Railway Track & Structures - August 2008 - (Page 18)

TTCI R&D thermal expansion and contraction of a bridge girder. Measured high strain rate values were 10 to 28 pounds-per-inch. Similar measurements on ballasted track range from 20 to 40 pounds-per-inch. BNSF bridge A third test under the Strategic Research Initiatives program was carried out in 2007 on a BNSF 400-foot steel bridge in Pueblo, Colo., as Figures 3 and 4 show. The bridge was fully anchored with no expansion joints. Rail forces, rail and span temperatures, track-to-span displacements and bridge bearing displacements were monitored for six months. Daily rail temperature reversals of nearly 70°F were recorded. Daily span temperature swings reached 45°F. As with previous tests, the results increased the understanding of the longitudinal restraint characteristics between cwr and the bridge spans. In addition, rail force increases due to span expansion at the approach end of the 210foot truss were measured at 45,000 to 50,000 pounds. These results emphasize the need to maintain good lateral track resistance and proper rail neutral temperature on bridge approaches subject to span expansion. Figure 3, top, shows the BNSF 400-foot steel bridge tested in 2007. Figure 4 is a schematic of the same bridge. Accelerated Service Testing. TTCI technicians waited for a cold morning when the rail was well below neutral temperature resulting in tension to cut the rail, and then measured the changes in rail force as the tension was relieved. The measurements helped to quantify characteristics of the cwr-to-bridge connection. Figure 2 summarizes test results. The chart illustrates the change in longitudinal rail force along the bridge before and after the rail cut. The change in force is due to the longitudinal restraint of the rail to the bridge girder. The high linearity indicates that the longitudinal restraint is fairly constant across the bridge, with the slope of the linear fit indicating the longitudinal restraint value. Designers have traditionally relied on rail anchors or special fasteners to 18 Railway Track & Structures August 2008 provide longitudinal restraint between cwr track and the bridge structure. This test has indicated that on bridges with a smooth interface between the bridge and deck, while the fasteners may provide a strong bond between the rail and deck ties, longitudinal restraint is weakest at the tie-to-girder interface. For the bridge configuration tested, only about 20 percent of the restraint is realized between the tie and girder, with about 80 percent between the rail and tie. Bridges with rivets or bolts protruding from the top surface of the span will likely have much higher resistance. Testing on the steel bridge at FAST has confirmed previous observations on open track that high strain rate applications such as the instantaneous release of tension during a rail break under thermal tension, produce lower longitudinal restraint values than gradual strain rate applications, which would be applied by Analytical model TTCI researchers have developed an analytical model for bridge longitudinal forces based on examples from the literature.3,4 The model incorporates cwr longitudinal restraint values from full scale bridge testing carried out under this and previous research programs and from a number of cwr tests carried out on open track.5,6 The model is being used to investigate forces induced by thermal displacement on the track and bridge superstructure, including potential for track buckling in hot weather. Additionally, the effects of a cold weather rail break are being examined. Modeling results As discussed above, bridge expansion during hot weather can add compressive forces to the rail by transmitting forces www.rtands.com http://www.aar.com http://www.rtands.com

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Railway Track & Structures - August 2008 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D Wish List: What Chief Engineers Really Want in M/W Machinery Keeping Ballast in Shape Amtrak Turns to Sophisticated System to Manage NEC Assets Supplier Profiles Products and Literature People Calendar Sales Representatives Website Directory Advertisers Index Professional Directory Classified Advertising Chicago Perspective

Railway Track & Structures - August 2008

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