In most cases the construction of bridges is unnecessary on account of
the steep grades the trucks can take and because they can negotiate
sharp curves, which make it easier to avoid expensive bridge work.
Where they are absolutely necessary a serviceable bridge is made of
cribwork.
The Esary Logging Company of Camano Island, Washington, operates over a
crib bridge 175 feet long and 15 feet high. The sub-structure of this
bridge is made of logs laid alternately crosswise in tiers. Six by
twelve inch plank are laid diagonally on the cribbing and four by twelve
inch plank are placed on crosswise to the road on top. This makes a
bumpy surface. A better one could be made with cross-ties placed on the
cribbing with fore and aft planking on top. A guard rail is placed on
all bridges.
Short bridges up to eighty or ninety feet in length are constructed by
the use of two large logs hewn flat on the upper surface. The logs
should be at least thirty-six inches in diameter and perfectly sound.
They are placed at the proper gauge and the regular road on cross-ties
constructed on top. On such short stretches this type of bridge has been
operated over without supports. It is not used, however, for long
stretches. The long bridges are, of course, constructed of bents or
piling but are very seldom used in connection with motor truck
transportation on account of the expensive construction and because they
are usually unnecessary.