dotted lines A' b C', and A E C, by any passing load, we insert the
counter braces marked R.

The braces A a and C c, must support all of the weight of the bridge
and its load within the parallelogram B a c D--and the next set of
braces, B b and D b, sustain that part of the load which comes over
the centre of the bridge. Consequently the braces must increase in
size from the centre towards the abutments. The rods resist the same
pressure in amount as their braces--but being vertical, do not need
the increase, given to the braces on account of their inclination--but
increase simply with the strain upon them, from the centre to the ends
of the truss.

There are many forms of small bridges differing from those enumerated,
in various minor details, but sufficient has been said to give the
reader a fair idea of the strains upon the different parts, and how to
arrange and proportion the materials to resist them.




PRACTICAL RULES AND EXAMPLES IN WOODEN BRIDGE BUILDING.


In any case that may arise, we must determine approximately the gross
weight of the bridge and its load--as a basis, and then we can proceed
as follows--in case of a Howe, Pratt, or Arch Brace Truss.


=To find the dimensions of the Lower Chord.=