InTrans / Aug 14, 2018

Superstructure load path testing of two open-deck timber bridges

As part of the Association of American Railroads‰Ûª (AAR) Timber Bridge Life Extension program, two open-deck timber bridges were field tested on a Southern Pacific line located in southwestern Texas. This is AAR‰Ûªs first field investigation of timber railroad bridge performance under heavy axle load traffic. As such, the emphasis is on assessing factors that
may contribute to faster degradation of timber bridges. Based on the initial results, the following conclusions can be made concerning the load path behavior and existing condition of the superstructures:

l Stringers within a chord acted independently to resist applied
loading. The chord did not behave as a unit. In some cases, this
may lead to overstressing of individual stringers.
l There was little continuity of deflection between chords of
adjacent spans.
l The bearing condition between ties and stringers and between
stringers and caps was not uniform, causing unequal load
distribution on the stringers. This may lead to premature
degradation of the structure.
l There is potential for significant improvement in timber bridge
life and behavior as a result of changes in design and
construction details, which will be developed from this research.

The primary objective was to evaluate the static and dynamic load paths in two bridges scheduled for strengthening in 1996. Data was obtained from revenue service trains as well as a test train These tests represent the first phase of a two-phase test in which load path information will be used to determine the effectiveness of two strengthening techniques: (1) replacing sawn timber stringers with glued laminated stringers and (2) using both new glued laminated stringers and, a ballasted-deck. Upon completion of SP strengthening operations in
1996, both bridges will be retested.