Cross arms are now almost limited to carrying polyphase circuits in
areas where appearance is not of paramount importance. They are also
used where lines cross each other or make abrupt turns at large angles
to each other. They are used as alley or side arms in which the greater
part of their length extends on one side of the pole to provide adequate
clearances where pole locations may be affected by limited-space rightsof-
way.
Loadings
The cross arm acts as a beam, supported at the point of attachment
to the pole, and must be capable of being subjected to vertical loadings
from the weight of the conductors (encased in ice) and a 225-lb worker
(specified as an additional safety measure). It is also subjected to horizontal
loadings stemming from winds and from tension in the conductors
where the tensions on each side of the pole do not cancel each other;
e.g., where spans or conductors are not the same on each side of the pole,
at dead ends, bends, or offsets in the line, or where consideration is given
to conductor breaking contingencies.
Stresses
The same principles for determining
stresses in beams as were
applied in the case of poles may also
be applied to cross arms.
Bending Moment
The total bending moment M is equal to the sum of all the individual
loads multiplied by their distances from the cross section under
consideration. Ordinarily, the weakest
section should be at the middle of the arm where it is attached to
the pole. At the pin holes, however, the cross section of the cross arm is
reduced and may, under unusual circumstances, be the weakest point in
the cross arm. The determination can easily be made by computing unit
fiber stress at the several points. Like the pole, the cross arm acts as a
beam and the same formula for determining stresses may be employed.
Double Arms
When fiber stresses approach the maximum safe values for a particular
kind of wood (always keeping in mind a factor of safety of 2),
two arms or double arms are used. Ordinarily, these are found at dead
ends, at points where loads are greatly unbalanced (such as large offsets
or bends in the line), and at intermediate points along a long line to limit
damage in the event that conductor breaks create severe load unbalances
on the supporting structures.
The two arms are placed one on each side of the pole and bolted
together near the ends, and often at intermediate points. Properly constructed,
with spacers of wood or steel between the arms, the structure
created would act as a truss with strengths of 10 to 12 times that of a
single arm, or 5 to 6 times that of the two arms considered individually.
Since such quality trusses may not always be constructed in the field,
prudence dictates that only the ultimate fiber strength equivalent to that
of two cross arms be considered. Where the loadings on the arms may
exceed their fiber strengths, the arms may be guyed, as shown in Figure
5-8, or steel arms may be substituted
for wooden ones.
Douglas fir and long leaf yellow pine are the most popular kinds of
wood used for cross arms, though other kinds may also be found in
use. Their ultimate bearing strengths.
Cross-arm Braces
Cross arms fastened to poles
are usually steadied in position by braces. Flat braces, usually flat strips
of galvanized steel bolted to the cross arm and fastened to the pole by a
lag screw, are most commonly used. The support given the cross arm by
the flat braces is questionable, and is usually neglected in determining
the effects of loads on the cross arm. Where the cross arm is not symmetrically
loaded on each side of the pole, the brace on the load side is
in compression and is of little benefit because of its slenderness; the brace
on the other side is in tension and aids in transmitting some of the load
to the pole, but does not reduce the bending moment.
For heavier loads, a preformed brace made of angle iron, of larger
cross section than the flat brace, aids in supporting the loads on the cross
arm. Here, the moments acting on the cross arm are usually computed
about the point of attachment of the angle brace to the cross arm.
For alley or side arms, an angle-iron brace is used; usually it is a
straight length with the ends adapted to be fastened to the arm and the
pole.
The brace transmits a considerable part of the load on the arm to
the pole; when all the load on the arm is beyond the brace, the vertical
compressive stress on the brace may be even greater than the load. The
points of attachment at the cross arm and at the pole are important; the
smaller the angle between the pole and the brace, the greater the load
transmitted to the pole. On the other hand, as the length of the brace
increases, the slenderness ratio (the brace acting as a slender column)
becomes larger and the effectiveness of the brace diminishes.
