D F x A B
and D H = ---------
A F
[TeX: $DH = \frac{DF \times AB}{AF}$]
This style of structure may be used up to 50 feet, but it is not
employed for spans exceeding 30 feet in length. It is very customary
to make the braces in pairs so as to use smaller 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, suspend the
points B and D from them, and start the braces B b and D b--and, if
the truss were longer, would continue on in the same manner as far as
needful. To prevent the. truss from altering its form, as shown by the
dotted lines A' b C', and A E C, by any passing load, we insert the
counter braces marked R.
The braces A a and C c, must support all of the weight of the bridge
and its load within the parallelogram B a c D--and the next set of
braces, B b and D b, sustain that part of the load which comes over
the centre of the bridge. Consequently the braces must increase in
size from the centre towards the abutments. The rods resist the same
pressure in amount as their braces--but being vertical, do not need
the increase, given to the braces on account of their inclination--but
increase simply with the strain upon them, from the centre to the ends
of the truss.
There are many forms of small bridges differing from those enumerated,
in various minor details, but sufficient has been said to give the
reader a fair idea of the strains upon the different parts, and how to
arrange and proportion the materials to resist them.