THE BERLIN IRON BRIDGE COMPANY,
in its construction it is not apt to get out of repair, as would be the case of a complex structure. Our patent Parabolic Truss particularly
commends itself upon these two points, viz : stiffness and simplicity.
Each part of the bridge is designed to do its special work, and every part helps to support as well as strengthen. A piece of string
has little or no strength of itself when drawn perfectly straight, but if allowed to deflect even a small distance, its strength is very materially
increased—the same with a piece of timber. Take, for example, a 12x12 piece of timber, supported at its ends, and having a span of 20
feet; this timber will stand a given load, under these conditions, but if the ends are rigidly held, and the timber is curved between so that
there is a rise at the center of three feet, its supporting power is more than tripled. This explains why the Parabolic Bridge is stronger,
better, and stiffer than the old style bridge, with parallel chords. Neither chord of a Pratt Truss will support a pound taken by itself
alone, but with a PafsJboflwBridge either chord will not only support itself but will support a large load besides. Unite these two at the
ends, one to resist the other, and you have the strongest form of truss known.
One particular merit in the Parabolic Truss is that each part of the bridge is designed to do its own separate work.
In iron bridges there are two loads to be resisted; first, the vertical or live load, and next, the horizontal or wind load—this latter in
long spans with narrow roadways being often more than the vertical load. In our Parabolic bridge the vertical load is resisted by the main
trusses, and the horizontal or wind load by a chord placed beneath the floor, and tangent with the vertical truss at the center, and designed
especially for this purpose. There is no other bridge built which provides for this wind load, and where the span is of any considerable
length, and the roadway of medium width, it amounts from 25 per cent to 100 per cent of the live road. This often reduces the live load
capacity of the truss one half, unless especially provided for, as in our Patent Parabolic Truss Bridge.
The Parabolic truss is simplicity itself. The chords have a uniform section throughout their length, and, therefore, a Parabolic
bridge is stiffer than a Parallel cfcord bridge, where the sections are lighter in the end panels. There are no temperature strains in the
Parabolic bridge, as the parts are so proportioned that each expands its regular proportion under the changes of temperature. Every part
of the bridge admits of exact analysis and calculation.
The capacity of iron highway bridges varies with their location. ' Large city bridges which are built to carry heavily loaded truck
teams and continuous traffic, must of necessity be heavier and stronger than country bridges, where the loads are light and the traffic
infrequent. For this reason the capacity of bridges, or what is equivalent to the same, the unit strains in the different members of a
bridge, vary with the different locations.
City bridges should also be proportioned for an Aveling & Porter steam road roller, concentrating a weight of 14,000 pounds on the
EAST BERLIN, CONNECTICUT, U. S. A.