Stresses
The forces acting on a pole stem from the vertical loading occasioned
by the weight it has to carry and from the horizontal loadings
applied near the top of the pole. These latter are exerted by the conductors
as a result of uneven spans, of offsets and bends in the lines, and of
the pressure of wind blowing against them. Both vertical and horizontal
loadings include the effects of ice collecting radially about the conductors.
The vertical force on the pole is the dead weight of the conductors
with their coatings of ice, cross arms, insulators, and associated
hardware. This vertical force exerts a compressive stress that may be
considered uniformly distributed over the cross section of the pole. This
loading, however, is almost always overshadowed by the requirements
of the horizontal loadings, and is usually not given further attention.
Even a very light pole can safely carry the dead weight of a multicircuit,
large-conductor line.
Assume relatively long spans of 200 ft, six primary conductors on two
cross arms, and one heavy neutral (for two three-phase feeders) and three secondary
conductors on a rack; moreover, the cross arms are two sets of double
arms, with insulators and associated pins and hardware.
Assume no. 4/0 copper or 397,500-cmil ACSR (250 cmil × 103 copper
equivalent), the largest usual distribution conductor, with a radial half-inch coating
of ice (at 57 lb/ft3).* Assume a light pole, class 5 pine, with a minimum (top)
circumference of 19 in (disregard the effect of the larger, lower cross-sectional
areas because of taper; i.e., assume a cylindrical column).
From wire manufacturers’ tables, no. 4/0 copper wire has a diameter of
0.528 in and a weight of 640.5 lb per 1000 ft; and 397,500-cmil ACSR has a diam
eter of 0.806 in and a weight of 620.6 lb per 1000 ft. The total weight, including
ice, for 200 ft (100 ft on each side of pole) for the copper conductors is 2120 lb
plus 500 lb allowed for four cross arms, insulators, etc., or 2620 lb. For the ACSR
conductors, it is 2840 lb, plus the same 500 lb, or 3340 lb.
Wind Pressure
In arriving at M, if P is the wind pressure on the length of the conductor,
including its coating of ice, and h the distance or height from the
ground at which the circular cross section is to be determined, the total
moment for the several conductors that may be supported is the sum of
the values of Ph for all the conductors.
To this must be added the wind pressure on the pole itself. Here,
the longitudinal cross section of the pole may be broken down into a
rectangle and a triangle, as indicated.
Equipment on Poles
Poles supporting transformers, capacitors, regulators, switches, or
other equipment support loads which are principally compressive in
nature; the equipment also adds to the wind loading. Since the center
of gravity projects out from the pole, a moment is created on the upper
portion of the pole, pivoting about its lower point of support. Ordinarily,
the pole class selected for supporting the conductors, with its factors
of safety included, is capable of carrying this additional load safely. For
larger-scale equipment installations, however, a pole one class greater
than that adequate for the conductors is specified.
The forces acting on a pole stem from the vertical loading occasioned
by the weight it has to carry and from the horizontal loadings
applied near the top of the pole. These latter are exerted by the conductors
as a result of uneven spans, of offsets and bends in the lines, and of
the pressure of wind blowing against them. Both vertical and horizontal
loadings include the effects of ice collecting radially about the conductors.
The vertical force on the pole is the dead weight of the conductors
with their coatings of ice, cross arms, insulators, and associated
hardware. This vertical force exerts a compressive stress that may be
considered uniformly distributed over the cross section of the pole. This
loading, however, is almost always overshadowed by the requirements
of the horizontal loadings, and is usually not given further attention.
Even a very light pole can safely carry the dead weight of a multicircuit,
large-conductor line.
Assume relatively long spans of 200 ft, six primary conductors on two
cross arms, and one heavy neutral (for two three-phase feeders) and three secondary
conductors on a rack; moreover, the cross arms are two sets of double
arms, with insulators and associated pins and hardware.
Assume no. 4/0 copper or 397,500-cmil ACSR (250 cmil × 103 copper
equivalent), the largest usual distribution conductor, with a radial half-inch coating
of ice (at 57 lb/ft3).* Assume a light pole, class 5 pine, with a minimum (top)
circumference of 19 in (disregard the effect of the larger, lower cross-sectional
areas because of taper; i.e., assume a cylindrical column).
From wire manufacturers’ tables, no. 4/0 copper wire has a diameter of
0.528 in and a weight of 640.5 lb per 1000 ft; and 397,500-cmil ACSR has a diam
eter of 0.806 in and a weight of 620.6 lb per 1000 ft. The total weight, including
ice, for 200 ft (100 ft on each side of pole) for the copper conductors is 2120 lb
plus 500 lb allowed for four cross arms, insulators, etc., or 2620 lb. For the ACSR
conductors, it is 2840 lb, plus the same 500 lb, or 3340 lb.
Wind Pressure
In arriving at M, if P is the wind pressure on the length of the conductor,
including its coating of ice, and h the distance or height from the
ground at which the circular cross section is to be determined, the total
moment for the several conductors that may be supported is the sum of
the values of Ph for all the conductors.
To this must be added the wind pressure on the pole itself. Here,
the longitudinal cross section of the pole may be broken down into a
rectangle and a triangle, as indicated.
Equipment on Poles
Poles supporting transformers, capacitors, regulators, switches, or
other equipment support loads which are principally compressive in
nature; the equipment also adds to the wind loading. Since the center
of gravity projects out from the pole, a moment is created on the upper
portion of the pole, pivoting about its lower point of support. Ordinarily,
the pole class selected for supporting the conductors, with its factors
of safety included, is capable of carrying this additional load safely. For
larger-scale equipment installations, however, a pole one class greater
than that adequate for the conductors is specified.
