Railway Track & Structures - August 2008 - (Page 17)

TTCI R&D CWR on steel bridges Figure 1, left, shows the 1,650-foot open deck steel bridge tested in 2003-2004. Figure 2 graphs the rail force results. Researchers use model to study behavior of cwr on long open deck steel bridges. ransportation Technology Center, Inc., researchers have carried out a series of bridge tests and developed an analytical model to investigate the interaction of continuous welded rail on long open deck steel bridges. Many railroads routinely use rail expansion joints, combined with minimal rail anchoring, to protect long open deck bridges from thermal effects of cwr. Because expansion joints are costly to install and maintain and induce high impact loads on the bridges they are designed to protect, the effects of potential removal of these joints are under investigation.1 As a steel bridge expands and contracts due to thermal effects, relative displacement is introduced between the bridge and track. Depending on the stiffness of the rail-to-bridge attachment, undesirable forces may be introduced into the bridge T and/or track. When additional hot weather rail compression forces are introduced at a bridge approach, the risk of track buckling may be increased. Under normal conditions, rail thermal forces alone are rarely an influence on structures with virtually all of the rail thermal expansion and contraction constrained within the rail.2 However, if the rail breaks, all internal thermal rail force (tension) is released in the form of either a gap between broken rail ends or restrained by fasteners and ties, which transfer some portion of the load to the superstructure. One design goal is to minimize the broken rail gap if the rail break should occur under a passing train, while minimizing excessive longitudinal forces from the rail break from affecting the bridge structure. While it may not always be possible to control high thermal forces and ensure acceptable cold weather rail gaps without rail expansion joints, in some cases, the number of expansion joints can be reduced or potentially eliminated. However, there are cases where expansion joints are necessary to balance conflicting design goals. CN bridge Figure 1 shows a CN, 1,650-foot-long, open deck steel bridge near Edmonton, Alberta, tested in 2003 and 2004. The test was performed to quantify thermal performance before and after the number of bridge expansion joints was reduced. Reducing the number of expansion joints, such that the length of unanchored rail increased from 950 to 1,450 feet, shows no negative effects in terms of rail longitudinal forces or displacement. Cwr behavior appears to be adequately controlled with the two remaining expansion joints and risks of track buckling and rail breaks are not increased significantly. Steel bridge at FAST In January 2007, TTCI carried out a simulated cold weather rail break test on the steel bridge at the Federal Railroad Administration’s Facility for by Richard Joy, Duane Otter, and David Read, TTCI. Railway Track & Structures August 2008 17 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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