T = --------- = 240 tons, or 537600 lbs.,
8 x 15

[TeX: $T = \frac{240 \times 120}{8 \times 15 = 240$}

for the two Lower Chords, and 1/2 of this, or 268800 lbs. for one
chord. The Tensional Strength of timber for safety may be taken at
2000 lbs. per square inch of section, and hence the area of timber
required to sustain the above strain will be

268800
------ = 134.4 sq. inches.
2000

[TeX: $\frac{268800}{2000} = 134.4$]

But this chord has also to sustain the transverse strains arising from
the weights passing over it, and, as in the case of a Locomotive, the
weight of 20 tons on 2 pair of drivers, (or 10 tons for one truss,)
may be concentrated on the middle point of a panel--the chord 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 " "