DESIGN - Prestressed Concrete Transmission Pole Structures - page 41

applied loads, including prestressing forces. The zero tension capacity is
controlled by the physical dimensions of the pole section and the prestress
level, but it is not a function of concrete strength. Zero tension strength is
similar to the crack reopening moment except that it ignores the frictional
effects of the rupture surfaces within the concrete.
Shear and Torsion
Shear and torsion loads seldom control concrete pole designs. However,
for concrete poles with very short embedments (i.e., socket pile founda-
tions or short rock embedments), the concentrated shear forces from the
moment transfer can be signifi cant and should be accounted for in the
pole design. High shear and torsion loads can be developed from conduc-
tor or guy wire attachments or under broken wire conditions.
Column (Critical Buckling) Loads
When unusually large vertical loads are encountered, such as those that
will be present in guyed structures and H-frames, the buckling capacity
should be checked. A useful reference on the subject is ASCE (1997).
Defl ection
The Purchaser should specify the defl ection limit and the associated
load cases to be considered in the pole design. In locations where appear-
ance or clearance is important, the Purchaser should specify any addi-
tional required defl ection criteria.
Prestress Losses
The magnitude of the prestressing force in the pole is not constant but
decreases with time. This decrease in the prestressing force is referred to
as the prestress loss. Some prestress losses are instantaneous and some
are time-dependent. Instantaneous losses are due to elastic shortening,
anchorage slippage, and friction in the case of post-tensioning. Time-
dependent losses are mainly attributable to shrinkage and creep of con-
crete and steel relaxation.
Experience has shown a detailed analysis of losses for prestressed con-
crete poles may not be necessary except in unusual circumstances. Lump
sum estimates of losses are commonly used. Depending on the materials
used, 15 to 25% for total losses are common design assumptions. A good
source for information on the calculation of prestressing losses is PCI
(2010).