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.