er scantling, and gain
in lateral stiffness--the two pieces forming one brace by being
properly blocked and bolted together. Below is given a table of
dimensions for the various parts of this style of structure:

Span. Rise. Bolster. Stringer. Braces. Rod.
No. Size.

15 6 12 x 12 12 x 12 2--5 x 6 1-1/8
20 7 14 x 14 12 x 13 2--5 x 8 1-3/8
25 8 14 x 14 12 x 15 2--6 x 8 1-1/2
30 10 14 x 14 12 x 18 2--6 x 9 1-5/8

Single Beams under each rail firmly braced laterally, and trussed by
an iron rod, (or preferably by two iron rods,) and a post on the under
side of the beam. The deflection of the rod is usually taken at 1\8 of
the span. Pl. II., Fig. 1, represents this style of trussing a
beam--which is generally used for spans of from 15 to 30 ft. Below is
a table of dimensions for this truss with single and double rods; if
double rods are used only half the given section will be necessary for
each one of the pair.

Span. Rise. Stringer. Post. Rod. Rods.
Feet. In Feet. (single.) (double.)

15 1-7/8 12 x 12 6 x 8 2-1/8 diam. or 1-1/2 diam.
20 2-1/2 12 x 14 7 x 8 2-1/2 " 1-3/4 "
25 3-1/8 12 x 16 8 x 8 2-3/4 " 2 "
30 3-3/4 13 x 18 9 x 9 3 " 2-1/8 "

It is as well to tenon the post into the beam, and also strap it
firmly with iron plates--and the end should be shod with iron to form
a saddle for the rods to bear upon.

Now if we should make a bridge, on the plan of Fig. 3, Pl. I., 75 or
100 feet, or perhaps more, in length, the braces A F and F C, would
not only be very long but very large and heavy, and one chief
requisite in a good bridge is, to have all the beams so proportioned
that they will resist all the strains acting upon them, without being
unnecessarily large. It now becomes necessary to have a different
arrangement of the parts of the truss in order to obtain increased
length of span.

Suppose we have a span, of 40 feet, as represented in Fig 2, Pl. I.
Now instead of running the braces from A C until they meet in a point,
as before we stop them at a, and c, and place the straining beam, a c,
between them to prevent those points from approaching,