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Having calculated the bending moment M and allowable
z max
stresses   on strength conditions (6.20) the required resistance

moment of a cross-section can be found
M
W  z max .
z
 

Depending on the chosen shape of a cross-section the required
resistance moment can be obtained in a different cross-sectional
area F , i.e. at different material expenditure on the beam. This
raises the question of a rational shape of the cross-section when the
area is as small as possible.
As the criterion of rationality the specific moment of resistance
 is taken, that depends on the shape and is independent of the
z
size of cross-section
W
  z .
z 3
F
Table 6.1

y y y y y

z z z z
Form z

Circle Square Rectangle Ring Double T
h/в=2 с=0,7 №10...70 b
0,911 …
 0,141 0,167 0,236 0,294
z 1,39

The larger the value  is, the more rational section is. The
z
value  for some shapes of cross-sections is shown in the table
z
6.1.
Analyzing the data in the table 6.1, we see that the farther the
larger part of the cross-section from the neutral axis is and located

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