When putting a truss together in its proper position, on the
abutments, 'false works' must first be erected to support the parts
until they are so joined together as to form a complete
self-sustaining truss. The bottom chords are first laid as level as
possible on the false works, then the top chords are raised on
temporary supports, sustained by those of the lower chord, and are
placed a few inches higher at first than their proper position, in
order that the web members may be slipped into place. When this is
done the top chords are gradually lowered into place. The screws are
then gradually tightened, (beginning at the centre and working towards
both ends,) to bring the surfaces of the joints into proper contact,
and by this method, the camber forms itself, and lifts the lower
chords clear of the false works, leaving the truss resting only upon
its proper supports. The subjoined Table will be found useful in
estimating the strains on a truss when proportioning a bridge for any
moving load.

Table of weights per running foot of a bridge, (either of wood or
iron,) including weights of floor, lateral bracing, &c., complete, for
a single track.

Clear Weight of
Span. Bridge.
Tons. lbs.

25 .266 596
30 .281 629
40 .313 701
50 .343 768