[Illustration: Pl. II. with Fig. 1., Fig. 2., Fig. 3., Fig. 4.]

=Counterbraces.= Now, as to the necessity of Counterbracing, there are
various opinions. The object of it is to stiffen the truss and check
vibrations. If a load be placed over any panel point, it causes that
portion of the truss to sink, and produces an elevation of the
corresponding panel point at the other end of the truss--thus
producing a distortion, which change of form is resisted by proper
counterbraces. The strain to which this timber is subjected is caused
by the moving load on one panel only--and requires only scantling of
the size of the middle braces. These counterbraces should not be
pinned or bolted to the braces where the cross--as their action is
thereby entirely altered--but it is well to so confine them as to
prevent vertical or lateral motion.

=Shoes.= Formerly it was the custom to foot the braces and counters on
hard wood blocks on one side of the chord, the vertical rods passing
through and screwing against a block on the other side--thus the whole
strain tended to crush the chord across its fibres. This is now
remedied by the use of cast iron blocks, bearing on one side of the
chord, but having tubes extending through to the other side, where the
washer plate for the bolts fits firmly on their ends, forming a
complete protection, as all the crushing strain is received on the
block itself.

=Width.= It now becomes necessary to determine upon the width between
the two trusses. For a single track bridge for a railroad, 14 ft. is
the usual width adopted, and for a highway bridge, from 12 to 16 ft.
When a double track is required, three trusses are usually employed,
with a width for each roadway of 14 ft. for railroads.

=Bolsters.= Large timbers 12 x 12, or thereabouts, are laid on the
bridge seats of the abutments to support the ends of the trusses, one
of these should be directly under each of the extreme panel points. A
panel point is the intersection of the centre line of a brace
produced, with the centre line of a chord. The rise of a truss is the
vertical distance between the centre lines of the upper and lower
chords.

=Camber.= Were a bridge to be framed with its chords perfectly
horizontal, it would be found to fall below the horizontal line on
being placed in its proper position, owing to the closing up of the
joints in the upper parts of the structure, and opening of joints in
the lower parts, as well as to the