A manual of the principles and practice of road-making: comprising the location, construction, and improvement of roads, (common, macadam, paved, plank, etc.) and rail-roads. By W. M. Gillespie ...

440 APPENDIX F.
4. Wedging -!up the ends of the lower chords. This produces an
initial strain of compression, which the stress of the load must
overcome before it begins to bring a strain of extension upon this
lower chorld. The lower chord then acts somewhat as an arch.
An objection is that it makes the strength of the bridge depend
upon the resistance of the abutment.
5. Double roadway. In important bridges it is best to have each
track separate to prevent a one-sided strain.
6. Durabiity. An uncovered wooden bridge is seldom safe for
more than eight or ten years. If covered, sided, and well painted,
it may last thirty or forty years. Some have been used fifty or
sixty years.
WOODEN ARCH BRIDGES.
A beam resting on two supports, sustains a load by the compression of its upper fibres, and the extension of its lower fibres. If
we confine the ends of the beam by immovable obstacles, these will
be substitutes for the tension of the lower fibres, which may therefore be removed without lessening the strength of the beam, as may
also the extreme portions of the upper fibres. So too a board
laid on two supports will bear a certain weight. Bend it up and
confine its ends and it will bear a much greater weight. This
principle may be adopted in building bridges of considerable span.
Strong, cheap bridges may be made by forming an arch of planks.
One such, with a span of 130 feet, rise 14 feet, was formed of 3'
plank in 15 layers and 30" wide. Three locomotives on it caused
a deflection of only'". The roadway may pass either over the
top, resting on posts and struts, or be at the springs and thus act as
a tie-beam,.beirg suspended from the arch. It is then called a
" Bowstring" bridge.
Perhaps the strongest and cheapest form of bridge, where abutments can be obtained, would be a parabolic arch, increasing in
cross-section froml crown to spring, according to stress, and stiffened by counter-bracing. The counter-braces may be wedged
down, as in Long's bridge, and thus made very stiff, as well as
strong.
Double or parallel arches are always bad. Suppose the "neutral
axis" to pass near the middle of the lower arch rib. Only half the