compression of the parts. To
obviate this defect, it is usual to curve the chords slightly in a
vertical direction, by elongating the upper chord, so that the bays or
panels are no longer rectangular but of a trapezoidal form--and, as a
consequence, the inclined web members are slightly lengthened, and the
verticals become radii of the curve. The amount of deviation from a
horizontal line is called the Camber.

A table of Cambers for different spans will be found further on, as
also a table of multipliers, by which to multiply the camber in order
to find the elongation of the upper chord. Part of the Camber table is
taken from Trautwine's Engineer's Pocket-Book, (which should be the
inseparable companion of every engineer,) and part was calculated for
this pamphlet, according to Trautwine's rules. The table of
multipliers is Trautwine's.

=Diagonal Bracing.= In order to stiffen a bridge, it should have the
two Trusses braced together at the Lower Chords always, at the Upper
Chords when practicable--and in case of a deck bridge, where the
roadway is supported on the upper chords, it is as well to have rods
for vertical diagonal braces, their planes being perpendicular to the
axis of the bridge. The more usual form is similar to the web members
of the Howe Truss--the rods from 3/4" to 1" in diameter, and the
braces of 6" x 7" scantling, footed on wooden blocks, usually. It is
more usual to have the tie rods of the horizontal diagonal bracing,
and the braces themselves, meet in a point about midway of a Truss
panel on the centre line, nearly, of the chord. This will generally
give a half panel of diagonal bracing near each end of the truss--and
it is very usual to have the diagonals foot at their intersection
there against a cross timber interposed between the trusses, while the
tie rod prevents any spreading.

=Floor Timbers.= The general dimensions of the transverse floor beams,
when about 3 feet apart, from centre te centre, are 8" x 14", the
largest dimension being the depth. The stringers should be notched to
the floor beams about 1" or 2", and should be about 10" or 12" x 14".
The cross ties should be 18" to 24" apart, from centre to centre, and
be 3-1/2" x 6".

Large, heavy bridges require no fastening to connect them with their
seats, but light bridges should be fastened, as the spring on the
sudden removal of a load, (as when the last car of a train has
passed,) may move it from its proper position.

=