the general strength required by the object cf the sti^ucture, and
upon the class of frame adopted. In the first class, in which the
roadway is usually above the frames, any requisite number of nha
may be used, and they may be placed at equal intervals apart,
or else be so placed as to give the best support to the loads wliich
pass over the bridge. lu the second class, as the frame usually
lies entirely, or projects partly above the roadway, &c., if more
than two ribs are required, they are so arranged that one or two,
as circumstances may demand, form each head of the bridge, and
one or two more are placed midway between the heads, so as to
leave a sufficient width of roadway between the centre and adja-
cent ribs. The footpatlis are usually, in this case, eitlier placed
between the two centre ribs, or, when there are two exterior ribs,
between them.
597. The manner of constructing tlie ribs, and of connecting
them by cross lies and diagonal braces, is the same for bridge
frames as for otiier wooden structures ; care being taken to ob-
tain the strength and stiiFness which are peculiarly requisite in
wooden bridges, to preserve them from the causes of destructi
bility to which they are liable. In frames which exert a lateral
pressure against the abutments and piers, the lowest points of
the frame-work should be so placed as to be above the ordinary
high-water level ; and plates of some metal should be inserted at
those points, both of the frame and of the supports, where the
effect of the pressure might cause injmy to the woody fibre.
