[TeX: $\frac{300000 \times 100}{96} = 3333333$]

The dimensions of the chord and splicing would be found in the same
manner as for a Howe Truss.


=Suspension Rods.= Fig. 1, Pl. III., represents an elevation of a
Pratt Bridge. Now, it is evident that the first sets of rods must
support the weight of the whole bridge and its load, which we have
found to be 300000 lbs. Each truss will have to sustain 150,000 lbs.,
and each end set of rods 75,000 lbs. Now, if there are two rods in
each set,--each rod will have to bear a strain of 37500 lbs., and this
will have an increase due to its inclination, so that the strain on it
must be found by the following proportion:

Height : diagonal :: W : W' or

12 : 15.8 :: 37500 : 49375 lbs.

Referring to the Table for bolts, we find that 2-1/8 gives a strength
a little in excess, and will be the proper size. The next set of rods
bear the weight of the whole load, less that due to the two end
panels, and so on. Fig. 2, Pl. III, shows the manner of applying the
rods. The bevel block should be so fitted to the chord that it will
not have a crushing action.


=Counters.= Top and bottom chords are always used in this bridge, and
consequently the counter rods have only to sustain the movable load on
one panel. The weight of the moving load cannot be more than 2000 lbs.