Railroad Tie Lateral Resistance on Open Deck Plate Girder Bridges

Abstract

On open-deck railroad bridges, the crossties (sleepers) are directly supported by the bridge superstructure and anchored with deck tie fasteners such as hook bolts. These fasteners provide lateral resistance for the bridge ties. Currently there are no provisions to assist in the calculation of lateral resistance provided by railroad ties on open-deck bridges, and as a result there are no specific requirements for the spacing of deck tie fasteners. This has led to different design practices specific to each railroad, and inconsistent fastener spacing in existing railroad bridges.

A research plan was conducted to experimentally quantify the lateral resistance of timber crossties on open-deck plate girder bridges using different wood species and types of fasteners. Experimental tests were conducted on five different species of timber crossties (beech, sycamore, southern pine, Douglas-fir, and oak) with three different types of fasteners (square body hooks bolt, forged hook bolts, and Quick-Set Anchors). A structural test setup simulated one half of an open-deck bridge with a smooth-top steel plate girder, and hydraulic actuators to apply both vertical and horizontal load to a railroad tie specimen. The three main contributions to lateral resistance on open-deck bridges were identified as friction resistance between tie and girder due to vertical load from a truck axle, resistance from the fastener, and resistance from dapped ties bearing against the girder flange. Initial testing isolated each component of lateral resistance to determine the friction coefficient between tie and girder as well as resistance from just the fastener itself. Additional testing combined both vertical load and fastener to determine whether or not the overall resistance is simply the sum of the friction and fastener resistance. Results indicated that friction resistance varies based on the magnitude of vertical axle load, species of wood, and creosote retention in the tie, while fastener resistance varies based on type of fastener and lateral displacement of the tie. An approximation of the lateral resistance as a function of lateral displacement was established depending on the vertical load, type of hook bolt, and coefficient of friction between tie and girder. The approximation was used in a structural analysis, which modelled a section of railroad track as a beam supported by non-linear springs spaced at discrete distance. Based on anticipated lateral loads, the analysis was used to determine a preliminary chart for a safe and economical fastener spacing for a railroad track based on type of hook bolt, creosote retention, tie species, and curvature of bridge.