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Normal stress from exposure to bending moment determined by
the formula
M y
  , (4.1)
м
S (r  ) у
н н

where y - the distance from the neutral line to the point where we
determine the stress; r - the radius of curvature of the neutral
н
layer; S  F e - static moment sectional area curve beam with
н
respect to the neutral line; e - eccentricity, or the coordinates of
the center of gravity sectional area relative to the neutral line; F
- cross sectional area of timber ,
F
r  dF , (4.2)
н

F r
y
where r  r  – the radius of curvature of an arbitrary layer
н
with coordinate y.

Figure 4.4

In general r do not coincide with the center line radius of
н
curvature, i.e. neutral bending line curved beam does not pass
through the center of gravity of the cross section, but shifted by
an amount e to the center of curvature of the beam.
From (4.1) it follows that the strain  varies in height section is
м
not linearly, as for straight beams, and on hyperbolic. Major
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