Bolts
The stability of a cross arm and its strength rely heavily on the
strength of the bolts through which the stresses are transmitted. The
distribution of stresses on the through bolt holding the cross arm to
the pole are shown in Figure 5-9a. The vertical load on the cross arm is
transferred by the bolt to the pole.
areas where appearance is not of paramount importance. They are also
used where lines cross each other or make abrupt turns at large angles
to each other. They are used as alley or side arms in which the greater
part of their length extends on one side of the pole to provide adequate
clearances where pole locations may be affected by limited-space rightsof-
way.
Loadings
The cross arm acts as a beam, supported at the point of attachment
to the pole, and must be capable of being subjected to vertical loadings
from the weight of the conductors (encased in ice) and a 225-lb worker
(specified as an additional safety measure). It is also subjected to horizontal
loadings stemming from winds and from tension in the conductors
where the tensions on each side of the pole do not cancel each other;
e.g., where spans or conductors are not the same on each side of the pole,
at dead ends, bends, or offsets in the line, or where consideration is given
to conductor breaking contingencies.
Stresses
The same principles for determining
stresses in beams as were
applied in the case of poles may also
be applied to cross arms.
Bending Moment
The total bending moment M is equal to the sum of all the individual
loads multiplied by their distances from the cross section under
consideration. Ordinarily, the weakest
section should be at the middle of the arm where it is attached to
the pole. At the pin holes, however, the cross section of the cross arm is
reduced and may, under unusual circumstances, be the weakest point in
the cross arm. The determination can easily be made by computing unit
fiber stress at the several points. Like the pole, the cross arm acts as a
beam and the same formula for determining stresses may be employed.
Double Arms
When fiber stresses approach the maximum safe values for a particular
kind of wood (always keeping in mind a factor of safety of 2),
two arms or double arms are used. Ordinarily, these are found at dead
ends, at points where loads are greatly unbalanced (such as large offsets
or bends in the line), and at intermediate points along a long line to limit
damage in the event that conductor breaks create severe load unbalances
on the supporting structures.
The two arms are placed one on each side of the pole and bolted
together near the ends, and often at intermediate points. Properly constructed,
with spacers of wood or steel between the arms, the structure
created would act as a truss with strengths of 10 to 12 times that of a
single arm, or 5 to 6 times that of the two arms considered individually.
Since such quality trusses may not always be constructed in the field,
prudence dictates that only the ultimate fiber strength equivalent to that
of two cross arms be considered. Where the loadings on the arms may
exceed their fiber strengths, the arms may be guyed, as shown in Figure
5-8, or steel arms may be substituted
for wooden ones.
Douglas fir and long leaf yellow pine are the most popular kinds of
wood used for cross arms, though other kinds may also be found in
use. Their ultimate bearing strengths.
Cross-arm Braces
Cross arms fastened to poles
are usually steadied in position by braces. Flat braces, usually flat strips
of galvanized steel bolted to the cross arm and fastened to the pole by a
lag screw, are most commonly used. The support given the cross arm by
the flat braces is questionable, and is usually neglected in determining
the effects of loads on the cross arm. Where the cross arm is not symmetrically
loaded on each side of the pole, the brace on the load side is
in compression and is of little benefit because of its slenderness; the brace
on the other side is in tension and aids in transmitting some of the load
to the pole, but does not reduce the bending moment.
For heavier loads, a preformed brace made of angle iron, of larger
cross section than the flat brace, aids in supporting the loads on the cross
arm. Here, the moments acting on the cross arm are usually computed
about the point of attachment of the angle brace to the cross arm.
For alley or side arms, an angle-iron brace is used; usually it is a
straight length with the ends adapted to be fastened to the arm and the
pole.
The brace transmits a considerable part of the load on the arm to
the pole; when all the load on the arm is beyond the brace, the vertical
compressive stress on the brace may be even greater than the load. The
points of attachment at the cross arm and at the pole are important; the
smaller the angle between the pole and the brace, the greater the load
transmitted to the pole. On the other hand, as the length of the brace
increases, the slenderness ratio (the brace acting as a slender column)
becomes larger and the effectiveness of the brace diminishes.
Bolts
The stability of a cross arm and its strength rely heavily on the
strength of the bolts through which the stresses are transmitted. The
distribution of stresses on the through bolt holding the cross arm to
the pole are shown in Figure 5-9a. The vertical load on the cross arm is
transferred by the bolt to the pole.
