Civil Engineering Reference
In-Depth Information
(1876)
The Ashtabula Bridge was constructed during 1863-1865 over a stream in Ashtabula,
Ohio. The bridge collapsed on December 29, 1876 after eleven years of service.
The conceptual design of the Ashtabula Bridge was carried out by Amasa
Stone, the president of the Cleveland, Painesville and Ashtabula Railroad, which later
became a part of the Lake Shore and Michigan Southern Railway. The bridge was a
simply supported, parallel chord, Howe truss with a span of 47 m (154 ft) consisting
of 14 panels of 3.4 m (11 ft) each. The depth of the truss was 6 m (20 ft) and the
center-to-center spacing of the trusses was 5 m (17 ft). The bridge was a deck
structure carrying double track railway. Stone employed Joseph Tomlinson to
determine the member sizes and prepare the fabrication drawings. Tomlinson carried
out the task assigned to him and supervised the entire fabrication. However, he did
not participate in the supervision of the construction.
The diagonals subjected to compressive forces consisted of several built-up
iron I-sections. The top chord consisted of segments, two panels long, fitting between
the lugs on the iron casting. The beams carrying the timber floor rested directly on the
top chord, thus introducing flexure in the top chord of the truss. The position of each
track was such that the live load of one train was carried predominantly by the truss
near to that track.
Failure of the bridge took place at about 7:30 p.m. on December 29, 1876
during a severe snowstorm. A train with two steam locomotives was crossing the
bridge heading west at an estimated speed of 20 to 25 km per hour (12 to 15 miles per
hour). As the first locomotive was about to complete the crossing, the bridge began to
fail. The second locomotive, its coal tender and eleven cars fell 20 m (65 ft) into
Ashtabula Creek. The accident was a national tragedy causing 80 deaths.
On January 12, 1877, the Legislature of Ohio appointed a joint committee to
investigate the cause of failure. A report to the legislature was made on January 30,
1877. In addition, the coroner's jury appointed an engineer to investigate the causes of
the disaster. A third major investigation of the failure was carried out by Charles
MacDonald.
Each investigation independently concluded that the failure occurred in the
second and third panels of the south truss, though the investigators were undecided
whether it was the top chord, or the compressive brace that initiated the failure. The
Joint Committee concluded the bridge failed because of inadequate inspection. The
coroner's jury made the same point, adding that iron bridges were in their infancy and
that an experiment should not have been made on a bridge with such a deep chasm.
Charles MacDonald suggested that a fatigue crack originated at a flaw in the lug and
propagated under repeated stress cycles.
