we shall have after deducting allowances (288-128) 160 square inches
area, giving an excess over 134.4, the area demanded, sufficient to
cover allowances for any accidental strain.


=Upper Chords.= The upper chords are compressed as forcibly as the
lower ones suffer tension--owing to the action and reaction of the
diagonals. In this case the compression is 268800 lbs., and as 1
square inch of section will safely bear 1000 lbs., we have for the

268800
area required, ------ = 268.8
1000

[TeX: $\frac{268800}{1000} = 268.8$]

square inches,--three pieces 8" x 11" will give 264 square inches and
this area will require no reduction, as the whole chord presses
together when properly framed and is not weakened by splicing. So far,
the calculations made would apply to either of the three Bridges
mentioned, as well as to a Warren Truss. But now, to obtain the
dimensions of the web members, so called, of the Truss, it is
necessary to decide upon the specific variety. The form of Bridge in
more general use in the United States is called the Howe Truss, from
its inventor, and in spans of 150 feet, and under, is very reliable;
for spans exceeding 150 ft. it should be strengthened either by Arch
Braces or by the addition of Arches, as the heavy strains from the
weight of bridge and load bearing on the feet of the braces near the
abutments, tend to cripple and distort the truss by sagging, although
the Baltimore Bridge Co. have built a Wooden Howe Bridge of two