TTCI R&D Recent developments in:
Alternative bridge ties for open deck steel bridges by Duane Otter, Ph. D., PE and Richard Joy, PE, principal engineers, TTCI.
TTCI researchers investigate lower life-cycle cost crosstie options for open deck bridges.
A
s railroad traffic increases, railroads are looking for longer lasting ties for their bridge decks. Railroad bridge engineers have noted increasing expenses and decreasing life of traditional solid sawn timber bridge decks in revenue service. This has prompted interest in alternatives that might provide lower life-cycle costs and has led Transportation Technology Center, Inc., to investigate alternatives to solid sawn timber ties for open deck bridges. For open deck bridge applications, ties in many cases serve as structural members, in addition to performing the functions of a conventional track tie. Such ties are sometimes referred to as span ties. For ties to perform adequately as structural members on open deck bridges, additional bending strength, shear strength and limited deflection are Figure 1: FFU bridge ties installed on a Class 1 railroad.
12 Railway Track & Structures
May 2013
required. Until recently, lack of design guidelines hindered development of alternative ties for open deck bridge applications. Under the direction of bridge engineers from Class 1 railroads, TTCI has been working in three areas to encourage development of improved bridge ties: (1) in-track testing of ties at the Facility for Accellerated Service Testing (FAST) and in revenue service, (2) laboratory testing of ties to determine suitability and (3) development of recommended practice for bridge ties in conjunction with committees of the American Railway Engineering and Maintenance of Way Association (AREMA).
Revenue service installations
Based on acceptable performance in laboratory testing by a Class 1 railroad and TTCI, one railroad made a pilot installation of fiber-reinforced foamed urethane (FFU) ties on an open deck steel bridge in December 2011 (see Figure 1). FFU is essentially a fiberglass material and ties made of this material have been used in Japan for a number of years. For this installation, the ties were dapped. However, no provision was made to provide relief for the protruding rivet heads on the top flange of the span. Instead, the ties were allowed to settle onto the steel under the weight of passing trains. The ties were installed using the railroad’s standard bridge tie insertion machinery. They are similar in weight and size to timber ties that have traditionally been used in this application. Girder spacing on this bridge is 6.5 feet center-to-center. For the girder spacing, the ties function as span ties, but the bending moments are much smaller than on spans with wider girder spacings, commonly up to approximately nine feet. TTCI is monitoring performance of the ties on this bridge. Periodic measurements include tie www.rtands.com