Railway Track & Structures - February 2009 - (Page 36)

BRIDGE APPROACH SETTLING the car suspension can filter out forces at frequencies which could transmit to the track, so no clear conclusion can be made about this aspect. It would, however, raise the question of whether forces large enough to cause abnormally greater track settlement could be generated when not even a slight vertical sensation is apparent from inside a passing train. These analyses found that any vertical dynamic forces generated by wheel loads crossing an abrupt track stiffness change are not large enough to be sensed aboard a passing train or to be measured by instrumented wheelsets. Both simple analysis and computer modeling indicate that the ballast and subgrade on a bridge approach are not subjected to increased vertical dynamic forces due to the stiffness change. Observations of the track profile at bridges indicate that approach settlement is often not greater than in track farther away, which suggests that these approaches were not subjected to higher vertical forces. Thus, the results from all five methods pointed to the same conclusion—that even the largest changes in track stiffness do not generate dynamic loads of practical significance, and otherwise have no practical effect on track settlement or ride quality at bridge approaches. To clarify, the conclusion is not that changes in vertical stiffness along the track never affect track settlement. The computer modeling results shown in Figure 5 clearly indicate such an effect is not only possible, but likely, through highly uneven loading. Even the initial work of the Talbot committee, first published in 1918, indicates this possible effect (see AREA, 1980). The conclusion is that a change in stiffness at one location (the bridge end) does not result in the generation of dynamic loads that cause or affect track settlement across a bridge approach. As Figure 5 indicates, a change in track stiffness affects track loading only in the vicinity (within about 4-5 feet) of the stiffness change, not on the adjacent 50 feet of track. Further, the localized variation in track loading at a bridge end actually causes a reduced loading on the track immediately adjacent to the bridge, with an offsetting increase in load occurring on the bridge structure. Figure 9. General arrangement of a highway bridge approach slab. pound freight car crossing a 2,000 to 10,000 (lbs./in./in.) change in track stiffness at 50 mph. As shown, these 41,500pound wheel loads cause an elevation difference of 0.15 inches.3 However, the rail cannot bend abruptly in a step, so the actual occurrence in track will more closely resemble Figure 8(b), a ramp. This ramp would be 0.15 inches high, but its length is needed to define the slope (and thus the severity) of the bump. Using simple beam deflection equations, an approximated shortest length can be roughly estimated by assuming a fixed end beam, which is actually more rigid than rail in track, but with no support between its ends, which is clearly more severe than the actual conditions. The result is illustrated in Figure 8(c), which shows that the rail requires about five feet to make this bend. Thus, under load this stiffness difference creates a ramp in track about 0.15 inches high and five feet long. How severe is this? Figure 8(d) shows it is equivalent to a 1 in 400 ramp. What effect might a 1:400 ramp have on a passing train? One way to estimate this is through a comparison with track surfacing runoff requirements, the minimum length for transitioning from the end of a section of track that has been surfaced and raised to the adjacent track that was not surfaced. These criteria for one large railroad show that a runoff of 1 in 331 is sufficient to create a smooth ride at 60 mph. Thus, the example stiffness bump would allow an equally smooth ride at about 70 mph—as a probable worst case. If so, what is the likelihood that this track profile would produce vertical dynamic loads sufficient to cause noticeably accelerated track settlement on a bridge approach? The train ride test. Another simple but revealing observation is the train ride test, that is, what a person can sense aboard a moving train while traveling over a bridge and bridge approaches. When the track at a bridge (either open deck or ballast deck) is well-surfaced, no vertical roughness can be felt, and only a change in sound will be apparent while traveling over the bridge. Thus, the change in stiffness clearly has no effect on ride quality. Regarding vertical force level, 36 Railway Track & Structures February 2009 What can be done? With the evidence clearly indicating no practical effect from any stiffness difference, what then causes the commonly-seen drop in track elevation on a bridge approach? And what can be done to prevent it? Bridge structures are typically supported by deeply-driven piles or by piers sitting on bedrock or some other hard layer a design intended to result in minimal settlement. Off the bridge, track is supported by a layer of crushed rock on top of an earth subgrade - a structure which typically experiences at least moderate levels of settlement over time. Thus, where these two different types of structures meet, at the bridge ends, an elevation difference (a bump) will typically develop sooner or later. As this elevation difference grows it often has adverse effects on ride quality and can lead to accelerated track settlement on the bridge approach. Attempting to prevent bridge approach settlement will not solve the problem. It will just move the problem farther away from the bridge. The general method needed is similar to that for accommodating any difference in elevation along the www.rtands.com 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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