Railway Track & Structures - February 2009 - (Page 35)

Figure 7, above. Top-of-rail profile at the ballast deck bridge with stiffness data shown in Figure 2. Vertical lines near the 170- and 260-foot points indicate bridge ends. Figure 8, right. How an abrupt 2,000 to 10,000 change in track modulus appears in track. that crossing a stiffness change (by itself) produced dynamic loads sufficient to cause accelerated track settlement or to adversely affect ride quality. The study could have ended here, as to warrant acceptance, good science (and engineering) require that any claim be supported with sufficient evidence. But that wouldn’t have answered the question, “Could it be true?” Here are five ways to address that question. Computer modeling. Figure 4 shows results of modeling the vertical dynamic forces below the ties when 286,000pound freight cars cross a 10,000 to 2,000 (lbs./in./in.) stiffness change (between tie 192 and 193) at 50 mph. The graph shows a force increase on the bridge end, followed by a decrease adjacent to the bridge abutment, leveling off to a value somewhat below that on the bridge by the third tie (No. 195) from the bridge end. In the approximately 13 feet past the bridge end shown in the graph, no forces are higher than nominal (or near static) level. Wheel force measurements. Data were examined from vertical force measurements made with instrumented wheelsets on the Northeast Corridor between Washington and New York. Figures 5 and 6 are representative examples of data collected at bridges. Each shows forces, filtered at 50 Hz, over a 400-foot section of track, with the bridge locations marked below the graph. The static wheel load is about 17,000 lbs. The train is running from left to right, in Figure 5 at 108 mph and in Figure 6 at 88 mph. In both cases, the forces before the bridges, over the bridges and past the bridges remain nearly steady, generally oscillating within +/-15% of static wheel load (up to about 20,000 lbs), which would be considered the nominal dynamic force range when traveling at these speeds. In other words, the forces generally remain within what could be called the noise level (marked by the red horizontal line). The figures show that if the bridge locations were not marked, there would be no indication from the force measurements that a deviation or change of any kind was www.rtands.com present in the track. According to the railroad, no alterations were made to the track to change stiffness on the approaches or on the bridges. Similar results were found at other bridge locations Examining bridge approach profiles. A limited observation of bridge approach track surface profiles showed considerable variation in appearance, even on the same route, where traffic was the same and bridge construction was similar. Profiles such as those shown in Figure 7 were common, in which top-of-rail elevation near the bridge was sometimes lower than in track farther away, but often not. While not conclusive, these observations indicate that bridge approaches often do not settle at a much greater rate than the rest of the track does, which suggests that vertical forces on bridge approaches are often not significantly greater than those which typically occur. (The increased settlement shown on the bridge ends in Figure 7 is consistent with the higher forces shown in Figure 4 at the last two ties on the bridge.) Simplified analysis.Without access to such technology as instrumented wheelsets or track-train dynamics computer programs, is there a simpler way to get a general idea of how large the stiffness difference effect might be? The following method relies on the readily available beam-on-elasticfoundation track deflection equations and other information to accomplish this.2 Figure 8(a) shows the change in deflection from a 286,000Railway Track & Structures February 2009 35 http://www.rtands.com

Table of Contents Feed for the Digital Edition of Railway Track & Structures - February 2009

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.