\fdO'^ - Old Forge Coal Mines

308 RAILROAD STRUCTURES.
(1060) From Art. 1809 we find the safe shearing
stress across the grain for long-leaf yellow pine to be 500 lb.
per sq. in., and to resist a shearing stress of 30,000 lb. will
,30,000 = 60
^^
sq.
.
in.
.
Ans.
require an area of — '
(1061) From Art. 1809 we find the safe shearing
stress across the grain for white oak to be 1,000 lb. per sq.
in., and to sustain a shearing stress of 40,000 lb. will require
^40',000 ,^ . .
an area of = 40 sq. in. Ans.
(1062) A uniform transverse safe load of 36,000 lb. is
equivalent to a center load of 18,000 lb., which, with a factor
of safety of 4, is equivalent to a
72,000
center breaking load of
lb. 72,000 X 12 = 864,000. The constant for transverse
breaking loads for spruce is 450. (Table 52, Art. 1 802.)
— — -^^ = 1,920, the cube root of which is 12.4, the side
4o0
dimension in inches of a square beam, which will safely bear
the given transverse load, (Art. 1 805.)
For tension, we find in Art. 1810 the safe working
stress for ordinary bridge timber is 3,000 lb. per sq. in.
The given beam has a pulling stress of 20,000 lb., and to
•
• •
-11
20,000 ' ^ ^ TTT
resist this stress it will require =6.6 sq. in. We
must, accordingly, add this to the area of the beam required
to sustain the transverse load alone. We determine the
increased size as follows: 4/12.4* -f- 6.6 = 12.6; hence, the
beam is 12.6 inches square. Ans.
(1063) See Art. 1813 and Fig. 629.
(1064) The total load upon the bridge is 6,000 X 20 =
120,000 lb. Of this amount the two king-rods sustain onehalf,
or 60,000 lb., which places upon each king-rod a load of
^^-^— = 30,000. This, with a factor of safety of 6, would
be equivalent to an ultimate or breaking load of 30,000 x
C = 180,000 lb. By reference to Table 53, Art. 1813, we