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the section has two shelves and a wall. All these three parts
simultaneously bend in the plane xOy , therefore, under bending
they have the same curvature; that’s why bending moments that
occur in each of these parts are proportional to their moments of
inertia with respect to the axis z :
1 M M M
 1  2  3 .
 EJ EJ EJ
1 2 3
As J is a very small value in comparison with J and J wall
3 1 2
influence can be neglected and assume that the two shelves
perceived all load. Then
M M
1  2 ,
J J
1 2
hence
M J M J
M  z 1 ; M  z 2 , (6.25)
1 2
J  J J  J
1 2 1 2
where M  M  M – full bending moment. For shear forces Q
z 1 2 1
and Q arising in the shelves, we can write expressions similar to
2
(6.25):

QJ QJ
Q  1 ; Q  2 , (6.26)
1 2
J  J J  J
1 2 1 2
where Q Q  Q – resultant shear forces that equal to the
1 2
external force P . The line of action of the resultant passes through
the bending center S .
To avoid the beam torsion, it is necessary to fulfill the
condition:
 M  Q h  Q h  0 ,
1 1
2 2
s
or considering the formula (6.26),

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