Civil Engineering Reference
In-Depth Information
3 Planning and
Preliminary Design of
Modern Railway Bridges
3.1 INTRODUCTION
The primary purpose of railway bridges is to safely and reliably carry freight and
passenger train traffic within the railroad operating environment. It is estimated that,
in terms of length, between 50% and 55% of the approximately 80,000 bridges (with
an estimated total length of almost 1800 miles) in the North American freight rail-
road bridge inventory are composed of steel spans (Unsworth, 2003; FRA, 2008).
Structuraland/orfunctionalobsolescenceprecipitatestheregularrehabilitationand/or
replacement of steel railway bridges. Many of the steel bridges in the NorthAmerican
freight railroad bridge inventory are over 80 years old and may require replacement
due to the effects of age, increases in freight equipment weight, and the ampli-
fied frequency of the application of train loads. Bridge replacement requires careful
planning with consideration of site conditions and transportation requirements in the
modern freight railroad operating environment.
Site conditions relating to hydraulic or roadway clearances, as well as the geotech-
nicalandphysicalenvironment(duringandafterconstruction),areimportantconcerns
during planning and preliminary design. Railroad and other transportation entity
operating practices also need careful deliberation. Interruption to traffic flow in rail,
highway, or marine transportation corridors and safety (construction and public) are
In 1910, it was estimated that there were about 80,000 metal bridges with a cumulative length of
1400 miles on 190,000 miles of track (see Chapter 1). In 2008, it was estimated that there were about
77,000 bridges of all materials with a cumulative length of 1760 miles on 191,000 miles of track. In 2008
the cumulative length of steel bridges was estimated as 935 miles.
In 1910, locomotives typically weighed about 300,000 lb (see Chapter 1). Over the next few decades the
weight of some heavy service locomotives increased by over 50%. The weight of typical locomotives
currently used on North American railroads approaches 450,000 lb.
Trains with loads causing many cycles of stress ranges that might accumulate significant fatigue damage
did not occur until the latter half of the twentieth century when typical train car weights increased from
177,000 lb to over 263,000 lb on a regular basis.
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