must be
so proportioned as to safely bear, as a horizontal beam, this weight.
Suppose we take three sticks of 8" x 12", to form the chord (the
greater dimension being the depth,) we shall have 3 x 8" x l2" = 288
square inches area of section, and

allowing for splicing 72 square inches,
" " foot blocks, 24 " "
" " bolts, 24 " "
" " washers, 8 " "

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
Trusses of 300 ft. span, 30 ft. rise, and 26 ft. wide, without any
arch, but it has a wrought iron lower chord, and is only proportioned
for a moving load of 1000 lbs. per ft. run. [Vide Vose on R.R.
construction.]

In order to ensure uniformity in strength in the chords--but one joint
should be allowed in a panel--and that should come at the centre of
the panel length--but in long spans this cannot always be done.

=Web Members.= We will now proceed to calculate the web members of a
Howe Truss of the fore