The workingman's advocate. [volume] (Chicago [Ill.]) 1864-1877, 1871-08-05
American and English Iron Bridges

American and English Iron Bridges.
The Railroad Gazette has the following
highly interesting information on this ques
ton Of The strength Of iron bridges:
" Some persons enormously suppose that
the more iron thore is in bridge the Stron-
ger it will be. But a little reflection will
show that it is only the iron that is work
ing, or, in other words, that is actually
strained by the load, that contributes to the
strength of the structure. All the rest is
dead weight, and merely weighs down the
bridge. In very short spans this is not dis
advantageous, as it tends to diminish vibra-
ton but In long spans, where the weights
Of the spans much exceeds that of the load
passing over it, every pound Of iron that
does net contribute to the strength Of the
bridge is positive injury. To illustrate
this mole clearlY-if one bridge weighs l25
tons and another 250, and BOZ b are strain
ed by the rolling load Of 10000 pounds
per square inch, the lighter is the stronger
of the two. But if 125-ton bridge be
strained 10000 pounds per square inch,
while the 25O-ton bridge is strained only
5000 pounds per square inch, then the
latter has really double the strength and
double the life Of the former for half the
iron may corrOde away, and then the work
ing area Of the bar will be equal. 1t is
not clearly perceiving this fact-that the
strength Of the bridge depends upon the
working area Of its parts-that has led our
English friends to make such heavy bridges.
In several plans, if tt, strain per square
inch is alike for similar loads, they must all
be Of the same strength, provided the con-
nections are equally perfect. Some take
more iron than others to effect the result,
but the result is the same.
The lightness of American bridges is due,
First, To the concentration Of material
along the lines Of strain, which enabled a
lighter web system to be used, and hence
higher truss.
Second, To this greater height Of truss,
which throws less leverage on the upper
and lower chord system, hence requires less
iron in their members
Third, To the use Of eye and pin con-
nections, instead of riverts, BY which there
is no waste Of metal to compensate for the
deduction Of rivet holes.
American bridges are sitter vertically,
and better braced latterly, than English
bridges, their greater height giving less
DETECTION under load, and allowing Of
overhead bracing, as well as that below the
track.
But the less quantity Of iron required
to do the work is not the whole explanation
Of the less cost Of American as compared
with English bridges. A second and
equally important reason is the less amount
Of manual labor required to construct and
erect tbem-owing to the general use Of
machinery in forming all the parts. Fog
Iish bridges are made Of low-priced iron,
and require a great deal Of it, and great
deal of hand labor in constructing and erec-
ting.
American bridges have all their principal
parts formed by machinery. They are Of
exact, uniform dimensions, in similar spans,
and hence perfectly interchangeable, like
the parts of the locks Of the American
rifles, or Of sewing machines. Hence ma
chine labor can be applied to their manu-