Class Y,

and for a similar 200-ft. deck-span

For a single-track 200-ft. through-span, designed by a contracting bridge company and checked by the author,

The detailing thereof, however, was ultra-economical. It is but fair to state that the 375-ft. span is about two feet wider than the ordinary single-track bridges of such span lengths, which causes the denominator of the fraction to increase somewhat. It is evident, though, that the assumption of any fixed value for

is unwarranted, because the weights per foot of trusses and laterals for spans of Classes Z and U of the Compromise Standard System will vary by from, say, 33 to 40 per cent, according to the span length; consequently the values of

would vary likewise.

In cases of structures for crossings where there is danger from washout, it may be truly economical to use metal unsparingly in the design, in order to ensure the metal-work going together readily and with the least possible delay; and in extreme cases it would be eminently economical to adopt a cantilever design, and thus reduce the risk of washout to a minimum by the expenditure of a considerable amount of extra metal for the superstructure.

There is another economic feature of design, which, unfortunately, has been overlooked continually, viz., that the most economic structure is the one for which the first cost, plus the capitalized cost of annual deterioration and repairs, is a minimum. A proper consideration of this economic feature would cause the use of better details, larger sections of main members, more efficient and rigid sway-bracing, and a greater minimum thickness of metal.