Now it is necessary for us to have some data from which to determine
the approximate weight of the bridge, and also its load. These can be
found by comparing weights of bridges in common use, as obtained from
reports. In a small bridge of short span, the weight of the structure
itself may be entirely neglected, because of. the very small
proportion the strains caused by it bear to those due to the
load;--but, in long spans, the weight becomes a very important element
in the calculations for strength and safety--inasmuch as it may exceed
the weight of the load.

In all Bridges of 120 ft. span, about 1/3 of a ton, per foot run, will
be the weight of each truss for a single track, including floor
timbers--transverse bracing, &c. If the bridge were loaded with
Locomotives only, the greatest load would be, on the whole bridge--160
tons = 1.33 tons per ft. run of the bridge or .666 tons per ft. run of
each truss. Now if we make the rise of the bridge 15 ft., and divide
the span into 12 panels of 10 ft. each, we shall have for total weight
of bridge and load 240 tons, or for a single truss 10 tons to each
panel.


=Lower Chords.= Now to find the tension on the Lower Chords,

W x S
T = ----- and supplying values, we have
8 h

[TeX: $T = \frac{W \times S}{8 h}$]

240 x 120