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 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.