Page 42 - 4749

the allowable stress is taken less than the limit. To do this, the
safety factor for liquid limit n and limit strength n are
L st
introduced.
For plastic materials (both tensile and compressive) stresses are
allowable


   t . (2.27)
n
L

For brittle materials in tensile and compression allowable stress
 t  С

   st ;    st . (2.28)

t с
n n
st st

The value of the safety factor is chosen by taking into account
the existing experience of operating structures and machines, using
the existing rules of structural design.
Usually n is selected from the range of 1,7 3,5, and n -
L st
from the range 2 5.
To assess the strength it is necessary to compare the actual
stress in the dangerous section of the rod with allowable:
N

  x    . (2.29)
x
A
Using this equation, we can solve the following problems:
- check the strength of the rod, i.e., knowing the load and the
size of the cross-section of the rod, determine the actual
stresses and compare them with allowable (checkout
calculation); actual stress should not exceed more than
allowable 5%;
- determine the size of the cross-section of the rod (project
calculation)

42