WHAT IS FACTOR OF SAFETY?
It is the ratio of ultimate strength of
the material to allowable stress. The term was originated for
determining allowable stress. The ultimate strength of a given
material divided by an arbitrary factor of safety, dependent on
material and the use to which it is to be put, gives the allowable
stress.
In present design practice, it is
customary to use allowable stress as specified by recognized
authorities or building codes rather than an arbitrary factor of
safety. One reason for this is that the factor of safety is
misleading, in that it implies a greater degree of safety than
actually exists.
For example, a factor of safety of 4
does not mean that a member can carry a load four times as great as
that for which it was designed. It also should be clearly understood
that, even though each part of a machine is designed with the same
factor of safety, the machine as a whole does not have that factor of
safety.
When one part is stressed beyond the
proportional limit, or particularly the yield point, the load or
stress distribution may be completely changed throughout the entire
machine or structure, and its ability to function thus may be
changed, even though no part has ruptured.
Although no definite rules can be
given, if a factor of safety is to be used, the following
circumstances should be taken into account in its selection:
1. When the ultimate strength of the
material is known within narrow limits, as for structural steel for
which tests of samples have been made, when the load is entirely a
steady one of a known amount and there is no reason to fear the
deterioration of the metal by corrosion, the lowest factor that
should be adopted is 3.
2. When the circumstances of (1) are
modified by a portion of the load being variable, as in floors of
warehouses, the factor should not be less than 4.
3. When the whole load, or nearly the
whole, is likely to be alternately put on and taken off, as in
suspension rods of floors of bridges, the factor should be 5 or 6.
4. When the stresses are reversed in
direction from tension to compression, as in some bridge diagonals
and parts of machines, the factor should be not less than 6.
5. When the piece is subjected to
repeated shocks, the factor should be not less than 10.
6. When the piece is subjected to
deterioration from corrosion, the section should be sufficiently
increased to allow for a definite amount of corrosion before the
piece is so far weakened by it as to require removal.
7. When the strength of the material or
the amount of the load or both are uncertain, the factor should be
increased by an allowance sufficient to cover the amount of the
uncertainty.
8. When the strains are complex and of
uncertain amount, such as those in the crankshaft of a reversing
engine, a very high factor is necessary, possibly even as high as 40.
9. If the property loss caused by
failure of the part may be large or if loss of life may result, as in
a derrick hoisting materials over a crowded street, the factor should
be large.
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