The manual tower spotting template must be used subject to a
“creep” correction for aluminum conductors. Creep is a nonelastic conductor
stretch which continues for the life of the line, with the rate of elongation
decreasing with time.
For example, the creep elongation during the first 6 months
is equal to that of the next 91/2 years. All conductors of all materials are
subject to creep, but to date only aluminum conductors have had intensive
study.
Creep is not substantial in other conductors, but the
conductor manufacturers should be consulted. The IEEE Committee Report,
“Limitations on Stringing and Sagging Conductors,” in the December 1964
Transactions of the IEEE Power Group discusses creep, and the reader should
examine that report.
Creep causes a continuous slow increase in the sag of the
line which must be estimated and allowed for. The aluminum-conductor
manufacturers will furnish creep-estimating curves, and most sag tension
computer programs now available are capable of calculating sags with and
without creep.
These curves are at approximately constant temperatures,
around 15.5 to 21#C, and plot stress against elongation, one curve for each
period of time, 1 h, 1 day, 1 month, 1 year, 10 years, etc. The values are
integrated values for the period and are considered to be reasonable estimates.
The temperature used is a reasonable average of the year’s temperature across
the center of the United States.
Precise values for creep are impossible to determine, since
they vary with both temperature and tension, which are continuously varying
during the life of the line. From Fig. 3 of the committee report in Ref. 53, it
is found that a 1000-ft span of 954,000-cmil 48/7 ACSR when subjected to a
constant tension of approximately 18% of its ultimate strength at a temperature
of 15.5#C will have a sag increase in 1 day of approximately 5.5 in; in 10
days, 13 in; in 1 year, 27 in; in 10 years, 44 in; and in 30 years, 52 in.
Unless it is known that the line will have a life of less
than 10 years, not less than 10 years’ creep should be allowed for. Creep has
come into consideration in transmission-line design only during the past 35
years, and to date no standards have been established for handling it.
Probably the simplest approach is to check all close
clearance points on the profile with a template made with no creep allowance
and to specify higher structures at these points if the addition of liberal
creep sag infringes on the required clearances. It is possible to prestress the
creep out of small conductors, but for large conductors this requires time and
special tensioning facilities not normally available.
Also the time lost in constructing an EHV line will more
than pay for the extra structure height required to compensate for the creep. Prestressing
changes the modulus of elasticity, and this new modulus should be used in the
design.
The vertical weight supported at any structure is the weight
of the length of conductor between
low points of the sag in the two adjacent spans. For bare-conductor
weights, this distance between
low points can be scaled by using a template of the sag at
any desired temperature.
The maximum weight under loaded conditions should be scaled
from a template made for the loaded sags. For most problems, the horizontal
distance may be taken as equal to the conductor length.
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