History and Development of Steel Railway Bridges - Design of Modern Steel Railway Bridges - page 8

The failure of a cast iron girder railway bridge in 1847 ∗ stimulated an interest in
wroughtironamongBritishrailwayengineers. † Britishengineerswerealsoconcerned
with the effect of railway locomotive impact on cast iron railway bridges. In addition,
many were beginning to understand that, while strong, cast iron was brittle and prone
to sudden failure. Concurrently, American engineers were becoming alarmed by cast
iron railway bridge failures, and some even promoted the exclusive use of masonry
or timber for railway bridge construction. For example, following the collapse of an
iron truss bridge in 1850 on the Erie Railroad, some American railroads dismantled
their iron trusses and replaced them with wood trusses. However, the practice of
constructing railway bridges of iron was never discontinued on the B&O Railroad.
European and American engineers realized that a more ductile material was
required to resist the tensile forces developed by heavy railroad locomotive loads.
Wrought iron ‡ provided this increase in material ductility and was integrated into
the construction of many railway bridges after 1850. The use of cast iron for railway
bridge construction in Europe ceased in about 1867. One of the last major railway
bridges in Europe to be constructed in cast iron was Gustave Eiffel's 1600 ft long
Garonne River Bridge built in 1860. However, cast iron continued to be used (primar-
ily as compression members) in the United States, even in some long-span bridges,
for more than a decade after its demise in Europe. §
1.2.2 W ROUGHT I RON C ONSTRUCTION
Early short- and medium-span railway bridges in the United States were usually
constructed from girders or propriety trusses (e.g., the Bollman, Whipple, Howe,
Pratt, and Warren trusses shown in Figure 1.2) . The trusses typically had cast iron
or wood compression members and wrought iron tension members. ∗∗ United States
patents were granted for small- and medium-span iron railway trusses after 1840 and
they became widely used by American railroads.
The wooden Howe truss with wrought iron vertical members (patented in 1840)
was popular on American railroads up to the 1860s and used on some railroads upto
the turn of the century. †† The principal attraction of the Howe truss was the use of
wrought iron rods, which did not permit the truss joints to come apart when diagonal
members were in tension from railway loading. However, the Howe truss form is
∗ This was Stephenson's cast iron girder bridge over the River Dee on the London-Chester-Holyhead
Railroad. In fact, Stephenson had recognized the brittle nature of cast iron before many of his peers and
reinforced his cast iron railway bridge girders with wrought iron rods. Nevertheless, failures ensued
with increasing railway loads.
† Hodgekinson, Fairbairn, and Stephenson had also performed experiments with cast and wrought iron
bridge elements between 1840 and 1846. The results of those experiments led to a general acceptance
of wrought iron for railway bridge construction among British engineers.
‡ Wrought iron has a much lower carbon content than cast iron and is typically worked into a fibrous
material with elongated strands of slag inclusions.
§ J.H. Linville was a proponent of all-wrought-iron truss construction in the early 1860s.
∗∗ Wrought iron bridge construction provided the opportunity for using riveted connections instead of
bolts. The riveted connections were stronger due to the clamping forces induced by the cooling rivets.
†† During construction of the railroad between St. Petersburg and Moscow, Russia (ca. 1842), American
Howe truss design drawings were used for many bridges.