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250 STRENGTH OF MATERIALS.
Crushing and Shearing Load of Beams Crosswise on the
Fiber.
Too much crushing load must not be allowed at the ends of
the beams where they rest on their supports, as all kinds of
wood has comparatively low crushing strength when the load is
acting crosswise on the fiber.
Approximately, the average ultimate crushing strength of
wood, crosswise of the fiber, is as follows :
—
White oak, 2000 pounds per square inch.
Pitch pine, 1400 pounds per square inch.
Chestnut, 900 pounds per square inch.
Spruce and pine, 500 to 1000 pounds per square inch.
Hemlock, 500 to 800 pounds per square inch.
. The safe load may be from one-tenth to one-fifth of the
ultimate crushing load. When the wood is green or water-
soaked, its crushing strength is less than is given above.
Example.
How much bearing surface must be allowed under each
end of the beam mentioned in Example 2, providing it also
has 10 as a factor of safety ? The crushing strength of spruce
crosswise on the fiber is 500 pounds, and using 10 as factor of
safety, the load allowed per square inch must be only 50 pounds.
The beam is 8 inches wide, and half of 4685 pounds is sup-
ported at each end ; thus the length of bearing required under
2342
each end will be —~
x g
= 5.85 inches. Thus, the least bearing
allowable should be about 6 inches long.
When beams are heavily loaded and resting on posts, or
have supports of small area, either hardwood slabs or cast-iron
plates should be placed under their ends, in order to obtain
sufficient bearing surface for the soft wood.
The same care must be exercised when a beam is loaded at
one point; the bearing surface under the load should at least
be as long as the bearing surface of both ends added together.
Short beams are liable to break from shearing at the point
of support, especially when loaded throughout their whole length
to the limit of their transverse strength.
The ultimate shearing strength for spruce, crosswise of the
fiber, is 3000 pounds per square inch (see page 273). Safe load
may be 300 pounds per square inch.
In the above example the beam is 8" X 12" = 96 square
inches, and its center load is 4685 pounds, or 2342^ pounds at
93421/
each end. The shearing stress is
96
2
= 24.6 pounds per
square inch. Hence, the factor of safety against shearing is
about 100, and there is not the least danger that this beam will
give way under shearing; but such is not always the result.
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