed by using steel hardware, whereas the second railing system was built by using glulam timber components. Four full-scale crash tests were performed, and the bridge railing and transition systems were determined to be acceptable according to the current safety standards in NCHRP Report 350.

Plans for crash-tested bridge railings for longitudinal wood decks on low-volume roads

The plans for crashworthy bridge railings for low-volume roads were developed through a cooperative research program involving the USDA Forest Service, Forest Products Laboratory (FPL); the Midwest Roadside Safety Facility, University of Nebraska-Lincoln (MwRSF); and the Forest Service, National Forest System, Engineering. Three railings were developed and successfully tested in accordance with National Cooperative Highway Research Program (NCHRP) Report 350 Test Level-1 requirements. The fourth system was developed for a lower test level based on criteria developed by the Forest Service for single-lane bridges on very low-volume roads. For the convenience of the user, full drawing sets are provided in customary U.S. and S.I. units.

Timber bridge initiatives in the United States

Timber has been used for bridge construction in the United States for hundreds of years. Despite the proven suitability of timber for short- and medium- span bridges, its use as a bridge material has declined in the 20th century. This has been due in part to a lag in the development of new timber bridge technology compared to other materials, and an unfamiliarity with timber on the part of many bridge designers and contractors. Since 1988, two legislative acts have been passed by the US Congress to establish national programs aimed at improving timber utilization in transportation structures. This paper briefly describes these timber bridge initiatives and summarizes activities in the areas of demonstration bridges, research and development, and technology transfer.