Civil Engineering Reference
In-Depth Information
pace for another 80 years following inception in the 1820s. During that period, due to
continually increasing locomotive loads, it was not uncommon for railway bridges to
be replaced at 10-15 year intervals. The associated demand for stronger and longer
steel bridges, coupled with failures that were occurring, compelled engineers in the
middle of the nineteenth century to engage in the development of a scientific approach
to the design of iron and steel railway bridges.
Americanrailwaybridgeengineeringpracticewasprimarilyexperientialandbased
on the use of proven truss forms with improved tensile member materials. Many early
Town, Long, Howe, and Pratt railway trusses were constructed without the benefit of
a thorough and rational understanding of forces in the members. The many failures of
railwaybridgetrussesbetween1850and1870attesttothis.Thisempiricalpracticehad
served the burgeoning railroad industry until heavier loads and longer span bridges,
in conjunction with an increased focus on public safety, made a rational and scientific
approach to the design of railway bridges necessary. In particular,American engineers
developed a great interest in truss analysis because of the extensive use of iron trusses
on U.S. railroads. In response, Squire Whipple published the first rational treatment
of statically determinate truss analysis (the method of joints) in 1847.
The rapid growth of engineering mechanics theory in Europe in the mid-nineteenth
century also encouraged French and German engineers to design iron and steel rail-
way bridges using scientific methods. At this juncture, European engineers were
also interested in the problems of truss analysis and elastic stability. B.P.E. Clapyron
developed the three-moment equation in 1849 and used it in an 1857 postanalysis of
the Britannia Bridge.
∗
Concurrently, British railway bridge engineers were engaged
in metals and bridge model testing for strength and stability. Following Whipple, two
European railway bridge engineers, D.J. Jourawski
†
and Karl Culmann, provided
significant contributions to the theory of truss analysis for iron and steel railway
bridges. Karl Culmann, an engineer of the Royal Bavarian Railway, was a strong and
early proponent of the mathematical analysis of trusses. He presented, in 1851, an
analysis of the Howe and other proprietary trusses
‡
commonly used in the United
States. The Warren truss was developed in 1846,
§
and by 1850 W.B. Blood had devel-
oped a method of analysis of triangular trusses. Investigations, conducted primarily
in England in the 1850s, into the effects of moving loads and speed were beginning.
Fairbairn considered the effects of moving loads on determinate trusses as early as
1857.
J.W. Schwedler, a German engineer, presented the fundamental theory of bending
moments and shear forces in beams and girders in 1862. Earlier he had made a
substantial contribution to truss analysis by introducing the method of sections. Also
in 1862, A. Ritter improved truss analysis by simplifying the method of sections
∗
The design of the Britannia Bridge was based on simple span analysis, even though Fairbairn and
Stephenson had a good understanding of continuity effects on bending. The spans were erected simply
supported, and then sequentially jacked up at the appropriate piers and connected with riveted plates
to attain continuous spans.
†
Jourawski was critical of Stephenson's use of vertical plate stiffeners in the Britannia Bridge.
‡
Culmann also analyzed Long, Town, and Burr trusses using approximate methods for these statically
indeterminate forms.
§
The Warren truss was first used in a railway bridge in 1853 on the Great Northern Railway in England.
