Railway Track & Structures - March 2009 - (Page 19)

resilient pads to concrete ties or using wood ties in concrete tie track. Figure 2 shows the effects of damping (as shown with different tie types) on dynamic forces. The traffic and IJ designs for each case are the same. The lower accelerations show the effects of the foundation on IJ dynamic performance. The foundations with more damping have lower accelerations. Transitioning from open track to higher damping track must be gradual enough to avoid creating an additional discontinuity at each end of the IJ. Maintenance Preventive track surface and running surface maintenance can greatly affect the life of the rail joint. The typical rail joint (including bonded IJs) is less stiff (vertically) than the surrounding rail. Since the foundation is the same as the rest of the track and the rail joint may generate dynamic loads from the running surface gap, the joint will require surfacing more often. Vertical slotting of rail joints (especially IJs) will increase average surface life, but more training in proper techniques is needed. Proper slotting removes metal that has flowed into the rail end gap, preventing the running surface from chipping out. Figure 3 shows an example of poor slotting practice. The rail on both sides of the joint has been sloped down to the end post, leaving a three- to fourinch low spot that will generate high dynamic forces. By not removing any of the end post, a dynamically worse situation was created. A semi-automated system is needed to ensure that the flowed metal is removed, while minimizing the effect on running surface profile. This suggests a fixture that will hold the grinding wheel vertical and limit the depth of cut. Rail stress management TTCI analyzed the effects of rail longitudinal forces on IJ epoxy stress to illustrate the effects of installation and rail stress management practices on IJ service life. Table 2 shows the maximum epoxy shear stresses from two cases.2 The first has no longitudinal force in the rail. The second has 200,000-pound tensile longitudinal force in the rail, which corresponds to winter conditions in main line track. Table 2 shows that lowering the average and maximum tensile force in the rail will reduce the epoxy shear stresses. Lower epoxy stresses will result in a longer service life for the joint. For a mechanical joint, the lower tensile forces may keep the bolts from directly bearing load. Consider managing the rail neutral temperature at IJ locations by balancing the risk of track buckling against the harmful effects of high tensile forces on rail and IJ components. When stressing the rail during an IJ installation, take care to avoid over-tensioning the rail and creating an unnecessarily high neutral temperature, especially if an in-track www.rtands.com Railway Track & Structures March 2009 19 http://www.rtands.com

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Railway Track & Structures - March 2009 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D NRC Smashes Attendance Bridge Inspection Battling Bugs Ballast Fundamentals Vital to Keeping Track in Shape Products and Literature People Calendar Sales Representatives Advertisers Index Website Directory Professional Directory Classified Advertising Chicago Perspective

Railway Track & Structures - March 2009

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