While timber is ubiquitous as a building material, it is commonly associated with lightweight residential construction. However, in a number of areas of the United States and in a number of applications, timber construction is an important part of the infrastructure. Bridges make an interesting and important point in this regard. In the National Bridge Inventory (NBI), 41,743 timber bridges are included in the inventory with an additional 42,102 steel bridges with timber decks. The number of timber bridges also is a significant percentage of the total bridge inventory in many states. Colorado, Nebraska, Montana, and North Dakota are the four western states with 20 percent or more of the bridge inventory built with timber main spans. In the southern portion of the country, Louisiana is notable with 42.1 percent of the bridge inventory built of timber with Arkansas, Alabama and Mississippi with 20 percent to 35 percent of the bridge inventory with timber main spans.1
Other important applications of timber are in the utility and railroads where much of the infrastructure uses timber. Telephone poles are a familiar application of large timber sections and represent a significant installed investment base in a critical application. Similarly timber railroad bridges are common throughout the country for most of the short and many of the longer bridges. Timber bridges are used on all types of rail lines including important main transcontinental rail lines. The large number of these structures makes it critical that the limited maintenance funds available are targeted to structures in greatest need of repair. In both cases many of the installations are between 50 and 100 years old, and are still necessary for daily operation. Replacement of the structures with concrete or steel is not cost effective and repair is becoming increasingly difficult because of limited availability of the large timber sections required for these structures. Repair or replacement-for-cause provides a means with which to maintain safety and reliability of the infrastructure while the railroads and utilities compete in an unregulated environment.2
Of the number of total number of timber bridges, more than 47 percent are classified as structurally deficient in the NBI. Based on the methodology of the NBI, deficiency of these bridges is based on visual inspection and may overstate the significance of visual defects. In spite of the reduced load ratings of many of these bridges, the rural locations of them increase the likelihood that overloaded trucks use the bridges on a regular basis. Thus, the lack of failures may indicate excessive conservatism in the evaluation of some of the structures.
While timber is an uncommon material for critical highway bridge structures, rail traffic depends on a large number of long and short timber bridges. Some of the bridges are nearly 100 years old. They have been in continuous use although they typically have been either partially or extensively rebuilt at periodic intervals. On spur lines, maintenance has been less consistent with the result that equipment and personnel may be at risk. Recent changes in the business environment of the railroads have resulted in operational changes, which significantly affect the use of timber railroad bridges. These changes have made evaluation of the safety and reliability of timber rail structures critical to the efficient operation of railroads. A similar situation also exists outside the United States, with raised inspection concerns regarding older timber railroad bridges.3
The most important operational change affecting timber has been an increase in the allowable axle loadings for railcars. For many years axle loadings remained constant at 30 tons. Double stack container trains now regularly run 35.7 ton axle loadings with an additional 10-20 percent increase in axle loadings projected by the turn of the century.4 This increase is due to the competitive environment in which railroads function and due to changes in rail dynamics technology that have allowed trains to operate safely with higher axle loadings. The result is that many aging rail structures undergo a dynamic proof test each time a piece of modern rail equipment passes over a timber bridge. Safety concerns would dictate the replacement of many of these bridges; however, maintenance funding is affected by the same competitive environment, which affects the rest of railroad operations.
Sufficiently conservative bridge design and maintenance could ensure the reliability of these aging timber rail structures. However since this level of conservatism in bridge design and maintenance is unsustainable in the current business climate, alternative strategies that ensure operational safety of the railroad must be identified. The strategies must center around a repair-for-cause approach to bridge maintenance. Structures can no longer be scheduled for a complete rebuild at a predetermined time, instead repair and replacement must be scheduled based on a priority-based decision making process. The bridges, which are unsafe or have significant deterioration, must be identified in a low cost and efficient manner based on a sound technological basis, modern inspection, and evaluation. When repairs are made, critical components of the bridge which have deteriorated must be identified for replacement or retrofit. Since the large timbers required for replacement of these railroad structural members are becoming less readily available and the replacement of these timbers is a time consuming and costly task, in-situ repair strategies are of great interest. Thus, technologies that can enable repair without timber removal are highly desirable. This preliminary study begins to address these needs by considering in-situ repair strategies, utilizing composite materials, for timber rail bridge components.
Several approaches previously have been applied to the repair of wooden timbers. However, these approaches have not been specifically developed for in-situ application. Procedures include the replacement of severely degraded timbers, epoxy repair approaches, the addition of reinforcing plates to the sides of exposed timbers, and the addition of a fiberglass wrap (bandage) around damage locations.5,6,7,8,9 Unfortunately, the reinforcing plates and the wrap can only be readily applied, in-situ, to exposed timbers which means that many of the structural timbers must be removed prior to repair. Figure 2.1 shows a laboratory single-span timber bridge. The span timbers are seen directly below the ties and are the focus of this research project. Five parallel span timbers are shown in this case, of which only the outside of the outer two timbers would be readily field assessable for the application of shear patches. This accessibility issue would negatively also effect the application of on-site applied composite wraps, and the central timbers would have to be removed for access or replacement.
Replacing timbers means that the structure must be unloaded during the required period of time. In applications such as timber railroad bridges, the large, high quality timber required is becoming more difficult to obtain. Thus, repairs have become much more desirable. Present repair procedures have the disadvantage that the plates or wrap can only be applied, in-situ, to exposed timber. Further, many of the composite material "patches" have been applied without consideration for the type of performance degradation that has occurred. Degradation can show up in a variety of modes, however, the most common mode for the span timbers seems to be a loss of flexural stiffness. This reduction in flexural stiffness often is related to substantial cracking in the middle of the beam as can be seen in Figure 2.2. Since the aspect ratio (length to height) of the span timbers is relatively low, flexural stiffness is strongly related to shear performance of the beam.
Also, the application to the exterior of the wood timber is complicated by preservatives used on the wood, which degrade the bonding performance and by large splits in the timber which are not filled, or healed, during the process.10 Thus the effectiveness and efficiency of these patches has not been high. Finally, many of the patches are found to not be aesthetically appealing as they are exposed and in open sight.
Structural performance of the wraps and plates also has been shown to degrade with time. This can be from direct exposure to the sun, in addition to problems related to the ingress of moisture and subsequent problems during freeze-thaw cycles.