Railway Track & Structures - February 2009 - (Page 37)

railroad, whether it’s a one inch or 100 foot difference; create a ramp. And if the ramp is long enough, the difference in elevation will have little, if any, significant effect on train operation. Thus, the solution appears to be in finding a way to control bridge approach settlement so its minimal nearest the bridge and about the same as the rest of the track at the other end. To estimate the prevalence of bridge approach roughness, the Transportation Technology Center, Inc., commissioned a survey to sample railroads from North America, Australia, and Europe on this subject.4 Responses from 11 railroads, including four major North American carriers, indicated that approximately 50 percent of bridge approaches were said to develop a low approach which adversely affected ride quality, frequently required a speed restriction, and generally required above average surfacing maintenance. Responses indicated the formation of low spots on bridge approaches ranging from 0.25 to 4 inches in depth and from four to 50 feet in length. In addition to requiring more frequent surfacing, it was noted that correcting track surface at bridge approaches is often more difficult and a more exacting task than at other locations. At open deck bridges, the track raise must transition in at the exact elevation, since the track on the bridge is not movable, and thus there is little room for error. Even at ballast deck bridges, there may be little allowance left for further raise, likewise requiring a careful transition into the bridge ends. Thus, developing a practical method for reducing the frequency of surfacing at bridge approaches would be desirable for this reason, as well. Figure 10. Top of rail profile at a ballast deck bridge. The highway approach As with railroads, settlement of approaches is common at highway bridges as well. Results from a survey of 48 state departments of transportation indicated that about 25 percent of all highway bridges in the United States are estimated to have approach settlement problems.5 Most newer highway bridges are built with an approach slab, as shown in Figure 9.6 The approach slab has one end supported by the bridge abutment, with the remainder supported in the same manner as the adjacent pavement. So, as the roadway settles, a ramp naturally forms. Even with this arrangement, however, rough spots can still occur at the ends of the approach slab. Although experience shows that its not a universal cure for approach settlement and roughness, this design does correctly target the problem and apparently works well enough that nearly all states commonly use it. Whether some variation of this design would benefit railroad bridges is not clear, as track and pavement construction differ considerably, as do railroad and highway maintenance processes. No single solution to the bridge approach problem is likely to be found, as bridge approaches differ in construction and in the nature and degree of the settlement that occurs there. What will likely be needed is to determine several categories which will cover conditions found at most bridge approaches and then search for solutions that address conditions for bridges in each category. One way to categorize approaches is by where the majority of the settlement occurs within the track support. Direct observation shows that at some approaches, most of the settlement occurs in the subgrade, while for others, it’s in the ballast section, and in a third group, the settlement appears about equally divided between the two. Thus, installing something like an approach slab or otherwise treating the subgrade will not work at a location where most of the settlement occurs within the ballast section. And even for bridges where subgrade settlement is clearly the main problem, the settlement can occur for different rea- www.rtands.com Railway Track & Structures February 2009 37 http://www.ensco.com/jointbar http://www.ensco.com/jointbar http://www.rtands.com

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Railway Track and Structures - February 2009 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D Railroads Stepping Up Use of Technology to Locate Rail Flaws Rail Lubrication Realizing Great Potential M/W Challenges: Track Settlement at Bridge Approaches Supplier Profiles Products and Literature People Calendar Advertisers Index Sales Representatives Website Directory Professional Directory Classified Advertising Chicago Perspective

Railway Track & Structures - February 2009

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