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from the neutral layer tangential and normal stresses arise that is at
these points a simplified plane stress occurs. At the point 3, which
is at the neutral axis a normal stress is absent, and a tangential
stress obtains maximum magnitude, i.e., the point is in a state of
pure shear.
Continued practice of using beams shows that in the most cases
supporting strength at points 1 and 5 defines performance
capability in general. So, strength test, design calculations or
determination the allowable bending moment are carried out
provided strength in the most normal stress states
M
  z max    . (6.20)

x max W
z
If the beam is loaded by big concentrated forces and its length
is small, or these forces are applied close to the support, there is a
need to check the strength of the beam at the point 3. Strength
condition in the largest tangential stresses
Q S
  y max z     . (6.21)
max
bJ
z
In calculating the beams with a thin-walled section (double-T
profile), and in the case when in a section of the beam at the same
time the maximum bending moment and the maximum shear force
arise or their values are close to the maximum, we should check
the strength of the beam at points 2 and 4 in principal stresses .
Conditions of strength in principal stresses are recorded depending
on the chosen strength theory. For example, a third theory

 III        ,

eqv 1 3

or considering the formula (3.19), we obtain

(6.22)
2
III
2
    4   .

еqv x
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