Fatigue behavior of full-size soild-sawn timber railroad stringers

Abstract

As part of the Timber Bridge Life Extension Program sponsored by the Association of American Railroads (AAR), the objective of this thesis is to investigate the fatigue behavior of large solid-sawn timber beams. Specifically, the main flexural components, or stringers, from timber railroad bridges are examined. Field observations indicate that severe horizontal cracking develops in these members after many years of service under high-tonnage operations. It is believed that the presence of large horizontal cracks prior to service are responsible for a more rapid fatigue strength deterioration of these members, and predisposes the stringers to fail considerably before their expected design life. The focus of the investigation is to determine the causes of horizontal cracks that are present in the stringers prior to being placed in service. Special attention is given to large horizontal cracks that form preferentially in critical shear locations, which have proven to significantly reduce the overall strength and performance of solid-sawn stringers when subjected to fatigue loading environments. In addition to these objectives, the data obtained from the tests is used to develop preliminary unit shear stress design values to be incorporated in the Manual for Railway Engineering published by the American Railway Engineering and Maintenance of Way Association (AREMA). Results from this study indicate that the presence of pre-test horizontal cracks reduces the fatigue resistance of the solid-sawn timber stringers. The relative size, location, depth, and orientation of these cracks has a direct influence on the failure mode and strength of wood. Pre-test cracks, specifically dominant horizontal cracks that appear in critical locations in regards to shear strength, are directly responsible for the rapid deterioration of fatigue resistance in the solid-sawn timbers.