The mechanical effects of short-circuit currents in - Montefiore

profiles in Figure 2.14 it follows with respect to the axis
Os-Os:
(2.59)
O s
O
O s
A s
3
d b
d b
J s = Zs
=
12
6
Ms Ss d Os σ
y
d O
z
d
S s
b
M s
O s
Figure 2.14 Stresses in sub-conductors
2.2.6. Superposition of stresses in conductors
2.2.6.1 Superposition of the stresses in main and subconductor
The time scale of the forces between the main
conductors and the forces between the sub-conductors
as well as the relevant frequencies of main and subconductor
differ. The maximum of the total stress in the
conductor follows from the time scale of the stress
caused by the forces in the dynamic system. To make
the method suitable for practical use in the standard,
stresses σm and σs are calculated separately in the
equations (2.5) and (2.6) and superposed according to
Figure 2.15. In sub-conductor 1, the stresses σm and σs
are subtracted; in sub-conductor 2, they are added up to
the total stress σtot:
(2.60)
σ
tot
= σ
m
+ σ
In the main conductor in Figure 2.15 with both ends
supported and one spacer in the span and also with both
Figure 2.15 Superposition of stresses in main and sub-conductors
s
2
S s
σ s
S s
x s
M s
M s
27
ends fixed, σm and σs occur at the same locations. Other
types of supports or continuous beams or
asymmetrically mounted spacers will give different
locations for σm and σs. Nevertheless equation (2.60) is
used and the results are on the safe side. Although in
equations (2.5) and (2.6) plasticity is considered, the
superposition is done.
2.2.6.2 Superposition of the stresses caused by
orthogonal moments
The force between main conductors perpendicular to the
force between sub-conductors or the consideration of
dead load, ice, wind in addition to the short-circuit
forces give orthogonal moments M1 and M2 in the
conductors. Figure 2.16 shows the situation for
conductors with rectangular and circular profiles.
The stresses σ1 and σ2 are perpendicular to the crosssectional
area and are moved into the plane. In the
neutral fibre, σ1 + σ2 = 0 holds, e. g. in point A. The
maximum stress is in the points H + and H − which have
maximum distance from the neutral fibre:
– rectangular profile, Figure 2.16a:
σ ( )
(2.61) max = ± σ1max
+ σ2max
– circular profile, Figure 2.16b:
σ ( )
(2.62) max = ± σ′ 1 + σ′ 1
Equation (2.61) is valid for U- and I-profiles, equation
(2.62) for tubes.